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Ferryman posted 11-02-2007 04:50 AM ET (US)
When did Mercury go to electronic fuel injection on the 90-HP four-stroke outboard? How difficult is it to strip and clean the carburetors on a 90ELPT four-stroke
I tried searching the Mercury website but nothing could be found.
itl posted 11-02-2007 07:19 AM ET (US)
I believe that EFI came to Mercury 90-HP four-stroke motors in model year 2005.
jimh posted 11-02-2007 08:01 PM ET (US)
For many years Mercury was selling a 90-HP four-stroke motor which utilized a power head they purchased from Yamaha. This model of 90-HP began with a carburetor fuel induction system. This engine was replaced by an all-Mercury 90-HP four-stroke motor called the "FourStroke 90-HP EFI", also called the 90-HP L4NA, also called the "Veradito" 90-HP, and also "Le Verado Petit". These Verado-related motors always had electronic fuel injection.
I have a collection of Mercury outboard catalogues. They show the following:
2001 model year: 90-HP appears to be the Yamaha motor, with carburetor, and no SmartCraft.
This same motor continues to appear through the 2005 model year.
A catalogue marked "Effective July 2006" shows a completely different motor, an all-Mercury 90-HP EFI four-stroke with SmartCraft, This was about the time when Mercury dropped their designation of model years.
This leaves the possibility that there may have been a "2006 model year" Mercury 90-HP based on the old Yamaha motor, but I do not recall if it had carburetor or electronic fuel injection. Perhaps someone can confirm this with either a catalogue citation or by owning that particular motor.
Ferryman posted 11-03-2007 03:13 AM ET (US)
Thanks
Jim
My motor is a 2004 90HP with carbs
When I typed the serial number in it through up the following designation.
MERCURY (4 STROKE B.WHALER)
Serial Number: 1B006745 Model Number: 1F90412DC
The sticky label with S/No shows 90 ELPTS
It has 2005 at the top then on the bottom is has the CW mark and 2004.
My dealer tells me it was made in 2004 as a 2005 model.
I am guessing this is the Yamaka 4 stroke powerhead with carbs.
jimh posted 11-03-2007 08:10 AM ET (US)
After some more research, I see that in 2006 Yamaha introduced EFI to the 90-HP four-stroke, and I also note that in the Boston Whaler catalogue for 2006 the Mercury four-stroke motor is noted as having EFI. It appears that for one year or so Mercury did have a 90-HP EFI four-stroke motor using the Yamaha power head. That was just prior to the introduction of their own made-in-Fond-du-Lac all-Mercury 90-HP EFI motor in July 2006.
There has been a lot written about problems with the carburetors on the Mercury 90-HP four-stroke motor. Here are some prior discussions about problems with the carburetors in the Mercury 90-HP four-stroke motor which was actually made by buying the power head from Yamaha. These motors were sold up through the 2005 model year:

Mercury 90-HP Four-Stroke: Carburetor Problems
http://continuouswave.com/ubb/Forum3/HTML/009936.html
This is a very good discussion with some photographs showing the Mercury motor with the made-in-Japan Yamaha power head and Mikuni carburetors. This article has first-hand advice on how to make repairs to the fuel enrichment circuit of the carburetor.
Mercury 90-HP Four-stroke Unreliable; Carburetor Problems
http://continuouswave.com/ubb/Forum3/HTML/009067.html
Carburetors are used on all sorts of motors, even current day NASCAR race cars which develop 800-HP, and they are a very mature and proven technology. In outboard motors they work well because almost all of the time they're being used at ambient temperatures in the 70- to 90-degree range, and close to sea level, so there are no special problems with fuel-air mixture ratios needing to be changed. If you do a lot of boating on mountain lakes at 6,000-foot elevation in winter, you probably will find that EFI is a big improvement. But if you boat in the summer and close to sea level, carburetors work just fine.
If you read the discussions I mention above, you will see that a number of explanations have been offered to explain the problems with the Mercury 90-HP four-stroke motor. Let me briefly review and comment:
--All carburetors are bad: this is a crazy conclusion. There are millions of engines running just fine with carburetors. You don't have to trade your motor to get EFI to have it run properly.
--Don't blame Mercury, it is Yamaha's fault. This is the Mercury apologists point of view. The irony is that Mercury loved these Yamaha motors so much they even went to court to force Yamaha to keep supplying them to Mercury after Yamaha tried to stop selling them.
--Blame the gasoline. This is a more reasonable conclusion. Modern fuel has many odd blends, and use of ethanol as an oxygenate in the fuel has caused the fuel to be hygroscopic. Water contamination of the fuel is a likely cause of problems. Use of fuel additives to condition the fuel against deterioration and water contamination is often recommended.
--Blame the operator. Poor practices and procedures in storing the motor can cause problems in any motor's fuel system. Carburetor fuel systems often drain themselves when a motor is tilted 90-degrees from the operating position, leaving the carburetor bowls dry or with small volumes of fuel in them. Poor priming can cause problems with restarting. Evaporation of the fuel can leave residue in the fuel system, and this is especially true in a carburetor, where the fuel is openly vented to the atmosphere.
If I owned one of these Mercury 90-HP motors which had a Yamaha power head, I would buy the Service Manual for it and become familiar with the fuel induction system. There often are many hoses and connections in these multiple carburetor fuel systems, and any leaks or blockages can cause problems.
Ferryman posted 11-03-2007 12:20 PM ET (US)
Well my local dealer reckons that my carbs need stripping and cleaning as the boat has not been run all summer, what with shipping it over from the USA.
He also reckons the best way to clean out al the galleries in the carbs is to use an Ultrasonic Cleaning tank and fortunately I just happen to have one that I use for cleaning firearms so that solves one problem.
I am looking to buy a shop manual but the only one I can find is the SELOC manual. How good are these or can anyone recomend a better manual.
I have worked on carbs on both cars and bikes before so am pretty certain I can do these myself
jimh posted 11-03-2007 12:43 PM ET (US)
Here is another long discussion about problems with the carburetors in the Mercury 90-HP four-stroke motor. They appear to require frequent cleaning.

2004 170 Montauk 4 stroke carburetor problems
http://continuouswave.com/ubb/Forum8/HTML/002726.html
My local Mercury dealer is cleaning out old stock on Service Manuals. I believe I saw a copy of the Mercury Service Manual for their 90-HP four-stroke motors on the clearance table for half price, or $30.
Ferryman posted 11-03-2007 01:31 PM ET (US)
Hi Jim
I am actually in the USA next week, and will try to get a manual why I am there.
Does your dealer have a website?
I will also post the VHF radio and Humminbird Fishfinder to boater9384@yahoo.com that I offered to the first member to respond to my offer.
GreatBayNH posted 11-03-2007 06:42 PM ET (US)
I own the 2006 Mercury Fourstroke 90HP EFI with Yamaha power head. Are there any documented issues others have experienced with this engine? I have only owned it two years and have been pretty pleased with its overall performance.
-Seth
swist posted 11-04-2007 07:36 AM ET (US)
I run the same engine and have had no problems in 3 years, BUT you gotta be manic about the fuel system - I installed a 10 micron water separator, I run Stabil and Quickleen year around . When the engine goes in for yearly maintenance, the mechanic pays particular attention to the carbs. They are drained and dried for the offseason. I keep an eye on the gas, but obviosuly have less control over that - Maine still doesn't have Ethanol in gasoline, so that may be helping.
I would comment though, that I would still prefer to have an EFI engine. Offshore in the Northeast is an environment where you need 100% confidence in your equipment. I certainly have that in the Montauk, but with the history of this engine, any time I even imagine it burps, I get a little nervous. In my previous boat with a tried and true large block Yamaha 2-stroke, it never even occured to me that the engine might fail, and indeed it never did.
swist posted 11-04-2007 07:37 AM ET (US)
Also, I sure I know the answer to this one, buit I don't suppose it's possible to retrofit an EFI system to the 90 carbed 4-stroke? I would imagine there are just too many differences.
bigjohn1 posted 11-04-2007 08:01 AM ET (US)
I am betting you could surely retrofit but at what cost? That would be a mighty stiff bill in parts alone. But back to your hypothetical question, It appears that for 1-2 model years, Mercury offered your exact engine with an EFI system. I could certainly imagine it was the EFI system off the 115 that I and a few other forum member now have.
jimh posted 11-04-2007 10:02 AM ET (US)
As mentioned, Yamaha made a similar motor with a 115-HP rating. This motor seems to have had electronic fuel injection. It is not clear to me if these Yamaha made four-stroke motors had a fuel injection system manufactured by Yamaha, or if Mercury fitted their own system onto the power head. It would be good to hear from some experts on this. I tend to think that the fuel injection was from Yamaha, since the Mercury motors lacked SmartCraft controllers which were used on all other Mercury motors with fuel injection made by Mercury.
From what I have seen, there are some advantages to having a fuel injection system, but they are mainly related to starting and storage considerations. I will explain my thinking.
In a carburetor system the fuel in the carburetor bowl is subject to evaporation to the atmosphere when the motor is not operated for long periods of time. When the fuel evaporates it may leave behind a residue which may cause problems in future operation. In contrast, in a fuel injection system the fuel system is generally closed the atmosphere, and there is an attempt to recover fuel vapor and return it to the fuel system by a fuel vapor separator. Also, fuel injection systems tend to be more prone to problems with debris and seem to have been designed with more filtering in the fuel system to block debris.
Carburetor systems also suffer when the engine is tilted up for storage. It is common that fuel can drain from the carburetor bowls when they are tilted. Even in normal operation this causes problems because the fuel system needs extensive re-priming in order to refill the carburetor bowls.
Carburetor motors suffer in comparison to electronic fuel injection because the need frequent re-priming to get them started, and they tend to have fuel evaporate from them during long periods of non-use. Heck, even short periods of non-use!
Electronically controlled fuel injection systems offer the possibility for more sophisticated control of the fuel air mixture, but this is not always taken advantage in all units. Many electronic fuel injection units are not particularly sophisticated and lack any sort of feedback circuit from the exhaust. They operate in an open control loop mode and just meter fuel according to pre-set parameters. Some systems have inputs for temperature sensors and barometric pressure sensors to make dynamic adjustment to the fuel air mixture. But there is no guarantee that every electronic fuel injection system has such sophistication, and some, particularly early systems which do not appear to have any sort of re-programmable stored instruction set, probably do not have much sophistication. They just meter fuel based on throttle input and not much else.
Both electronic fuel injection and carburetor systems offer enhancements for warm up periods and for acceleration enrichment. In a carburetor the fuel-air mixture ratio is fixed by the size of the jets, while in an electronic fuel injection system it is fixed by the design of the controller.
Once a motor is warmed up, operating in normal ambient temperatures, and at normal elevations (like sea level), I do not think there is a world of difference between carburetors and electronic fuel injection as fuel delivery system, and much depends on the design of each system. Overall, electronic fuel injection is generally preferred for the benefits it conveys as mentioned above.
It seems that in four-stroke outboard motors with multiple carburetors and moderate horsepower, there are more reported problems with carburetor clogging than usually heard. This may be due to the very small jet sizes being used. In the Yamaha engine there are four cylinders and each has its own carburetor. This is unlike the normal automobile engine where a single carburetor serves all cylinders. Each outboard carburetor must be smaller and also have smaller jets, particularly those used for idle speed operation. On this basis it is easy to see that there could be more problems related to blockage.
swist posted 11-04-2007 12:13 PM ET (US)
Agree with much of what you say - I had also thought that the reason for multible carbs in outboards was redundancy, but that doesn't seem to be the case if I understand the many threads detailing with the problems people have had - if they have a carb problem, the whole engine seems to go out. It would seem that whatever ails one carb simultaneously hits the others soomer or later, usually sooner.
jimh posted 11-04-2007 12:20 PM ET (US)
In a two-stroke engine, multiple carburetors are very common and necessary, I believe. Each cylinder in a two-stroke operates like its own little self-contained engine. So each one gets its own carburetor.
In four-stroke outboard motors it seems like designers have gone to these long intake manifold paths with separate chambers for each cylinder. This is probably done to get better performance from the motor--they're more highly tuned than the four-stroke motors in your old Chevrolet. So, again, you wind up with one carburetor per cylinder. I don't understand why it is so common in four-stroke outboards and so unusual in four-stroke automobile engines. Heck, even my old GMC fuel-injected engine only has one fuel injector serving all eight cylinders.
Maybe some knowledgeable engine designer can explain why marine four-stroke motors love individual carburetors and also individual fuel injectors.
boatdryver posted 11-08-2007 12:11 AM ET (US)
Although I'm not an engine designer, I think the reason for one carburetor per cylinder in outboards might be because of the difficulty in acheiving a consistent air-fuel mixture in a vertically oriented intake manifold long enough to serve several cylinders.
JimL
sosmerc posted 11-08-2007 11:41 AM ET (US)
Ah....the old Merc "tower of power"...6 cylinders inline being fed by 3 carbs.....the genius of old Karl!
fisherman posted 11-14-2007 05:06 PM ET (US)
I purchased my 170 in May 2006. It has the Merc 90 str efi serial # 1b346183. Does this motor have a Yamaha Powerhead or is it a true Merc?
Thanks
Ray
jimh posted 11-14-2007 11:11 PM ET (US)
All the Mercury 90-HP four-stroke motors were made with power heads from Yamaha until the introduction of the "Veradito" motors. I believe the Veradito motors began to appear in early 2007.
But the process of identification is made difficult because there does not seem to be any clear model name or model number distinction for these two completely different models. They are usually just referred to as "90-HP FourStroke" models in Mercury literature.
You can easily tell the difference because the cowling shape of the new Verado-derived 90-HP "FourStroke EFI" or Veradito motor is very different from the cowling shape of the other 90-HP motor which utilized the Yamaha power head.
If you have a c.2006 90-HP four-stroke Mercury motor and it is designated with EFI, you probably have the Yamaha power head with the Yamaha fuel-injection system.
fisherman posted 11-15-2007 09:50 AM ET (US)

Thanks Jim.
Ray
fisherman posted 11-22-2007 03:47 PM ET (US)
Jim:
Not that I questioned your wisdom, but since you threw in the protective word "probably," I decided to send an e-mail to Mercury. Their reply was as follows:
"The 2006 90EFI 4 stroke did not yet have the capability for the Smartcraft System. At that time, we did have a joint-venture with Yahama. Regards, Mercury Service".
Ray

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Author	Topic: Mercury 90-HP Four-stroke Change to EFI
Ferryman	posted 11-02-2007 04:50 AM ET (US) When did Mercury go to electronic fuel injection on the 90-HP four-stroke outboard? How difficult is it to strip and clean the carburetors on a 90ELPT four-stroke I tried searching the Mercury website but nothing could be found.
itl	posted 11-02-2007 07:19 AM ET (US) I believe that EFI came to Mercury 90-HP four-stroke motors in model year 2005.
jimh	posted 11-02-2007 08:01 PM ET (US) For many years Mercury was selling a 90-HP four-stroke motor which utilized a power head they purchased from Yamaha. This model of 90-HP began with a carburetor fuel induction system. This engine was replaced by an all-Mercury 90-HP four-stroke motor called the "FourStroke 90-HP EFI", also called the 90-HP L4NA, also called the "Veradito" 90-HP, and also "Le Verado Petit". These Verado-related motors always had electronic fuel injection. I have a collection of Mercury outboard catalogues. They show the following: 2001 model year: 90-HP appears to be the Yamaha motor, with carburetor, and no SmartCraft. This same motor continues to appear through the 2005 model year. A catalogue marked "Effective July 2006" shows a completely different motor, an all-Mercury 90-HP EFI four-stroke with SmartCraft, This was about the time when Mercury dropped their designation of model years. This leaves the possibility that there may have been a "2006 model year" Mercury 90-HP based on the old Yamaha motor, but I do not recall if it had carburetor or electronic fuel injection. Perhaps someone can confirm this with either a catalogue citation or by owning that particular motor.
Ferryman	posted 11-03-2007 03:13 AM ET (US) Thanks Jim My motor is a 2004 90HP with carbs When I typed the serial number in it through up the following designation. MERCURY (4 STROKE B.WHALER) Serial Number: 1B006745 Model Number: 1F90412DC The sticky label with S/No shows 90 ELPTS It has 2005 at the top then on the bottom is has the CW mark and 2004. My dealer tells me it was made in 2004 as a 2005 model. I am guessing this is the Yamaka 4 stroke powerhead with carbs.
jimh	posted 11-03-2007 08:10 AM ET (US) After some more research, I see that in 2006 Yamaha introduced EFI to the 90-HP four-stroke, and I also note that in the Boston Whaler catalogue for 2006 the Mercury four-stroke motor is noted as having EFI. It appears that for one year or so Mercury did have a 90-HP EFI four-stroke motor using the Yamaha power head. That was just prior to the introduction of their own made-in-Fond-du-Lac all-Mercury 90-HP EFI motor in July 2006. There has been a lot written about problems with the carburetors on the Mercury 90-HP four-stroke motor. Here are some prior discussions about problems with the carburetors in the Mercury 90-HP four-stroke motor which was actually made by buying the power head from Yamaha. These motors were sold up through the 2005 model year: Mercury 90-HP Four-Stroke: Carburetor Problems http://continuouswave.com/ubb/Forum3/HTML/009936.html This is a very good discussion with some photographs showing the Mercury motor with the made-in-Japan Yamaha power head and Mikuni carburetors. This article has first-hand advice on how to make repairs to the fuel enrichment circuit of the carburetor. Mercury 90-HP Four-stroke Unreliable; Carburetor Problems http://continuouswave.com/ubb/Forum3/HTML/009067.html Carburetors are used on all sorts of motors, even current day NASCAR race cars which develop 800-HP, and they are a very mature and proven technology. In outboard motors they work well because almost all of the time they're being used at ambient temperatures in the 70- to 90-degree range, and close to sea level, so there are no special problems with fuel-air mixture ratios needing to be changed. If you do a lot of boating on mountain lakes at 6,000-foot elevation in winter, you probably will find that EFI is a big improvement. But if you boat in the summer and close to sea level, carburetors work just fine. If you read the discussions I mention above, you will see that a number of explanations have been offered to explain the problems with the Mercury 90-HP four-stroke motor. Let me briefly review and comment: --All carburetors are bad: this is a crazy conclusion. There are millions of engines running just fine with carburetors. You don't have to trade your motor to get EFI to have it run properly. --Don't blame Mercury, it is Yamaha's fault. This is the Mercury apologists point of view. The irony is that Mercury loved these Yamaha motors so much they even went to court to force Yamaha to keep supplying them to Mercury after Yamaha tried to stop selling them. --Blame the gasoline. This is a more reasonable conclusion. Modern fuel has many odd blends, and use of ethanol as an oxygenate in the fuel has caused the fuel to be hygroscopic. Water contamination of the fuel is a likely cause of problems. Use of fuel additives to condition the fuel against deterioration and water contamination is often recommended. --Blame the operator. Poor practices and procedures in storing the motor can cause problems in any motor's fuel system. Carburetor fuel systems often drain themselves when a motor is tilted 90-degrees from the operating position, leaving the carburetor bowls dry or with small volumes of fuel in them. Poor priming can cause problems with restarting. Evaporation of the fuel can leave residue in the fuel system, and this is especially true in a carburetor, where the fuel is openly vented to the atmosphere. If I owned one of these Mercury 90-HP motors which had a Yamaha power head, I would buy the Service Manual for it and become familiar with the fuel induction system. There often are many hoses and connections in these multiple carburetor fuel systems, and any leaks or blockages can cause problems.
Ferryman	posted 11-03-2007 12:20 PM ET (US) Well my local dealer reckons that my carbs need stripping and cleaning as the boat has not been run all summer, what with shipping it over from the USA. He also reckons the best way to clean out al the galleries in the carbs is to use an Ultrasonic Cleaning tank and fortunately I just happen to have one that I use for cleaning firearms so that solves one problem. I am looking to buy a shop manual but the only one I can find is the SELOC manual. How good are these or can anyone recomend a better manual. I have worked on carbs on both cars and bikes before so am pretty certain I can do these myself
jimh	posted 11-03-2007 12:43 PM ET (US) Here is another long discussion about problems with the carburetors in the Mercury 90-HP four-stroke motor. They appear to require frequent cleaning. 2004 170 Montauk 4 stroke carburetor problems http://continuouswave.com/ubb/Forum8/HTML/002726.html My local Mercury dealer is cleaning out old stock on Service Manuals. I believe I saw a copy of the Mercury Service Manual for their 90-HP four-stroke motors on the clearance table for half price, or $30.
Ferryman	posted 11-03-2007 01:31 PM ET (US) Hi Jim I am actually in the USA next week, and will try to get a manual why I am there. Does your dealer have a website? I will also post the VHF radio and Humminbird Fishfinder to boater9384@yahoo.com that I offered to the first member to respond to my offer.
GreatBayNH	posted 11-03-2007 06:42 PM ET (US) I own the 2006 Mercury Fourstroke 90HP EFI with Yamaha power head. Are there any documented issues others have experienced with this engine? I have only owned it two years and have been pretty pleased with its overall performance. -Seth
swist	posted 11-04-2007 07:36 AM ET (US) I run the same engine and have had no problems in 3 years, BUT you gotta be manic about the fuel system - I installed a 10 micron water separator, I run Stabil and Quickleen year around . When the engine goes in for yearly maintenance, the mechanic pays particular attention to the carbs. They are drained and dried for the offseason. I keep an eye on the gas, but obviosuly have less control over that - Maine still doesn't have Ethanol in gasoline, so that may be helping. I would comment though, that I would still prefer to have an EFI engine. Offshore in the Northeast is an environment where you need 100% confidence in your equipment. I certainly have that in the Montauk, but with the history of this engine, any time I even imagine it burps, I get a little nervous. In my previous boat with a tried and true large block Yamaha 2-stroke, it never even occured to me that the engine might fail, and indeed it never did.
swist	posted 11-04-2007 07:37 AM ET (US) Also, I sure I know the answer to this one, buit I don't suppose it's possible to retrofit an EFI system to the 90 carbed 4-stroke? I would imagine there are just too many differences.
bigjohn1	posted 11-04-2007 08:01 AM ET (US) I am betting you could surely retrofit but at what cost? That would be a mighty stiff bill in parts alone. But back to your hypothetical question, It appears that for 1-2 model years, Mercury offered your exact engine with an EFI system. I could certainly imagine it was the EFI system off the 115 that I and a few other forum member now have.
jimh	posted 11-04-2007 10:02 AM ET (US) As mentioned, Yamaha made a similar motor with a 115-HP rating. This motor seems to have had electronic fuel injection. It is not clear to me if these Yamaha made four-stroke motors had a fuel injection system manufactured by Yamaha, or if Mercury fitted their own system onto the power head. It would be good to hear from some experts on this. I tend to think that the fuel injection was from Yamaha, since the Mercury motors lacked SmartCraft controllers which were used on all other Mercury motors with fuel injection made by Mercury. From what I have seen, there are some advantages to having a fuel injection system, but they are mainly related to starting and storage considerations. I will explain my thinking. In a carburetor system the fuel in the carburetor bowl is subject to evaporation to the atmosphere when the motor is not operated for long periods of time. When the fuel evaporates it may leave behind a residue which may cause problems in future operation. In contrast, in a fuel injection system the fuel system is generally closed the atmosphere, and there is an attempt to recover fuel vapor and return it to the fuel system by a fuel vapor separator. Also, fuel injection systems tend to be more prone to problems with debris and seem to have been designed with more filtering in the fuel system to block debris. Carburetor systems also suffer when the engine is tilted up for storage. It is common that fuel can drain from the carburetor bowls when they are tilted. Even in normal operation this causes problems because the fuel system needs extensive re-priming in order to refill the carburetor bowls. Carburetor motors suffer in comparison to electronic fuel injection because the need frequent re-priming to get them started, and they tend to have fuel evaporate from them during long periods of non-use. Heck, even short periods of non-use! Electronically controlled fuel injection systems offer the possibility for more sophisticated control of the fuel air mixture, but this is not always taken advantage in all units. Many electronic fuel injection units are not particularly sophisticated and lack any sort of feedback circuit from the exhaust. They operate in an open control loop mode and just meter fuel according to pre-set parameters. Some systems have inputs for temperature sensors and barometric pressure sensors to make dynamic adjustment to the fuel air mixture. But there is no guarantee that every electronic fuel injection system has such sophistication, and some, particularly early systems which do not appear to have any sort of re-programmable stored instruction set, probably do not have much sophistication. They just meter fuel based on throttle input and not much else. Both electronic fuel injection and carburetor systems offer enhancements for warm up periods and for acceleration enrichment. In a carburetor the fuel-air mixture ratio is fixed by the size of the jets, while in an electronic fuel injection system it is fixed by the design of the controller. Once a motor is warmed up, operating in normal ambient temperatures, and at normal elevations (like sea level), I do not think there is a world of difference between carburetors and electronic fuel injection as fuel delivery system, and much depends on the design of each system. Overall, electronic fuel injection is generally preferred for the benefits it conveys as mentioned above. It seems that in four-stroke outboard motors with multiple carburetors and moderate horsepower, there are more reported problems with carburetor clogging than usually heard. This may be due to the very small jet sizes being used. In the Yamaha engine there are four cylinders and each has its own carburetor. This is unlike the normal automobile engine where a single carburetor serves all cylinders. Each outboard carburetor must be smaller and also have smaller jets, particularly those used for idle speed operation. On this basis it is easy to see that there could be more problems related to blockage.
swist	posted 11-04-2007 12:13 PM ET (US) Agree with much of what you say - I had also thought that the reason for multible carbs in outboards was redundancy, but that doesn't seem to be the case if I understand the many threads detailing with the problems people have had - if they have a carb problem, the whole engine seems to go out. It would seem that whatever ails one carb simultaneously hits the others soomer or later, usually sooner.
jimh	posted 11-04-2007 12:20 PM ET (US) In a two-stroke engine, multiple carburetors are very common and necessary, I believe. Each cylinder in a two-stroke operates like its own little self-contained engine. So each one gets its own carburetor. In four-stroke outboard motors it seems like designers have gone to these long intake manifold paths with separate chambers for each cylinder. This is probably done to get better performance from the motor--they're more highly tuned than the four-stroke motors in your old Chevrolet. So, again, you wind up with one carburetor per cylinder. I don't understand why it is so common in four-stroke outboards and so unusual in four-stroke automobile engines. Heck, even my old GMC fuel-injected engine only has one fuel injector serving all eight cylinders. Maybe some knowledgeable engine designer can explain why marine four-stroke motors love individual carburetors and also individual fuel injectors.
boatdryver	posted 11-08-2007 12:11 AM ET (US) Although I'm not an engine designer, I think the reason for one carburetor per cylinder in outboards might be because of the difficulty in acheiving a consistent air-fuel mixture in a vertically oriented intake manifold long enough to serve several cylinders. JimL
sosmerc	posted 11-08-2007 11:41 AM ET (US) Ah....the old Merc "tower of power"...6 cylinders inline being fed by 3 carbs.....the genius of old Karl!
fisherman	posted 11-14-2007 05:06 PM ET (US) I purchased my 170 in May 2006. It has the Merc 90 str efi serial # 1b346183. Does this motor have a Yamaha Powerhead or is it a true Merc? Thanks Ray
jimh	posted 11-14-2007 11:11 PM ET (US) All the Mercury 90-HP four-stroke motors were made with power heads from Yamaha until the introduction of the "Veradito" motors. I believe the Veradito motors began to appear in early 2007. But the process of identification is made difficult because there does not seem to be any clear model name or model number distinction for these two completely different models. They are usually just referred to as "90-HP FourStroke" models in Mercury literature. You can easily tell the difference because the cowling shape of the new Verado-derived 90-HP "FourStroke EFI" or Veradito motor is very different from the cowling shape of the other 90-HP motor which utilized the Yamaha power head. If you have a c.2006 90-HP four-stroke Mercury motor and it is designated with EFI, you probably have the Yamaha power head with the Yamaha fuel-injection system.
fisherman	posted 11-15-2007 09:50 AM ET (US) Thanks Jim. Ray
fisherman	posted 11-22-2007 03:47 PM ET (US) Jim: Not that I questioned your wisdom, but since you threw in the protective word "probably," I decided to send an e-mail to Mercury. Their reply was as follows: "The 2006 90EFI 4 stroke did not yet have the capability for the Smartcraft System. At that time, we did have a joint-venture with Yahama. Regards, Mercury Service". Ray