1998 Mercury 100 2+2 Motor: Rough Acceleration

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Author Topic:   1998 Mercury 100 2+2 Motor: Rough Acceleration

Mike Ault posted 05-30-2008 12:35 AM ET (US)
I recently purchased a VIP Fish Ski with a 1998 Mercury 100-HP two-cycle outboard motor. I am having some motor acceleration [problems]. The mechanic says that this model was designed to run on the top two carburetors until 1,750-RPM. After exceeding that [engine speed], all four carburetors [are] operating. [Asked for a confirmation of the mechanic's representation of the operation of the Mercury 100-HP motor using the 2+2 cylinder technique.] [How can the problem of] rough (uneven) acceleration [be solved]?
Is any factory literature and manuals available for this engine?
Mike
jimh posted 05-30-2008 08:03 AM ET (US)
Mercury publishes service manuals for their engines. You should be able to get a manual for a 1998 engine. I recently got one for a 1976 Mercury, so I think they still have them available for older engines. Usually you can buy the manual from an authorized Mercury dealer's parts and service department. The Mercury service manuals are nicely written and well illustrated.
The mechanic's description of the engine's operation is correct. It is design to provide a simple implementation of displacement on demand. At lower speeds it runs on only two cylinders. When the engine speed increases the other two cylinders begin operating. As the mechanic explained, this is accomplished by the carburetor design. I believe that there are no low-speed fuel orifices ("jets") on two of the carburetors. Those cylinders do not get any fuel until the engine speed reaches the transition point.
This engine is somewhat notorious for having an abrupt transition when it changes to four-cylinder operation from two-cylinder operation. These engines are known as a 2+2 engine. The technique of shutting off cylinders in multiple cylinder two-cycle engines is fairly common, although this engine is somewhat unique in shutting off half the cylinders. Other engines which used this technique, notably some Yamaha two-cycle engines with six cylinders, only shut off one or two cylinders, and this made the change much less noticeable.
Like many two-cycles engines with somewhat primitive fuel and ignition systems, this engine has a lot of raw power but it is not known for smooth operation or freedom from carburetor problems.
Tohsgib posted 05-30-2008 11:15 AM ET (US)
It would really help if you described what "rough" is. They all are a tad rough in the transition period and most find that nailing it until you plane makes it much smoother.
bill705 posted 05-30-2008 12:28 PM ET (US)
Mike:
I've got the same engine and I would recommend getting the service manual and going through the link & sync procedure.
I don't know if the engines come from the factory with the oil pump set to supply extra oil or not since they have you run the first 6 tanks of gas premixed to supply extra oil.
There are two scribed lines on the oil pump arm, one of which should align with a mark on the pump body when the throttle is closed. Mine was setting right between the two so it was getting more oil than it should. I was a little leery about reducing the oil supply but that's been three or four years ago now and it's still doing fine.
The low speed jets on the lower two carbs are fixed and they run too lean to support combustion but they get enough for lubrication at the low speed.
There is an accelerator pump on the side of the engine that could need adjustment. The diaphragm on mine split but you would know that because it sprays fuel all over the engine.
Bill
Mike Ault posted 05-30-2008 01:15 PM ET (US)
Tohsgib:
"Rough" means very uneven accelleration from idle progressing through to full throtle. From replies, I now assume that this is due to the transition from 2 to 4 cylinders. I was left to wonder if this transition can be "smoothed-out" through tuning.
As a "though within a thought" here, I wonder if this 2+2 means you have a decent trolling capability at idle due to reduced hp delivered to the prop. Any thoughts?
Mike
brisboats posted 05-31-2008 04:58 PM ET (US)
Mike, That motor is known to be excellent at trolling just rough at the transition. Others have commented that with their particular hull the motor isn't an ideal match because the best cruise speed falls in the range where the 2+2 transition occurs. Tune it, try it and see if it works for you.
Brian
jimh posted 06-01-2008 09:17 AM ET (US)
I believe the ignition is provided to all cylinders all the time. If a cylinder has a fuel charge that will ignite, the cylinder is "on," and if a cylinder has a fuel charge what won't ignite, that cylinder is "off." The fuel charge is determined by the carburetor design. Mercury has implemented some technique where the carburetors for two cylinders don't produce a combustible mixture until the engine speed is at a certain RPM range. I don't know the exact details of this, but it is often cited as being related to the arrangement of the fuel orifices, in particular to the lack of any idle speed jets. I don't know how you would adjust ("tune") this engine to run any differently.
The speed range where the engine is designed to transition to four cylinders from two cylinders occurs at an engine speed that is normally not useful for most moderate speed planing boats. The typical outboard boat will be in displacement mode with engine speeds from idle to about 1,500-RPM. Above 1,500-RPM most boats are in a transition to hydroplane from displacement mode. This is precisely the region where more power is needed, so the motor kicks in two more cylinders. Once on plane most boats typically will be running with the engine at 2,500-RPM or higher. With the Mercury 2+2 design this should be enough engine speed to keep the motor in the four-cylinder mode. The result is that in most situations on outboard boats the engine is seldom asked to operate in the region of 1,500 to 2,500-RPM, so having a bit of rough running in that range is not a problem.
If your boat set-up requires that you have to run the engine continually in the range of 1,500 to 2,500-RPM, the Mercury 2+2 is not going to be a good engine for you. You might consider changing the propeller pitch to see if you can adjust this situation so the boat doesn't need that engine speed.
In more modern motors the technique of operating a portion of an engine's cylinders in a different combustion mode than the other cylinders is stil used, but it is now done with much more sophisticated controls. The result is the transition is much less noticeable.

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Author	Topic: 1998 Mercury 100 2+2 Motor: Rough Acceleration
Mike Ault	posted 05-30-2008 12:35 AM ET (US) I recently purchased a VIP Fish Ski with a 1998 Mercury 100-HP two-cycle outboard motor. I am having some motor acceleration [problems]. The mechanic says that this model was designed to run on the top two carburetors until 1,750-RPM. After exceeding that [engine speed], all four carburetors [are] operating. [Asked for a confirmation of the mechanic's representation of the operation of the Mercury 100-HP motor using the 2+2 cylinder technique.] [How can the problem of] rough (uneven) acceleration [be solved]? Is any factory literature and manuals available for this engine? Mike
jimh	posted 05-30-2008 08:03 AM ET (US) Mercury publishes service manuals for their engines. You should be able to get a manual for a 1998 engine. I recently got one for a 1976 Mercury, so I think they still have them available for older engines. Usually you can buy the manual from an authorized Mercury dealer's parts and service department. The Mercury service manuals are nicely written and well illustrated. The mechanic's description of the engine's operation is correct. It is design to provide a simple implementation of displacement on demand. At lower speeds it runs on only two cylinders. When the engine speed increases the other two cylinders begin operating. As the mechanic explained, this is accomplished by the carburetor design. I believe that there are no low-speed fuel orifices ("jets") on two of the carburetors. Those cylinders do not get any fuel until the engine speed reaches the transition point. This engine is somewhat notorious for having an abrupt transition when it changes to four-cylinder operation from two-cylinder operation. These engines are known as a 2+2 engine. The technique of shutting off cylinders in multiple cylinder two-cycle engines is fairly common, although this engine is somewhat unique in shutting off half the cylinders. Other engines which used this technique, notably some Yamaha two-cycle engines with six cylinders, only shut off one or two cylinders, and this made the change much less noticeable. Like many two-cycles engines with somewhat primitive fuel and ignition systems, this engine has a lot of raw power but it is not known for smooth operation or freedom from carburetor problems.
Tohsgib	posted 05-30-2008 11:15 AM ET (US) It would really help if you described what "rough" is. They all are a tad rough in the transition period and most find that nailing it until you plane makes it much smoother.
bill705	posted 05-30-2008 12:28 PM ET (US) Mike: I've got the same engine and I would recommend getting the service manual and going through the link & sync procedure. I don't know if the engines come from the factory with the oil pump set to supply extra oil or not since they have you run the first 6 tanks of gas premixed to supply extra oil. There are two scribed lines on the oil pump arm, one of which should align with a mark on the pump body when the throttle is closed. Mine was setting right between the two so it was getting more oil than it should. I was a little leery about reducing the oil supply but that's been three or four years ago now and it's still doing fine. The low speed jets on the lower two carbs are fixed and they run too lean to support combustion but they get enough for lubrication at the low speed. There is an accelerator pump on the side of the engine that could need adjustment. The diaphragm on mine split but you would know that because it sprays fuel all over the engine. Bill
Mike Ault	posted 05-30-2008 01:15 PM ET (US) Tohsgib: "Rough" means very uneven accelleration from idle progressing through to full throtle. From replies, I now assume that this is due to the transition from 2 to 4 cylinders. I was left to wonder if this transition can be "smoothed-out" through tuning. As a "though within a thought" here, I wonder if this 2+2 means you have a decent trolling capability at idle due to reduced hp delivered to the prop. Any thoughts? Mike
brisboats	posted 05-31-2008 04:58 PM ET (US) Mike, That motor is known to be excellent at trolling just rough at the transition. Others have commented that with their particular hull the motor isn't an ideal match because the best cruise speed falls in the range where the 2+2 transition occurs. Tune it, try it and see if it works for you. Brian
jimh	posted 06-01-2008 09:17 AM ET (US) I believe the ignition is provided to all cylinders all the time. If a cylinder has a fuel charge that will ignite, the cylinder is "on," and if a cylinder has a fuel charge what won't ignite, that cylinder is "off." The fuel charge is determined by the carburetor design. Mercury has implemented some technique where the carburetors for two cylinders don't produce a combustible mixture until the engine speed is at a certain RPM range. I don't know the exact details of this, but it is often cited as being related to the arrangement of the fuel orifices, in particular to the lack of any idle speed jets. I don't know how you would adjust ("tune") this engine to run any differently. The speed range where the engine is designed to transition to four cylinders from two cylinders occurs at an engine speed that is normally not useful for most moderate speed planing boats. The typical outboard boat will be in displacement mode with engine speeds from idle to about 1,500-RPM. Above 1,500-RPM most boats are in a transition to hydroplane from displacement mode. This is precisely the region where more power is needed, so the motor kicks in two more cylinders. Once on plane most boats typically will be running with the engine at 2,500-RPM or higher. With the Mercury 2+2 design this should be enough engine speed to keep the motor in the four-cylinder mode. The result is that in most situations on outboard boats the engine is seldom asked to operate in the region of 1,500 to 2,500-RPM, so having a bit of rough running in that range is not a problem. If your boat set-up requires that you have to run the engine continually in the range of 1,500 to 2,500-RPM, the Mercury 2+2 is not going to be a good engine for you. You might consider changing the propeller pitch to see if you can adjust this situation so the boat doesn't need that engine speed. In more modern motors the technique of operating a portion of an engine's cylinders in a different combustion mode than the other cylinders is stil used, but it is now done with much more sophisticated controls. The result is the transition is much less noticeable.