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Author Topic:   Engine coughs at mid power
ChrisCT posted 06-01-2002 04:53 PM ET (US)   Profile for ChrisCT   Send Email to ChrisCT  
Checked plugs on 85 70 HP Yammy, plug wires, fuel line etc... All seem fine. Will check plugs themselves to see if fouled tomorrow. Removed water separator/fuel filter - and it seemed to be full to the top with gas. Does that hold water and gas with water on bottom? Was thinking that it may have water in it and I should dump it - but since it had a gas odor (not too strong) I did not know where or if I should dump it...

I treat the engine nice, mechanic thinks its good, give it premium fuel, idle it a lot to see if I can run it through this new issue etc. It started last weekend and not sure what to try next. Can get new plug wires (53 bucks a piece) but mechanic thinks (based on speaking w/ me) that is not the issue. Any ideas - considering I will have it only to end of season? Will replace the easy, probable causes if you all have ideas. Should I go buy the spark plug testers and compression tools?

By the way when I do get it past the rough part 7-12 mph it runs great - also runs great up to that speed. I am stumped and don't want to take it out of the water if I don't have to. I need a mechanic that makes house calls to Stratford. I have no fears of getting stuck w/ my kicker but am frustrated.

Clark Roberts posted 06-01-2002 05:00 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Might be low speed jet clogged on one of carbs.... above about 1600rpm the high speed jet takes over... just a thought! Clark
ChrisCT posted 06-09-2002 06:21 PM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Update - Motor seemed to work through (*so far) whatever was making it caugh at mid power. Yesterday first run started w/ same problem, but after getting out to some a good fishing spot it stopped doing it. Today - no problems at all. If it was a clog in the carbs - could it have cleared itself out?
Just wondering if the sudden better running gives any new ideas of the issue?

Also - what does one do with the water fuel separator when there is nothing to loosen at bottom. Dump it to be sure it is all gas? Where?

By the way caught Sea Robbins (many), Fluke - the thin one w/ teeth (1), Harbor Blues (5), Striper (1). First success on the boat for us!!! All caught at base of Housatonic River about 3/4 of a mile out near a rocky area.

ChrisCT posted 07-26-2002 07:29 AM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Finally took boat out of water to take it to mechanic - yes it still coughs at mid power. Apparently nothing wrong can be found. Compression good (a little low but normal for 17 yr old engine, a little gunk in fuel filter, but nothing else wrong.

When picking up boat my mechanic suggested I try a 15 inch prop instead of a 17 inch (received 2 17's w/ boat). When he asked about rpms - I told him that I noticed it never gets over 3800-4000 or so and we now think it may just be having a problem getting to correct rpms because of prop size combined w/ older motor. Picking up a 15 inch today and will try it this weekend. He said that hopefully the 15 may solve the stalling/coughing issue and also get my rpms up at near full power. Anyone ever heard of this issue before?

John from Madison CT posted 07-26-2002 08:07 AM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     

RPM's that low with a 17" prop, have nothing to do with the 17" Prop.

If you are only running 3800-4000RPMs (MAX), then there is something wrong with the motor (or possibly the Tach).

That alone should have told the mechanic something..loud and clear.

The Top RPM's for that motor are really 5K-5500RPM's. My 90hp Ran at 5300-5400RPM's. A 15" Prop will only give you 400 more RPM's , still way below spec..

Again, this has nothing to do with your Prop.. Find a competant mechanic. His comment about the 15" prop solving the coughing issue proves he's a moron.

Are you coming to the Rendevous this weekend?


John from Madison CT

jimh posted 07-26-2002 11:40 AM ET (US)     Profile for jimh  Send Email to jimh     
What kind of Whaler are your trying to push with the 70-HP Yamaha?
ChrisCT posted 07-26-2002 12:16 PM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Wish I was comfortable enough w/ the motor to come to Rendevous. Will make next one as new motor coming in mid September.

Jim - Its just a 1976 17ft Montauk.

Will test it w/ new prop anyway - nothing else has worked. So desperate I'll try anything. Wife - is getting to point where she is ready to let me buy a new motor now - but am awaiting a 4 month lease return from the local yacht club.

Also - fuel line seems fine, once past that 6-9 mph barrier motor runs like a charm, plugs are fine, super unleaded gas, new water filter, tuneup done, idles perfect, neutral high idle perfect - I'm stumped and so is mechanic... If this doesn't work - I'll beg the Yamaha mechanic to test it in water - approx 2 miles from their shop. Regular mechanic is merc, johnson, evinrude - father/son team. Went to high school w/ the son about 12 years ago. If prop seems to help or if its something easy the Yamaha service ends of finding will update all asap...

Bigshot posted 07-26-2002 01:08 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Something fuel related, get carbs cleaned and run a compression test.....again. The separator seems to me like the problem. Pour it into a glass jar and see if it has water in it. water will cause surging. Are throttle cables going all the way to make full throttle? Surging can also be worn lower unit gears.
Sammy posted 07-27-2002 09:02 AM ET (US)     Profile for Sammy  Send Email to Sammy     
I think Bigshot is on to the right area(s). Only other thing to add to his suggestion is trying a fuel additive like Sea Foam to clean out fuel system/carbs. Do the inexpensive/simple stuff first and work your way up.

Don't know anything about the Yamaha carbs but the 'stumble' during acceleration at low-to-mid range RPMs on MercCruiser I/Os with the GM block and Rochester carb is usually related to the carb's accelerator pump.

Bigshot posted 07-29-2002 10:08 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Try Techron.
Gep posted 07-29-2002 10:32 AM ET (US)     Profile for Gep  Send Email to Gep     
Where can I get Techron?
Bigshot posted 07-29-2002 11:48 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Any autoparts store, K-Fart, Wally world, bottle. Try Berryman's B-12 too.
ChrisCT posted 08-05-2002 07:32 PM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Darn - You guys were right again... New prob got me more RPM's but same coughing etc... Gonna see if I can get a more local repair shop tp come out for a test drive. Probably try Techron too. I hate it when you guys are right and my mechanic isn't - wish some of you were in the business.
ChrisCT posted 08-18-2002 08:19 PM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Update! Tapping the choke toggle switch a few times seems to get motor over that tough period and to faster speeds... This a bad practice for my motor? This indicate any specific problems or solutions?

I tap it when I'm about to try to rev up - or sometimes when its about to die. By the way caught 20 bluefish w/ my brother in law in about 2 hours! They were everwhere. from the base of the Housatonic river over toward Charles Island in Milford 5-7PM. A few waves (schools) went by in that time.

David Ratusnik posted 08-18-2002 08:59 PM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
gep Techron = Chevron gas station. In bottle or the gas. D
simonmeridew posted 08-19-2002 07:28 AM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
Does Techron work well on OMC's? '99 Johnson 70.
Clark Roberts posted 08-19-2002 07:38 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Chris, have mechanic check reed valves for blowback... could be a cracked reed and/or a blown gasket at reed plate...????
etstmeb posted 08-19-2002 09:38 AM ET (US)     Profile for etstmeb  Send Email to etstmeb     
You have water in your fuel, change the seperater and give your tank a shot of stablizer.
Hank posted 08-22-2002 10:52 PM ET (US)     Profile for Hank  Send Email to Hank     
The fact that you seem to be able to overcome the stumbling with a shot from your choke seems to me to indicate a lack of fuel. This could be a fuel pump problem. I'm not familiar with your Yammy, but presume that fuel pump power comes from your crankcase pulses. If it does, then your problem would manifest itself mostly at low speeds. Your pump could have a small leak, or you could be sucking in air in your fuel lines giving you a momentary lack of fuel. In any case since toggling the choke gives you a shot of fuel it seems to me if that cures the problem momentarily the solution lies in that direction.
Take the cowl off, pump the bulb and look for fuel leaks.


jimh posted 08-23-2002 10:13 AM ET (US)     Profile for jimh  Send Email to jimh     
How is the primer bulb working? Check the fuel lines. If the entire fuel system is 17 years old, it may need an overhaul.

Borrow a portable tank (from the mechanic, perhaps) with a hose fitting that mates with your engine. Take the boat for a test drive and see if the problem is still there. This test could isolate the problem to either the engine or the fuel supply.

ChrisCT posted 08-29-2002 09:25 AM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
Tried everything so far but the alternate gas lines. Primer bulb gets hard - very hard. Mechanic changed water filter. Ran some carb cleaner - can't remember kind - something from Walmart for 2 strokes. No fuel leaks anywhere. Spark plugs look good. I guess I'm just waiting for new engine to come in (next 2-3 weeks). Just hoping mechanic still gives me the 800$ or so for the Yammy as trade in.
Bigshot posted 08-29-2002 10:53 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Try the techron in a 1/2 gallon of gas. Run it till the jar(windshield washer,etc) is empty but carbs are not run dry. Let sit overnight, then run. This will not only clean the crap out of your carbs, it will decarbonize the engine.Yamaha recommends this to their techs.
Hank posted 08-29-2002 09:57 PM ET (US)     Profile for Hank  Send Email to Hank     
How much Techron per 1/2
gallon ? I have used Techron in my automobiles with good results but instructions on the container say use the 12 oz. bottle with up to 12 gallons of fuel.

Should the concentration used on a two stroke be higher ? Any idea how much to use ?


ChrisCT posted 08-30-2002 09:08 AM ET (US)     Profile for ChrisCT  Send Email to ChrisCT     
gonna try the Techron today - assuming Wally World has it. Boat filled with water as bilge pump failed and we got a ton of rain here - so I have to go there anyway. Thank goodness for my boat neighbors realizing my bilge pump was clogged and using there pump. I feel pretty safe w/ the whaler anyway - but had a few nightmares as a boating newbie this year. Will let you know if the Techron makes a difference.
Bigshot posted 08-30-2002 10:17 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Whole bottle in a 1/2 gallon of gas. Takes about 1/2 or so to do. Got nothing to lose. If VRO is disconnected, don't forget to add the oil:)

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