This should be an easy diagnosis....

[Uncle Daddy]

Yamaha 1999 C115 pre-mix carbed engine. Motor suddenly started running rough recently...it idles and runs (not under load) smooth enough but coughs occasionally and blows a puff of smoke. I replaced the spark plugs, installed new fuel filter cannister (no water), serviced the fuel filter at block, installed a new fuel pump (that seemed to help a little but not completely). Here's the easy diagnosis clue. Starboard bottom plug is more DRY and black...all three other plugs are tan and damp with fuel/oil mix(normal appearance). I switched the coil on this cylinder with the one above, but the lower plug continues to be significantly darker. If this is a carb/fuel gum problem, then stabilizer really doesn't protect you all that well because I use it religiously and I don't let it sit more than a month without running, usually every two weeks.

 

[rodbolt]

Re: This should be an easy diagnosis....

do a compression and spark test first<br /> then remove and clean the carbs.<br /> the cough is a lean condition in the crancase. its actually a backfire that tries to push the bottom of the piston back up when its on its way down.

 

[Ray Neudecker]

Re: This should be an easy diagnosis....

Stabilizer will not prevent the oil gas mixture from gumming the carbs if allowed to sit. It will help to maintain the octane and quality of the fuel. The best method of keeping the carbs operating properly is to drain them before putting up the motor. One of the real advantages of Yamah's oil injection has been that the oil was injected behind the carbs and the carbs never saw the oil. Rodbolt has most likely answered your question with his suggestions.

 

[Uncle Daddy]

Re: This should be an easy diagnosis....

Thanks very much. The amount of knowledge that you need to have to be a "good" boat owner is vast. I have a lot of mechanical experience but the more you know the more you realize how little you know. Anyway, what are the mechanics behind a "lean sneeze"...how does the fuel oil mix behind the piston ignite and how does a dirty carb create the condition for this to occur?<br /><br />Also, I found a post that referred to a Chevron discussion of gasoline oxidation and it really made the case of why you should keep a VENTED fuel tank topped up in storage...expansion and contraction of warm and cold air pumps new oxygen into the fuel tank (through the vent) and supplies the condition for fuel oxidation and gum formation. The more I know...

 

[Triton II]

Re: This should be an easy diagnosis....

I agree with you Uncle Daddy. I have a reasonable perception of how most things work and have been involved in boats for 30 plus years, but finding this forum site has increased my knowledge greatly in only a few weeks. People like JB, rodbolt etc have always responded quickly and knowledgably to most people's requests for help. Good on you guys!

 

[bigbrownbuku]

Re: This should be an easy diagnosis....

lean mix burns slower. still burning when the piston is on the down stroke as the intake port opens, incoming mix is being burnt in the crankcase before it gets to the cylinder.<br />a dirty carb promotes this condition by not fully supplying the engine with correct fuel/air ratio ie too lean....and the circle continues.

 

[yubbie]

Re: This should be an easy diagnosis....

Originally posted by Ray Neudecker:<br /> Stabilizer will not prevent the oil gas mixture from gumming the carbs if allowed to sit. It will help to maintain the octane and quality of the fuel. The best method of keeping the carbs operating properly is to drain them before putting up the motor. One of the real advantages of Yamah's oil injection has been that the oil was injected behind the carbs and the carbs never saw the oil. Rodbolt has most likely answered your question with his suggestions.

 

[yubbie]

Re: This should be an easy diagnosis....

NOW SIR, IM NOT QUITE CLEAR ON THIS. WE ARE TOLD BY THE DEALER, I CAN FAX YOU THIS IF NEED BE. FUEL STABILIZER SHOULD BE ADDED @ 1/2 OZ PER GALLON OF FUEL. ENGINE RAN WITH FRESH WATER UNTILL UP TO RUNNING TEMP AND STABILIZED FUEL HAS MADE IT'S WAY TO THE ENGINE. WERE APPLICABLE, ENGINE RPM SHOULD BE BROUGHT UP SLIGHTLY TO PROPERLY FOG ENGINE UNTILL IT STALLS. CARBS AND FUEL SYSTEM SHOULD BE FULL AS TO AVOID CONDENSATOIN ISSUES. ONLY THE VAPOR SEPORATOR SHOULD BE DRAINED TO AVOID O RING DAMAGE. NOW NOT TO SAY THAT ALL IN PRINT OR VERBALY TOUGHT IS CORRECT. ALSO, I LIVE IN CANADA WERE THERE ARE MANY ADDITIVES IN OUR FUEL.