Yamaha 90hp 2 stroke 1997 - misfire/rough idle at low revs and strong exhaust smell

[markr1]

Can anyone help with the below please?

I run a Yamaha 90hp 2 stroke and have had this for about 18months. Recently the engine seems to have developed a misfire at low revs when running at around 5 knots (no idea of RPM). There also seems to be a very strong exhaust smell mixed with a slight fuel smell at low revs as if not all fuel is being burnt. It may be that this is more noticable at low revs because there is little wind to blow it away and it is in fact happening all the time!

Initial startup from cold, starts very easily with a some smoke for about 30 secs. Quite often there is no need to pump the bulb.
Runs well at full revs
When engine warm and running slow appears that there is a misfire and runs quite lumpy with the exhaust smell as described above
Engine serviced new plugs and fuel filter about 4 months ago, engine ran fine.
Problem with fuel/water separator about 2 months ago, changed from the bulb type to the oil filter style of separator. Not sure, but the misfire I believe may have happened around this time. Is it possible that the separator is the culprit?

New ignition coil assemblies and plug assemblies fitted about 4 weeks ago did not resolve the misfire.
All Plugs are wet and have a light fuel/oily coating after use.
Checked TDC and this appears ok.
Any advice gratefully received, would be nice to get this great engine running better.
Thanks

Mark

 

[mark hunt]

Re: Yamaha 90hp 2 stroke 1997 - misfire/rough idle at low revs and strong exhaust sme

i have the same problem on my 2008 Yamaha 90 2 stroke after having the carbs rebuilt. i installed myself. runs fine at high rpms. seams to use a lot of fuel. need help
mark

 

[robert graham]

Re: Yamaha 90hp 2 stroke 1997 - misfire/rough idle at low revs and strong exhaust sme

Sounds like bad/cracked fuel pump diaphragm causing raw gas to get sucked into motor causing very fuel rich/wet spark plug condition. New fuel pump is about $50, very easy 2 screw replacement....they do wear out after so many years. My 1999 90C Yamaha pump I just replaced last year, old pump still good so kept it for a spare.

 

[ragbul]

Re: Yamaha 90hp 2 stroke 1997 - misfire/rough idle at low revs and strong exhaust sme

Ya I had just posted the same thing about my 2001 115 2strk. The motor would not run at idle and I thought it was the carbs. Rebuilt both and same problem. I noticed the plugs were wet so the next thing I was thinking the fuel pump has a pin hole or something that causes fuel to suck back into the crankcase and foul the plugs. I am waiting on the fuel pump from ********** and will let everyone know if that was it. Also I hooked the fuel line directly up to the T between the two carbs and it would idle with me pumping it so through elimination I believe that is it. Also buy the whole fuel pump not the parts to rebuild it because it is cheaper.
good luck
Danny

 

[robert graham]

Re: Yamaha 90hp 2 stroke 1997 - misfire/rough idle at low revs and strong exhaust sme

Can anyone help with the below please?

I run a Yamaha 90hp 2 stroke and have had this for about 18months. Recently the engine seems to have developed a misfire at low revs when running at around 5 knots (no idea of RPM). There also seems to be a very strong exhaust smell mixed with a slight fuel smell at low revs as if not all fuel is being burnt. It may be that this is more noticable at low revs because there is little wind to blow it away and it is in fact happening all the time!

Initial startup from cold, starts very easily with a some smoke for about 30 secs. Quite often there is no need to pump the bulb.
Runs well at full revs
When engine warm and running slow appears that there is a misfire and runs quite lumpy with the exhaust smell as described above
Engine serviced new plugs and fuel filter about 4 months ago, engine ran fine.
Problem with fuel/water separator about 2 months ago, changed from the bulb type to the oil filter style of separator. Not sure, but the misfire I believe may have happened around this time. Is it possible that the separator is the culprit?

New ignition coil assemblies and plug assemblies fitted about 4 weeks ago did not resolve the misfire.
All Plugs are wet and have a light fuel/oily coating after use.
Checked TDC and this appears ok.
Any advice gratefully received, would be nice to get this great engine running better.
Thanks

Mark

The symptoms you're describing would point to a bad fuel pump diaphragm as a strong possibility. Think about the number of times that thing flexes at once every RPM....at WOT, maybe 5000 times a minute!...they can and will crack. Good Luck!