Yamaha F9.9C idle rough

[Duncanholmes]

Hi Folks,

I have joined today with a specific request for some help on a 2003 F9.9CMH unit that I use on my little ali boat as the main motor.


Motor does around 30 hrs per year, almost all of which is in temps below 50F, I have owned her for the last 8years and prior to that had dealer services.

The motor runs fine with the throttle open but is "lumpy" on tick over (950rpm) and whenever I check or change the plugs there is a marked colour difference in them, see photo

Fuel system was overhauled 12 months ago, ultrasonic clean on the carb, new main jet, service kit. all hoses replaced and new fuel pump as it was leaking past the diaphragm.
I believe the pilot valve is in the optimum, and I run the carb dry if its going to sit for more than a week between uses.

Cold starting is fine, warm starting is fine, will fire first pull on 90% of occasions

Compression is 125psi on both cylinders wet and dry, which I know is down on the spec (139), but I am not sure on the integrity of the gauge.

Both spark plug caps measure at 4.8kohm (4-6spec)

Exhaust Valve clearances were a little tight, these have now been set at spec (0.25)

I am running DPR6ea-9 plugs gapped at 0.9mm, its been suggested to run a hotter plug on that cylinder, but not sure this is a good idea?

This unit has the HT leads that run straight out of the bottom of the coil, so unable to do a simple test,

Beginning to think I am getting an intermittent spark on cylinder 2 (blacker plug), which may be causing the rough idle.

Would be really grateful for any thoughts as to cause or cure.

Thanks in advance

Edit, fitting new plugs doesn't remove the "kick" from the idle, so I don't believe the plug issue is the cause.

[99yam40]

did it run well after the carb cleaning?

If it did but now it is running rough,
My guess would be the carb needs attention again.
running a motor until it runs out of gas /dies does not get all of the fuel out of the float bowl, so residue is left after the fuel evaporates

You can test spark with the proper test equipment to see if it is sparking all of the time or not, if it is then fuel to air ratio is more than likely the problem(carb)

[Duncanholmes]

did it run well after the carb cleaning?

If it did but now it is running rough,
My guess would be the carb needs attention again.
running a motor until it runs out of gas /dies does not get all of the fuel out of the float bowl, so residue is left after the fuel evaporates

You can test spark with the proper test equipment to see if it is sparking all of the time or not, if it is then fuel to air ratio is more than likely the problem(carb)

The idle has been rough for a while, the work on the fuel system didn't sort it, but did sort out other issues I had.

Will look in to the spark testers, thanks

[TownsendsFJR1300]

The engine is not a US model and has one carb: https://www.boats.net/catalog/yamaha...lhb/carburetor

Agreed the carb, low speed jet sounds clogged, especially jet #9 which MUST BE REMOVED for cleaning.

But, one plug IS burning different from the other so there IS an issue there. With one carb, the color SHOULD BE THE SAME.

Might want to check that fuel pump again, or you have an ignition issue.

You can get spark testers cheap and easily see, (with the engine running), that the cylinder with the "black plug" is likely the culprit.


BTW, when storing, it's best to drain the carb at the bowl drain screw. Running it dry does NOT get all the fuel out.

[zenoahphobic]

Just a thought, that dark spark plug is oily up the thread much more than the other. This happens on two strokes where there is oil in the fuel.
I wonder if a bit of oil is leaking into that cylinder. It would not be the single.carb, the symptoms would show up on both plugs as stated previously.
Oil getting into combustion chamber can come from worn rings and piston, worn valve guides and seals, or faulty gaskets.
I would check those things. You may need to do a leakdown test. And check tightness of head bolts.
You said the exhaust valve clearance was tight, so that may indicate the valves have receded. If a valve doesn't fully close combustion can be erratic particularly at idle, as a valve may seal better under pressure at higher revolution and loads. A valve inspection and lapping may improve sealing.

[Duncanholmes]

Thanks for your replies, I have sourced some spark test caps, and will check that first.

Will let you know how it goes.