4Cyl 2Strk lower cylinder won’t fire. Has spark but low compression.

[johncapecod]

My 30 year old Yamaha 115 two stroke has one cylinder that doesn’t fire. If I pull that spark plug wire nothing changes but there is plenty of spark. The compression is 120/120/120/100. The carburetors have been cleaned and rebuilt and in the Venturi valves nothing seemed amiss. I switched the coils and the same cylinder has the problem. The motor dies if rev’d above 2000 RPM. I don’t see any oil leaks under the cowling. I got rid of the precision blend system and went with a premix because the oil pump was no longer available for this motor when this issue started. I guess what I’m wondering is if I should spend any more time on this motor (parts are no longer being produced)? There are a lit of these motors out there but the YouTube videos I’ve seen don’t address this particular problem. Hopefully, this forum will offer some guidance. Thanks for reading.

 

[robert graham]

100 psi on one cylinder shouldn’t keep it from running….could be a stuck piston ring on that cylinder….maybe something else causing poor running. I’d put a set of new or very clean spark plugs in, run the motor and then pull/ inspect plugs for how they’re burning, carbon, etc….might tell you something about what’s wrong.

 

[racerone]

Inspect reed valves.-----Perhaps one is broken / stuck open.

 

[99yam40]

also the lower crank seal may be letting air in leaning out the lowest cylinder

Not sure how many fuel pumps this thing has, or what crank case they are tied to.
but if a pump leaks fuel into a crankcase, that cylinder will be too rich and not fire/burn the fuel properly

 

[johncapecod]

100 psi on one cylinder shouldn’t keep it from running….could be a stuck piston ring on that cylinder….maybe something else causing poor running. I’d put a set of new or very clean spark plugs in, run the motor and then pull/ inspect plugs for how they’re burning, carbon, etc….might tell you something about what’s wrong.

That cylinder’s spark plug comes out wet. On the “Precision Blend” motors (where the oil is injected into the motor just after the carbs and before the venturi’s), there is a control feature that keeps the motor from running too fast if there is a problem with the oil pump or oil sending unit. I am wondering if anyone knows how to disable that feature.

 

[johncapecod]

Thanks for the responses! This is all very helpful.
On the “Precision Blend” motors (where the oil is injected into the motor just after the carbs and before the venturi’s), there is a control feature that keeps the motor from running too fast if there is a problem with the oil pump or oil sending unit. I am wondering if anyone knows how to disable that feature.

 

[racerone]

How did this motor run for you last season ??-----A wet sparkplug is due to a weak spark.----Good spark easily jumps a gap of 3/8" ( 1 cm ) on a test device.----Or fuel pump diaphragm is ruptured.-----Or lower seal is bad.-----Or reed valves have issues.----Jet missing in a carburetor.-----Not hard to figure out.

 

[99yam40]

Thanks for the responses! This is all very helpful.
On the “Precision Blend” motors (where the oil is injected into the motor just after the carbs and before the venturi’s), there is a control feature that keeps the motor from running too fast if there is a problem with the oil pump or oil sending unit. I am wondering if anyone knows how to disable that feature.

low oil level in tank on motor is the only part of oil system to set the alarm and RPM reduction.

But I do not believe only one cylinder would be affected if it was oil level problem
Not sure, but think it does a random spark interruption to limit RPMs

 

[QBhoy]

From what you’ve said…especially around it dying over certain rpms…I’d be looking at its charging system and battery. If you aren’t getting sufficient voltage and amps from the charging system or the battery is compromised…you’ll start loosing ignition ability and shortly after…lose it all together.

 

[racerone]

The charging system and battery may have nothing to do with making spark on these motors.

 

[99yam40]

and why would it only effect just one cylinder.
that makes no sense at all

 

[QBhoy]

The charging system and battery may have nothing to do with making spark on these motors.

Could be right. But I thought on a v4 yam, it would.

 

[flyingscott]

Look for wires rubbing somewhere? What year Yamaha 115? Are all the linkages moving smoothly? Anything broke? Bad oil sensor?

 

[johncapecod]

Thanks for replying. It’s a 1986. I removed main oil tank and oil pump and sending unit and switched over to running a premix. I’m thinking I might have two issues. One is, the sensor is telling the motor not to rev above 2500 RPM and the second one is low compression in the number four cylinder. Do you know of a way to disable that sensor since I no longer need it running a premix, and where to find it?
Thanks again for replying.

 

[flyingscott]

The sensor is in the tank. Do you have the tach with 3 lights? And yes that 100#s of compression is concerning. But that motor should have had 2 tanks I believe. You might need to buy a factor manual for your motor.

 

[johncapecod]

The sensor is in the tank. Do you have the tach with 3 lights? And yes that 100#s of compression is concerning. But that motor should have had 2 tanks I believe. You might need to buy a factor manual for your motor.

Yes it has a tach with 3 lights. The tank(s) was/were removed but the harness to the tack is still present in the motor and wiring is still connected at the other end.

 

[johncapecod]

Yes it has a tach with 3 lights. The tank(s) was/were removed but the harness to the tack is still present in the motor and wiring is still connected at the other end.