1990 Yamaha 90hp 2 stroke (third cylinder issue)

[Aaroncaplan8]

Hi all,

I have done a lot of diagnostics with my 1990 90hp Yamaha 3 cylinder 2 stroke so I wanted to see if people had any idea where my issue could lie.

Getting nothing out of the third cylinder right now. First and second cylinder are fine. Diagnosed this by taking off the plugs while the motor is running. When the first boot comes off, motor runs on second cylinder fine, when I take the second boot off, motor runs pretty bad on first cylinder alone, when I take third boot off nothing happens. Checked fuel pump, and the diaphragm is completely fine. Have changed spark plugs, and switched coil packs and the problem persists in the third cylinder. Compression is at an even 90-100psi on each cylinder. There is good spark from the spark plug as well. Just this morning took apart the whole third cylinder carburetor and it was clean but I still went through and cleaned everything and the problem persists. Fuel supply to the carbs is also not an issue.

Which leads me to this. I feel as though this has to be electrical and maybe the third cylinder is firing at the wrong time or there’s another issue causing this. If y’all had any input that would be greatly appreciated. Running out of ideas now. With these old two strokes the only real things I’ve learned are fuel, compression, and spark and since it seems that all three of them are fine, I’m thinking timing could be one of my only options. Thanks!

 

[tblshur]

which one is 90 psi?

 

[Aaroncaplan8]

All three cylinders are at about 95psi. Should have specified. But that may not be as accurate of a psi reading because the previous owner I got it from said they were at 120 across the board on his gauge. I’ve always read that having even compression across is more important than the number because a lot of times the actual number is based on the gauge and they’re not all perfectly accurate.

 

[tblshur]

what i have learned from here is good spark jumps a 3/8 " gap on a adjustable gap spark tester second compression seems low if your gage is accurate if those are good then you might need to look at cylinder walls. im sure someone more knowledge than me will chime in :joyous:

 

[racerone]

There are 4 things you need for that bottom cylinder to produce power.-------Fuel.----Compression in the cylinder.----Spark at the right time.-----Crankcase compression.------Simple trouble shooting finds the problem.-------So does spark jump a gap of 3/8" on all 3 leads , yes or no ?

 

[Aaroncaplan8]

Yes, spark jumped over 3/8 of an inch from the boot to the spark plug on all cylinders when I removed the boots.

 

[racerone]

Checked reed valves on #3 cylinder ?

 

[Doh]

Clean and Reclean the Carbs. Go down to Napa or simular, buy a Tip Cleaner and some Parts/Brake clean, and watch a You Tube about cleaning every jet.

Turn each Idle Jet out 2 Turns from Stop, and try that.

If you are still having issues, I would suspect Crank Seals. Bottom and Top, which is why the Center Cylinder is the best runner of the 3 of them.

 

[Aaroncaplan8]

After all that, I took carb three and put it on cylinder 2 and bam, cylinder two stopped working. Looks like although I disassembled the carb and cleaned it already, that the problem is still in the carb. Well good news is I’ve finally located the issue, now I’ll go back in and clean it with a full kit and if that doesn’t fix it, I may just replace the whole thing. Thanks so much for everyone’s input. I really appreciate it.