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1 cylinder won't run 2stroke twin

DAMN maintenance and repair thread; including Farkle Fests! :boohoo:
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billy jack
DAMN Wanabe
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1 cylinder won't run 2stroke twin

Post by billy jack »

Working on a 69 yamaha cs3 200cc 2 stroke twin. Similar to RD350, but piston port injection. Left cylinder runs ok. Can only get the right to run at around 3k rpm. If I open the throttle more or less no more combustion. Spark looks just like the other cylinder.both cylinders compression test about 90 psi. Spark plug is covered in fuel/oil mix after a few rotations. I swapped the carbs also. That covers fuel, spark, and compression as far as I know. Only difference between the two cylinder is the exhaust clamp threads on the faulty cylinder are gone so the pipe is pressed on and sealed with copper gasket goo. Had I similar issue with a single cylinder I never figured out as well. Any suggestions?
"that's NOT a chainsaw strapped to the back of your harley is it!"
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Hare
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Re: 1 cylinder won't run 2stroke twin

Post by Hare »

I am not familiar with this motorcycle specifically but this is general engine advice, apply to suit.


good job on the carbs and trouble shooting, I would of said main jet but swapping the carbs out covered that.



first off 90 psi is pretty low this could give lots of different problems, it will run but not for much longer without rings and or piston and rings or even more if it breaks a ring.

possibly sounds more like a stator or coil/coil wire/spark plug wire breaking down, that's what a failing stator acts like, if not dead. I would leave this as the last option after all else has failed. An intermittent electrical failure is the worse to diagnosis.


theres a good chance it has points if so this would be the cheapest place to start, maybe a good cleaning and setting of them would help if its a twin there would be a set on each side

First and for sure just replace the spark plugs with new, again cheap and eliminates another problem, spark plugs fail first when under compression, spark plug testers are just air tanks with a glass window, they screw the plug into it, pump it up and watch it spark at 100psi +
billy jack
DAMN Wanabe
Posts: 60
Joined: Mon Nov 21, 2011 1:52 pm
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Re: 1 cylinder won't run 2stroke twin

Post by billy jack »

Spark seems visually the same for both cylinders. I do intend on swapping the coils and wires along with new plugs.haven't gotten around to that yet...I can get just at 100psi on each cylinder with the electric starter, but not the kicker. Just waiting on 1 more oversize piston from eBay(NOS parts for a 69 yamaha are hard to find) then I'll have a whole new fully functional 1st oversize top end with a spare set of jugs :-)
"that's NOT a chainsaw strapped to the back of your harley is it!"
"It was when I strapped it on there..."
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