Blow by causing stalling?

so i've been having a problem with my bike stalling at idle, (starts and runs good) and nothing i've tried has helped; new pilot jets, cleaning the pilot jet passage several times with fishing line, new fuel screw, checked acv diaphragm, aluminum hot start nut, idle/fuel screw adjustments, cleaned petcock and fuel cap....


I just recently replaced the piston and rings on it, and the cylinder had some slight scoring, but I thought It would be fine. I talked to a friend of mine who does a lot of motorcycle work, and he said he has seen cases where the scoring causes Blow by which creates a rich fuel mixture in the cylinder head causing the bike to stall. I have also noticed that ive been losing oil slowly, but have no smoke out the exhaust. I do have moisture in my airbox that seems to be engine oil.


Has anybody heard of this? and is there any way I could verify this before I replace the cylinder?

You pull the head, measure the cylinder , piston, and rings, for wear.

You don't just replace parts cause someone said you should.

Your service manual gives you specific wear tolerances to look for so you know exactly the condition of your components.


There is no such thing as 'slight scoring'. You either have a damaged cylinder or you don't.


If you can feel the ridges by running your fingernail over the visibly marks. If you can feel them, you cylinder is damaged.

You might be able to hone them out, or even possibly use scotchbrite and WD 40, but you are now in  the territory where it should be looked at by a pro.


Have you adjusted the valves?

Have you inspected the valve seals, seats, and the valves themselves?

Have you inspected the auto decompression system for sludge in the mechanism? Bad spring?


This stuff does not last forever you know...

Edited by KRANNIE

On the 07-11's if they are overfilled with oil they will oil the air box while riding. I would double check your accelerator pump adjustments or carb before anything else.

i could feel the scoring with my fingernail, I took a ball hone to it, to cross hatch/deglaze, and it smoothed the scores a little bit, but I could still feel them. I adjusted my valves, but did not check for leaking seals. also my auto decomp seemed fine.


I have an 06, I've cleaned the carb 1/2 a dozen times, and I'm assuming the apump is good, because I have great acceleration and power throughout the throttle.


I did measure the cylinder and piston and they were in spec, rings had the correct gap too.

Edited by folger86

I never owned a 06' so I am not sure on oil in the air box. There is a passage above the pilot jet that you can use a fishing string on to clean. You should check the squirt at least per the manual. If you have excessive blow by you see smoke in the exhaust. But I doubt it causes it to stall. Put a new Iridium plug in it and give it a rip. Also check the oil is not hopelessly overfilled.

Daft question but is your idle set at ~2000rpm?

You realize that these cylinders are plated and not designed to be honed?   


If you have scratches you can feel you have broken through the plating and you need

to either replace it or replate it.   In some cases the replating costs near enough to the

price of a new cylinder (especially if there is damage) that it makes sense to buy the new one.


Honing just removes more of the plating and if you honed it enough to "smooth out the

scratches" there is probably very little if any plating left.   It will not last long like that.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now