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8hp Johnson bad stumble under load / WOT

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bcontento

Contributing Member
I was having all kinds of issues with the motor, but have since had a buddy with at least some decent know-how go through it. He rebuilt the carb (again), replaced the impeller, fixed the cam roller and pull start spring. The engine starts nicely and idles nicely, finally. In neutral, it'll throttle up 100% with no issue. In gear, it'll accelerate nicely to a low-mid speed but when it's cracked above "medium" it starts stumbling badly. Can't tell if it's missing, bogging, leaning out, etc.

I put up a quick video to maybe help hear the issue.

https://www.youtube.com/watch?v=saf6CGhVFX8

My buddy did say that to get the motor to idle, he had to turn out the mixture screw (the one circled in the pic below...that's not my carb but essentially the same layout) much more that he has on previous motors (like 7-8 turns instead of an expected ~2 turns).

carb_2.jpg

Are there more adjustments to be made on this little carb other than the idle speed screws? what can cause this kind of throttle/load response?

Is there a reliable order of adjustment for the 3 setting screws?

thanks all!
 
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Do a compression test.---Post the numbers.----Motors will rev to 100% in neutral on 1 cylinders so that proves nothing.----I suspect it is running on 1 cylinder.----Do not like the rattle either.
 
racerone said:
Do a compression test.---Post the numbers.----Motors will rev to 100% in neutral on 1 cylinders so that proves nothing.----I suspect it is running on 1 cylinder.----Do not like the rattle either.
Click to expand...

Compression on the top cylinder was around 100lbs and the bottom cylinder around 95lbs. Seems right to me. I've also got good spark.
 
bcontento said:
I was having all kinds of issues with the motor, but have since had a buddy with at least some decent know-how go through it. He rebuilt the carb (again), replaced the impeller, fixed the cam roller and pull start spring. The engine starts nicely and idles nicely, finally. In neutral, it'll throttle up 100% with no issue. In gear, it'll accelerate nicely to a low-mid speed but when it's cracked above "medium" it starts stumbling badly. Can't tell if it's missing, bogging, leaning out, etc.

I put up a quick video to maybe help hear the issue.

https://www.youtube.com/watch?v=saf6CGhVFX8

My buddy did say that to get the motor to idle, he had to turn out the mixture screw (the one circled in the pic below...that's not my carb but essentially the same layout) much more that he has on previous motors (like 7-8 turns instead of an expected ~2 turns).

Are there more adjustments to be made on this little carb other than the idle speed screws? what can cause this kind of throttle/load response?

Is there a reliable order of adjustment for the 3 setting screws?

thanks all!
Click to expand...

If You have to turn out the idle mixture screw much more than expected it suggests that the carb my still need some cleaning or that there is air in leakage. Check gaskets between the carb and the manifold and either side of the leaf plate.

Check the carb top cover for distortion or cracks. If your carb has a pump type primer (choke) check that too.

Reduce the idle speed to an absolute minimum while adjusting the mixture screw. Readjust it to a sensible idle speed when done.

Best not to touch the adjustment screw on the cam follower, unless you have to, as that can become brittle with age and easily broken.


BUT, BUT BUT.

At high speed your engine seems to be "4 stroking". The engine note changes from a steady "purrrrrr" to more of a rapid "pu, pu, pu " sound. This is usually indicative of the mixture being to rich!

I think you have to check the assembly of the carb very carefully against the diagrams in the workshop manual. Checking on gaskets, O rings etc

Float height and float valve operation too.

Not as good as an OEM manual but you may find the manual at http://162.144.28.33/lib/library.html which covers your engine, useful


In future always quote the full engine identity with any questions . Make, hp, year and in particular the full model number.
 
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