High rpm engine bobble

[Quadruplecccc]

I've got a 79' 80hp merc. that bobbles at 4500 rpms. Backed off to 4k it runs smooth. Last year the floats in carbs were somehow knocked out of adj. and were overflowing out the front of carbs. I slightly adjusted them to stop the overflow which worked.

Ran it a couple of times with no trouble this year and no trouble till this past weekend. Now it has a feel of running outof fuel at high speed.

Has great hole shot and takes off like a rocket untill it hits about 45k when it begins to bobble and lose rpms.

Now it got me thinkin, could the floats have moved again? If so what would the float measurements be so I can readjust them? Or could I maybe have a spark plug not firing right at high rpm. I have not yet changed them out.

 

[CharlieB]

Re: High rpm engine bobble

A weak fuel pump, air sucking into the fuel lines before the fuel pump, a restriction in a fuel line or fitting, any of these can cause what you describe.

DANGER, Will Robinson!! Continuing to operate a motor with a weak fuel supply WILL MELT PISTONS due to a LEAN fuel mixture at higher RPM.

Test the fuel supply, disconnect the fuel line between the fuel pump and carbs, point it into a suitable container and crank the motor, look for a strong pulsed flow. Any weakness, check ALL hoses and fittings for restrictions, rebuild the fuel pump.

Temporarily reinstall the fuel line with a clear hose, start the motor and look for any air bubbles passing through indicating air sucking into the system between the fuel tank and pump. Remove clear hose when finished.

 

[Quadruplecccc]

Re: High rpm engine bobble

My son reminded me that as soon as we hit a couple of waves, the bobble showed up right away.

I can't help thinkin the floats are bouncing. Am I just wishful thinkin here or could this be a possibility?

Still need to get out there and check fuel pump and connections but cant help to think I messed up when adjusting the floats since I hadn't had this problem before I touched them.

Please keep the thoughts comin , thanks

 

[Quadruplecccc]

Re: High rpm engine bobble

rebuilt the carbs and took her out for a run. runs much better . got it home , popped the cowling and noticed the bottom carb was wet around the throat. there was a small amount of fuel pooled inside cowling in front of bottom carb. apears to be coming out of the small tube in throat of that carb.
when I shot carb cleaner through it during rebuild it went into the bowl area.
what did i miss since I did both carbs the same? top carb looks fine.

 

[Maxz695]

Re: High rpm engine bobble

poor valve seat maybe. did you change the needle valve and the seat when you rebuilt or just the needle valve?