One cylinder has higher rpms..

[kbait]

'80 35hp Evinrude EL. Idles too high at lowest setting (Link/sync and all that good). Pull bottom plug wire and test lower cylinder = nice slow steady idle. Pull top plug wire and repeat test = much faster idle. I don't think it's a carb issue that would cause that difference. New plugs gapped correctly, and tested good spark on both leads. I switched the spark plugs and retested to same result. ?? Thanks for any thoughts..

 

[oldcatamount]

Re: One cylinder has higher rpms..

sounds like it may be a timing issue. If that motor has breaker point ignition, check the points, there may be one set that's burned a little more than the other thereby screwing up the timing a bit. (keep in mind that I'm just guessing here and someone will surley help us both out)

 

[Joe Reeves]

Re: One cylinder has higher rpms..

What are the compression readings?

 

[kbait]

Re: One cylinder has higher rpms..

110 and even.

It has CD ignition, and it all seems fine..

 

[AlTn]

Re: One cylinder has higher rpms..

fuel pump diaphram leaking into pulse cylinder?

 

[kbait]

Re: One cylinder has higher rpms..

I thought about the fuel pump too, but the top cylinder seems to be smooth and slow, and the pump is attached to that one. It's the bottom cylinder which runs too fast. Maybe a vacuum leak below powerhead only affecting the lower cylinder?? I welcome any thoughts..

 

[boobie]

Re: One cylinder has higher rpms..

Could have a bad lower c/s seal leaking air raising the rpm or else a bad crankcase leak. Seen this happen on snowmobiles before.

 

[Outsider]

Re: One cylinder has higher rpms..

You realize the motor has but one RPM reading at any point in time, right, the number of times the crank revolves per minute? And the pistons go up and down the same number of times in both cylinders? Not sure what you're testing, or how ...

 

[kbait]

Re: One cylinder has higher rpms..

I first found that the motor wouldn't idle down correctly. I pulled one plug wire and ran with fire on the other cylinder only, and switched to test the other cylinder only. When the top cylinder is firing alone, it idles slow and smooth. When the bottom is firing alone, it isn't so smooth, and noticeably higher rpms, with the throttle still at 'slow'. Air leak to bottom cylinder?? I've ruled out the carb, and fuel pump (fuel pump gets it's pulse from top (good running) cylinder. I'm gonna start it today and spray carb cleaner around the lower part of the powerhead to try and find a vacuum leak. Any other thoughts??

 

[Joe Reeves]

Re: One cylinder has higher rpms..

Although you feel you've eleminated the possibility of the problem being related to the carburetor....... I have at times found that occasionally the single carburetor models will develop a situation whereas the slow speed needle valve, when turned in ever so slightly too much will result in a loss of power/rpms to one cylinder.

Not too often on the 20/35/40 single carburetor models but I have had it happen. On the 1974 and some later 9.9/15hp models, it was/is possible to have one cylinder drop out altogether, continuing to run on one cylinder. A slight turn out (richen mixture) and the engine ran normally again as it should on both cylinders.

If the tip of the slow speed needle valve is worn slightly, a true adjustment would be impossible which could result in the above scenario.

Another possibility..... a slightly off center leaf valve, or a cracked leaf valve, faulty intake manifold gasket(s) = (crankcase pressure/vacuum problem).

Let us know what you find.