912 ul on a kit fox 4. Just completed it. Zero time engine. I synced it, but runs rough / dead spot around 3400-3800 rpms. Almost seems like needs a bigger jet.? I did put a air box in the front of the engine feeding both carbs. Should be better air flow. When I synced it, it doesn’t stay consistently synced through the entire throttle range. But not very far off though. Cables don’t seem to be binding or unequal lengths. Any thoughts?
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Re: Newly completed build. Runs rough at 3400-3800 rpms
by Garrett Wysocki » 6 years ago
The engine intake manifolds are not tuned for operation in the midrange RPM (different tube lengths between the front and back cylinders). A resonant frequency develops due to the pulsing between the front and rear cylinders, and reverberates through the engine crankshaft. So you have all of these resonating parts between 3,500 to 4,300ish. This isn't in the manuals because the expectation is that you will make use of the engine horsepower. It probably is not good for the gearbox either.
Thank you said by: RotaxOwner Admin
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Re: Newly completed build. Runs rough at 3400-3800 rpms
by Andrew Dunning » 6 years ago
Where are the carb vent tubes?
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Re: Newly completed build. Runs rough at 3400-3800 rpms
by RotaxOwner Admin » 6 years ago
Have a look at the video posted here...
Thank you said by: Capt. Siva,
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