Honda 'surging' at partial throttle

Sketch

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Got my replacement engine (thanks Jeremy/birddog). Got it all put together and things went pretty smooth. I'm being very careful to run at partial throttle, especially for the first 100 or so miles, and I'm experiencing a surge that makes me nervous about a problem and also makes the ride less than enjoyable. I am being subtly pitched forward and backward as I go down the road. It is not smooth. It's unsettling.

If I am at any partial throttle level, the net effect is that (like I already said), the power is surging - so 1 or 2 seconds of smooth power, followed by a fall-off in power, followed immediately by another 1,2 or 3 seconds of smooth power.

I don't think the first engine did this.

It's not the engager, it is definitely staying engaged. I have it permanently engaged and there is no disengagement going on.

I'm thinking (based on reading threads here):

oil sensor switch?
the large amount of grease I packed in the gearbox this time, affecting the clutch?
kill switch wiring, as Irish John mentioned in a thread?
the engine not firing on a cycle(s), for reasons other than kill switch or oil sensor?
fuel flow?

Now, as another clue, keep in mind this is ONLY during partial throttle. I tried running wide open for a short stretch and the ride becomes smooth, no surging.

Thanks in advance...
 
Sounds like the engine is running lean, most likely due to a carb problem. A simple carb cleaning will probably fix the issue.

Low oil sensor will shut the engine off, it will not produce surging. Same goes for kill switch issues, especially if the engine runs more consistently at WOT.
 
You aren"t going to like to hear this, but it may be the Governor? The Governor when it works as designed not only limits max. rpm, it also causes the rpm's to increase when the engine becomes loaded.

ocscully
 
You aren"t going to like to hear this, but it may be the Governor? The Governor when it works as designed not only limits max. rpm, it also causes the rpm's to increase when the engine becomes loaded.

ocscully

I hate to say it also, but that was what I was thinking.
 
I think there is a spray can of carb cleaner here. What is the easiest way to clean the carb? Do I have to take it all apart?
 
Can I really rule out fuel flow? I mean, I haven't inspected the filters.
 
I think there is a spray can of carb cleaner here. What is the easiest way to clean the carb? Do I have to take it all apart?

Yep - just spraying cleaner down the throat of the carb will do nothing.

You need to drop the fuel bowl, and force the cleaner in to every little orifice in the carb. The more you can dis-assemble and clean, the better.
 
No I'd go ahead and take the time to check out your filters just to be sure there is no problem. How does the motor idle? Does it run smooth and steady or does it surge? What I see/hear in your description sounds to me like the Governor doing its job.

ocscully
 
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