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post #1 of (permalink) Old 11-18-2002, 09:08 AM
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Hesitation on acceleration

I have a hesitation drop at 2000rpm under smooth acceleration on a flat surface. It also washes out at 3000rpm. I have rebuilt the carb. What should I check/adjust first? Other than these two problems the acceleration is strong and even.
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post #2 of (permalink) Old 11-18-2002, 09:20 AM
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Re: Hesitation on acceleration

I'd start with Fuel System, Carb (which you said you did) Fuel filter, make sure you have good flow, then check Timing, and then Ignition. maybe a bad coil.

brent
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post #3 of (permalink) Old 11-18-2002, 09:29 AM
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Re: Hesitation on acceleration

The accelerator pump on most carbs has several different settings, and some rebuild kits come with different springs for the pump. Is there a setting that will give more pump action? Is there a stronger spring you can use? Was the rebuild in response to the hesitation, or did it com about after the rebuild?
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post #4 of (permalink) Old 11-18-2002, 09:45 AM
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Re: Hesitation on acceleration

Isn't the accelerator Pump only for an initial squirt of fuel upon sudden acceleration not gradual? If you were to get on the gas from either idle or runing speed where you're wanting to accelerate hard, that's when your acclerator pump gives a squirt of fuel to keep up withthe increase air flow and lean fuel mixture. but just gradual acceleration withthe same throttle position doesn't use the accelorator pump. Atleast that's how I came to understand the working of my carb when I rebuilt it.

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post #5 of (permalink) Old 11-18-2002, 10:30 AM
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Re: Hesitation on acceleration

Actually, if the pump is working properly, any opening movement of the throttle will squirt in a proportional amount of extra gas. When I was in school close to 40 years ago, they taught us that the pump is needed because air, being less dense than gasoline, will accelerate faster when the throttle opens - the air velocity will increase faster than the fuel velocity, so the pump makes up the difference. If everything is working properly, opening the throttle will not cause a momentary rich condition.

Anyway, because there's a mechanical connection between the throttle and pump, they must move in unison, no matter what speed. If the change in the throttle is slow and small enough, the pump might not be needed, but you're going to get it anyway.

There are other things that will cause a stumble. For example, after the throttle is about 3/4 or more open, the mid-range circuit is pretty much out of the picture, as it works off the difference in pressure above and below the throttle plate. But, at low engine speed, there is not enough airflow through the carb to activate the power circuit, which is dependent on relatively high venturi vacuum. In that case, you can get an initial surge of power, followed by stumbles until the engine speed fights its way up.

And, as someone else pointed out, incorrect timing can also cause the stumbles. The loss of vacuum advance from the throttle opening, before engine speed is high enough to activate the centrifugal advance, can result in timing too retarded for efficient operation. In that case, the engine will stumble immediately and continuously until engine speed comes up.

It could also be that the vacuum advance is attached to manifold vacuum, when it should be on venturi vacuum. I guess that the reverse could also cause a problem.
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post #6 of (permalink) Old 11-18-2002, 10:54 AM
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Re: Hesitation on acceleration

AAAh! i see said the blind man. sounds like I was mostly there, but lacking on a few points. Thanks for filing in the missing info.

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