Author Topic: accelerator pump on carb  (Read 345 times)

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Online jmester

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accelerator pump on carb
« on: October 14, 2013, 10:24:48 pm »
This may sound foolish, but can someone help explain the function to me. And what happens when they go bad. Not real familiar with them. Have not come across many saws that have them.
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Offline sharkey

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Re: accelerator pump on carb
« Reply #1 on: October 14, 2013, 11:27:31 pm »
Jason,
No gas engine will accelerate off a lean carb.  Because of this, the newest generation of lean run carbs give the intake a shot of fuel to provide for the rpm transition. 

Have you ever worked on an older GM with a Q Jet carb?  Those carbs shot a stream of fuel into the intake to handle the transition.  The idea is the same on your saw just in a smaller amount. 

Online jmester

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Re: accelerator pump on carb
« Reply #2 on: October 15, 2013, 06:34:11 am »
I follow what you are saying. So if the is a hesitation from idle to part throttle , you should look towards the pump. If your low speed is set at the basic setting then. I know that on a float carb you can check the pump by looking down the carb and work the throttle open and you should see a squirt of fuel come in. I guess this does not work the same for non float style carbs.
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Offline 660magnum

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Re: accelerator pump on carb
« Reply #3 on: October 15, 2013, 10:03:21 am »
With a chainsaw or weed whacker with a regular Walbro/Zama carburetor, the low speed circuit has to be tuned richer than best running so that the engine can accelerate to high speed. Therefor the magic tuning for the low speed normally is to make the low speed as lean as possible and the engine still able to accelerate when you jerk the trigger.

With a accelerator pump carburetor, the idle needle can be tuned for best running at idle and the engine will still accelerate to high speed. Probably a EPA feather in the hat thingy?

I experienced a problem with a new chainsaw and it took me a moment to figure out what was wrong because I didn't know in the beginning that the carburetor was different. I have a Husqvarna 357XP with the HDA-199 carburetor and this is a accelerator pump carburetor. When I first bought the saw, it would quit on you in the lower mid range speed area unless you wiggled the trigger a little every now and then. I then decided that the chainsaw had a accelerator carb and that for some reason the midrange was lean and the accelerator pump would keep the chainsaw running. I finally decided to make the low speed a little richer and the midrange (poor lean) running problem went away.

With this knowledge of the operation of my 357XP I feel that I would know if the accelerator pump would not work? But I also see the possibility that a bad accelerator pump carb could be tuned to run the chainsaw with a bad pump by making the mixtures richer but the overall running characteristics would not be as good as original new condition.
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Offline Al Smith

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Re: accelerator pump on carb
« Reply #4 on: October 15, 2013, 06:30:08 pm »
If you look at a chainsaw carb the idle or low speed jet is on the engine side of the butterfly valve .To make the transition from idle to high speed it takes a pressure drop across the high speed jet which is on the outboard side .The achieve this the idle circuit has to provide enough fuel for at least a short period of time until the high speed cicuit can take over .

Not to change the subject but in the case of a torn boot the saw is getting air ahead of the butterfly .If you have never experianced it you would be surprised how fast an engine can run only using the low speed jet if a condion like that arrises .

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Re: accelerator pump on carb
« Reply #5 on: October 15, 2013, 07:58:39 pm »
Thanks, so the accelerator fuel comes in on the idle side of the carb and helps give the saw more transition fuel from idle to part throttle and from part throttle back to idle. So if you have to richen the low speed way past basic you could assume that you might have a pump problem?
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Offline 660magnum

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Re: accelerator pump on carb
« Reply #6 on: October 15, 2013, 08:03:03 pm »
True
We should share what we know... someone may learn...
That knowledge can live after us... and that "Pays It Forward".
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Online jmester

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Re: accelerator pump on carb
« Reply #7 on: October 15, 2013, 08:14:57 pm »
Thanks for the info guys.
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