Any carb gurus here?

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Any carb gurus here?

Post  Brandon D on March 17th 2015, 11:16 pm

Motor is a 460 with an Edelbrock Performer 460, Holley 770 street avenger with vacuum secondaries and everything else is stock to my knowledge.

Just rebuilt the carb due to letting the fuel tank run empty and the fuel filter was full of dirt and it started to run like crap.

I have the mixture screws set at 1.5 turns out, idle at 750 rpm, timing at 12* btdc and I put a vacuum gauge on it today and got 19" of vacuum at idle in park and 15.5" in gear.

My problem is it sputters under light throttle from idle up to about 1500 rpm and it has a split second dead spot from idle under WOT in park. But, if I give it light throttle from say 900 rpm there is no sputter and no dead spot under WOT, only from idle. It has #70 primary main jets and #75 secondary main jets. When I rebuilt it I put a 6.5" power valve back in it but the rebuild kit came with a 10.5" so I tried that with little to no improvement.
I heard there are two ways of determining what power valve you need. 1. Divide vacuum reading by two... 2. Subtract two from vacuum reading.

Questions:
Do I need a power valve that's closest to 7.75" without going over? Or one that closest to 13.5 without going over?
Would changing the main jets help?
Or does the problem sound like the accelerator pump? Which I know nothing about...

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Re: Any carb gurus here?

Post  supervel45 on March 18th 2015, 12:17 am

I would check your acelerator pump shooter and make sure no trash is stopping up the nozzles. Make sure it has a gasket under it, and the check ball or neadle is in place also, under the shooter. A 10.5 powervalve should be OK for the vacuum you have. I did not think they make powervalves higher than 10.5? How did the carb run before it got dirty?

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Re: Any carb gurus here?

Post  maverick on March 18th 2015, 12:30 am

Check adjustment of the accelerator pump linkage.  Look carefully as the throttle linkage is opened from idle.  If the throttle can be opened a bit before the pump begins to shoot you may have a stumble as you come off idle.

If it ran ok before you picked up dirt, why would you suddenly need different power valves or jets?
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Re: Any carb gurus here?

Post  Brandon D on March 18th 2015, 12:54 am

supervel45 wrote:I would check your acelerator pump shooter and make sure no trash is stopping up the nozzles. Make sure it has a gasket under it, and the check ball or neadle is in place also, under the shooter. A 10.5 powervalve should be OK for the vacuum you have. I did not think they make powervalves higher than 10.5? How did the carb run before it got dirty?

When I rebuilt it I made sure there was no trash or dirt in the shooter and the needle and gaskets are in place.

maverick wrote:Check adjustment of the accelerator pump linkage.  Look carefully as the throttle linkage is opened from idle.  If the throttle can be opened a bit before the pump begins to shoot you may have a stumble as you come off idle.

If it ran ok before you picked up dirt, why would you suddenly need different power valves or jets?

It has always had the stumble and flat spot and I will look at the linkage in the morning and see when it opens.

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Re: Any carb gurus here?

Post  supervel45 on March 18th 2015, 1:24 am

If the linkage is OK, you might try a larger shooter and or a more aggressive pump cam. There are also two cam screw holes on the linkage for the pump cam. You could move it to the other hole and see if it helps first. Check and see what the number is on your shooter is now, when you check your linkage. Are you running vacuum advance?

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Re: Any carb gurus here?

Post  butterbean on March 18th 2015, 7:30 am

X2 on what he said and also make sure that the accelerator pump spring has no slack in it
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Re: Any carb gurus here?

Post  DaveMcLain on March 18th 2015, 7:54 am

Those Avengers carburetors are notorious for excessively lean part throttle metering and it can't be easily cured without changing to a different style of metering block. It can't be the shooter because you are experiencing the problem when the throttle is not moving.


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Re: Any carb gurus here?

Post  rmcomprandy on March 18th 2015, 9:03 am

DaveMcLain wrote:Those Avengers carburetors are notorious for excessively lean part throttle metering and it can't be easily cured without changing to a different style of metering block.  It can't be the shooter because you are experiencing the problem when the throttle is not moving.


Dave is correct ... it can be fixed but, not easily. It requires recalibration of the idle / off idle system and co-ordination with the main system "start-up".

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