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Old 06-28-2013, 08:43 PM
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accelorator pump cam adjustment

Put on a new quick fuel 680 vs carb a few days ago. Ive got the idle and mixture all set now and she run great. Seems to have a lot more pull than the old edelbrock, however,, im trying to get rid of an off idle stumble. Ive read posts on pump cam adjustment and my head hurts now. The carb came with a pink cam set on #1 hole on the cam and #1 hole on the linkage. What is the effect of going to the #2 hole on the cam and the #2 hole on the linkage? Or #2 on the cam and the #1 hole in the linkage? I think it needs more of a pump shot. By changing cam holes on same cam does it change amount of shot? Duration of shot? Or what...... also the cam has a third hole.. more shot? Duration? And would it go in the #1 or #2 hole in the linkage?

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Old 06-28-2013, 09:03 PM
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go to the second hole on the cam.If you look at the shape you will see it will be a little more aggressive at the beginning.(steeper ramp) make sure the pump is adjusted properly.You also have the options to have a bigger pump and different size squirters.Where is the float level set ATM?
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Old 06-28-2013, 09:09 PM
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Do I use hole#1 or #2 on the throttle linkage? Fuel level is set at about 1/4 up the sight glass.
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Old 06-28-2013, 09:15 PM
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You have made sure the pump is operating correctly? It should shoot 2 streams of fuel the instant the throttle is moved, if it don't there could be a problem with the pump diaphram or the check ball in the pump housing.
Also make sure there is no fuel dripping from the boosters while the car is at idle. If there is then you don't have enough base timing and the throttle is opened too far at idle so it's off the idle slots and into the mains, if this is the case it too can cause a stumble off idle.
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Old 06-28-2013, 09:21 PM
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Base timing is at 14 and getting 2 streams from the booster. It just seems it needs a bigger shot.
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Old 06-28-2013, 10:19 PM
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If it's 14 and without vacuum advance that's too low.
What's the combo?
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Old 06-28-2013, 10:23 PM
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Old 06-29-2013, 02:26 PM
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A little more timing and a jump to a 35 pump nozzle from a 31 did the trick. No off idle bog now
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Old 06-29-2013, 02:49 PM
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post a video if you can
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Old 06-29-2013, 11:03 PM
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Are you using a vacuum advance? If so then disconnect it and plug it.
Reset the base timing to 12 degrees before top dead center.
Reconnect vac advance, make sure it's connected to a manifold source, this is a source that pulls vacuum at idle. When you connect it right away the idle will go up, bring it back down to around 650 rpm. Recheck spark timing. It should be around 20 to 24 degrees before top dead center.
Next, reset the idle mixture screws. Afterwards, take it for a test drive. You'll be amazed at the difference it made. When the base timing is too low as you had it, to get it to idle it moves the butterflies off the idle slots so it forces it to pull fuel from the main boosters. This will cause a tip in stumble as said.
This is another reason why more base timing is needed via vacuum advance. The higher timing means higher idle speed, so you can lower the idle speed back down to the idle slots so mixture screw adjustment comes back to life.
The next reason for adding it in with vacuum advance is it elimates the hot start problem as there is no vacuum when the engine is running so the additional timing the vacuum advance gives it doesn't come into effect until it starts.
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Old 06-29-2013, 11:47 PM
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14 was base timing. Total is 34 with full manifold vacuum advance. I added 2 degrees base and reset the idle mixture using a vacuum gauge. After that it still had a very slight stumble off idle. Still driveable but couldnt go full throttle from a stop, had to feather it a bit. The change to the 35 booster nozzle took care of the hesitation. It now pulls very strong from idle thru 6k. Man, what a difference from the Edelbrock that was on it. Now planning a change to 3.73 gears.
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Old 06-30-2013, 10:10 PM
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Something I missed here. If total is 34 with full vac advance that's too low.
Base and total are set without vacuum advance. When full manifold vacuum is used as most of us use on small builds we connect it after base and full timing is set. If the vac advance is adding say 10 degrees, that means full timing would be around 24 degrees with vacuum advance not connected and that's too low.
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Old 06-30-2013, 11:08 PM
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Base - 16 now was 14 vac advance plugged at idle
With vac added 26 at idle
34 at about 3000 rpm
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Old 07-02-2013, 07:50 AM
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Thanks for clearing it up. Looks good.
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Old 07-02-2013, 09:30 AM
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base timing is still too low, pull out some mechanical advance and set your base for about 20-22 with about 35 total. Then you'll really be amazed.

BTW, not that your new carb is bad by any means but that Edelbrock could have been tuned to perform flawlessly on this combo, its a very mild combo and pretty easy to tune one for. When you start getting into much larger racing cams it can have problems though.
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