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Old 09-04-2006, 11:13 AM
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Off idle stumble

I have a 71 Z28 Camaro RS with a 383 stroker engine, an MSD digital 6+ and a HEI distributor with an Accel Super coil in the cap. Ever since I installed the Digital 6+ box my engine has been stumbling on hard acceleration. When I installed it I wired everything per the MSD instructions including running the magnetic pickup wires away from the coil trigger wires and spark plug wires. I also had to rerun the ground wire from the coil to the negative battery terminal as the original owner had just run it to the firewall. I also installed an MSD performance cap and rotor kit and a set of Taylor Thundervolt wires. Like I said it has a very bad hesitation or stumble off idle under hard accelleration, if I just ease into the throttle there is no problem. However, I am building this car to run at the drag strip so I really need to be able to "get on it". Thanks in advance for any help you could give me, Bobby

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Old 09-04-2006, 12:12 PM
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Quote:
Originally Posted by TorqueJunky
I have a 71 Z28 Camaro RS with a 383 stroker engine, an MSD digital 6+ and a HEI distributor with an Accel Super coil in the cap. Ever since I installed the Digital 6+ box my engine has been stumbling on hard acceleration. When I installed it I wired everything per the MSD instructions including running the magnetic pickup wires away from the coil trigger wires and spark plug wires. I also had to rerun the ground wire from the coil to the negative battery terminal as the original owner had just run it to the firewall. I also installed an MSD performance cap and rotor kit and a set of Taylor Thundervolt wires. Like I said it has a very bad hesitation or stumble off idle under hard accelleration, if I just ease into the throttle there is no problem. However, I am building this car to run at the drag strip so I really need to be able to "get on it". Thanks in advance for any help you could give me, Bobby
What carb you got? You may need to step up on the squirter if it's a holley!
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Old 09-04-2006, 12:24 PM
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It is a Holley double pumper and I recently did a rebuild on it and also upgraded the main body from a 650 to 750, but this started before I did the rebuild. I thought it might be the power valve so I replaced it along with everything else in the rebuild kit. So with the MSD box I would need to go with a larger squiters???? How would I know how musch larger to go? (still kind of a novice when it comes to carbs) Thanks.

Last edited by TorqueJunky; 09-04-2006 at 03:40 PM.
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Old 09-05-2006, 02:40 PM
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There are two kinds of stumble off the line, a lean stumble or a rich bog...

If you are leaving behind a puff of black smoke, you're rich and your mechanical secondaries may be loading up the engine. This does not occur often, especially if you're driving a manual transmissioned car. With an automatic without a high stall, the double pumper is guarandamnteed to create a rich bog off the line.

A lean stumble will occur if: 1. your power valve is not calibrated to the engine vacuum: it should be approximately 2" below idle. If you're pulling 10" at idle, the PV should be an 8.5 or 7.5.... 2. your accelerator pump is not able to fill the hole created when you slam the butterflies open and allow all that fresh air to rush through the venturis... 3. you can buy a set of larger squirters and the hollow screw which holds them in place. Experimentation is the only way I know how to see the effect of an increase in squirter size will work. Plus, you can play with the cams on the accelerator pumps to bring the secondary squirter into play just slightly later in the stroke.

A bog (too much fuel) could also be the result of a timing curve which is not steep enough and a lean stumble can in part be the result of insufficient initial mechanical advance not in phase with the vacuum advance shutoff when the throttle's slammed open.

So: double check your initial timing lead.
double check the advance curve to see at what rpm you are at full mechanical advance.
put a vacuum gauge on the engine and see what you're pulling at idle, and install a PV which is stamped within 2-2.5 inches of your idle vacuum.
have a buddy check out your exhaust when you accelerate off idle hard, if it's black smoke you're loading up...normally a bog is a brruuunnnghhhh...rrrrrrrrrrr

a lean stumble will be a rrrun..uunnnnnnnnnn....rrrunnn. Lean stumbles can create an occasional backfire, bogs usually don't.

The addition of the MSD box shouldn't be the root cause of your problem. If anything it should make it simpler to get the car to launch because the spark is consistently hotter up to 3000rpm.

I hope some of this is helpful, and well realize there are probably other people on the forum far more knowledgeable than I, but I've had this problem before and solved it by taking it step by step.

good luck

Paul
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Old 09-05-2006, 03:33 PM
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accelerater pump needs more stroke maybe? i fixed the same problem on an edelbrock.
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Old 09-07-2006, 09:57 AM
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Thanks for your help guys, it was definately leaning out. I double checked the clearance on the accelorater pump arm and there was too much slack there. That got rid of most of it (about 75%). I had some extra squirers laying around so I put one on the drill press and drilled it out to .035". I installed it over the primaries and it worked great. So I went out and bought a set of directional #35 squirters and installed them this morning. That got rid of almost all of it (about 95%), so I hope that the rest of it is just going to be getting the timing just right. Thanks again for your help guys, very informative, I'll let you know if I have any further issues .....................Bobby
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Old 09-07-2006, 10:36 AM
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Quote:
Originally Posted by TorqueJunky
Thanks for your help guys, it was definately leaning out. I double checked the clearance on the accelorater pump arm and there was too much slack there. That got rid of most of it (about 75%). I had some extra squirers laying around so I put one on the drill press and drilled it out to .035". I installed it over the primaries and it worked great. So I went out and bought a set of directional #35 squirters and installed them this morning. That got rid of almost all of it (about 95%), so I hope that the rest of it is just going to be getting the timing just right. Thanks again for your help guys, very informative, I'll let you know if I have any further issues .....................Bobby
There should be no slack in the accelerator pump arm ,this assures you of instant pump shot, you just need a to be able to get a .010 gauge between arm and pump at Wide Open Throttle.
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Old 09-07-2006, 11:56 AM
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Sorry that is what I was trying to say, I adjusted it to .015" clearance per the Holley rebuild specs. Would the additional .005" help a even a little more?
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Old 09-07-2006, 01:04 PM
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Quote:
Originally Posted by TorqueJunky
Sorry that is what I was trying to say, I adjusted it to .015" clearance per the Holley rebuild specs. Would the additional .005" help a even a little more?
No,it don't really matter the setting as long as you have instant pump shot and the pump arm still has some slack at WOT. What you are shooting for is the most fuel delivery you can get without coil binding the spring on the pump arm. The spring compresses at instant WOT and keeps on delivering fuel as it decompresses, thus delivery of fuel is uninterupted to squirter up to the CC's capacity of the pump.
I have seen people take the little spring on the arm and adjust it to coil bind, and when they try to get instant WOT the gas pedal will not move, because the gas in the pump acts as a hydraulic lock, until the fuel can slowly dump through the squirters.
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