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Old 09-28-2010, 09:02 AM
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distributer on 5.7chevy

Needing some more help w/the project truck guys!! 1990 GMC w/5.7l, I have changed everything over to carb instead of tbi. Have also added comp,thumper cam,headers, and true dual exhaust w/flow masters. Sounds good and acts good through about 3500 rpms then bogs down through bottom of accelerator pedal, have tried two different carbs and didn't help! Also have a msd distributor but using the same factory connections! Do I have to use the old style distributor with points and all or can I remain with this one ? And could this be the problem w/the upper rpm's?

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Old 09-28-2010, 01:17 PM
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What carb is there now, what's your idle timing and total?

If it's running I doubt it's the distributor unless the weights are screwed up or the advance isn't working. Almost sounds like a carb issue. Bogs can be from over rich condition or secondary not opening.
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Old 09-28-2010, 03:02 PM
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Thanx for your help bubba, the idle timing is 8 degrees after tdc, not sure about the total? The fist carb that was on it was a holley 650 w/mechanical secondary, then I thought it was a carb problem also and went w/a new holley 750 w/vacuum secondary! Still same problem? Its stock bore and stock ports on heads. Only that has changed is the valve springs.
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Old 09-28-2010, 03:55 PM
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Make sure the timing is advancing. Watch the timing mark and bump the throttle. Should be around 32-34 degrees total.
What do you mean by "MSD distributor with same factory connections"?
Or the fuel pump is not strong enough for Holley when the secondaries are fully open. What kind of setup are you using for a fuel pump?
Or clogged fuel filter.
Or kinked fuel line.

vicrod
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Old 09-28-2010, 04:05 PM
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Quote:
Originally Posted by gmc1990rodder
Thanx for your help bubba, the idle timing is 8 degrees after tdc, not sure about the total? The fist carb that was on it was a holley 650 w/mechanical secondary, then I thought it was a carb problem also and went w/a new holley 750 w/vacuum secondary! Still same problem? Its stock bore and stock ports on heads. Only that has changed is the valve springs.
With that Cam you probably need about 16* BTDC Initial @Idle. You said you have 8* ATDC which would be severely retarded timing. Total timing = Initial + Mechanical about 34*-36* all in by 2500-3000rpm. And Vacuum advance connected to full manifold vacuum port.

Last edited by SSedan64; 09-28-2010 at 04:12 PM.
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Old 09-28-2010, 04:12 PM
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The distributor is an aftermarket made be msd, its a performance distributor that has the same set up and connections as the oem for 1990 5.7. And I will set the timing @ 16 btdc and try it! Keep you posted
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Old 09-28-2010, 04:23 PM
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The 750 vac secondary is s better choice for a street car. Check the total timing as mentioned too.
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Old 09-28-2010, 04:40 PM
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Quote:
Originally Posted by gmc1990rodder
The distributor is an aftermarket made be msd, its a performance distributor that has the same set up and connections as the oem for 1990 5.7. And I will set the timing @ 16 btdc and try it! Keep you posted
If you did away with the TBI & ECM/computer, you now have the wrong Distributor. No way it should hook up to the Factory plug as thats for a CCD/ComputerControlledDistributor. You should have a normal HEI, one wire hookup = +12V.
The ECM will be looking for Input from TBI sensors etc. & may be screwing the timing up.
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Old 09-28-2010, 04:47 PM
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Does it look like this? >> http://www.summitracing.com/parts/MSD-8366/?rtype=10
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Old 09-28-2010, 06:34 PM
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Yep that the distributor that I have! So I need the HEI right? thanx for everyones help!
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Old 09-28-2010, 08:33 PM
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Quote:
Originally Posted by gmc1990rodder
Yep that the distributor that I have! So I need the HEI right? thanx for everyones help!
If you're no longer using the TBI & ECM, yes.
You can use the Large cap type w/built in coil >> http://cgi.ebay.com/ebaymotors/CHEVY...Q5fAccessories or the Small type w/external coil >> http://cgi.ebay.com/ebaymotors/CHEVY...Q5fAccessories
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Old 09-29-2010, 02:59 PM
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Whats fuel pressure at? Still using the original pump? The stock tbi pump might have a little too much pressure without a regulator. Or its dying and under spec.
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Old 09-29-2010, 03:07 PM
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Still using stock pump that's running at 16psi, and I'm using a regulator set at 7psi. Although I've tried it at all pressures between 5-8psi
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Old 09-29-2010, 03:35 PM
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But is this distributor change necessary?
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Old 09-29-2010, 11:04 PM
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Quote:
Originally Posted by gmc1990rodder
But is this distributor change necessary?
Yes, it will not run right with a TBI style distributor in a carbed motor. The distributor you have has no timing curve in it, it has a locked rotor and the timing curve was controlled be the computer with feedback from the various engine sensors. You have no upper rpm power because you have not enough timing at any rpm above about 1500rpm.

All you are doing is running in "limp home mode" now.
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