Chev S-10 w/ v8 bad miss in high RPMs

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  #1  
Old 05-29-08, 07:45 PM
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Chev S-10 w/ v8 bad miss in high RPMs

I have a high performance 350 v8 in an s-10 truck. Basic specs are: NEW HEI distributor w/ Accell super coil and MSD 6al box (7k chip installed). Edelbrock Performer 600 carb/Perf. RPM intake. Stock 2.8/4.3 TBI in-tank pump w/ Mallory return regulator. Comp Cams XE274H cam. (230/236 @ .050").

Here's the problem: Engine idles great, throttle response is great with no load. Once the engine is under load it stumbles a little off-idle but picks up quickly. Revs up to 3500rpms okay but after that it misses and falls on its face. (sounds like the rev limiter is kicking in but I've checked and changed the chips between 6000, 7000, and 8000rpms and had no change). I have installed a vac. gauge on the intake, a fuel pressure gauge at the carb and verified readings. With timing set at 8-10* BTDC vacuum is 10-13" with idle set at about 875rpm and fuel pressure is a constant 5.5psi. I've checked the inner parts of the distributor and everything is as it was from the parts store except the ignition module was removed when the MSD was installed. Cap and Rotor are new. Things that were suggested to do was try running with Vacuum advance unplugged which I did try but didn't have much better results. Vac. Advance and mechanical advance operate properly as well. What do I do? This engine used to run really really well but since its sat for a few years without being driven its gotten a few bugs.. Suggestions or thoughts? I'm all ears.

Thanks in advance and sorry for the encyclopedia of a post.
 
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  #2  
Old 05-30-08, 08:13 AM
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it could be alot of things and it may be cheaper to have a shop look at it and diagnose it while its a simple carburated engine hooking up a scope can find a problem in the ignition system really fast and atleast narrow it down to a fuel or ignition problem.
if you still have the old ignition module would probably start by installing the stock module and bypassing the msd to see if theres any change.
 
  #3  
Old 05-30-08, 03:12 PM
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The ignition module is gone. If it is fuel related, I have it narrowed down to the carburetor. Ignition system is narrowed to either faulty plug wires or vacuum advance over-advancing the spark. The carburetor was rebuilt a year ago and the spark plug wires are New Accell 7mm. I talked to a guy at a local speedshop and he thinks that the vacuum advance is advancing the spark too far and to unplug the vacuum advance and plug it and try it that way. If I could afford to take it to a shop I would but I don't have the cash to do that. It can't be anything real major since it used to run really good.
 
  #4  
Old 05-30-08, 05:28 PM
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you stated that you had already unhooked the vac advance and it didnt run any better would suggest you hook it up to manifold vacum instead of ported vacum if its not already.
if you can get it to act up in park or nuetral you can usually tell if its a fuel related problem by using carb cleaner and enriching the mixture by squirting a small amount down the carb if its lean or running out of fuel it will start running better the second you enrich it with carb cleaner, do not use very much and you might want to have a fire extinguisher handy or some wet towels to smother out any flames that may occur if it backfires.
the only components of the ignition system that would actually cause the vehicle to fall on its face would be ignition coil, your msd module, or pickup assembly you might make sure that the coil ground strap is installed if you using the top mounted coil as it gets left out often usually destroying the coil and eventually the cap and rotor also.
 
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Old 05-30-08, 05:28 PM
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You forgot to say something real important: Did the car always do this ever since you 'built up' the engine? Or did it run beautiful, and suddenly start this, out of the blue?

As much as you said, we do not know everything. It never hurts to say too much.

How many miles on engine?"

What all was torn down and inspected? Cam for wear, valve springs to make sure valves do not float at high rpms, etc.

Are secondaries opening up when they should?

What do you know about the coil?

Have you looked for carbon tracking?, or arcing at night?

Under throttle/ load, as you should know, the advance actually backs off, rather than increase, to prevent detonation. Under load and low rpms, the advance curve backs off as centrifugal advance is also low then - and then as you continue to accelerate, even under load, the centrifugal advance will start increasing the advance curve some. If you let off the throttle at high rpms, you will be at maximum advance all the way around, as you are at least risk of detonation then.
 
  #6  
Old 06-03-08, 03:28 PM
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my 2 cents

It is possible you have valve float. Have you done anything withe the valve springs ?
Many many years ago a buddie shoehorned a V-8 into a porsche and it did the same thing at high rpms.
 
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