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Author Topic: Mystery Solved But Not Sure Why  (Read 3968 times)

Offline jspell

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Mystery Solved But Not Sure Why
« on: May 10, 2015, 04:04:42 PM »
I mentioned my dilemma on the main forum a few months back, but wanted to report on Spuds forum.  My combo is not injected, but is a 422" SBC with a HRE sheet metal intake and a dominator.  Engine ran great on the dyno, then bolted into frame and engine won't idle.  I've been chasing the problem for months, I completely changed all of the Mallory HyFire VII ignition to MSD 7AL3 to match the dyno setup (no change).  Checked and re-checked all wiring in the chassis and re-checked entire fuel system, purchased a new carburetor, nothing worked.  Today, I took off the 2" up-style zoomies and installed the exact down style collector headers (borrowed from engine builder) that were used on the dyno, presto...the engine runs like a champ...idles perfect.  Anybody have an idea why the zoomies don't work?  I forgot to mention I also have a vacuum pump system pulling 12 Hg.  The carb would never get on the idle circuit, you had to keep hitting the squirters to keep it running.
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Offline H.G. Wells

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Re: Mystery Solved But Not Sure Why
« Reply #1 on: May 10, 2015, 07:08:03 PM »
A little back pressure is a good thing.
Was the cam designed to run with zoomies or collector type headers?
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Offline jspell

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Re: Mystery Solved But Not Sure Why
« Reply #2 on: May 10, 2015, 07:30:37 PM »
Collector headers I presume, engine was in a door car when I purchased it...
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Offline H.G. Wells

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Re: Mystery Solved But Not Sure Why
« Reply #3 on: May 10, 2015, 08:25:35 PM »
I said back pressure, but more correctly collector headers offer a scavenging effect. I do not claim to understand all of the engineering behind it, but the exhaust pulse from the other cylinders helps to evacuate the spent gasses. When I have had cams ground for a particular combo the type of exhaust is one of the factors that determine the design.  Just as when having Spud lay out a fuel system that is one of his questions. Perhaps it is even more critical with a carburetor?
 
The sbc I had in my FED to start was out of a street rod and not really set up for my combo. Full exhaust and carbureted motor, put zoomies and a stack injector on it and it did not perform like it should. Had a new cam ground that tricked the motor into thinking it had more compression to make better use of the alky, and more free flowing exhaust and it woke right up.

I bet someone here who understands overlap and cam design can give a better explanation.
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Offline masracingtd1167

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Re: Mystery Solved But Not Sure Why
« Reply #4 on: May 11, 2015, 12:10:47 PM »
John that's an odd one ! I wonder if you had less manifold vac with the zoomies ? I think you will make more power with the collector headers !   
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