Higher Rpm stumble!
Thread Starter
Pro
Joined: Oct 2000
Posts: 629
Likes: 1
Higher Rpm stumble!
OK, I now have my Big Block (Dyno says 471 Horsepower) up and running in my '79 'Vette. I have no problems starting, driving and or idling. However, when I go to the local drag track I have a problem making a full out pass without a major stumble from 4500 rpm. It feels like someone is just reaching over and shutting off the ignition but it does continue to run. It seems like its running outa fuel to me but wonder if anyone has ever been through such a thing.
The carb is a quadrajet which I have jetted up 2 sizes to "75" on the primarys. The fuel pump is a Holley 110 gph, which I bought on Ebay, so hmmmmmmm.
Even without a full pass, its gone 12.98 @ 105, hehehe with my .505 reaction time. And a 1.84 -60 foot time!
I have no pressure gauge, but I guess I'm wondering should I try a new pump first or a different carb. Help!
The carb is a quadrajet which I have jetted up 2 sizes to "75" on the primarys. The fuel pump is a Holley 110 gph, which I bought on Ebay, so hmmmmmmm.
Even without a full pass, its gone 12.98 @ 105, hehehe with my .505 reaction time. And a 1.84 -60 foot time!
I have no pressure gauge, but I guess I'm wondering should I try a new pump first or a different carb. Help!
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
It sounds like the secondaries are leaning out the engine. The stock setup for the 1970 390 HP 454 was 78/49 main jet/rod combo and "AX" secondary rods. I think this would be a good place for you to start with your engine. Try this and see what happens.
BigBlockk
Later.....
BigBlockk
Later.....
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 787
From: Lebanon Township New Jersey
Can you rev it past 4500 in neutral? If not, then it is probably ignition related.
Also, 1.84 60ft time - on street tires?
BTW - your reaction time has nothing to do with your ET or 60ft time.
Also, 1.84 60ft time - on street tires?
BTW - your reaction time has nothing to do with your ET or 60ft time.
CF Community Team
Joined: Oct 2004
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From: Cape Cod, Mass.
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I had a similar condition with my 68 BB just a few days ago. Over 2500 RPM's it felt like it was running out of gas. It was the ignition coil. Good luck. 
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
did you try the simple basics first and check that the fuel bowl float levels are correct? It the float level is too low it could be fine on normal driving but start starving for gas at high RPM's and WOT when it is sucking it down pretty good.
