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Discussion Starter · #1 ·
I have a 81 Firebird with :
468" Olds (455" .060 over)
Comp Cams XR276HR-10 cam (224/230 @ .050 )
w/ 1.6 Harland Roller rockers
Forged Pistons
Edlebrock Heads (Triple port Job?)
Air Gap Dual Plane Manifold
750 cfm Holley HP Double Pumper
3.73 Positraction
28" tires / Slicks
Balanced rotating assembly

When I accelerate at a medium rate everything is Good up to 4500 rpm (new engine). If I stuff it, it goes thru first and second OK but t the top of 2nd or 3rd it spits fuel back up through the carburetor where you can see it and I get fuel on the windshield ( engine seems to almost stop and has some times? ).
Fuel pressure is between 6 & 7 lbs. Before the rebuild I ran this engine with a 1050 Holley Dominator on a single plane Edlebrock Victor Jr. and went 116 mph @ 11.31 (consistently) no problems.

Why is this happening? Any suggestions?

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Blowing mixture out the airhorn sure speaks to reversion. That could be crappy flow dynamic between the manifold and cam timing, but as mentioned sure could be an inability to seat the intake valves, hence my earlier question about what lifters are used with that cam, push rods, and valve springs, Edlebrock's springs and studs or someone else's.

If the fuel source is splash in the float bowls coming from the vent tubes which on a Holley are right above the airhorn, it could be a venting issue where the fuel refill is splashing up the vent tube. Hence Holley whistles in the float chamber and using lengths of tube to stand the vent higher or tie them together, as done on marine carbs with a Tee in the center that may connect to a longer tube to simply catch the splash using length.

Bogie
 
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Discussion Starter · #12 ·
Bogie, Thanks for reflecting on my problem. I do have a new/different intake manifold. i'm using an Edlebrock RPM Air Gap Dual Plane and was using a Victor JR. Single Plane Air Gap Manifold and a Holley-HP 750 cfm now. I was using a 1050 cfm Holley Dominator on the Victor Jr.? Could a 1" Spacer help?

I thing I'm using Edlebrock Studs and Comp Cams Push Rods from the Kit.
 

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750 cfm may not be enough carb. You might be pulling the max fuel the jets will allow. Try throwing 10% larger jets in carb.
My 70 boss 302 came with a 780 cfm vac secondary.
Or you could put in some heavier power valves, but these wouldnt like the sneeze, probably blow out the diaphrams.
 

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Race it, Don't rice it!
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“Lean Sneeze” is a a slang term I use to describe when a carburetor carb goes lean and pops back out the carb and kinda makes a sneezing sound.
It happens sometimes when the intake valve is open and the mixture fires or when the excessive fuel collects in the intake tract.
IE, carburetor backfire vs exhaust backfire.
 
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Discussion Starter · #18 ·
“Lean Sneeze” is a a slang term I use to describe when a carburetor carb goes lean and pops back out the carb and kinda makes a sneezing sound.
It happens sometimes when the intake valve is open and the mixture fires or when the excessive fuel collects in the intake tract.
IE, carburetor backfire vs exhaust backfire.
Thanks for the input. I'll explore this option with my mechanic, I gues if the intake valv e is open then the cam must be installed wrong?
 

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with the cam installed too advanced, the engine can act like it has a review limiter...I have had a couple cars come in like that, they run OK until they get to some higher RPM and then just lay over like a review limiter. Weak valve springs can give the same behavior.
 
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Discussion Starter · #20 ·
That is something to look at. The MSD6AL-2 Digital Ignition has a rev limiter but it's set at 6100 rpm .... I'll look at it. There's a Launch function I don't use but is set to 1500 rpm last I looked, recently. I'll check again. ....Thanks
 
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