Hi all. I have a 383 with aluminum heads that when first built ran 7.8 in the 1/8 in my 86 Z28. Motor is a stock 4 bolt block 30 over with Patriot 64cc heads. Air gap intake open and a 750 vacuum Holley. The last number of track passes it’s gone from the above to 9.0 and seems to be just sluggish at higher rpm (barley get to 5k now). Have checked valves and timing (35) at 3k same as first runs. It has stock tank with return line plugged as a new fuel pump without return was installed. Only thing I can think of is it’s running out of fuel at top end. Maybe the sock in the tank or vent line at the tank? These have never been touched. Please let me know your thoughts. I know I am leaving some engine stuff out but nothing has changed and just keeps getting slower.🤮🤮

 

Air flow restrictions, either into or out of the engine, might be involved.

 

If the engine is starving for fuel at high rpm causing the carb to lean out it will clearly shiw up indicated by a change in WOT AFR as seen on a AFR guage.
This is a better indication of whats really going on than say a fuel pressure guage is.
It need not be expensive.. I use a Narrow Band AFR guage with heated type NB Oxygen sensir.
No calibration errors... easy to install and use.
Clearly shows if when the fuel system cannot keep up to engine demand @WOT as the guage clearly leans out under WOT as rpm climbs.
If the fuel system is good the AFR guage will stay solidly in the typical "rich zone" on the guage @WOT. If its running out of gas @WOT the guage will clearly shift progressivly LEANER
.85 mv +/-. is normal for WOT AFR.

The stock fuel line is a very likely suspect.
Replace it and reroute it away from heat sourses.
Headers. trans etc.
On F-body cars ,with a engine mounted mechanical fuel pump, adding a electric "pusher pump" at the back eliminates all the BS with hit soak vapour lock in the summer by presdurizing the fuel feed line going up to the mechanical fuel pump.
A Carter P4070 electric fuel pump works great for this.

At the track, what is the MPH from drag ET slips?

Re activating that fuel pump return line , or reusing the stock fuel line as a return line really helps too.
Plumb the return line AFTER the carb fuel inlets.
A simple .,030" (1/32") bleed orrifice on the return line will circulate enough fuel all the time to help keep the fuel and pump a lot cooler too.
No regulator is needed.
Eliminating that return line creates all kinds of issues on these cars.

 

Low cost. Tells you a LOT.

 

Have you gotten anywhere with this?

 

Valve springs.

 

Thanks all for reply. Think I have it figured out. Faulty new fuel pump.

 

That's where I would have gone next.