Hey guys, I need some insight as to why my car is popping at the top end of the track. I figured it might be lean at the top so, I jetted the 750 vac up to 74 from 72. This didn't work. I also took out some timing. I dropped to 31deg from 34. This also did't work - popping increased slightly. I am using an HEI ingnition and I am starting to think that an MSD or a Jacobs ingnition may help, but I don't want to recklessly throw money at the car. I cross the line at 6000 rpm. Has this happened to any of you? If so, how did you fix the problem?
Thanks in advance,
Daniel :confused:

It could be valve spring related. A buddy had a similar problem with the recommend Comp springs for his cam. Springs with more seat pressure fixed it. I don't know of any way to test this without changing springs.

are you getting any more blow by than normal? I had that happen with a broken piston ring/top land before. Probably as Pat said...broke valve spring

Happened to me a few times when I was using nitrous. I was using a spark plug heat range that was too hot, the mixture was being lit up by the hot plugs (long before it ever fired).

Probably not your issue, but just something to consider.

-Bob Cunningham

Last Dec. I had the springs tested and they were all in good condition with appropriate seat pressure. They are double coil springs and my cam only has a .510 lift. I will open my valve covers and check for a broken spring. Are you guys sure it could not be the ignition cutting out at the top end. It has plenty of power down the track, but starts popping at about 30yds before the finish line. I was using a 50 shot of nitrous, but my plugs are a heat range colder than normal, and I was using a 50/50 mix of VP 112 and 93 pump gas.

Fuel pressure??? If I had that combo I think that I'd be running double pumper.

I've had the same problem. I fixed it by degrees.

1) Fuel starvation -- Bad fuel pump.

2) New valve springs (only 100# of seat pressure!)

3) Intake manifold -- I blocked off the exhaust cross-over on my stock low rise iron intake manifold and that did improve top end performance somewhat. The Edelbrock RPM AirGap helped a bunch more. But it's no fun in the winter...So Cal winter at that!!

4) Worn out cam -- Flattened exhaust lobe. Exhaust popping back through intake valve.


I'm engaging in a turd polishing endeavor. I'm buying parts that will go on the new motor and "trying" them on this old dog. I have learned a tremendous amount by step-wise fiddling.

check your plug wires, what fuel pressure were you running, nitrous pressure, and plug gap? Sounds fat on the nitrous.

Does it pop on a motor only pass?

I appreciate all your comments, as far as the fuel pressure - I run about 6.5 psi, nitrous pressure is about 900-950 psi. I haven't done a motor pass in a while. I guess that should be the first thing I will do next time out. Plugs are set at .035. Wires are less than a year old with less than 600 miles on them. There are no outward signs of any damage to the wires. The engine idles and runs smooth and strong, just pops in 3rd gear near 6000rpms.
Daniel

Do you check your plugs after a run? How do they look?

Since you now mention that you are running nitrous, I would give more consideration to the spark plug heat range. Do a web search on how to read plugs to tell if you have the right heat range.

-Bob Cunningham

Plugs look very good. No signs of any detonation. I check my plugs often. I have decided to take a chance and purchase a Jacobs Fire Control 1000 ignition. Even if this is not the fix, I felt it is a good investment to have a good ignition system. Do any of you have any experience with this ignition system? I am planning on ordering the box as well as the matching Jacobs coil. They are both on sale at Summit. $115 for the box and $45 for the coil.
Daniel

You say 6.5 lbs fuel pressure. Is that at idle, or are you checking this pressure at racing speeds? Many times the fuel press will be fine at idle, and if you only have a press guage under the hood, you will never know what it is when you are using fuel, at WOT.

RB

I've had this problem before and it turned out to be too lean a mix.

If the engine is too cold (like 150* or less) it will do the same thing - pop at the top end no matter how rich the carb is set up.

My engine temp was averaging 160-170 deg. I dropped the timing a bit and the temp. quickly rose to 200 degs. Even at this temp. the engine popped. If the mix is lean, would using a bigger fuel jet help, or I could richen up the mix by running through the mufflers. I could also run an air filter. This may also help to richen the mix.
What do you guys think is the best way to richen the mix.
Daniel

Are you running with at least the air cleaner base? I picked up .07 with the base only.

Originally posted by djgaleana:
would using a bigger fuel jet help Like RB1 said, are you sure you are maintaining fuel pressure at racing speeds? If your fuel pump is insufficient (feeding both carb and nitrous), then the biggest jets in the world won't help.

-Bob C.

Thanks for all your replies! I will now order a stronger fuel pump with a regulator. I just talked to my neighbor who had the same problem. He increased the fuel pressure and the popping stopped.

Fatter jets in the secondary, or heavier spring in the diaphram to open the secondaries a little slower. But you might want to check your fuel pressure first.

Before you spend a bunch of money on unnecessary parts you should figure out the problem.

You should figure out if its a N/A problem or a NOS problem. What do the plugs look like?, what was the bottle pressure at the end of the run?
If it was lean your plugs would show it.