Engine popping at high rpm

[ksr89]

We've been having a strange engine/electrical problem for weeks now. It only happens when the engine is hot. The car starts skipping/popping when it's up at full speed. We've changed coils, wires, spark plugs, rear pick up, alternator, ignition box (red one), fuel pump, carbs, and engine. We've changed just about everything we can and the problem still occurs. We've gone through every wire and checked to make sure all the connections are correct along with making sure all of the grounds correct. The problem seems to be getting worse week after week. The strange this is is that while on the track at full throttle it's making all the skipping and popping sounds but then it goes away for a few seconds and the engine gets back to normal then issue occurs again. It only happens when he's on the gas. We use just regular Sunoco 93.  No one at the track has any idea on what it is and I've been searching on here to find an answer and so far nothing. Any help would be great.

[Legends57x]

Not sure if this applies, but we had a similar issue earlier this year, unplugged tachometer and problem went away.  Sometimes tach go bad internally(solder joints, etc.)  However, intermttent misfire did it came back a few weeks later, installed brand new spark plug wire with brass ends properly crimped on and hasn't come back since. 

Hope this helps! 

[TG01]

Hate to sound silly here but I did not see the fuel filter listed as something changed? Is it possible the filter is dirty not allowing enough fuel through?
Usually that problem would result in a engine shutting down at high RPM but I did have a bobbed out Goldwing that a dirty filter caused all sorts of stress for me on?
It caused the carbs to overflow and then go crazy rich of all things.

[Winterracing]

check the fuel line that runs in the fuel cell.  I've heard of those getting sucked against the tank and it'll starve the motor of fuel

Dan

[VMS Motorsports]

Where are your grounds, and how are they attached?
How old is the fuel cell foam? Ethanol will cause it to come apart and clog fuel filters, or if using a higher pass through micron fuel filter, get into the carbs.
Do you run a hard-wired transponder? If so, is it wired to the fuel pump?

[IraceLegends77]

If your running pump gas take the top of the fuel cell off and look at the pick up line. The pump fuel eats this line up and will starve a motor.

[ksr89]

No we don't have a hard wired transponder, and the foam is less than one year old and our pick up tube in the fuel cell is positioned so that it can't get stuck to the side of the cell. We took apart the fuel filter and there's no debris in it. We have three grounds one from the fuel cell to the frame, one from the ignition box to the engine and the other is to that block under the dash where all the wires connect to. We believe we fixed the problem, it seems to be the circular metal piece that the pick up surrounds just needed to be replaced and now there's no more popping/skipping when on the throttle. Thanks for the help!

[s10custom]

Most folks around here wire the transponder with the fuel pump.  What are your thoughts

[VMS Motorsports]

Most folks around here wire the transponder with the fuel pump.  What are your thoughts

I used to run a hardwired transponder in conjunction with the fuel pump. when the transponder started going bad, it would mess up the fuel pump. Wasted 3-4 races trying to find the problem til the transponder died. Cut the wire and the car ran great.
I only run rechargeable now. Hardwired are not designed for racecars with alternators

[Legends57x]

Sorry Jim, but have to disagree with your last statement regarding running hardwired transponder with a car equipped with an alternator.  Ran an AMB hardwired transponder in my Superstock car for three years and same one for another year in my figure 8 car running at 14.5 volts with not one single problem.   We've run the Westhold hardwired transponder in our Legend dirt car going on four years now with no problem either.  I believe you had something else going on with you car.

[VMS Motorsports]

Sorry to disagree with your disagreement Mark, but Westhold confirmed this with me, that their transponders were not designed to run with a car with an alternator. That is why they have been re-designed 4 times in the last few years. The new ones are supposed to be fine, but I dont have the date that the latest generation started.

If the transponder finally died completely per the track officials being unable to score me, and the only thing I did was cut the wire and put on a rechargeable transponder and ran problem free the rest of the season, what, pray tell, was my problem if it wasn't the transponder?

[csm14701]

  I had a similar problem.  My car would run good in practice and the heat.  When the feature would come, my car all of a sudden broke up at high RPM.  It would pull 3/4 of the way down the straight, bobble , backfire then run normal again.  It would do this down every straight away and only in the feature.  It did so for several weeks. I then talked to someone about carb parts and realized my car was lean. Lean plus cool temps in the features (Night), meant it needed more fuel.  Changed the pilot jets, problem solved.  I really thought it was a electrical problem at first. 

[Zero Hero]

I am having the exact , I mean EXACT same problem.  I too have changed almost everything chasing this problem.  My next thing to look at (thanks to your post) is the fuel pick-up and the metal ring surrounding it.  My question is how did you know yours had gone bad?  What should I look for, I have never opened my fuel cell before.  Thanks, Matt

[phylzub02]

has anyone checked the valve adjustment . we had this problem when we had first bought a motor and did not know about valve adjustments . be sure too do this every 3-5 races ..

[Zero Hero]

Well I checked the pickup...no issues there, it looked fine.  Anyone have any other ideas?  I have tried 2 sets of carbs, traced every ground, new spark plugs, new fuel filter, new fuel pump, disconnected the tach, emptied the fuel cell and used fresh fuel....nothing seems to work.  The problem is difficult to diagnose because it doesn't occur on every lap.  I might go 3 laps where the car runs perfect, then the next 3 laps when I get into the throttle it stumbles, pops, runs extremely rough....then all of a sudden it will just start running strong again and run that way for a few more laps. Sometimes it will be the whole race and when that happens the motor is difficult to keep running at idle, other times I can run the whole race without issue?  HELP!!!