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Stumble/Hesitation Issue at High RPM


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Posted (edited)

Hello... I've been dealing/troubleshooting a stumble issue at 3500-4000 RPM and above and hoping to get some suggestions for where to go next.  Here's a little history on my setup:

  • Original L24 engine bored .30 over
  • Triple Weber DCOE 40 Carbs (currently running 55 idle jets, 135 main jets, and F11 emulsion tubes)
  • Electric fuel pump with regulator set to 5 psi
  • Pertronix Electronic Ignition with Flamethrower Coil (bypassing the ballast resistor)
  • NGK Iridium Plugs (gapped to .40)
  • Vacuum Advance NOT Connected
  • Timing is retarded at the (stock) distributor

The issue mainly seems to occur when it's hot outside and the car is warmed up.  It idles totally fine, but when I push it, I get a stumble/hesitation at around 3500-4000 RPM and above.  On a hot day, I don't notice the issue at all until the car gets hot... and on a cold day, I don't seem to notice the issue much at all.  It's really baffling me... I've tried different main jet sizes but it doesn't seem to help.  I also installed a heat shield for the carbs and I put in hood vents (to help dissipate the heat), but no dice.

 

I read somewhere that a rich condition could cause the stumble/hesitation when the engine is warmed up, but I experience the issue with all of the main jet sizes I've tried, all the way down to a 125 main jet, so I don't know if that is really the issue or not.

 

Any ideas/suggestions?  Thanks in advance!

Edited by eastcoastz
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you could fit a wideband o2 and gauge and then you'd know if it was a fuel problem. 

I've had problems like this with a couple of pertronix igniters. So I'd mainly try to get/borrow a 280zx distributor and see what that does for it. 

 

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I switched back to points and it's running much better.  When I removed the pertronix, I noticed that the plastic piece/ring at the bottom of the magnet piece was cracked... not sure if that had anything to do with the issue.

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