i just bought a 71 240z the guy just rebuilt the motor with about 2000 miles on the motor now. it has a L28 motor in it out of a 81 280zx also has all the drivetrain out of the 280. it has the stock 240 ignition with the 240 coil and distrbuter. he instaled carburetors form ZTheropy. thats about all that i know
So my problem is that the car misses/backfires when its at low rpm and the engine is under a load. past 3grand it runs great. it does it mostly when im crusing a 2500rpm or so then when i go to accelerate it misses/backfires. (its kinda hard to describe) it also does it when taking off from a stop. when i start from a stop the thing almost dies. for the first 10min of driving it runs almost perfect but it gets worse and worse the longer i drive it.
So i went through the whole carbs 2 times. cleaned them all out and checked the flotes nedel etc... i adjusted the mixture, synchronized the carbs, checked the timeing advance, changed the coil, checked the compression, and checked the plugs. NOTHING WORKS its gotten better but its still giving me alot of trouble.
i need help!!!!!!!!!
thanks

 

I suspect the dizzy. Remove the cap and check for play in the shaft. See if the breaker plate advances as it should and returns. Check the cap for cracks. It could be bad spark plug wires leaking to ground. The later E-12 dizzy works great if you can find a good one.

 

mine does the exact same thing!! I know what you mean, when you go into first gear does it seem like its going to die?? its driving me crazy!! pun intended!!