I have a stumble at 2500 and its killing me!!! Cant seem to shake it. Its a sbc 355 afr heads hsr intake 76 mm turbo accel dfi 83lbs injectors 255 pump msd wires dist and 6al2. Its in a 85 z28. Sometimes it will run perfect for a couple days. Then all of a sudden hit the key and the stumble is back. I sometimes can pull over shut it off, restart it and itll run good again. Could it be something in the ignition switch?? And help would be awesome.

 

It could possibly be the ingition switch. Though the correlation to engine speed makes that less likely. As with any pattern failure, I'd start by monitoring spark and injector pulse while driving to recreate the failure. You can watch spark by using an old plug wire to connect an in line spark tester where you can see it through the windsheild. To monitor injector pulse you may need to make up a test set with leads so that you can see either the noid or test light from the driver's seat. Watch for either signal to glitch when the stumble occurs.

 

Thanks for the reply. Im pretty sure its not in the tune. Cause i can shut the car off, and restart and itll run amazing!! Sometimes!!! I have shut it off on purpose a time or two with good results. Changed tons of parts. Nothing helps. Im just looking past something. I have a trunk mount batt. And have the ground in the back too. Not to motor. So thats a possibility. And i also have the charge wire to the starter off the alt. Another possibilty. Thanks for your help.

 

These electrical issue could be causing your trouble. The ECM and distributor need at least 11.5v to operate. When they don't get it, they will shut down. Since they are both solid state, they will shut off fairly quickly. I was going to suggest ignition switch but I would start by reconnecting the charge wire and making sure the engine is well connected to batt neg.

 

So you think i could be having a voltage drop at 2500rpm cause at around 2700 the car wakes up and takes off. It like loses spark for like a split second. Ive changed tons in the tune with no success.

 

I have the batt grond hooked to a bolt on the rear bumper and the big wire off the alternator hooks directly to the starter. Where it meets up with all the other stuff.

 

Nobody???

 

have u changed the cap an rotor.

 

Yeah multiple times. Msd!! I found something today that could be my problem. I had one 12v wire powering both the 6al2 and the msd distributor. It tee'd off to both. Shouldnt they both have their own 12v keyed power source???? Any help or hints or tips would be great!!! Thanks.

 

Having the dist and module powered by the same circuit shouldn't be an issue. Just be sure there's plenty of currenty capacity. The analog box(AL) draws fairly hefty current. It's strange that you have that catch right at 2,500rpm. The MSD switches from multiple spark discrarge to single spark ast about 3,000rpm. You might have a defective MSD module. Perform the spark test that I described earlier using an old plug wire or two to attach an inline tester so that you can see it while you drive. Watch for the spark to drop out right at the spot where you feel the stumble.

Then perform the injector pulse test. You could also be dropping crank referrence pulse to the ECM at that point.