Alright so about a month to two months ago now I swaped my k5 blazers tbi 305 with a carburated 383. Everything works great expect.......the ignition

First my ignition system consists of an MSD streetfire HEI distributor, MSD streefire CDI ignition box, 8.8 accel 300+ race wires, AC delco r44t spark <NOBR style="FONT-SIZE: 100%; FONT-WEIGHT: normal" id=itxt_nobr_1_0>plugs</NOBR>.

The motor idles like a dream but from 2500 to 6000 rpm the motor misses like crazy and falls flat on its face. So first we thought it was a carb problem, I fully rebuilt my holley 770, it still missed, i swapped the 770 with a brand new 670, it still missed, I put the 770 and 670 on my friends buick 430ci and it ran perfect, so the carbs are good. We over jetted, still missed. We under jetted, it still missed, we switched from 93 octane to 110 octane, still missed. Fuel is not the problem.

Then we thought the timing was off, we re-timed it almost 50 times, we took out the distributor at least 5 time and re-set it. Timing was not the problem.

We thought maybe the motor wasn't grounded properly, so we ran big fat ground wires from the motor to the frame and firewall to the frame, the alternator is brand new and cranking 14 volts at all times, and the batt is brand new.

Then we started looking at ignition parts. We looking at the motor running in the dark, at the time i had accel super stock 8mm wires, every wire was arching to the headers...so i replaced all the wires with accel 300+ 8.8mm wires and the arching was stoped. I replaced the spark plugs first with ac delco r43t's then with ac delco r45t's and finally we settled with ac delco r44t's because they werent fouling at all.

Then we thought maybe the ignition box was faulty (and i should also mention that the tach has not worked properly either, its a sunpro super tach II its very sluggish and cant keep up with the motor) so we disconnected the box and re-installed the parts into the distributor....The misses arent as bad anymore but it still misses a lot. Then we thought maybe it was the coil so we tried three diffrent coils (by the way all these ignition parts are brand spankin new MSD parts right out the box...distributor, ignition box ect....) it still missed with all the coils and even when we connected the tach wire directly to the coil negative it was still slugish and couldn't keep up with the motor.

I have no idea where to go from here...??? The box is good, the wires are good, the powers and grounds are good, the coil is good, i'm pretty sure the distributor is not defective, the plugs are good, the fuel system is good, the air intake is good, WHAT AM I MISSING?????????????????????

I'm truly stumped, my friends are all stumped, i even actually thought of putting the 305 back in!! If anyone has any ideas please let me know.

 

If Im not mistaken you have to use a non hei distributor with a carb motor swap.

 

GM put HEI distributors with carburators all the time.....
In my ignitions class I worked on a 1985 chevy truck with a carburated 350 and an HEI igntion system, it ran like a dream.

 

I had that happen also a couple times. Different things that caused it though. One was a distributor cap button had moved and wasn't arcing correctly. And the other ended up being a bad cam that had rounded lobes. I checked with a compression tester and then a leak-down tester. I ended up replacing the cam and lifters. Then it ran hard again. I guess there's a lot that can happen and unknown variables. Trial and error. Good luck to you!!

 

You need to find a shop that has a ignition scope. Have them look at the ignition pattern. Do you have a vacuum gauge?

 

Well actually the problem is now fixed. I always had a gut feeling that the timing was wrong. My buddy who has timed countless motors (and also timed this motor) had assured me that he timed it correctly. Problem is that his timing light is from about 1930 and I replaced the timing chain cover with a chrome cover and I got a aftermarket timing tab that bolted on to the side of the motor. This must have tripped him up. So last week I bought a timing light that has a **** that you can adjust for the advance, thus not needing the timing tab. you set the advance on the gun and make sure the mark on the harmonic balancer is at 0. Turns out that my buddy set the ignition advance to 40 degrees. I brought it down to 15 degrees and boom that motor has never run better. The missing is 100% gone, I was so happy it wasn't anything serious and I didn't have to bring it anywhere. dodged a bullet.