Ugly misfire

[Matches]

Hello all,

I have a 1970 with a replaced engine. Pretty common L24/E88 combo. I have Z Therapy round tops in that have been working perfectly as far as I can tell. They are reasonably new, and have low miles on them. I have done nothing with them save minor adjustments to the fuel mix per the instructions they come with.

Overall the car runs well. She starts easy, revs quickly, holds a steady idle at around 900 with out support from the idle screws. She runs and drives happily at all RPMs and throttle positions until about 4-4.5k rpms then quite savagely hits bad misfire.

I have done a lot of searching on this forum and found a lot of helpful stuff. Here is a quick list of what I have done so far:

New vacuum advance

New points

New coil

New condenser

New plug wires

New ignition rotor

New ignition cap

New spark plugs

New battery

New starter

Ignition timing at 7*

Removed and cleaned engine grounds

Tested for battery voltage at coil, good

Tested ground resistance at distributor breaker plate, good.

I had a Pertronix unit in, and I had the exact same issue. So I swapped back to points just to be sure. Problem still exists.

The distributor is old, but I don't know what to look for once it is too old. Should I be suspecting this?

Running out of ideas, help!

[Zs-ondabrain]

Turn up the fuel just a tad. From what I understand (and could be wrong) Backfiring around that RPM is a result of Lack of Fuel. Check w/ Z Theropy and see if they say the same thing or not.

My triples did it all the time till I gave just a smidge more fuel to 'em.

Dave.

Edited January 9, 2009 by Zs-ondabrain

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Thanks for the reply! Gonna go give that a shot.

[Matches]

Adding fuel didn't seem to help.

I began by giving the fuel adjustment knobs one turn for the richer and then using the maintenance screw to keep the engine idling. Went for a blast around the block and same misfire. I played with the fuel more after that, trying different settings. No change as far as I can tell.

So I just reset the fuel knobs and idle screws with the flow meter. I think now I will re-watch the "Just SUs" video to be sure I remember what I am doing.

[bigoak]

Mis-fire isn't usually a lack of fuel, it's a timing issue.

Make sure your advance is working properly. A wall at mid-RPM's likely means the advance isn't moving the spark ahead enough to keep up with the speed of the engine.

[Matches]

So how do I tell? I pulled the vacuum hose off and sucked on it, I can watch the breaker plate and all that stuff move and will hold so long as I keep vacuum on. I am not sure what a sticking plate would look like.

Thanks for the advice.

[bigoak]

Not super in depth on this...try a search on making sure the vacuum is drawing at rpm...

[Arne]

I think what Dave (Zs-ondabrain) meant was that your fuel pump might not be supplying enough fuel to feed the engine at higher RPM.

[doradox]

Does the car miss at 4K regardless of throttle position or gear you are in? Make a run from 2K to 4.5K in each gear at half throttle and again atwide open throttle and note any differences. If so, probably not fuel related. Timing typically does not cause a misfire except in gross cases. How old is the engine? Specifically the valve springs. A weak spring can cause valve float which will cause problems directly related to engine speed.

Steve

[Diseazd]

It could possibly be bad bushings in the distributor shaft. Grasp the rotor while still in the distributor and shake back and forth for play. Also, did you check for broken point connection (small wire in distributor) they get pinched and can short out. That would be a $1.00 fix. Let us know where you stand. Guy

[ChrisZ]

And while you have your hands on the dizzy, you might as well take it apart, clean it up and re grease it, check if the weights and springs work properly.

This could be an internal advance problem.

Good luck

Chris

[Matches]

I decided to purchase and install a remanufactured dizzy. Problem solved!

Thanks everyone!