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Runs really good till 4000 RPM

Paul_G

Paul_G

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The junk I am playing with is a 49 Plymouth with a 75 318 and a 4 speed. The car has a single point ignition system in it. I just rebuilt a thermoquad and put that in place of a Holley 600, Eddy RPM intake, B body iron exhaust manifolds through glass packs w/dual exhaust. I dont really run the thing over 4000 RPM, but it is running so well I wound it out. Then this problem. At 4000 RPM it starts to misfire, stumble and backfire through the carb. All the way to 4000 it runs really well. It pulls hard as long as it stays under 4000 rpm.

What I have done;
Timing is at 8° btdc, around 35° total w/vac adv plugged. Checking the total timing I noticed the mark is jumping around a lot. Loose timing chain?

Fuel pressure is at 7 PSI through an electric pump.

I am puzzled wether this is fuel or ignition related. How far can an old points system rev to?
 
Depending upon the set 6000 reliably. Years ago when good stuff could be had 8000.
If equipped with tach whats it do at or above 4000?
 
at 4000 it starts to stumble, 4500 it it is stumbling and backfire through the carb. Didnt take it any higher. It will do it in any gear. seems like 4000 is the magic number. Not speed related. WOT from a low speed it runs great, secondarys kick in and pulls great, till 4000.

Could it be the ignition coil breaking down? Plug wires look good, no bad spots and they are flexible, but no telling how old they are. They are thinner cheap replacement set.
 
Do you have a dwell meter? If not, give the point gap a careful check.
 
Do you know the point the mechanical advance goes full? What are the points gapped at? .017 is a good starting point. Do you have the equipment to check dwell? 21° preferably.
I suspect points bounce. Now to prove it.
 
Sure sounds like ignition- complete tuneup. Plug wires,cap,etc. Then maybe worry about stretched timing chain.
 
I had a 440 do the same thing last year. Turned out it was a problem in the distributor. Everything looked ok inside, but I installed another one and the issue went away.
 
  • The points could be bouncing at the higher RPM
  • Capacitor could be breaking down
  • Could be dirty contacts inside the distributor cap.
Start with the easy stuff first then report back.
 
I had this problem about 20 years ago. Used a dwell meter to set the dwell and she ran like a raped ape! Good Luck!!
 
A timing mark that jumps around can indicate too much wear in the distributor bushings, possibly affecting point operation as the shaft tilts or bounces.
 
Man, the electronic/mag pickup stuff is SO MUCH better for just this reason. Good luck.
 
69Bee said:
A timing mark that jumps around can indicate too much wear in the distributor bushings, possibly affecting point operation as the shaft tilts or bounces.
Click to expand...

This is a good possibility. Easy to check with a dwell meter. If the dwell starts jumping all over when you rev the motor up high. Or pull the distributor out and any wear in the bushings will be evident in shaft play.
 
Replace the condenser first, many times this is the culprit B4 troubleshooting any further.
 
Picked up a points and condenser set today. I will change those out next chance I get. I set the points dwell about 6 months ago. Never thought to check it again.
 
A little grease on the rubbing block would help that. I still have an old tube of "distibutor grease" on the shelf. God only knows why.
 
Could this be the problem? The slot in the oil pump drive shaft is not right. I pulled the shaft out, it has a nice snug fit between the cam gear and shaft gear. The slot in the shaft is worn on the ends and not clean cut.



This is a pic of the slot in the oil pump shaft from this 318. The slot looks wallered out.

318_shaft.jpg


This is a pic of a 440 shaft I have. The slot is uniform.

440_shaft.jpg
 
Last edited:
I'm pretty sure dwell should be 30-34,not 21
stated above
 
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