I've been chasing a high-rpm miss in my 427/390 for a couple years. It may have existed far longer but I didn't see it until optimizing the timing curve; it doesn't affect regular driving as I rarely rev into the 4000 range.

Using a timing light (both Innova and c.1980s dial-back Craftsman) the strobe becomes sporadic and the timing mark fluctuates pretty wildly once you hit ~4000 rpm. In my pursuit of this, I've tried 3 sets of points (reg. spring and HD spring), 2 condensers, 3 coils, 2 sets of plug wires, new distributor cap & rotor, replaced spark plugs with NGKs, and replaced the GM nylon timing set with a Cloyes roller set. The distributor has been completely disassembled and checked for bushing wear (minimal) and the points plate is not wobbly in its mount (the dwell at 30°, is very steady up into the low-4000s). I think the only component I haven't replaced is the ballast resistor.

Two things I'm contemplating are bypassing the ballast resistor to see what effect that may be having on the coil's energy "filling" abilities and increasing the dwell to various settings (with timing re-adjustment) to see how that affects the miss. I understand that bypassing the ballast risks burning the points - how immediate is that likely to happen?

I'm thinking it's an ignition issue because the carb behaves well in all other situations - however, that's the same thing with all the ignition parts too!

 

The coil will be happy with the extra current of bypassing the resistor, you have the possibly of burning the points up if you let it sit with the key on and the points are closed.

What you could do is get or make a jumper wire with alligator clips, start it normally, rev it up and observe your miss with your timing light, then while still idling, attach your jumper from a known good hot to your coil (effectively bypassing the resistor) and then redo your test.

This would let you see if the resistor is making a difference without having to take anything apart or risking burning up your points.

 

Put a scope on it and see your ignition trace normally is and what it is when it is breaking up, try the jumper wire across the resistor and see how it affects the trace and go from there.

Bill

 

Just get a new resistor - not that expensive. That said, as I understand, the type of resistors that were used on old style breaker point ignition systems weren't "smart". The resistance any particular resistor created, measured in ohms, is constant. So..... the amount of resistance you are experiencing at 1000 RPM - for example - should be the same at 5000 RPM. So..... have you measured the resistance of your resistor? Is it within factory spec range? If yes - my guess is that you have a gremlin elsewhere.

 

I don’t think it will harm the points for a short test. Just don’t turn the key on without the engine running.

A funny story when I was in high school in the late 60s, I and a friend went trout fishing one day and my friend dropped my keys in the river and we were about 3 hrs from home. I made my way to a phone and called a Plymouth dealer who gave me to the service manager. He said without the key tag numbers he couldn’t hep me. He said why don’t you just hot wire it. I told him I didn’t know how, And he proceeded to explain it to me, I guess hoping I would not become a teenage auto thief. He told us to run a wire from the + battery to + coil although maybe he should have told us to the input side of the resister instead. But he probably assumed we were too ignorant to know where or what the ballast resister was. He said run the fan and lights all the way home to pull the voltage down a little and we would be fine. So we stripped out some aftermarket speaker wire and got the car running and drove it home without a problem.. Didn’t seem to harm the dual points at all as I know I didn’t change them until some months later.

 

As I recommended in a PM, he needs a scope on it to see if it is ignition and where/when. A scope will get to the bottom of it. Or near the bottom. I cannot imagine the ballast resistor has anything to do with it but jumping it out and taking it for a ride will not hurt a thing.

Dan

 

The resistor measures 2.3 ohm when sitting on a bench. The factory rating is supposed to be 1.8 ohm. I haven't looked at it at different rpms but should be easy enough to do.

OK, you've just thrown a wrench in my diagnostic skillset. I have zero experience with oscilloscopes -- though I've seen their use on YouTube if that counts for anything! If I decide to pursue this route, what should I look for in buying one? Any recommendations for something that I can't foresee using very much? Discontinued is also good if it's findable on eBay.


I'll see about jumpering over the resistor and see what happens. Just concerned it would let out the magic smoke!

 

Jumping or bypassing the resistor for testing will not cause any issues.

 

I agree with Bill Pilon and dplotkin, that a scope would help in your diagnosis.
Having taught oscilloscopes for Allen test, I am a staunch supporter of scopes. They are a window into the engine and are fantastic diagnostic tool.
I just helped a friend who had two cylinders not firing, 7 & 2, I put my scope on the engine and it showed he had only 6 out of 8 patterns. So I knew right away it was in the distributor, he has Pertronix's and two magnets fell out. My point being I found his problem without putting a hand on the engine.
I wrote a paper on scopes, if anyone wants a copy send me your email address.
As to the issue, I would guess that there is not enough available voltage to fire the plugs at higher RPM.
Possibly too much resistance in the secondary or not enough voltage/amperage in the primary.

 

Sent email via PM - thanks!

Do you have an idea where not enough voltage may be coming from? (Could this be a resister or dwell thing?)
The primary and secondary you refer to are coil-based?

 

I have a Sun scope that I have had for several years and use it quite often as well as my distributor machine, whenever I do a tune-up on my car I pull the distributor and change points and condenser, set the dwell, check point tension and check the max advance while it is set up in the machine.

I would like to receive a copy of the paper you wrote on scopes. My e-mail address is: billpilon@comcast.net
Bill

 

What about over 4000? Free rev it to at least 5000 and check that dwell doesn't vary more than 2 degrees, or rig up the dwell meter in the car with the test lead extensions, take it for a drive and take it to the redline while observing dwell.

With a 5300 redline the standard 19-23 oz. points should be perfectly adequate. If you're positive that the points (hopefully a brand name made by SMP) are okay you could have a wobbly breaker plate. Try pressing down on the edge of it. If it moves the clearance is loose allowing it to wobble. You'd be surprised how much bending load it on it as the points open.

Duke

 

Are you using an emissions rotor and is the drive end gear shimmed to under 10 thousandths? The white colored rotors made after 1975 by Delco have a shortened tang which make high rpm runs more difficult.

 

The end play is right about .008". I'm not familiar with an "emissions rotor" - is that the little one? Mine looks like a standard round rotor with the square and round pins in the bottom - and it's an off-beige looking thing (am colorblind, so it could be light green).

 

An emissions rotor is the newer style with a shorter tang. At high rpm your coil may not have enough EMF to jump the plug gap. Older made Accel, MSD , or Delco 409s have the longer tang .

 

Might you have a measurement length for the tang for comparison with what I have? I googled "emissions rotor" but didn't come up with much that was usable.