Is this a manual or automatic set up? What is your hot idle rpm? I assume you are running a numbers match Holley 735? If it is an auto, a quick fix is to leave it in drive or reverse when you turn the key off and then go to park. If a manual you can always leave it in gear and kind of kill it with the clutch. If it is an auto car and will not turn off while in gear, we can go further. One thing that could improve it would be to close the secondary blades about 1/8 of a turn on its stop. Turning it counterclockwise an 1/8 of a turn. This will force you to open the primary curb idle stop a bit. Or you can experiment going the other way and open it 1/8 of a turn and then close the primary side. Note you start position and only turn the secondary in small amounts. A little goes a long way on the secondary stop screw.
These seem like bandaids, but the truth of the matter is many high compression engines of this era had some run on problems. The slower the hot idle rpm the less likely it will run on. Again that is why the electric solenoid to control the hot curb idle was introduced. To slam the blades shut when the key was turned off. A good friend of mine owned a 1971 Boss 351 Mustang in the mid 70’s. Completely stock. It had run on problems from the get go. He had the hot idle set at around 1,000 rpm and had to dump the clutch. I was young and inexperienced at this time. Would be interesting to get in a time machine and go back. Maybe it was a bit misadjusted! Who knows!!
Thank you ALL for the wonderful input.
So, here’s where we are now.
We feel the timing is “dead on” for initial timing.
The carburetor is set well (we adjusted the jets, springs, and rods)
She has TONS of power now, but still knocks and pings on highway or under load.
We believe it’s the distributor. We tried several times to make adjustments to it… but no luck.
Here’s the curve before and after adjustments.
Your screen shots are hard to read, but from what I can see the distributor is set at 10 degrees initial and 38 degrees total. That would probably work fine with good 1960’s mid - grade 95 octane gasoline. Unfortunately you are stuck with 92 octane.
What you need is an adjustable vacuum advance from Summit racing so that you can continue to run 10 degrees advance on initial, but limit the total to perhaps 34 - 35 degrees.
Also, the centrifugal advance is doing too much. There is a second position that limits centrifugal advance. You need to disassemble the distributor to make the swap, or use two of the heavier centrifugal springs to get the desired results.
What Royce said. A new distributor will do nothing to change this. One quick test that you need to run. Disconnect and plug the vacuum line running to the distributor and see if you still have pinging. If it goes away you are getting too much vacuum advance and you can adjust it by adding shims or turning the little screw inside the vacuum tube. If it still pings you need to change the distributor to the narrower of the two windows to limit maximum centrifugal advance.
Thank you.
We finally got it to run without the knocking and pinging.
Here’s what we did.
We adjusted the vacuum advance (using the allen wrench) to only give about 10 degrees advance (it was going up 20 degrees initially)
Then, we retarded the timing down to -10 degrees (estimated since our timing light doesn’t do negative)
We then added the vacuum advance and the timing stood at around 0 degrees at idle.
We haven’t checked yet, but we estimate that the car runs at total timing around 20-30 degrees.
I know these numbers aren’t optimal according to what we’ve read and what everyone is saying, but I was FINALLY able to open up the secondaries and we even got a good second gear “chirp” out of her last night.
+1. either your timing light is broken or your harmonic balancer has “turned” a bit in the rubber collar. If those numbers are accurate, I doubt the car would even run at idle.
If the elastomer on your damper has failed, the ring can turn, relative to the crank. That means the timing marks won’t line up any more, but it also means the counterweight on the damper is not in the right place, and can snap your crank.
Best to find TDC on your #1 piston and verify your timing marks, then set things correctly. Dealing with a bad damper, or misadjusted timing can both destroy your engine very quickly. Timing is not about guesswork, and this isn’t something that ‘no one’s figured out’. Better to just make sure things are right, so you can enjoy your car for many years to come! =)
Thank you. We did find it (with my cousin Chris who’s an amazing mechanic) via the first piston. We’re sure of it.
The problem, in my opinion, is the timing curve of the distributor.
But my son is having too much fun “tinkering” and… well… that’s the point of us getting the car. So I’m going to let him keep playing.
Okay so you have verified that at TDC on cylinder 1 (front cylinder on passenger side) the timing pointer is aligned with the 0 line one the damper. Next disconnect and plug the vacuum line going to the distributor. Loosen the hold down clamp, and rote the distributor so you have 6 degrees advance at another 650 RPM. This is the factory spec. Anything less will make it run like crap and overheat. Then connect the vacuum line. The timing should not change. If it does you have the vacuum line connected at the wrong place. This verifies that initial timing is working properly.
Next, disconnect and plug the vacuum line again. Now it’s time to verify mechanical centrifugal advance. The idea is to observe the change in timing as RPMs increase. As you increase RPM note the change in advance. Some where around 2800 RPM it should stop advancing. You will probably see something like 32 to 38 degrees. If so, your distributors mechanical advance is working properly.
The vacuum advance should be connected to the ported vacuum port on the carb. It is on the passenger side of a Holley or the upper of the two on the front of an Edelbrock . your first driving test is with it disconnected and plugged.
Time for road tests. The engine needs to be fully warmed up. Hopefully you won’t have any pinging. Then you can try advancing initial or base timing by 2 degrees (8). Keep advancing the timing in two degrees increments. Most likely it will start pinging again at about 15 degrees or less. Once you find the threshold, adjust back by a degree or two.
Vacuum advance is only intended to fine tune efficiency at part throttle cruise. Reconnect the vacuum line and road test. If you get pinging when you go up a slight grade or tip on the throttle it means you are getting too much advance. Tighten the spring by adjusting the screw inside the vacuum canister.
This was with an initial advance of 10 degrees and NO vacuum advance.
She drove fine during normal driving, but knocked and pinged on load and on highway.
So, we retarded it and added the vacuum advance.
After some tweaking (-10 degrees retarded and vacuum advance), we had a great run. She had tons of power and did NOT knock and ping.
I thought it was fixed, until I started her up the next day. She couldn’t make it up the driveway.
Thinking it was just that she wasn’t warm enough, we let her warm up the next day and tried again. But she still barely ran. Kept sputtering and dying.
We advanced her a bit and it helped some. But not enough.
I truly believe it’s the distributor that’s causing these problems. And if we had one with a better curve, we could move her back to say 6 degrees as you mentioned.
When my son gets home from school today, we’ll try your notes.
Thank you again
You have to sort out one system at a time. Disconnect and plug the vacuum advance then try it at 6 degrees. Don’t connect vacuum until mechanical is sorted.
Okay. Last night, we set it to 6 degrees at idle and then put the vacuum advance back on.
She definitely ran better on the streets, but didn’t get a chance to try her out on the highway.
Will try today.
Any advice on a new distributor?
Any recommendations?
Also, you need to test first with vacuum disconnected and plugged. I suspect that one problem may be that you are inadvertently running manifold vacuum instead of ported vacuum.
The factory manual has a chart with both mechanical and vacuum specs. Once you get mechanical advance dialed in you can start on vacuum advance.
The distributor is “frankensteined” from the original one and the new one and, honestly, I just would prefer something I know works well.
I want to stop wondering if the problem is the distributor and I’ll never get rid of that doubt with this one.
Also. We’ve NEVER run vacuum advance. So this is our first time.
We’re running it from the drivers side port off the Edlebrock carb.
We’ve also tuned it using the allen wrench to the best of our ability.
