I was checking the timing on my small block Camaro yesterday and noticed that the timing doesn’t return to initial like it should. I set my initial timing at 14 degrees but then when I rev the engine and allow it fall back to idle, the timing light shows that the timing now is at 16 and sometimes 18-20 degrees. If I rev the engine quickly, sometimes it will come back to the original setting, but most of the time it does not. I have an aftermarket HEI distributor. Do I have to replace the distributor?
Jeff Smith: This issue could be caused by several different issues. Because you mentioned you have an aftermarket HEI, we’ll assume that it may be only a few years old. If the distributor is original, factory HEI distributors from the 1970s and later were famous for issues with misfires that grounded through the shaft. Let’s start with the older, used HEI’s and then work our way to the newer models. There are several things you can try to determine the real issue.
With an older HEI, start your diagnostics by removing the distributor cap and rotor. Look inside the rotor for brown deposits inside the rotor. This is a common problem where the HEI misfires internally. Often the spark burns through the rotor into the mechanical advance mechanism on its way to ground. The brown material is iron oxide that has burned off the HEI shaft on its way to ground. Inspect the mechanical weights to see if they move freely on their pivots. Likely they will not move freely. This can be because the posts are highly worn from the spark energy burning them away. The brown iron deposits are the result of this oxidation. If the posts are worn, this could cause the weights to stick. If the posts are worn, you must replace the entire shaft, which might cost more than just buying a new distributor. This is a very common problem but you might be able to find a used distributor with a good shaft.
If the pins for the mechanical weights are a good shape, it’s possible that all you need to do is remove the springs and weights, clean and lubricate everything with white lithium grease to ensure the weights move freely. Often this is all that needs to be done. If the distributor is stock you might also consider adding a performance weight and spring package. There are several available through various companies like Accel, Moroso, Spectre, and Summit Racing to name a few. For example, Summit sells an inexpensive weight and spring kit (PN G-5212). Don’t be afraid to mix and match the springs to get the curve you desire.
Another issue that can cause the HEI to be slow to recover back to base timing is a bit more complicated, but easy to repair. We had an issue once with an Accel distributor that exhibited the same kind of problem. We removed the distributor and the weights and springs. Moving the advance shaft, the pin seemed very tight inside the slot and not easy to move as it should be. We removed the distributor gear roll pin and removed the gear and separated the shaft from the distributor. Even with the shaft out, it was still tight to move the limit pins within the slot that limits the total mechanical advance.
The advance slot main piece is retained by two small C-clips over the pins where the advance weights pivot. With the small C-slips removed, we were able to disassemble the shaft from the plate and found the original grease used by the manufacturer had hardened and restricted movement of the pins in the slot. We cleaned all the old grease and re-lubed it with white lithium grease which returned the system to total freedom of movement. We re-assembled the distributor and put it back in the car. We noticed that the mechanical advance now responded very quickly to engine rpm. In fact, it reacted so quickly that we had to add a slightly heavier spring to create the mechanical advance curve that we desired.
Other good ideas to help tune up your HEI include an adjustable vacuum advance canister. Many enthusiasts don’t realize just how important vacuum advance is to street performance. For any engine that spends time on the street, it is to your engine’s benefit to not only use the vacuum advance, but to be able to tune the amount of advance and its rate – or how quickly the advance is added and how much. Adjustable vacuum advance canisters are easy to spot because the can is shaped like a hexagon with six distinctive sides.
You can adjust these cans with a small 3/32-inch Allen wrench inserted into the vacuum nipple. By turning the adjustment clockwise, this increases the amount of advance. Turning the Allen screw counterclockwise reduces the amount of advance the can will deliver. Make adjustments two turns at a time as this should make a measurable difference.
Many enthusiasts don’t take the time to look closely at their parts to evaluate what’s really going on. Often they just assume the parts are defective or broken and buy a new one. If you take the time to evaluate the parts you have, you can often find the simple issue and repair the problem while spending very little money.
And that’s something everybody can appreciate.
Thank you for the valuable information regarding the tuning of an HEI distributor. I’m at a point with my first build where I need to tune my basic carburated SBC which has an Accel HEI distributor. This will be complicated as the car was an auction vehicle and unfortunately, I have no specs on the cam. Is there one manual that would assist me in attempting to set timing/advance with no info on the cam shaft ?
Robert in Gloucester , MA
I replaced my distributor and I still can not get the timing right.On my 3500 chevy with a 454 motor.
I have a 1985 corvette that had an hei ignition system which I replaced with an msd direct ignition system (6015). The problem I have is that since my factory ecm expects to see information from the hei system but doesn’t, my pcm sets a code 43. Is there anything I can do with the est wire (white) and the bypass wire (tan) to address this problem? I know it’s an oddball question.
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