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Timing corrections for elevation change

I’m currently restoring a 1965 Pontiac LeMans/GTO with a (pretty much stock) 326. Among my most annoying and debilitating problems while driving is that the engine has this weird hiccup/buck/seizure problem while I’m stopped at red lights or stop signs. It has gotten to the point that it seizes (rpm drops and engine seems to stop for half a second before picking back up to normal) multiple times before I get going again and occasionally even dies on me!

I’m just learning about all the ins and outs of car restoration and maintenance, but I have recently changed the fuel and air filters, treated the gas for water, cleaned and checked the gaps in the plugs, changed the oil, and set the idle mixture. None of them helped. I have heard that elevation impacts a car and should be adjusted for in the timing (I live at about 4200ft.) My questions are: How much should the timing be adjusted for elevation? Advanced or retarded? And could there be a different cause to these seizures? Thanks!

If your timing and dwell angle are at standard recommended, I do not believe it to be the source of the problem. Improper choke, or failed accelerator pump is where I would look first.

Timing and dwell both set at standard, I actually have an Edelbrock Performer series 1450 carb (650 cfm, I think) with no choke (probably overkill, but I didn’t have the money for another one at the time). I’ll check into that though, thanks!

With no choke, the hiccup/buck/?? problem will be consistent until the engine is fully warmed up. Then, the idle settings of the carb need to be set for the altitude. If you drive it down to a lower altitude, the idle settings of the carb will need to be re-adjusted to prevent a lean condition that can burn valves and pistons.

My carb should be set at idle-best right now. Tested it with a pressure gauge to make sure I had the max compression I could get, and the plugs didn’t look like the mixture was too rich or too lean… the problem is pretty consistent throughout all tempuratures, although it tends to be slightly less prevalent when it’s warm. Not much better though.

I would check the spark plug wires, too. I didn’t see them on your parts list, although they may be new. They can cause temporary misfires at any temperature if they’re arcing. You may be able to use WD-40 to test the theory if the problem is consistent. I hate the stuff, but it does have a few uses, and isolating the electrics for this kind of test is one - it’s not any kind of long term fix.

Come to think of it, coils can do that, too.

Has this car ever run right with this carb? It may be poorly set up. I don’t understand why you’d get a carb without a choke, that doesn’t sound like one that would be set up correctly for a mild street engine.

Also, you said ‘Tested it with a pressure gauge to make sure I had the max compression I could get’. What do you mean? You ran a compression test? That’s good to do, but doesn’t have anything to do with your carb or ignition problems.

Timing is not your problem, but to answer your question, you can safely advance the timing 1 degree per 1000 feet of elevation…If you are restoring it, install a proper QuadraJet carb and it will idle like it should…GTO?? Not…

Texases: When I got the car, it had a Chevy 350 racing engine and this carb on top. I used the same carb to save some money, and didn’t really know the difference between manual, electric, and no choke at the time. Now that I look at it, it looks like the guy I bought it from just got rid of the manual choke… gonna have to check into that.
As far as the pressure comment, I was taught (through research and my own mechanically inclined friends and family) that the idle mixture affects compression. I was simply saying that I already checked that area.

Caddyman: Thank you! I had already heard that rule of thumb, so I’m glad someone could help confirm it.
Also, I said LeMans/GTO because in '65, the only difference was a small package applied to the car, of which my car has about half of… therefor, 50/50 LeMans/GTO. If I decide to commit either way, then I’ll call it either-or.

Thanks everybody for all the input! This gives me a few good directions to try.

gtoguy - well, idle mixture shouldn’t affect compression, it’s checked with the throttle held wide open. If this carb was on a high-output 350, it’s probably not well-suited to your 326. Do you know what carb came originally on yours? 2 or 4 bbl?

2 bbl, I’m pretty sure.

I think it affects vacume not compression. On my old Corvair, I used to need to use a vacume guage to properly set the two carbs in it. Can’t remember how or why and threw the book away 30 years ago, but do remember having to use the vacume guage on it.

Ah, yes! That’s what I meant. Sorry for the confusion…

OK. You may want to check some GTO references. The '65 GTO only came with the 389, and you’ll probably get better answers if you just call it a Tempest. Great looking car, neighbor had 2 of them!

The 326 Tempest engine came in both 2v and 4v flavors…250 and 285hp respectively…Is the intake manifold on your engine stock? Not likely with the Carter (Edelbrock) carb…

It’s stock. Not the one that came with the engine though. We scrapped the 2 bbl version for a 4 bbl while we had it rebuilt to give it a little extra hp.

The '65 GTO’s came originally with a Carter AFB carburetor, not a Rochester, unless you got the tri-power setup, which was , of course, 3 Rochester 2 barrels. The GTO 389 also had a higher compression ratio, and supposedly had (slightly) better connecting rods and crankshaft.

The Edelbrock carburetor looks for all the world to me like a clone of the Carter AFB, so it’d probably be perfect for a 389, but I’d expect it to be too big for a 326.

The car should not be too much for this engine. I’ve got several small displacement engines running this series of carburetor and they’re fine after a little metering rod and jet tweaking. One even has 2 of them on it.

Offhand, it sounds to me like it’s loading up from being too rich but there’s a lot of unknowns here.
How far out are you running the idle mix screws and have you pulled a few spark plugs to read them?

I would also not get too carried away with ignition timing. This is an older engine running on unleaded gas with likely (?) no EGR system. You could end up trashing an engine if you put too much advance into it.

Look at the wires inside the distributor for shorted or opened circuits.

There’s a vacuum advance mechanism inside the distributor that can flex these wires causing all kinds of problems with the ignition system.


gtoguy, do you have a shop nearby that knows how to tune carbs? OK’s right, this carb could work if it gets setup right. But you need somebody that knows how to do that, few do these days.