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Old 06-26-2020, 09:39 PM
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1979 corvette tune up

So same friend with the 79 corvette and I were talking some more about his car. He’s a young 20 something college kid (one of our interns) and doesn’t really have any kind of automotive mentor. Loves his car, has big plans and wants to learn.

So I’d like to help him get his car running the best it can.

He says it runs hot at low speeds. Not overheating just hot, hard to keep under 200 until road speed comes up.

My first thought is a proper tune up with a proper timing curve. Looking it up on the inter webs it looks like it has the typical early emissions lazy ignition timing. I think fixing that is probably the best place to start. Get the base timing up around 12-14 and 32-34 in around 3,000 rpm.

I haven’t seen the car yet, but it sounds like he is running a stock fan with no clutch, so if that’s true, the fan should be pulling plenty of air. I suggested a fan clutch after the hot running issue is under control.

So my questions are:
Am i on the right track? I don’t want to steer him wrong.

What’s the best way to limit mechanical advance on the stock HEI? Assuming his is adding about 30 degrees (based on the 4 degrees initial spec), we need to limit to about 20 if we bump the initial. I have a stock HEI I can play with on the bench.

My plan is to get the initial up, limit to 32-34, and get the vacuum advance on full manifold and see if the low speed hot condition gets better.

Any and all input is welcome. I’d like to teach him right, not use him as a leaning tool.

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Old 06-26-2020, 10:12 PM
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Plan sounds good.

i would have expected that application to have a clutch fan already, so check, it may just be worn out.
There is a quick and easy way to limit the mechanical advance on an HEI using a machine screw, I'll link it, it's in the Hotrodders Wiki, Engine section, "Hotrodding the HEI distributor". Pics of the modification too

https://www.crankshaftcoalition.com/...EI_distributor

May need to fab up a vacuum advance limiter too.
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Old 06-26-2020, 11:17 PM
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The Corvette tends to idle hot not a lot of air flow space under those hoods.

Those early emission engines are hampered by insufficient idle advance and lean mixtures as well. I think your idea of getting the idle advance up has merit in helping this. Given those emission heads with their lazy burn and low compression I don't really think that limiting max advance to 32/34 degrees really is of much concern. Those old low compression heads often like a peak advance over 40 degrees so I wouldn't restrain myself from getting the base advance up where it needs to be and correct the idle mixture and speed then see where this is as to how the engine likes it. My thought is not to do too many changes at once as its easy to get lost as to what changes are producing which effects.

You need to kerp in the back of your mind as to how many miles are on the timing set. If it is worn out it will defeat any-other tuning changes you make.

Bogie
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Old 06-27-2020, 01:27 PM
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Good point about the condition of the engine. That’s something I can check when I see it. Can get a feel for how much slack is in the timing chain and see from there.

So if we are going to go at this one step at a time, where would you start?

Base timing? Vacuum advance?

Good to know that the engine might be ok with lots of timing. I was being purposely cautious because I don’t know how easily he will pick up on pinging. I can ride with him but when I’m gone .....
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Old 06-27-2020, 01:37 PM
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If you fear the owner may not know or realize detonation...deliberately over-advance the initial timing on a short test drive with you in the car, so you can alert him and familiarize him with the sound without staying on the throttle and hurting it.
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Old 06-27-2020, 01:53 PM
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Easy enough.

I’m thinking I’m going to start with base timing. See if that helps, so long as there is room on the top end.

Going through the chart I found, I don’t see any combination of weights that would get 30 degrees mechanical anyway. According to this, 24 is about the top.

https://www.hotrod.com/articles/back...i-distributor/

So if that is true, then it may only be running 28 from the factory.
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Old 06-27-2020, 05:38 PM
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So that list is clearly not complete.

I went to my dead 350 and looked under the rotor. First thing right off is that it was completely shagged so I’m glad I replaced it when I got the new motor


But beyond that, it has a 409 center piece and 186 weights. Neither of which are on the list. So now to look up what that combo might be. Just so I know. So much for using this one as a parts donor ....

Check out how bad this sucker was though.
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Old 06-27-2020, 08:02 PM
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Here are some articles from the wiki that will help you guys out.....

https://www.crankshaftcoalition.com/...EI_distributor

https://www.crankshaftcoalition.com/...oting_ignition

https://www.crankshaftcoalition.com/...ive_wiring_101

https://www.crankshaftcoalition.com/...op_dead_center

https://www.crankshaftcoalition.com/..._a_distributor
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Old 06-27-2020, 08:18 PM
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I don't think I've ever seen advance weights trashed that bad, with the pivot hole nearly worn clear through the weight.

I'm surprised the rotor mount piece wasn't frozen to the shaft locking the advance up...I've seen that a few times.

The HotRod list of weights and cams was FAR from complete....i think i remember reading there are something like 486 different factory combinations spread across the entire GM products line-up..
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Old 06-28-2020, 12:24 PM
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Quote:
Originally Posted by BogiesAnnex1 View Post
The Corvette tends to idle hot not a lot of air flow space under those hoods.

Those early emission engines are hampered by insufficient idle advance and lean mixtures as well. I think your idea of getting the idle advance up has merit in helping this. Given those emission heads with their lazy burn and low compression I don't really think that limiting max advance to 32/34 degrees really is of much concern. Those old low compression heads often like a peak advance over 40 degrees so I wouldn't restrain myself from getting the base advance up where it needs to be
X2... If it isn't boiling over, it's not really too hot... the emissions EGR systems are set up to make the engine idle hot... if don't have to pass emissions testing, remove it and save it for the next buyer of the car...

Add a temp. controlled fan clutch to get fan noise down... use a stock GM steel fan... if it has an aftermarket fan with aluminum flex blades, don't stand where the blades can get you when they break off...

Max ignition advance of 28 - 34 degrees is for engines with 52 - 64cc 'fast burn'/Vortec heads... those old '79 bathtub chamber 76cc heads combined with dished pistons need 36 - 40 degrees advance to fire off that huge combustion space in time for it to make max power...

Need to find out if it's the emissions crap L48 165 netHP at 4,000 RPMs engine or the crap L82 180 - 205 netHP at 5,000 RPMs engine... or if the engine has been modified or replaced...
.
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Old 07-30-2020, 09:31 PM
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So he was finally able to get home and drive the car back.

It is the low performance L48. Emissions tag says 4 degrees advance. Leastwise, that is what was originally. He said that the previous owner said something about it being rebuilt. As for condition all I know is that it runs smooth and appears well maintained. Clean outside and inside the valve cover, what little you can see.

There is definitely something not completely right though. The choke doesn’t appear to be working at all. It took a lot of pedal pumping to get it to fire and feathering it to keep it running. Started off in a low lopey idle with some black coming out of the exhaust, but was idling smoothly after a short time.

Checking the vacuum connections there is some goofy stuff going on. There is a vacuum line coming out of the choke housing that goes nowhere and it is an active vacuum leak. Small leak, but definitely leaking vacuum. I have no idea where this line is supposed to go as there are no obvious missing connections.

My best guess is that the distributor is hooked up to ported vacuum as per the diagrams I have been able to find. But there is vacuum present at idle, indicating more that there is something not right.

There is some mysterious canister on the back of the carburetor. Looks like maybe a big vacuum connector, but i didn’t get a chance to check it out while it was running.

I’m all ears for ideas. I’ve never seen a vacuum line come out of a choke like that so I’m a little lost.

Where would you start?
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Old 07-30-2020, 10:33 PM
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That’s a hot air choke that indeed has a small vacuum leak inside. The vacuum is used to pull air from that connection on the top back of the carb (has a plugged hose on it) through a choke stove in the intake (below the choke cap). The heated air makes the bi-metallic coil in the housing expand and open the choke. That front vacuum pot is the choke pull off. Used to open the choke blade when you first start it. Just enough (about 1/4 inch) to let it run on the high idle. That rear pot is a secondary dampener.
Nice thing about that style choke is the cap on the side is easily converted to an electric choke.
The choke stove tubing generally rots out. Could be that way on yours.

These guys will have parts to restore it or replace it with electric.

https://quadrajetparts.com/

Last edited by RWENUTS; 07-30-2020 at 10:38 PM.
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Old 07-31-2020, 07:02 AM
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Forgot to mention that the cap is held on by 3 screws that can be loosened enough to rotate the cap so the choke coil riches or leans the setting. Yours likely has been rotated all the way lean. Which in your case is a good thing seeing that none of it is working or hooked up.
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Old 07-31-2020, 07:21 AM
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Ok. So it is the air source for the heated choke. Got it. So leave the factory vacuum leak. (Man that seems like a bad design. I would have expected a filtered air source at least!)

The choke may work. I was assuming it didn’t because it started so hard, but then last night I remembered that he said something about when it is cold it will go on the high idle which he commented “was like 3000 rpm”.

I might look at the choke and see if I can get it working a bit so that it starts better. I do have an electric conversion I could part with too if he wants to do that route.

So I think I’ve developed a plan that is quick and easy.

1: change distributor to full vacuum. Going to cap the line it is on and tee into the line for the vacuum modulator. This is quick and easy and easily reversed. Reset idle and see if it helps anything. Will try to reset or at least check idle at this time too.

2: adjust base timing up to 10-12 and see how it runs. Might look at the weights and center piece and see what they are and how they look for condition. From there try to figure out how much timing is in the distributor and check that against my guess of 10-12 initial.

3: find a recurve kit for the HEI, or at least lighter springs. Try setting max timing around 36 and she if it is happy and let the initial fall where it may.

4. Convince him that his fan clutch is shot and he needs a new one. It feels shot to me. He’s reluctant to spend much money on it because he is saving for an LS swap. I might just buy him one as a gift if it isn’t too expensive. We will see. Looks like a bear to put on so I’ll let him handle that. Lol

Open to input.
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Old 07-31-2020, 08:11 AM
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The air is filtered that is pulled through the choke. That capped vac line at the top back of the carb is the air source. With the air cleaner on it’s only source is filtered air. A tube runs down to the stove from it.
That choke won’t work without a good stove and the tubing to hook it up. Better to tie it open or replace with electric.
Try 14 initial.
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