1967 Corvette L71 Running Hot

[Charles F.]

[Edward J.]

Re: 1967 Corvette L71 Running Hot

Chuck, although the 215/220 range is not a over heated engine, it may be slightly higher than normal. I would check things like the fan clutch operation, and if you have a infrared gun to see what the real temp is of the engine just point the infrared gun at the metal thermostat housing just to is if the gage is accurate. Make sure that the foam seals around the perimeter of shroud are in place that are suppose to be there.

[Guest]

Re: 1967 Corvette L71 Running Hot

If all above was done and no changes I would check your Vacuum Advance unit and your timing at the distributor to start

[Charles F.]

[Larry M.]

Re: 1967 Corvette L71 Running Hot

Chuck, although the 215/220 range is not a over heated engine, it may be slightly higher than normal. I would check things like the fan clutch operation, and if you have a infrared gun to see what the real temp is of the engine just point the infrared gun at the metal thermostat housing just to is if the gage is accurate. Make sure that the foam seals around the perimeter of shroud are in place that are suppose to be there.

I agree...........all good suggestions. A few more thoughts:

1. Check fan blade position in the shroud to meek sure it is not to far in or to far out.

2. Fan clutch may be a newer one with a higher "engage temperature than original. Not certain how to test this, but if it is a newer replacement, it may be incorrect for the engine. There are now some Schweitzer units available that will engage at a lower temperature

3. Use a quality IR gun to get the correct engine temperature.

4. I would completely map out both the vacuum advance and the centrifugal advance curves to see what they are really doing. The 9 BTDC seems a bit high for the engine per the original factory initial advance.......which was 4 or 5 BTDC. Advance curve may have been changed. Full time vac advance should always be best.

Larry

[Charles F.]

[Larry M.]

Re: 1967 Corvette L71 Running Hot

If you have 9 BTDC initial and 36 BTDC total mechanical advance, you do not have the original factory curve. So you need to find out what you do have.

Larry

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Larry, when you say full time vacuum advanced is best, are you speaking of using manifold vacuum or ported?

[Larry M.]

Re: 1967 Corvette L71 Running Hot

Larry, when you say full time vacuum advanced is best, are you speaking of using manifold vacuum or ported?

Manifold..........

Larry

EDIT: Ported advance was done to help improve idle emissions, not for engine performance or engine (lower) temperature.

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Larry, if I simply moved the vacuum tube off the ported carb inlet and put a T on the manifold inlet shared with choke pull off, would I have to make any timing changes? I would think with full time vacuum that I would have a lot more idle timing than normal. Would I need to dial back the initial timing? I am not a timing expert other than I have turned a distributor and listened by ear to what I thought was the best running state of an engine.

[Edward J.]

Re: 1967 Corvette L71 Running Hot

Chuck set your base timing as factory says. and plug vacuum hose into choke pull off as you said using a vacuum tee for full time vacuum for advance. look at some of the vender sites to see what if any seals were used to seal the shroud against the radiator. You want to pull as much air through the front of radiator.

[Larry M.]

Re: 1967 Corvette L71 Running Hot

Larry, if I simply moved the vacuum tube off the ported carb inlet and put a T on the manifold inlet shared with choke pull off, would I have to make any timing changes? I would think with full time vacuum that I would have a lot more idle timing than normal. Would I need to dial back the initial timing? I am not a timing expert other than I have turned a distributor and listened by ear to what I thought was the best running state of an engine.

The factory vacuum advance is 15 degrees advance at 15.5 inches HG and 0 degrees at 8 inches HG. You should first verify that that is what you have on the car. You can use a dial back timing light and a Mighty-Vac to pull the vacuum. But if the idle speed increases when you do this test, you need to temporarily pull the idle speed back to what it was before you started.........this is so that the RPM change and the centrifugal advance change does not influence the timing change you are seeing.

You also need to ensure that the engine is pulling at least 17.5 inches of HG vacuum at idle to fully pull in the vacuum advance. This is commonly referred to as the " SWC Duke 2 inch Rule". If the engine at idle does not produce 17.5 in HG vacuum the vacuum advance will be unsteady and vary......and this may be an issue for you. I am not sure your engine produces this vacuum at idle.......but you can/must check.

If you changed from ported to full time manifold vacuum as recommended, I would back off the initial advance to the original 4-5 BTDC factory spec. At least at the start. But unless the engine produces enough vacuum to completely pull in the vacuum advance can + an additional 2 inches HG safety margin, I would not make the change. I would find a different vacuum advance unit for my distributor that works for me. This information is available, but I would have to search for it in my files. A few of the forum guys, like Duke, probably know it for memory.

But as I stated earlier, you need to also determine exactly what centrifugal advance your distributer has. This way the best solution can be determined.

As an example, my 1967 327/350 HP car currently has 6 BTDC initial advance and a vacuum can (manifold vacuum) that provides and additional 16 degrees of advance at idle. Total of 22 degrees at idle. I dropped the initial timing a few degrees from factory 10 BTDC just because I did not want to have too much total mechanical advance at 5000-6000 RPM WOT. Right now my total mechanical advance (initial + centrifugal) at 5000 RPM is around 36 degrees. I should have 40 degrees advance per the factory, so I am losing a few horsepower just to be safe with the current gasolines at the pump. My engine also makes way more vacuum at idle than the vacuum can needs for full vacuum advance, so my idle timing is rock steady. My car also runs cool (180 F).

You should have manifold vacuum to the vacuum can...........but perhaps your vacuum can specs and actual engine vacuum......... make this difficult. And that is what you are seeing as a problem.

So run the tests I mentioned above and see what you have........and then we can advise better. But if you want to try it sooner, cut your initial advance back to 5 BTDC and go with manifold vacuum.........and see what happens. You could also just leave things alone.......but that will not help your engine temperature issue.

Larry

[Larry M.]

Re: 1967 Corvette L71 Running Hot

According to Duke Williams and Lars Technical Paper on this subject, the 1967 427/435 engine should use the NAPA/Echlin/Standard Motor Parts "B20" vacuum advance can. This can provides the following advance:

16 degrees advance at 11-13 inches HG

0 degrees advance at 5-7 inches HG

Bua as stated earlier, you first need to measure the actual vacuum your engine produces at normal curb idle. This way the best selection can be confirmed.

Larry

[Doug M.]

Re: 1967 Corvette L71 Running Hot

Make sure heat riser is gutted or wired open

[Charles F.]

Re: 1967 Corvette L71 Running Hot

I thought the heat riser may be an issue so I put in the spacer and eliminated it altogether. When I first purchased the car the timing was retarded about 6 degrees. Needless to say, it was overheating in very little time. I seem to think this has more to do with timing and A/F since I have literally addresses pretty much all that can be addressed. The switch to manifold vacuum seems to have helped for almost all but did not do much for me. I think I need to test the vacuum and the canister, as reduce the initial timing. That will be my next attempt to correct this issue.