1967 Corvette L71 Running Hot

[Joe T.]

Re: 1967 Corvette L71 Running Hot

I did all of the above. Didn't cool down until I switched to a B28 vacuum and used the light springs from the 928G set. I have 12 degrees at idle (900 RPM) with vacuum disconnected, 28 degrees connected and 40 degrees connected at 2500 RPM. This is on a 1966 427 425 HP.

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Joe, I assume because you have 28 connected that you are running manifold vacuum? Did you ever have it hooked to ported?

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Joe, what was your average high temp and what is the avg temp with the changes you made?

[Joe T.]

Re: 1967 Corvette L71 Running Hot

Manifold. Was running as hot as yours. Maybe on 80 degree days so hot it would boil over. Drove me crazy with new radiator, hoses, seal between shroud and radiator, different thermostat and new stock coil. The GM replacement coil was breaking down when it got hot. It runs between 150 and 180 now, May hit 200 or 210 in traffic on a 85 degree day.

[Charles F.]

[Carl S.]

Re: 1967 Corvette L71 Running Hot

Hi Charles. Interestingly enough I am soon going to change from ported vacuum to manifold vacuum on my 67 L71 and am anticipating much lower temperatures this summer. I have had my car since April of 1981 and have been fighting the heat problem for years. It was only recently that (A huge thanks to Duke and John and Lars) I learned of the advantage of moving your vacuum source for your distributor vacuum cannister from ported to manifold. The other thing that is just as important as switching the source is switching the vacuum cannister itself. As some have mentioned you need to (at the same time) change your vacuum cannister to a B20/B26 which you can buy from Rock Auto for $10.37 under the part number VC181 made by Standard Motor Products. As Duke and John and Lars have explained, when you do both of these things, your idle will likely go up and must be re-adjusted back down to between 900rpm and 750rpm.

In addition you should take a vacuum gauge and monitor it while you adjust the idle mixture screws (located on each side of your main metering block on your center carburetor) to obtain the maximum manifold vacuum at idle. This will help to insure you meet Duke Williams' two inch rule. In addition there are many other things you can do to your engine and engine compartment to help lower the temperature. In 38 years of ownership I have tried most everything I have ever heard of or could think of. Finally this summer I get to try the ported to manifold switch and I have already bought my B26 vacuum cannister in anticipation of the move. Good Luck with the switch and let us know how it goes. C.J.

[Joe T.]

Re: 1967 Corvette L71 Running Hot

You can get a B28 can and a 928G spring kit from Rock Auto. Part #4V1053 for the can. It can be fustrating but sometimes the little things work.

[Duke W.]

Re: 1967 Corvette L71 Running Hot

The OE L-71 centrifugal curve is start @ 900, 30 @ 3800. Converting to full time vacuum advance REQUIRES replacing the OE 15" 201 15 VAC with a 12" B26 in order to pass the Two-Inch Rule.

The above ASSUMES an OE camshaft, which should produce about 14" Hg at 900 idle speed.

You have to verify all of the above before you proceed. In order to get the final destination you need to know the starting point.

The best way to set timing is to use a dial back light and set it in the range of 36-40 a few hundred revs above the maximum centrifugal point with the VAC disconnected. Then with a proper VAC installed that meets the Two-Inch Rule go through the idle speed mixture adjustment procedure with a target idle speed of 850-900.

Read my San Diego National Convention tuning seminar for more info... easy Web search.

Duke

[Charles F.]

[Larry M.]

Re: 1967 Corvette L71 Running Hot

To do things correctly you also need to establish/find exactly what centrifugal advance curve you have. Is it original or modified? You want initial timing somewhere around 5-10 degrees BTDC and a total mechanical advance (initial and centrifugal) at 36-38. Initial is just that INITIAL. So make sure your RPM is low enough so that centrifugal is not coming in and changing things. And the vacuum can is disconnected and the vacuum line is plugged during this phase of setting the mechanical timing. Then as Duke says keep running up the RPM until the centrifugal advance is "all in". You want the total mechanical advance to be 36-38 BTDC.

If your car won't idle low enough to properly set the Initial Advance, you can alternately do it by running up the RPM (again with vac can disconnected) to where no further advance occurs and then adjust the distributor until the total is 36-38 BTDC. But you really need to see exactly what the centrifugal advance is doing at all the various engine RPMs. So I would recommend making a table of centrifugal vs RPM from 750, 1000, 2000, 3000,4000 RPM.

Vacuum advance can and will add to the initial and centrifugal advance BUT it will drop to zero or near zero at high engine loads and at WOT (Wide Open Throttle). That makes sure the total timing from all sources is not too high to cause detonation in the engine. Vacuum advance changes with engine load and engine RPM. That is why the vacuum can specifications have a START or BEGINS number and a FINAL or ALL IN number. Your original factory vacuum can BEGINS to provide advance at 8 inches HG and is ALL IN at 15.5 inches HG. This is not a good match for your engine, since the engine likely produces about 14-16 inches HG vacuum at idle. So you therefore need to change to a vacuum can that more closely matches your engine. The B20/B26 and the B28 are two cans which are probably much better for you.


As Duke states his papers and tech presentations spell it all out. Suggest your get a copy to help you.

Larry

[Charles F.]

[Charles F.]

Re: 1967 Corvette L71 Running Hot

I got it off fellas. Thank you!

[Carl S.]

Re: 1967 Corvette L71 Running Hot

No, you do not have to take the distributor out of the engine block in order to remove the vacuum cannister. With the vacuum line removed from the cannister hook up a vacuum pump to the tip of the cannister and pump up the vacuum until the slim silver bar which protrudes out of the back of the cannister into the distributor begins to move and then keep pumping unti it maxes out. Leave the bar fully retracted and that should then give you access to the final screw which you cannot reach and cannot currently see. Undo the screw, take it out and then remove the cannister. Then release the vacuum pressure and you are good to go.

[Duke W.]

Re: 1967 Corvette L71 Running Hot

Thanks Duke. I am going to run a vacuum test on the engine today to see if I have the 14-15” of vacuum at the manifold. If so, I will order the appropriate canister per your instructions to get the full vacuum advance at idle. I think the only thing that I am a little unsure of is where to set the initial timing without vacuum. Are you saying without the vacuum advance can hooked up that the total timing advance should be 36-40? I assume that because manifold vacuum is strongest at idle that Vacuum advance offers no additional timing at higher rpms or WOT? Otherwise I guess I would have almost 16 more degrees added to initial and centrifugal for almost 56 degrees. Am I on the right track?

At WOT (wide open throttle) manifold vacuum is only about 1.0" Hg, so the vacuum advance is fully retracted. The reason to removed and plug the VAC signal time when either setting initial timing to total WOT timing is to SIMULATE WOT without any load on the engine.

Given the OE 30 @ 3800 max OE centrifugal, which you have not verified, initial timing for 36-40 total WOT advance will be in the 6 to 10 range. I prefer the total WOT advance method to set "timing" because if the centrifugal starts at below idle speed your settings won't be accurate and getting some SHP engines to idle stably long enough to set initial at less than the centrifugal start point can be a problem.

To determine the points of max centrifugal increase engine revs (VAC disconnected and plugged) until the mark stops advancing, then add a few more hundred revs and blip the throttle to ensure that it no longer advances as you approach max revs. For the novice this test should be done at least three times in a row, and if the results aren't consistent there is something wrong with your technique or something wrong with the distributor.

Duke

[Charles F.]

Re: 1967 Corvette L71 Running Hot

I will be putting everything back together in about an hour. Do I have to leave off the chrome shroud to access the distributor lock down bolt? Just wondering if I put it all back together or without the chrome shroud so I can access the bolt. Never adjusted the timing so never tried to access the bolt. Knowing this may save me the time of assembling everything twice. Thanks