Overheating and backfiring

[Oliver S.]

Re: Overheating and backfiring

At 550 rpm there's no perceptible rpm drop and timing change when removing and plugging the vac hose.
Since there's vacuum produced (vacuum gauge shows this) the vac isn't working ... at least at this rpm. Since there's a rpm drop at higher revs the vac is at least working.
Should the vac perceptible contribute to timing at idle - here 550 rpm - for a stock '65 300hp, Carter AFB engine configuration?

Oliver

[William C.]

Re: Overheating and backfiring

The first question would be on a vacuum gauge how much vacuum are you getting at idle? The second question is what are the numbers stamped on the vacuum assembly just behind the can itself? (On the flange that mounts the can to the distributor)?

[Bob H.]

Re: Overheating and backfiring

Is your ignition a Pertronics?

I am not an expert by any means as evidenced by the many questions I have asked on here and gotten great answers and solutions.

Over the years I have converted several of my cars to the single wire Pertronics with good results. I had a 67 427 400 that ran perfectly and amazingly had no overheat problems even here in Florida. I decided to convert it simply for reliability. Once converted everything was fine until I drove it the first time a longer distance. It started to overheat and coming to a stop would not idle. After cooling it was fine until I drove it a longer distance. Thinking there was an issue with the Pertronics, I point the points back in and all was fine. I used that Pertronics kit in another car later on with no issues at all.

I am just wondering what I and maybe your previous owner could have done wrong in the installation of the kit? If this is what you have maybe try points again?

There is a lot of discussion around vacuum. Also guys, could he have a intake manifold gasket leak that gets worse when the car warms up?

[Jack H.]

Re: Overheating and backfiring

PRESUMING the ignition system is factory stock, then you have a Delco 111076 distributor with 1116236 vac advance. Cracking open your copy of the 1965 Corvette Shop Manual Supplement, we find the distributor specs to be:

Cent Advance: 0-degrees @ 750 RPM, 15-degrees @ 1500 RPM, 26-degrees @ 4100 RPM
Vac Advance: 0-degrees @ 4" HG and 16.5-degrees @ 8.2" HG

So, as Clup asked, what's the vac pressure you see at idle? If it's 4" HG or less, then what you observed (no advance at idle AND your idle is 750 RPM or lower) is what it's supposed to be.

From the tune-up chart in the book, 8-degrees BTDC is spec'd to be the nominal setting with the allowable range being 6 to 12-degrees BTDC. The 4-degrees BTDC is the nominal spec'd for the base engine...

Most folks setup more aggressively than you've got things dialed in, typically in the 8-10 degree range...

[Oliver S.]

[Duke W.]

Re: Overheating and backfiring

The 300 HP engine didn't have the "236" VAC. That was SHP/FI engines only.

I don't have the specs/number for the '64 250/300 HP VAC, and I don't know if the OE VAC is properly matched to the 300HP/manual trans combination.

These damned threads go on and on because we never get any test data and OE specs, which is why I am reluctant to even get involved in these discussions anymore. In this case we finally got some specs- at least partially, but nothing accurate on the VAC.

What the OP (or someone else) needs to do is post all the spark advance map specs from the '64 Shop Manual Supplement. Then the OP needs to test that the centrifugal and VAC are functioning properly, and the manifold vacuum at idle speed (SPECIFIED) in neutral with a MT and in Drive with PG needs to be stated.

Only then can anyone make any reasonable diagnosis or recommend new parts.

Duke

[Oliver S.]

Re: Overheating and backfiring

@Duke, sorry for being to unspecified with my answers.
Please consider that I'm facing some non-Corvette specific challenges:
I'm not that experienced with cars and some techincal terms in English must be translated and then their German meaning must be understood by me.
The gauges and devices used in European use dimensions for which I must find a suitable converter.

Oliver

[William C.]

Re: Overheating and backfiring

OK, back to the top, this is a '65 300 hp car. That means it has full time (not ported) vacuum to the distributor. The idle vacuum is rated at 16-20 inches of mercury. that means the vacuum advance normally will be fully in at engine idle speed. the distributor should be a #1111076, and the original vacuum advance would have a stamp of 238 on the mounting flange. This vacuum advance unit is fully advanced (20 degrees crank) at 14 inches of vacuum, so should be fully engaged at idle. That is why the factory requires that the timing be set with the vacuum advance DISCONNECTED to obtain an accurate timing setting. Set the timing with the vacuum unit disconnected and the line plugged, and then reconnect the vacuum line. Idle speed should increase immediately, and if you recheck the timing with the vacuum connected, you should see about 20 degrees more than the setting with the vacuum advance not connected. This is perfectly normal, and will result in a cooler running engine under most if not all conditions.

[Duke W.]

Re: Overheating and backfiring

Okay, now we've got some basic OE specs and data. IIRC that ...238 VAC was a one year deal only, and it's been discussed in the past few months. It changed in '65 and again in '66 which carried over to '67. Why did the design keep changing one might ask? TO GET IT RIGHT!

The '66-'67 VAC was 16@12", which worked with PG, but was more aggressive than necessary for a manual trans, but with the somewhat higher fuel octane available back then it wasn't an issue.

I recommend the B22 (16@15") for 250/300 HP with manual trans and the B20 or B26 (16@12") for P/G.

Search the archives using the B-number to get representative brands and part numbers. It doesn't make any difference what brand you buy as long as the B-number is correct. All, including "Delco" brand are made by Standard Motor Products.

Duke

[Stuart F.]

Re: Overheating and backfiring

If I didn't know much about the beast, and not too mechanically inclined, and someone told me to "plug the line". Well, I might just do that instead of capping off the port. It's just semantics I know, but I've often wondered about that phrase - being an old retired Tech Writer I guess I can't help it.

Stu Fox

[Oliver S.]

Re: Overheating and backfiring

Thanks all for you help.
I ordered the B22 recommended by Duke. Tomorrow I'll know how long to wait for it. Corvette America has a backlog with these parts as their partner dealer here told me.
Futhermore, I had a quick glance at the VAC mounting flange. What I was able to read was "238" and "24". The last digit was almost covered by the distributor cover. So perhaps still the original one - but now out of spec.


Oliver

[Duke W.]

Re: Overheating and backfiring

24 degrees was really too much, which is why the VAC was changed to a 16 degree spec for '66 and '67. That ...238 was a one year odd ball for '65, and yours is probably original and 45 years old. They don't last forever and there are probably a lot of vintage Corvettes running around with non-functional/out-of-spec VACs because nobody ever checks them or even look at the distributor beyond dwell angle.

I have heard reports that various new VACs don't meet spec, so you should check that the plunger is fully deployed at about 15" Hg. vacuum before you install it.

Check the VAC maximum advance after it's installed. With the VAC disconnected, set the initial timing in the range of 8-12 at no more than 500 RPM (Start with 12, but back it down if you hear detonation in subsequent road testing.)

Then raise engine revs until you observe advance. According to the specs this should occur at about 750 RPM.

Reconnect that vacuum advance and set the idle speed to 500. You should read about 16 plus the initial timing value on the tab. Go through the idle speed mixture adjustment procedure. Normal warmed up idle vacuum should be about 18-19" at 500 RPM in neutral.

I recommend you use this opportunity to disassemble, clean, and inspect the distributor. I doubt if it's been touched since it was assembled 45 years ago. This is a relatively simple process for an amateur mechanic. It has been discussed numerous times on this Board, and the basic procedure is covered in the '63 Corvette Shop Manual.

Duke

[Gerard F.]

Re: Overheating and backfiring

For what it is worth, here is my original vacuum can on a 67/327/300/4spd:



MS 355 15

This is after a little plating job. It still works very good.

What is the current replacement number for this one?

[Donald L.]

Re: Overheating and backfiring

[quote=Gerard Fuccillo (42179);436522]For what it is worth, here is my original vacuum can on a 67/327/300/4spd:



MS 355 15

This is after a little plating job. It still works very good.

What is the current replacement number for this one?[/quot

I believe MS 355-15 = B20

[Duke W.]

Re: Overheating and backfiring

The B20 (nominally 0@6" 16@12") is functionally equivalent to the ...355. Keep in mind that nominal specs usually include tolerances of about +/- 1-2 degrees and +/- 1-2" Hg. The 12" maximum is best for PG, which will idle in Drive at less manifold vacuum than a manual trans idling in neutral.

"One size fits all" in the case of OE, but the B22 (0@8", 16@15") is better "tuned" to the manual trans 250/300 HP applications and will have less tendency to generate transient detonation when you open the throttle at low revs.

Whether the VAC is stamped 15 or 16 to indicate the maximum crank advance not important. Most 15s would be within producion tolerance of 16s and vice versa.

Duke