'65 Timing and Tuning

[Ronald D.]

I would appreciate information and guidance on the proper timing and tuning on my stock '65.

First, let me baseline the discussion by saying that the car is a stock '65 with the base 250hp engine and powerglide transmission. As I know the history of the car, the engine is orginal without modifications or a rebuild in its past. So, the cam is stock, etc. The WCFB carb has been rebuilt recently and seems to operate correctly. The distributor's mechanical advance and vacum advance have been altered or adjusted in the past but I don't know enough to specify exactly how or when. However, I believe the distributor to be original.

The car is demonstrating a rough idle when warm and a sluggish acceleration from a stop under moderate to WOT acceleration. Because of these issues, I have begun to trouble shoot the car's timing and tuning. However, as I have only a basic knowledge of tuning I would greatly appreciate some guidance.

Let me tell what I know and ask some basic questions. I have placed a new set of points and a new condenser in the distributor. I have set the dewll to 30 degrees at an idle speed of 750 rpm. The dwell reads steady at 30 degrees as the engine rpm is increased to 3,000 rpm. I believe this indicates no problem with the distributor bearings, ie. no shaft wobble. The distributor cap is new and shows no sign of arcking at the inner terminal tabs. Also, the spark plugs are new and per spec. The plug wires are new and of the correct resistance.

My first question is one of correctly identifying the distributor. The shop manual suggests 6 different distributors in its specifications section. For example the base distributor may be identified with the reference number 1111076. How can I determine the model number of my distributor? This may be important as the shop manual gives different centrifugal advance specifications for different distributor models.

My next questions go to timing the engine. I first did this with the vacum advance hose disconnected and plugged. I set the timing to the "O" mark on the reference tab. I assumed this was 0 degrees as the other specified mark on the tab was the letter "A" with line graduations between these marks. One basic question is what degrees of advance do the "0" and "A" marks on the timing tab correspond to? What degrees of advance do the incremental lines between these marks refer to? I know this is basic stuff but I don't want to assume something wrong, especially since the shop manual specifies timing at 1500 and 4100 rpm for centrifugal advance.

Believing I set the timing correctly at 0 degress for 750 rpm, I then checked the operation of the centrifugal or mechanical advance with the vacuum advance disconnected and vacuum line plugged. As I increased the rpm the distributor advanced the timing such that the timing was at the upper limit of the timing tab at 3,000 rpm. However, as I don't know the significance of the timing marks on the timing tab, I don't how much of advance this was as I accelerated the engine. I believe there should be certain levels of advance for different rpm as the shop manual specifies. Can someone offer an opinion as to the correctness of the mechanical advance I saw?

I then went to check vacuum advance by connecting the vacuum advance canister to the vacuum line. This did not change the advance of 0 degrees at 750 rpm nor did the advance at 3,000 rpm change. Am I correct in believing that the distributor/VAC was not advancing the engine? Is there a better check?

Well, this is where I am at now... basic stuff with basic questions. Any advice would be appreciated to help me trouble shoot the rough warm idle and sluggish accel problem as well as understand my setup.

Cheers

[William C.]

Re: '65 Timing and Tuning

The distributor for a midyear corvette should have a metal band around the housing just above where the hold-down clamp is no the housing. To check vacuum advance you need a manifold vacuum source or a vacuum hand pump like a mityvac to apply a known vacuum to the can and check for the timing advance. The '076 Distributor is the proper one for a base engine car, so you are off to a good start, next step is to verify function. Rough idle when warm is many times a product of today's gasoline, especially if you still have the manifold heat crossover active. Lots of info in the topic in the archives...

[Donald L.]

Re: '65 Timing and Tuning

The 65 initial timing for base engine w. PG is 6*-10* BTC. The idle timing ( initial timing + VAC timing) should be in the range of 20*-26* w. the VAC connected. Since your idle timing has not changed w. the VAC connected your VAC is either defective or your vacuum source ported.

Perhaps since your distributor has not been serviced other than tuneup parts you should have it "blueprinted" to optimize the ignition curve, lubed and checked for housing wear from the tach gear.

[Duke W.]

Re: '65 Timing and Tuning

Google the GM Heritage Website and download the '64 information. It includes the AMA specs, which have vast technical info including everything you need to know about the spark advance map.

If you need a new VAC use a B20 or B26, which has the best specs for 300HP/PG, and is equivalent to that used on the '66-'67 300 HP engine.

If you don't understand these B-numbers, a search of the archives will yield a wealth of information.

"A" on the balancer tap means advance, and each hash mark is two degrees.

Duke

[Ronald D.]

Re: '65 Timing and Tuning

Gents, thank you for this great advice.

William, the Mityvac confirmed that the VAC was indeed bad and needed replacing. With its replacement my idle vacuum rose from 5 to 18 in-hg. This also dramatically affected idle timing and produced a smoother running engine. I still have a rougher idle than I think is appropriate and I will address the manifold heat crossover flow which is active although I have a pass through heat riser in place.

Donald, I timed the engine to your specs and confirmed the total idle timing with your numbers. The engine is more responsive under all throttle conditions. I am still curious as to why the shop manual gives the initial timing as 0 degrees BTC. I will also take your advice and have the distributor blueprinted.

Duke, thanks for the reference information. I will do my homework. The bad VAC was designated as B1, not correct. It was replaced with one designated as B26. A good change. Also, thanks for answering the freshman question of the hash marks on the timing tang. I was almost too embarased to ask.

Everyone, thanks for all your advice as this helps greatly in getting me farther down the road.

Cheers!

[Duke W.]

Re: '65 Timing and Tuning

If you want to improve performance/fuel economy, advance the initial timing to the 12-14 degree range. If it doesn't detonate, you're good to go. If it does detonate, back the timing down in two degree increments until the detonation goes away.

Once you've got the initial timing dialed in, go through the idle speed/mixture adjustment procedure in the 1963 Corvette Shop Manual for the WCFB.

It should be done idling in Drive with a target idle speed of 450-500. Be sure to set the parking brake firmly and chock two wheels.

If the idle still has some roughness or uneveness, check spark plug wire resistance, and they should be no more than 5K ohms/foot. Cylinders occasionally misfire, but it shouldn't be so frequent that you can feel it, and the culprit is often leaky or high resistance wires.

Modern engines can detect misfires with sensors that look at crankshaft torsional vibration anomalies. If two many misfires are detected, you get a check engine light and excess misfires code.

Duke

[Ronald D.]

Re: '65 Timing and Tuning

Duke, thanks for the additional info.

Running with your information, last night I performed several test runs with the car. I settled on 10 degrees BTC initial timing as this produced a lightly audiable ping under WOT and heavy loading (running up a steep hill). This setting should be good for nearly all my normal driving. I will perform the idle speed/ mixture adjustment.

However, after setting the initial timing I decided to perform a compression test. I am doing this with the engine warm so far. Only got the left bank done last night with readings of 160, 160, 160, 150 psi. These readings seem high, may have a guage issue, but I was glad to see a relative degree of consistency. The right bank will be done next but my guage will give a level of difficulty due to access issues from the heater hoses, battery shield, etc. Also, I am thinking of doing the test with the engine cold given this situation. Comments?

Again, thanks for all your help.

Cheeers.

[Jack P.]

Re: '65 Timing and Tuning

Gents, thank you for this great advice.

William, the Mityvac confirmed that the VAC was indeed bad and needed replacing. With its replacement my idle vacuum rose from 5 to 18 in-hg. This also dramatically affected idle timing and produced a smoother running engine. I still have a rougher idle than I think is appropriate and I will address the manifold heat crossover flow which is active although I have a pass through heat riser in place.

Donald, I timed the engine to your specs and confirmed the total idle timing with your numbers. The engine is more responsive under all throttle conditions. I am still curious as to why the shop manual gives the initial timing as 0 degrees BTC. I will also take your advice and have the distributor blueprinted.

Duke, thanks for the reference information. I will do my homework. The bad VAC was designated as B1, not correct. It was replaced with one designated as B26. A good change. Also, thanks for answering the freshman question of the hash marks on the timing tang. I was almost too embarased to ask.

Everyone, thanks for all your advice as this helps greatly in getting me farther down the road.

Cheers!

If you still have 43r spark plugs, they are too cold. Upgrade to 45RS plugs. They are hotter and better with today's fuels.

Jack

[Duke W.]

Re: '65 Timing and Tuning

Duke, thanks for the additional info.

Running with your information, last night I performed several test runs with the car. I settled on 10 degrees BTC initial timing as this produced a lightly audiable ping under WOT and heavy loading (running up a steep hill). This setting should be good for nearly all my normal driving. I will perform the idle speed/ mixture adjustment.

However, after setting the initial timing I decided to perform a compression test. I am doing this with the engine warm so far. Only got the left bank done last night with readings of 160, 160, 160, 150 psi. These readings seem high, may have a guage issue, but I was glad to see a relative degree of consistency. The right bank will be done next but my guage will give a level of difficulty due to access issues from the heater hoses, battery shield, etc. Also, I am thinking of doing the test with the engine cold given this situation. Comments?

Again, thanks for all your help.

Cheeers.

The specified cranking compression pressure for the 327/250 is 160 psi with a max variation of 20 psi high to low. If your right bank is similar to the left bank, you have a healthy engine.

Cranking compression should always be done with the engine hot - all spark plugs removed, the throttle blocked wide open, and a fully charged battery. Crank the engine until the readings reach maximum, which is usually 3 to 5 cycles.

Compression gage readings can be all over the map depending on the gage. The most important thing is consistency, not the absolute number.

The best spark plug heat range for all road driven vintage Corvette engies is AC "5" such as the currently available R45 for small blocks or equivalent in another brand.

AC no longer offers non-resisitor plugs, which were OE in '65, but equivalents of the out-of-production non-resistor AC 45 are available in other brands.

There are many spark plug discussions in the archives, but basially all you need is a "two-doller" spark plug equivalent to AC heat range "5". Everything else is just fiddling around at the margin.

Duke

[John H.]

Re: '65 Timing and Tuning

I am still curious as to why the shop manual gives the initial timing as 0 degrees BTC.

Ron -

The tune-up specs in your Owner's Manual show 4* BTDC for initial timing, and the tune-up specs in Section 6 of the '65 Shop Manual Supplement show 4* nominal, with a range of 4*-10*. The only place I show 0* in the Shop Manual is the centrifugal advance at 750rpm, in the Specs section in the back of the Shop Manual, and that's not the initial timing setting.

[Jim D.]

Re: '65 Timing and Tuning

For some reason, 65 seems to be an odd year for the distributor. Here's a thread addressing this issue. The judging says one thing, the truth is another. https://www.forums.ncrs.org/showthre...acuum&uid=4278

[Ronald D.]

Re: '65 Timing and Tuning

Everyone, thanks again for the solid advice.

Jack, yes, I was running with the 45RS plugs which all came out of the engine with a tan color on the tip and no apparent oil fouling or other abnormality. I intend to replace with another set of 45RS if I can source them. Otherwise, I will take Duke's advice on a substitute.

Duke, out of sheer convenience I repeated the compression test last night on a cold engine but per your procedure. On a repeat of the left bank I got similiar readings but a general decrease of about 5 psi on each for the cold engine compared to the prior hot engine results. A test of the right bank gave cold readings very similiar to the left hand bank. All cold test readings were between 145 and 155 psi. This was encouraging but I will repeat it at operating temperature now that I have a guage which makes taking the readings easier.

John, my ignorance here. I mistook the 0 degree centrifugal advance spec for the initial timing spec. My mistake explains the discrepancy. I'll learn up on this but bottomline is I believe the engine is timed correctly now with the distributor operating about right with a new VAC. I still want to have the distributor blueprinted as I am not confident that it has the correct centrifugal weights or springs.

Jim, thanks and I will check out that thread but I don't know the significance of your comment yet.

Again, everyone THANKS! Got more to do as you all suggest.

Cheers!