Initial timing issue

[Lorne G.]

I know that timing issues have been discussed in great length but I could not find any similar issues to mine in the archives.

i am tuning my recent acquistion a 1967 small block 300hp which runs well and starts well but I was shocked to see the initial timing is 32 degrees. The timing marks on the balancer and tab line up at TDC so I assume the distributor gear must be out 180 degrees which was probably done when the pertronix conversion was done.

Someone told me there is about 27 degrees difference if the gear is out 180 degrees so doing the the math my initial timing is really about 5 degrees which is close to factory specs. Does this seem logical or is there any other reason for my initial timing reading of 32 degrees?
My total timing is close to 60 degrees.
Lorne

[Frank D.]

Re: Initial timing issue

You're checking the initial with the vacuum advance line plugged ? Correct ?

[Joe L.]

Re: Initial timing issue

I know that timing issues have been discussed in great length but I could not find any similar issues to mine in the archives.

i am tuning my recent acquistion a 1967 small block 300hp which runs well and starts well but I was shocked to see the initial timing is 32 degrees. The timing marks on the balancer and tab line up at TDC so I assume the distributor gear must be out 180 degrees which was probably done when the pertronix conversion was done.

Someone told me there is about 27 degrees difference if the gear is out 180 degrees so doing the the math my initial timing is really about 5 degrees which is close to factory specs. Does this seem logical or is there any other reason for my initial timing reading of 32 degrees?
My total timing is close to 60 degrees.
Lorne

Lorne------


The distributor gear being "180 degrees out" will not cause a problem as you've described. The only thing this will cause is for the orientation of the distributor to be "non-standard". This is really only an issue for Corvettes since the tachometer drive has to be oriented properly and the distributor vacuum control has to be clear of the distributor RF shielding supports.

[Joe L.]

Re: Initial timing issue

You're checking the initial with the vacuum advance line plugged ? Correct ?

Frank-------


Plugged or not will not make much difference since 1967 engines used PORTED vacuum for the vacuum advance. If the engine has been previously converted to full-time vacuum advance, then it will make a difference.

[Timothy B.]

Re: Initial timing issue

Hi Joe,

I'm not sure when the ported vacuum advance showed up on all engines but in 1967 for 300hp it's full manifold vacuum advance. The engine should have a initial setting of 6*BTDC with vacuum advance (#355) 15*@12" hooked to full manifold vacuum.

[Lorne G.]

Re: Initial timing issue

The initial timing was checked with vac line plugged. The vac line is also hooked up to full manifold vacuum. I have tuned several Corvettes but never experienced a problem like this.

Would the Pertronix III conversion have anything to do with this?

[Duke W.]

Re: Initial timing issue

The '66-'67 300 HP engine has full time vacuum advance unless equipped with AIR, in which case it is ported.

To the OP: Report the numbers stamped on the VAC mounting bracket. You may have to partially remove the cap to see them all.

Initial timing must be measured with the VAC signal line disconnected and plugged. Verify that the plug wires are indexed IAW the CSM. The #1 wire should be along the right edge (as viewed from the driver's seat) of the cap window. If not, reindex the wires correctely and set the initial timing at 8.

IIRC the OE centrifugal starts at 700 and should be 30 @ 5000. Verify with your CSM or AMA specs. Initial must be set at a speed lower than the start point, and you should verify the start point with a timing light.

When you refer to "total advance" you must be specific as there are several "totals".

Total centrifugal
Total idle (initial plus full vacuum advance)
Total cruise (initial plus centrifugal at cruise RPM plus full vacuum advance)

The OE CR, which is maybe 9.7 true, can tolerate ligher springs in the distributor to bring all the centrifugal in at 3000-3500, which will improve low end torque.

Duke

[Stuart F.]

Re: Initial timing issue

Lorne:

My 63 has Pertronics II. I recently did a rebuild which included tightening up the tolerances on the drive gear and the Pertronics pickup gap. As was posted a few months back, I had trouble getting the distributor installed so that the initial timing would be correct and the distributor would be ideally positioned midway between the manifold and the plug wire bracket. I had the gear on correctly with the dimple in line with the rotor, but was not able to get it set up right, i.e. either too far advanced or too far retarded. I eventually, against my better judgement, had to reposition the gear 180 degrees off in order to get it to run and time up with the 12 degrees initial I was after. Even then, the VAC was tight against the manifold, but at least it ran properly. Understand, I had the Pertronics II unit in the distributor for a number of years before without any problems and it lined up well. I don't recommend altering your setup based on my experience, in fact, I intend to try to get mine to work with the gear installed correctly some day. I'll be on a long tour most of the summer so it will have to wait until fall.

Stu Fox

[Duke W.]

Re: Initial timing issue

The easiest way to solve all these Pertronix issues is to throw the damned thing away and install a set of points.

Duke

[Lorne G.]

Re: Initial timing issue

Duke,
The Vac canister was replaced with a B26 can from Napa when the Petronix conversion was done. The wires are indexed correctly and initial timing was measured at 750 rpm. with vac disconnected and plugged.
Aside from a slight surging at low speed around 1500 rpm while cruising, the car starts, accelerates, and idles great. Perhaps the springs are too light?
Lorne

Lorne

[Steve G.]

Re: Initial timing issue

Lorne,
It doesn't matter the change in the tooth orientation when it comes to reading timing with your light. It is showing you the crankshaft position (according to marks on the dampner and timing tab) at point of firing. The time of firing is altered by the gear of course, but you are reading the end result. Whatever the gear changed it is what you are reading. It doesn't give you a false reading. I can drop the dist in anywhere I want, turn the gear any which way and, except for the physical constraints the vac canister places, I can adjust my wire position in the cap and set my timing to whatever I want it to be.

If this engine was running fine, but when you put your light on it you found 32* of advance I would say that it is not actually running 32*, the problem is in the reading.

At 32* initial it would be extremely hard to start, if it would start at all. It would be evident it was firing against itself when cranking. It would ping and knock like crazy when you drove it.

What method did you use to confirm the balancer and timing mark accuracy? If you have confirmed without a doubt that the marks are correct, look to a problem with your light or hookup.

Steve

[Lorne G.]

Re: Initial timing issue

Stu,

I read the Pertronics III instructions that came with the car and I see no mention of any pickup gap required. A new mounting base plate is installed in the distributor and the Pertronics unit is mounted onto the base plate. The Pertronics II and III are supposedly different styles then the original Ignitor models that required a specific gap.
This Pertronics unit may have been installed incorrectly and causing the timing problems but one would think that the performance would be poor if installed incorrectly and this is not the case at all.

[Lorne G.]

Re: Initial timing issue

Steve,
I fully agree with you that the car is not actually running at 32 degrees but closer to OE specs but for the life of me I can't figure out what is causing the false initial timing reading. I thought my timing light was the problem but a mechanic using his own light had the same 32 degree reading.


I will go back and recheck the timing marks on the balancer and timing tab.

Lorne

[Stuart F.]

Re: Initial timing issue

Lorne;

The II unit is very similar to the I unit as when I made the change, all I did was change the pickup and the coil.

I don't attribute any of my problems to the Pertronics unit and it will be a cold day in he double hockey stick before I go back to points.

Stu Fox

[Duke W.]

Re: Initial timing issue

Duke,
The Vac canister was replaced with a B26 can from Napa when the Petronix conversion was done. The wires are indexed correctly and initial timing was measured at 750 rpm. with vac disconnected and plugged.
Aside from a slight surging at low speed around 1500 rpm while cruising, the car starts, accelerates, and idles great. Perhaps the springs are too light?
Lorne

Lorne

750 is above where the centrifugal starts. You have to set initial below the speed that it starts. A 300 HP engine should idle butter smooth at 500 @ 18-19" in neutral (450-500 in drive with PG). If not, someone may have installed a high overlap hot rod cam. Measure and report manifold vacuum at 500 and 750.

Look at the OE spark advance map specs in the CSM or AMA specs. Then determine what the map actually is on your engine with a dial back timing light. Maybe a bunch of things on the engine have been modified in the past 48 years. The only way to know what you have is to take the measurements I recommend.

Google duke williams san diego corvette and download the pdf of my 2012 National Convention presentation. That will teach you how to optimize the spark advance map for any OE engine. If you engine has been modified, then you have to use other general rules described.

Duke

Duke