DMG64

Hello all i am having timing problems with my fathers corvette. the engine is a stock 350 with pertronix ignitor 3 in it. I have read lots on timing but my problem is that it was running fine last week with initial timing at 12 degrees and would only max out at 20 degrees at 3K rpm. It had steady vacuum at 20 during the whole rpm sweep. the springs it had were very stiff, Well I thought it needed lighter springs so i placed an accel gold and silver spring in it and changed out the weights from an advance curve kit i bought. After starting it back up the timing was at 18 degrees during idle and would be at 36 degrees at around 1500 rpm. pinging was noticable when i attempted to place it any lower than 16 degrees, after replacing the original springs and weights in it and turning it back on i cannot get the initial timing back to 12 degrees. It idles fine around 18 degrees but if i attempt to retard the timing it starts pinging. So my father wants it back to the way it was but it seems like i cant do it even if i place the original springs and weights in the distributor. It was running on regular unleaded. I plan of placing premium in it. Any help would be appreciated. David

Gary's '66

What made you start fooling with it in the first place? I assume something must have precipitated it so there must have been some sort of issue with the car to start with. Is he running a vacuum advance can? If so, what is it and have you tested it via sucking on the hose or more accurately using a Mightyvac? 36° @ 1500 rpm is way too much advance.

Mike Ward

No C2 Corvette came with a 350. Are you sure it's supposed to use regular (97) octane fuel?

What year and what engine option is the car?

Gary's '66

Mike, I think he means motor is in stock form, not stock to the car OR maybe he meant 350hp?

DMG64

no, it's not a stock engine. The car is a 64 but the engine is a early 70's 350 C.I. motor in stock form. I messed with it to improve the timing so i can have it at the recommended 36 degrees at 2500-300 rpm for this engine. Yes i know that timing changes from engine to engine but I figured that 20 degrees at 3K rpm was to little advance. It does have vacuum advance canister that has had the vaccum nipple capped for a long time. It has run fine with it capped, but i have never attached it to the full manifold vacuum. The engine ran fine with it capped so that's how we have ran it for months.

Mike Ward

It's possible that the engine cannot tolerate 87 octane fuel unless the mechanical advance is all but inhibited and the vacuum advance is disabled. You'd be leaving a lot of HP on the table if so.

The recent changes you made to the advance curve may have uncovered this.

DMG64

I believe you may be on to something Mr. Ward. I removed all the 87 octane and places 4 gal's of 93 octane in it. I will try it tomorrow morning with the original springs/weights and see if it regains original specs. If it does not I have a previous accel distributor with points that was previously in the car (and it ran well). Do you all think it would be worth it to place that back in, or am I just opening more cans of worms?, BTW. if I cannot remedy this timing problem does anyone know of a reputable mechanic in San Antonio,TX that makes house calls. Thanks

DansYellow66

What is your idle speed? I suspect that when the light springs were installed your timing at idle increased because the centrifugal advance was already kicking in. Take a rubber band and secure the weights in the closed position and set your inital advance. Then check your total advance at 3000 rpm.

The lightest springs are not always the best. I think (FWIW) that all advance in by 3000 - 3500 rpm works fine on the street. Also, the medium springs will stablize centrifugal advance to prevent it kicking in at idle which can cause an erratic idle or "trailer hitching" action.

Also, I assume you have a dial back timing light. Double check every timing setting because I think everyone with a dial back light can admit to forgetting to zero it out at one time or another.

And yes - run premium. A fairly stock, stock cam engine may have fairly high cylinder pressures due to mild cam timing. It could be more sensitive to octane than a high compression, 30-30 cam engine.

DMG64

my idle speed was around 800 rpm. i tried the rubberband trick but my rotor kept pushing the rubberband off the weights so i just taped the weights down with masking tape. The stock (heavy ) springs and weights were installed and i still had the same initial timing around 18 degrees. If i tried to retard it, it started knocking. I was not using an adjustable timing light but and acturon inductive light. I'm hoping the premium gas will help. Do you think that reinstalling the distributor a tooth off (retarded) might help the timing? How about if I place the spark plug wires one spot down (ie: place the number one wire in the number eight spot, place the number eight in the number four spot, etc) ? Or am i just asking for trouble?Thanks once again. David

Mike Ward

This is difficult to understand. Are you confusing retarding with advancing?


The 'tooth off' concept does not change timing. It allows a different range of movement for the housing only. This is not your problem.

DMG64

i don't think i'm confusing the two. when my initial timing was 18 degrees (no nocking), but when i tried to retard it to 14-16 degrees it would start nocking at idle. I was hesitant to even go above 1000 rpm for fear of damage. Thanks for clarifying the tooth off idea. I thought it might be n option worth considering. I assume the same goes for the spark plug wires also?

Usafstingray

Here's a useful article for you. It is based the stock breaker point distributor. The optimal curve will be between the light and medium springs in the accel spring and weight kit. I'd go medium with your application unless you have a big cam. The curve in the set is useless. Google the correct one based on the accel part number. You need the right VAC, tapped into the vacuum off your carb's metering block. If your dizzy has a limiter bushing, make sure the bushing is actually installed. Remember to set initial timing with the vac disconnected and cap the port on the carb. Good luck.

http://www.metroli.org/pdf/2012%20Na...ingseminar.pdf

Mike Ward

Don't fool with that either. If the #1 plug is firing at the desired degrees BTDC, moving plug wires, rotor or anything else will make things worse not better.

Tiros

Put in the lightest springs, or one medium and one light.
Disconnect and plug vac advance if you have it.
Rev up to 3000, and adjust distrib. for 34 to 36 advance.
The initial will fall where it may. Even if the springs are allowing some advance at idle, you don't care, since this method sets the top of the curve. If the engine wants some mechanical at idle, it doesn't matter... What matters is how fast the curve comes into the target rpm range. The quicker the better, limited by pinging.

Personally I run light springs, but if you get into pinging, try the next heavier, using the same method.

I never worry about initial.

btw: Advance is a larger number, retard is smaller. More advance leads to pinging.

tbarb

thank you...

DMG64

Well i tried it with the premium gas and it still knocks if i retard the timing it less than 16 degrees. The darn engine seems to like 18 degrees BTDC. If i follow Tiros method for timing shouldn't I be concerned with detonation when the engine is revving that high while adjusting the distributor? any thoughts? BTW the advance plastic bushing was in place. I appreciate all the feedback that everyone is providing.

JohnZ

Describe what you're calling "knock". "Knock" (detonation) only occurs when the engine is under load, and it's not under load when you're fiddling with the distributor. Are you sure it's not an exhaust leak or a mechanical noise?

DMG64

The knock that i hear only occurs sporadically when the engine is idling and timing is less (retarded) than 18 degrees. In the span of 10 seconds the engine may knock around 3-4 times. The knock sounds similar to tapping on a piece of solid wood with the pad of a fingertip ( as silly as this sounds). It is a dull tapping noise. I do have mechanical lifters and the valvetrain has around 500 miles on it. There is no evidence of the balancer slipping. I am fearful of revving the engine up to 3K rpm and setting the timing to 36 degrees for fear of catastrophic damage.

Mike Ward

That's not pinging/detonation.

DMG64

I think i have to realize when it's time to call in people with experience with these issues. I just need to call around to a mobile mechanic. I appreciate everyones insight and comments on this issue. I will let you all know what the verdict is when the mechanic looks at it. Thanks. David