Initial Timing Advice

[Squirrely Brother]

After figuring out the problem with fuel and fire the truck runs good. I put a light to it and it was at 14 degrees initial timing with the distributor vacuum unplugged (vacuum was stable at 21 hg @ 950 rpm). I was concerned the initial timing was too high, so I backed it down to 12 degrees, but now power seems lacking. Is it bad to put it back to 14 degrees timing? I understand every engine is unique with initial timing and power is good, but I don’t want to mess up the engine. Any constructive advice would be appreciated. Thank you.


1991 350 long block
Edelbrock dual plane intake
HEI distributor
Edelbrock 1406 carburetor

 

[Ricko1966]

Depends on the distributor, and the vacuum advance can, and if you have functioning egr. What do you have for total?

 

[Squirrely Brother]

I plead ignorance to total timing. I’ve watched a few youtube videos and they just weren’t clicking. It’s just a run of the mill ac delco hei distributor with a msd coil and msd control module. I’ll look at videos again until it makes sense.

 

[Squirrely Brother]

So, if I set my light at 36 degrees with rpms @ 2500 I should be at tdc? At least that’s what I learned from CorvetteHop on youtube.

HPTuners software tuning is a heck of a lot easier than a carburetor.

 

[Ricko1966]

What you want is 50 total with vac advance if you have working EGR 46 if no EGR. Where ever you have it timed now with advance can disconnected and plugged, now hook the vac can up to manifold vacuum.How much did it change? That's your vacuum number, write it down disconnects and plug again rev the motor until the timing mark quits moving , how many degrees did it move? Write it down.add vacuum number and centrifugal number. Subtract from 50 or subtract from 46 depending on egr. The sum is what need for initial.time it there, hook the vacuum advance back to manifold vacuum and go for a test drive. No pinging? Your done.

 

[Squirrely Brother]

@Ricko1966 You’re awesome. I can’t thank you enough.

 

[Ricko1966]

That was super brief and basic. I can go through it better later. Super busy right now. But I hope you understand. You need to know what your vac and centrifugal total than set base timing to total 46 when it's all in.

 

[Ricko1966]

Post up your numbers when you get them.Also post what rpm the centrifugal is all in by.

 

[Squirrely Brother]

Will do. It’ll be a minute. I couldn’t find the chalk, so I’m headed to a 2nd Walmart. Thanks again.

 

[Squirrely Brother]

@Ricko1966 After going to 3 Walmart’s to find chalk I’m spent. I’ll set the timing as you instructed in the morning then post the numbers. Gave a great night.

 

[75gmck25]

This is my timing procedure.
- Get a cheap dial back timing light and tachometer.
- I spliced into my HEI tach wire (in dash tach) to get a place to connect the tachometer. If you don’t have an in dash tach this terminal is probably open and you can just connect there.

- disconnect and plug the vacuum advance line and turn the idle down low (650 rpm?) so you have no vacuum or mechanical advance.
- Then use the setback light to verify base timing. Just turn the wheel on the light until the balancer line points to zero on the timing tab, and read the number on the light. 4-12 degrees is common, but it should be fine with up to 16 degrees BTDC.
- now raise the rpm gradually and watch the timing move. Hold it at several rpm points and turn the dial to find the timing (1500 rpm, 2000 rpm, etc., up to about 3000 rpm). Write these values down. It will probably max out at about 3000-3500 rpm, and the value will be about 18-20 degrees higher than the base timing you measured. This total should not exceed about 36 degrees. Having it come in faster (at a lower rpm) will help your acceleration; however, many stock HEIs will have a slow/lazy advance curve. It may not max until 3500 rpm.
- now let the engine go back to idle, and reconnect the vacuum advance line. If you have a vacuum pump you may want to use that on the line so you can pull and measure vacuum at the same time.
- as you pull full vacuum you will see the engine timing advance and reach a maximum. The max will probably be about 18-20 degrees more than your base timing. Write this value down.

There are several types of vacuum advance canisters, including those that are adjustable. The key point you are looking for is to get max advance without light throttle ping. Vacuum advance is primarily for better gas mileage, so keep that in mind. Your engine would perform fine without it, but gas mileage would suck. Stock canisters advance about 20 degrees, and there are others that max at 12-15 degrees. Your engine may tolerate a total of 54-56 degrees (base + mechanical + vacuum), but you don’t want to get carried away and have detonation (pinging) hole a piston.

Bruce

 

[Squirrely Brother]

@75gmck25 Thank you, Bruce. It’s a shorter work day, so I’ll do the timing after work. I have an Innova 3568 timing light with advance and tach, a hand vacuum pump and a vacuum gauge I’ve been using to dial in the carburetor. I didn’t realize I needed the idle at 650. I’ve kept it at 950, because I didn’t install a ac solenoid when I installed the Edelbrock carburetor. I’m not going to lie, I’m thoroughly confused right now after reading advice and watching youtube videos; mostly about the vacuum advance portion. But, I’m sure when I’m working on it things will click.

 

[Matt69olds]

Depending on the advance springs, you may not notice any difference with the idle at 650 or 950. Setting the idle as low as possible will ensure no centrifugal advance at idle speed.

While most (if not all) HEI provide about 20 degrees of centrifugal advance, the rate at which some advance is different. There is dozens of differently shaped weights and cams, plus different springs, all designed to control advance curves. There are hundreds of different combinations. I found a partial listing on a Pontiac forum.

If you get some spark knock with the vacuum advanced hooked up, don’t pull timing out. You can weld the vacuum advance slot to limit travel, or easily make a stop to limit plunger travel.

Don’t give up on this. Too many people are content to drive around with messed up timing curves or no vacuum advance because they are either too lazy or too stubborn to make it work. Those same people are the ones that complain about lousy fuel economy or performance.

 

[82sbshortbed]

I just pinched the vacuum advance hose and set my gun to 10* and moved distributor till mark was at 0 on balancer.

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Starts easy and runs great.

 

[Ricko1966]

Matt is pretty right on what he's telling you. But there is still more to consider which is why I wanted your base numbers you have now, with your mystery distributor. Let's get those numbers so we can dial those in. Your distributor could have no centrifugal, maybe 18 degrees maybe 34 degrees. And there are countless vacuum advance cans with different specs not just for how much they advance but at what vacuum they work at.Set it up like I told you yesterday, get the numbers I asked for yesterday, then we can start to get into intricacies and fine tuning.There is no perfect setup just a bunch of compromises for your intended use, weight, compression,camshaft, etc.and no matter what anyone tells you vacuum advance is very important on a street car.