Timing with a .... timing light?

[Wes Harden]

Turning the distributor is not how you adjust for total advance. Put it back to 12°, if it still pings put it at 10°. Preignition is destroying your pistons.
To adjust for total advance you need to change the vacuum can, some are adjustable. And/or change the springs and/or weights in the mechanic advance.

Did you prove the timing marks are correct ?

 

[78 GMC Jimmy]

Turning the distributor is not how you adjust for total advance. Put it back to 12°, if it still pings put it at 10°. Preignition is destroying your pistons.
To adjust for total advance you need to change the vacuum can, some are adjustable. And/or change the springs and/or weights in the mechanic advance.

Did you prove the timing marks are correct ?

Ya, timing marks are good. I checked. Cam type is unknown.

 

[78 GMC Jimmy]

OK, I will go back to 12 and see how it goes.

 

[Blue85]

You have to treat initial, vacuum and mechanical advance separately. Tuning it is easiest with a tachometer and vacuum gauge, but you can get pretty far with guessing on those numbers.

Initial timing is for starting the engine and timing for engine lugging (low RPM, high load). This is set with the distributor hold-down clamp. When you mess with this, you change the timing everywhere.

Mechanical advance is reacting to the speed of the engine. As speed increases, there is less time on the power stroke, so the fire gets lit earlier. With the vacuum advance disconnected, increase the engine speed until the timing stops increasing (probably something like 3000RPM). Record this number to know the total timing it's bringing in. Maybe record it at a few engine speeds to refer to as you chase your pinging or bogging issues. There are weights with different shapes to adjust the curve of the timing increase and different springs adjust the RPM range of that curve.

Vacuum advance is reacting to the load on the engine. It's mostly to improve economy. At idle, connect the vacuum advance can to manifold vacuum and record how many degrees the timing changes. This is the maximum advance it provides (since vacuum is high at idle). If the timing suddenly jumps as you move the throttle off-idle, you are connected to a ported vacuum source. Some of the cans can be adjusted for more or less response by sticking an allen wrench in the vacuum line and adjusting (clockwise to increase timing). There are also limit plates to set the total vacuum advance limit. Some of them are not adjustable.

Now that you know the characteristics of the timing under different conditions, you need to identify the RPM and vacuum where the pinging occurs and then you can make the appropriate adjustments.

 

[78 GMC Jimmy]

You have to treat initial, vacuum and mechanical advance separately. Tuning it is easiest with a tachometer and vacuum gauge, but you can get pretty far with guessing on those numbers.

Initial timing is for starting the engine and timing for engine lugging (low RPM, high load). This is set with the distributor hold-down clamp. When you mess with this, you change the timing everywhere.

Mechanical advance is reacting to the speed of the engine. As speed increases, there is less time on the power stroke, so the fire gets lit earlier. With the vacuum advance disconnected, increase the engine speed until the timing stops increasing (probably something like 3000RPM). Record this number to know the total timing it's bringing in. Maybe record it at a few engine speeds to refer to as you chase your pinging or bogging issues. There are weights with different shapes to adjust the curve of the timing increase and different springs adjust the RPM range of that curve.

Vacuum advance is reacting to the load on the engine. It's mostly to improve economy. At idle, connect the vacuum advance can to manifold vacuum and record how many degrees the timing changes. This is the maximum advance it provides (since vacuum is high at idle). If the timing suddenly jumps as you move the throttle off-idle, you are connected to a ported vacuum source. Some of the cans can be adjusted for more or less response by sticking an allen wrench in the vacuum line and adjusting (clockwise to increase timing). There are also limit plates to set the total vacuum advance limit. Some of them are not adjustable.

Now that you know the characteristics of the timing under different conditions, you need to identify the RPM and vacuum where the pinging occurs and then you can make the appropriate adjustments.

Cant I just buy this, set the initial, and call it good?

Amazon.com

Is that not going to work?

Also, and old school mechanic friend of mine said that he just advances them until they engine kicks back when trying to start it, and then backs it off a little and calls it good. Thoughts on that?

 

[Wes Harden]

the amazon chinesium distributor will fit and run, whether it has the correcting curve is a guess at best. It also will probably quit in 6 months.

You can time an engine that way in pinch, but I wouldn't leave it that way for long. I thought you were after perfect ? The old school method is far from perfect.

 

[Blue85]

I wouldn't put a lot of faith in a $50 distributor. Keep some walking shoes in the truck.

So why will a new one time any better than an old one? Sure, the advance mechanism could be sticking, but you would see that if it's not consistently coming back to the same mark. You don't know anything more about the new curve than you do the old one.

The RIGHT way to curve a distributor is using a dyno and wideband oxygen sensor. The WRONG way is to advance till it pings (or won't start) and then turn it back. I'm sure the latter is done more often.

 

[78 GMC Jimmy]

the amazon chinesium distributor will fit and run, whether it has the correcting curve is a guess at best. It also will probably quit in 6 months.

You can time an engine that way in pinch, but I wouldn't leave it that way for long. I thought you were after perfect ? The old school method is far from perfect.

Ya, the Chinese distributor would probably have "sum ting wong" with it before long.
I am trying to get the perfect timing, but I cant find a way to achieve that. Everyone says different ways to do it, none of the numbers in the book match what I read online, and I have no idea what camshaft I have. I try it one way and it performs well, but the numbers are WAY off. I try it by the book and I lose power and it pings. I have tried verifying timing marks and they are correct. I have tried higher octane fuel, and I have gapped the plugs and fixed the carb. I cant fix it! I wish it was new enough that I could run the OBD2 codes and then just replace the crank sensor or O2 sensor.

 

[78 GMC Jimmy]

How about this: Turn the distributor until I get 36 total advance at 3K RPM and just leave it like that. As long as it doesn't ping and has good power, then I should be OK if my goal is max power and I don't care about fuel economy. Is that correct?

 

[Blue85]

I assume this is a continuation of this thread: https://ck5.com/forums/threads/pre-ignition-starting-problem-ripping-hair-out.349339/. So you already bought the new distributor 2 months ago? You never explained how you verified the timing marks, or how you verified neither the vacuum or mechanical advances were sticking.

 

[Blue85]

OK, so I blocked off the vacuum advance and changed my timing from 16 to 12.

Then I connected the vacuum and checked the total advance with my timing light set to 30.

Timing marks said something around 5 after TDC @3K PRM.

So I turned the distributor to get the marks to zero (@ 3K RPM).

Rechecked the initial timing, and its back at about 16.

Seems fine I guess, but when the engine lugs it pings. 2K RPM 4th gear, if I press the throttle it pings between about 25 and 40% throttle, but doesn't ping outside of that throttle range.

With the light set to 30 and getting a reading of 5 ATDC implies it's firing at 25 BTDC. But you went in a circle there - ending where you started. Your motivation is chasing 30, but the correct way to get there is changing the mechanical advance. The next question is whether or not timing is still increasing a 3k RPM. If so, lighter springs would increase the timing. If not, you may need different shaped weights.

However, if it's pinging you shouldn't be adding timing. It sounds like your issue is below 3k RPM anyway.

 

[78 GMC Jimmy]

I assume this is a continuation of this thread: https://ck5.com/forums/threads/pre-ignition-starting-problem-ripping-hair-out.349339/. So you already bought the new distributor 2 months ago? You never explained how you verified the timing marks, or how you verified neither the vacuum or mechanical advances were sticking.

I had returned the distributor. Verified timing by cranking it over with finger over the #1 cylinder until it popped my finger away, and then looking at the timing mark and distributor rotor position. Timing was at 0 and rotor was pointing toward #1. Vacuum advance works. I can see the timing marks change when plugging it in. Mechaical advance has been cleaned and and lubed, and the timing marks change when I rev it up without the vacuum hooked up.

 

[Blue85]

Verified timing by cranking it over with finger over the #1 cylinder until it popped my finger away, and then looking at the timing mark and distributor rotor position. Timing was at 0 and rotor was pointing toward #1.

Nope, that's not conclusive. TDC means position of the piston, not the rotor. After your finger pops off, then you turn to a tool down the sparkplug hole to get it exact (or pull the oil pan to look up). Remember that the compression stroke is half of a crank revolution and there's no way you can hold back the full compression to the top. The finger trick is a bit of a time saver, but also verifies you're finding TDC compression, not TDC exhaust. The difference is easy to tell with the valve cover off, but that extra work isn't worth it if it isn't already off.

 

[Wes Harden]

It has been explained here several times, exactly how to achieve the correct total timing. Many of us are/were GM techs, when these trucks were new, and many of us learned timing with point ignition distributors. There are other members here that race, and pay special attention to timing advance curves, be cause that where you win or lose, all things being equal.
you do what you want, it is your truck. We are only trying to help, but it seems you don't like our answer, maybe we are not explaining well idk.

 

[78 GMC Jimmy]

It has been explained here several times, exactly how to achieve the correct total timing. Many of us are/were GM techs, when these trucks were new, and many of us learned timing with point ignition distributors. There are other members here that race, and pay special attention to timing advance curves, be cause that where you win or lose, all things being equal.
you do what you want, it is your truck. We are only trying to help, but it seems you don't like our answer, maybe we are not explaining well idk.

I had pulled the valve cover as well, but I forgot top list that. They are the chrome valve covers with the bolts that have a T at the top, so they can be easily removed by hand in 30 seconds.

 

[Fastereddie]

What they’re trying to say is to verify your timing marks, otherwise you’re shooting in the dark. After you popped your finger off the spark plug hole with compression, you need to stick a small screwdriver or something in the hole and turn the crank
By hand as you feel the piston come up to the tip top of its travel. Right when it stops and changes direction, your timing mark should be zero. If it’s not, your balancer has slipped or it could be an incorrect timing tab for that balancer.

 

[mendoman]

Set the timing with the vacuum disconnected- and make sure to temporarily plug the vacuum port.

Adjust the distributor until you have a max of 34 degrees. Rev up to around 3500 to make sure you've found the max. Again, this is done while the vacuum is disconnected.

Turn the engine off. Crank it over to see how it starts. If it starts fine, plug the vacuum advance back in. DO NOT recheck the timing after this. Take it for a drive.

Enjoy.

 

[Chickntrk]

My advice is to stop worrying about “total timing” at 3000 rpm. Find TDC, verify timing marks are correct and set base timing. Hook vacuum back up and go for a drive, if it pings back it down, if it doesn’t, drive it.

 

[78 GMC Jimmy]

I set the max timing to 36 degrees. Then when tried to start it, it wouldn't turn over. I turned the distributor little by little clockwise until it would start properly. Then I drove it and it would ping a little under load. So I turning it clockwise a little more. I don't think it was pinging after that, but I couldn't be sure so I went just a little further. Diffidently no pinging and its got enough power and gets up and goes real good. Might be able to fine tune it to get a little more power. During the max acceleration test the rear driveshaft front U-joint blew up. Started with major vibration followed by a loud BANG as the driveshaft put another dent in the underside of the body of the truck. Drove home in 4 wheel drive with the rear driveshaft resting/spinning on the parking brake cable. Took forever to get home going 5 MPH. Fortunately damage was mostly to the U-joint itself and I get it back on the road very quickly.

 

[Blue85]

the rear driveshaft front U-joint blew up.

That sucks. Hopefully you can get it together without too much headache.

Please stop trying to tune "max timing" by turning the distributor, re-read the advice in this thread and tackle things in an orderly fashion. Otherwise, we're all going to check out and spend time helping people that can be helped.