vacuum advance issues

[Erratic67]

I changed my dist. cap, rotor, control module, and adjustable vacuum advance module on my Chevy 350. My advance vacuum module was bad so that's the reason for the change, other parts were for a quick tune up. I had no problems with a rough idle or backfire before the changes, all plug wires and firing order have been checked and verified to be correct. I followed the instructions for the new vacuum module when installed by turning it counter clockwise until bottoming out then installed it, I attempted to turn it clockwise as instructions indicate if problems exist but it does not seem like the allen screw is turning. I've disconnected and plugged my vacuum module but no change in performance either. My tach. is very erratic and sometimes not indicating RPM's either. Could all my problems be due to now having vacuum before and now having it?
Do I now have to reset my initial timing?
How do I set advanced timing in correlation with other timing?

Any help on this matter would be greatly appreciated as I am clueless at this point.

[storm9c1]

If it runs badly (backfiring) with vacuum advance unhooked, then yes, you need to reset your initial timing. Sounds like it need to be more advanced.

However, I am concerned about the erratic tach. Assuming it worked fine before, that tells me something is going on with the coil, module, or wiring.

[crazy longhorn]

Originally Posted by Erratic67

I changed my dist. cap, rotor, control module, and adjustable vacuum advance module on my Chevy 350. My advance vacuum module was bad so that's the reason for the change, other parts were for a quick tune up. I had no problems with a rough idle or backfire before the changes, all plug wires and firing order have been checked and verified to be correct. I followed the instructions for the new vacuum module when installed by turning it counter clockwise until bottoming out then installed it, I attempted to turn it clockwise as instructions indicate if problems exist but it does not seem like the allen screw is turning. I've disconnected and plugged my vacuum module but no change in performance either. My tach. is very erratic and sometimes not indicating RPM's either. Could all my problems be due to now having vacuum before and now having it?
Do I now have to reset my initial timing?
How do I set advanced timing in correlation with other timing?

Any help on this matter would be greatly appreciated as I am clueless at this point.

Before changes,what issues did you have?

[Erratic67]

It would not start but would turn over just fine, that's the reason I changed my ignition coil and module. It ran fine with a little hesitation prior to not starting and the tach worked fine also.

[storm9c1]

I think you may have multiple issues. So first, let's get your base timing. If you don't have a timing light, you need to get one or borrow one. A digital dialback light is the best tool for this job unless you have degrees marked on your balancer.

Then disconnect and plug your vacuum advance port at the carb.

Then start it up. What I do on an unknown engine that won't idle well is slowly turn the distributor until I hear the engine settle down. If you need to blip the throttle, do so to keep it running, but then let it idle down and turn the distributor a little more until it settles down and runs smoothly.

Once it idles on its own, then check the timing with a light. Set it to 8 degrees at idle.

Next step is to make sure mechanical advance is working. Slowly ramp up the RPMs to 3000 and dial back the light or watch the timing marks if degrees are on the balancer and make sure it slowly advances. It should stop advancing by 3500RPM or so. Make note of when it stops advancing and what degrees it stopped at. This is your total mechanical advance. You want to keep this between 28-34 degrees. Most SBC engines like 30-32.

Next step is to take it for a test drive. If it backfires, bump it to 9 degrees initial, and repeat until the backfires are gone. Hopefully it's no more than 12 degrees initial at this point. And hopefully you won't have more than 34 degrees at 3500 RPM. Never drive it hard if you hear it pinging. This means you've gone too far.

Next step is to bring the vacuum advance into the picture. But first, let's make sure you can get this far. If you make it here, then post back and we will explain the vacuum advance further. We need to make sure all of the above steps are sane first. And we need to make sure you can even keep it running. If not, then something else is wrong, possibly electrical. You can always swap caps (if HEI) to eliminate the coil and center button. Then swap ign modules from your old one. Shouldn't have to do that with a new unit, but if you bought a cheapo distributor, it could be bad out of the box.

If you want more details, a good primer is in the FAQ section, but I don't want to burden you with worrying about springs and weights yet. Not until we know what you currently have.

http://67-72chevytrucks.com/vboard/s...d.php?t=290498

[Erratic67]

Thanks for the advice .... No issues with start up and It idles fine, when I begin giving it gas is when it begins to run rough and if I punch it that's when it backfires. I have not checked my timing with the light yet, my dist. Cap manufacture is MSD. If I disconnect my vacuum port from my carb. It performs the same, no difference. I have not taken it on the road to see, only troubleshooting done is in my shop.

[Erratic67]

I checked my timing at idle with my vacuum port on the carb. & the vacuum advance module disconnected and plugged. With the timing advance knob at 0, it is running between 0-1 on the before side. I could not increase to an accurate higher RPM for further timing checks cause my Tach. is not working properly.

[storm9c1]

0-1 ?? Really?

That needs to be advanced to at least 4 degrees to drive at all without backfiring. And at least 8 for best results.

If you didn't know the terminology, BTDC aka "before top dead center" is the same as "advanced". So turn the distributor until you see 8 BTDC on the timing tab with the light (keep the light dial at 0 for now). Your idle will go up, so back off the idle stop screw on the carb to lower it to normal. Then take it for a test drive again with the vacuum advance disconnected. I bet your results will be much better. Once we get this worked out, we can bring the vacuum advance back into the mix. And we can map out your timing curve using the dial on the timing light. But not yet.

Most (if not all) engines run poorly with timing set to 0. Exception: computer controlled engines get set to 0 because the computer advances the timing. I didn't ask but I assume this is NOT a computer controlled engine, right? It's a conventional carb and HEI distributor?

[Erratic67]

Thanks again for this valuable info and words of wisdom (BTDC) which was unknown to me. I am new to this as it is my first restoration and mechanical experience, I have usually swallowed my pride and just taken it to a professional but have decided to learn myself and receive knowledge and life experiences from others to build my own.
I'm going to advance my timing as you indicate and see what results are achieved, and you are correct, (no computer control) just the conventional type. I will report my findings and we can progress from there. Very appreciative for your help thus far, thanks.

[storm9c1]

Learning is excellent. And when you gain enough knowledge, share it with others to complete the cycle. That's the only way old iron is going to stay on the road for generations to come. Our American heritage is important. And so is our passion for older vehicles.

[Erratic67]

I advanced my timing with the dial on 0, until I achieved 8, afterwards adjusted my idle. As i increased my RPM,s it began to shake and miss, when I stomped on the gas it began to backfire. I then increased my timing in increments of one until I reached 11 where it currently is without any change, I took it out for a spin and at approx 20 mph it began to miss/sputter terribly not letting my exceed that speed due to performance.
Question I have is .... how do I know that it was at TDC when I increased the timing to 8 originally, should I have found that position first prior to increasing? I originally checked timing from where it was and increased from that point. Looking forward to your feedback.

[geezer#99]

Just a thought!!
You are advancing the distributor counter clockwise, are you?

[storm9c1]

Hmmmm. That thing should be running right with 8-11 degrees of advance. Something is wrong.

You do have your timing light attached to the spark plug wire for cylinder #1 right? See pic below.

So if the timing light dial is on zero and your timing was on zero, that should be TDC. This assumes your timing marks are right on your balancer. A balancer can shift.

So.....what you may need to do next is verify TDC. Once you verify that, all timing advance is based on TDC.
Instructions here: http://67-72chevytrucks.com/vboard/s...d.php?t=290498
You don't HAVE to buy a piston stop. You could just used a wooden dowel or similar in the spark plug hole, turn the crank by hand, and wait for the dowel to stop moving. That's TDC. Some people also have success just feeling the compression in the cylider by covering the spark plug hole with their finger and rotate the crank, but it's better to have a helper for this to rotate the crank for you. TDC is when you feel compression stop during the up stroke. Make sure the TDC timing mark on the balancer is at 0 in the timing tab (+/- 2 degrees).

Another troubleshooting step is really important -- check your spark plug firing order! Do it now!



I'm a little baffled at what's going on with your setup. Almost sounds to me like you got a defective distributor (since your tach is bouncing around). You may need to swap parts with your old distributor. But let's get you in a sane state first.

[Erratic67]

Round 4 ...... I verified TDC and my timing is dead on 0 on my harmonic balance, removed my dist. Cap and verified rotor is facing #1 cylinder. Yes, when I checked timing previously I had the light attached to #1 too. I left everything as is and did not try to start at this point until I hear back from you. Next step please and as always thanks for taking the time to help.

[Erratic67]

I attempted to send a photo of my firing order which seems different from yours, hope I did it correctly.

[Erratic67]

That's how it originally was before making any changes so I suspect it to b correct.

[storm9c1]

Looks like the same firing order to me. Just might be clocked slightly different. As long as the rotor (under the cap) points at the cylinder #1 spark tower in the cap (where ever that tower is located) when cylinder 1 is at TDC on the compression stroke, it's right. It doesn't matter how precisely the cap is clocked in relation to the picture. What matters is to make sure the rotor and #1 spark plug tower line up when #1 is at TDC.

Now that we verified TDC, and your plug wires are in the right order, and you have it advanced to 11 degrees (with the vacuum advance canister plugged) and it still backfires, that tells me that something else is wrong. I'd swap your old distributor back in since you said it runs fine (sans the vacuum advance). Get the timing set the same way. Keep the vacuum advance disconnected and see if it still runs badly. If it runs fine, then we can almost guarantee you got a new distributor that is bad. I'm running out of ideas here.

[Erratic67]

Hallelujah .... After setting my total advance timing at 35* everything fell into place. Initial timing was set at 11, total advanced timing 2500 RPM's at 35*, all backfiring! shaking and sputtering is GONE & running smooth. When I completed the total advance that brought my initial timing up also. My tach is still moving all over but it is a (el-cheapo), which may be the problem.
Now moving on to the vacuum advance which I am not excited about doing since everything is running wonderful at this point. Both my carb. & vacuum advance remain disconnected and plugged. How do I proceed with setting this correctly without upsetting what I have already accomplished?
Surely could not have made it this far without your support and expertise, very thankful !!

[storm9c1]

Not sure how you got there. You said earlier that initial timing was at 11* so at that point your total timing should have been somewhere around 35* already at 2500+ RPM. Were you looking at total timing before instead of initial? Or is the initial now a higher value since you set total? What is it? Anyways, OK, I'll take it! Glad it's running right. The numbers make sense for a typical SBC. So I would leave it there.

So for the vacuum advance, it's a little trickier but we will get you there.

Warm up the engine. Choke has to be off. Plug the vacuum advance back in to a full manifold vacuum source. This is usually the lowest port on the carb. The upper port is "ported" vacuum and we don't want that one. The RPMs should go up, so back off your idle stop screw on the carb to return idle to normal. Then check timing at idle speed. Subtract this new timing value from 11* in your head (or whatever your initial timing value discovered earlier). This is how much your canister is adding. Hopefully it will be 8-10*. If it is not, then we need to play with the allen screw you mentioned earlier to get it to 8-10* additional advance at idle. No more. No less. That canister cannot not pull in more than 10 degrees otherwise you may start pinging at cruising speeds, especially with initial timing at 11*.

[Erratic67]

Did not have much time today in regards to honey do's, but I did sneak away long enough to check my timing again. I hear a very so slight knock at idle since adjusting timing yesterday, beings that I increased my total timing at 35*, my initial timing went from 11 -> 19, could this be the reason for the knock and should I lower it? Don't want to move forward with advanced timing until I have this corrected so I don't have to backtrack, what is your opinion?

[storm9c1]

Initial at 19 is too much mechanical (with vacuum advance unhooked) IMO. I wouldn't go more than 12. Sounds like that distributor has a terrible timing curve if you need to run 19 initial to get 35 all in. What make and model distributor is this?

[Erratic67]

I have no idea what make/model the motor or distributor is, neither did the individual I purchased it from. When I began adjusting my initial timing initially I kept increasing it to stop the backfiring which never did until I set my total timing, maybe I should have set my total when I was at 8 initial. I'm going to go back down on my initial and check total based on that. What indication should I be looking for to ensure initial timing is set correctly?
Unfortunately I'm working and will not get around to doing this until Friday, please bare with me. Once again thanks for taking the time to help.

[fleetsidelarry]

Originally Posted by Erratic67

When I began adjusting my initial timing initially I kept increasing it to stop the backfiring which never did until I set my total timing, maybe I should have set my total when I was at 8 initial. I'm going to go back down on my initial and check total based on that.

I'm sorry, Erratic, but that doesn't make sense. the total you're after is the combination of what's in your distributor (mechanical [centrifugal] advance) + initial (don't use vacuum advance for this test). the 2 values (total and initial) are directly linked together by the value of the mechanical which is the same once it is all in

right now you're saying that you are at 35 degrees total at 19 initial. that means your distributor has 35-19 or 16 degrees in the mechanical IF you determined the total correctly.

if you drop your initial to 8 degrees you should have 8 + 16 or 24 total. but you're saying it doesn't run right 'til 35 total, right? that means you need more mechanical advance in the distributor mechanism

maybe you're not revving the engine enough to wring all the mechanical out.

[crazy longhorn]

I have to agree with Storm and Fleetside....it sounds like the timing curve is off. you might try revving to around 3500, to see if it advances more. I will toss in another thought....I have seen broken wires on the pick up coil, cause intermittent/erratic issues. They seem to run fine, till the vac advance "rocks" the the plate, then the wire loose contact Longhorn

[Erratic67]

Thanks for the additional input .... If you begin from the start of this post you will understand issues I was having in the begining and now current. Trying to decifer fleetside is way over my head at this point. Thanks to storm, I have gained a lot of ground and very much appreciate.
1. Yes timing was set with vacuum advance disconnected and remains that way.
2. Pick up coil was tested and proven to be fine.
3. My tach. is not working correctly so total was set at 35* when the mark on the harmonic balance stopped rising when excellerated, "RPM was unknown"
4. I didn't check total when originally at 8 BTDC, which may have stopped the backfiring if I would have set the total @ 35* then.
5. If i choose to decrease initial back to 8 then reset total back to 35*, what will that hurt & do I need to find TDC all over again?