Distributor Advance Expertise Needed

[Road Grabber]

I recently had a vacuum diaphragm rebuilt for my dual point distributor installed in a 1970 383. This Prestolite, I believe, has a "10" stamped on the diaphragm arm. It has a spring with washers inside the canister for adjustment - just for some background information.
,
My situation is this. I installed a timing tape. With my timing set at just 5 degrees before, I revved the engine and discovered my total advance was 50 degrees. I can't set my timing using total advance because it puts my timing much to retarded to run right.

I disconnected the advance hose hooked (to port) and watched the timing again. I saw that the mechanical advance total moved 28 degrees without using the vacuum advance. When I hook up the vacuum advance it jumps way up to 48-50 degrees total.

I have always thought the 383 BB are supposed to be set 36 total advance.

The distributor needs some kind of adjustment.

Do I need to make adjustment on the mechanical advancement? Vacuum advance? Is there something wrong with this distributor?

Years ago I knew where I could have my distributor setup for 14 initial advance and 36 total which worked great in my 440 engine but now I don't know anyone who can work with setting up distributors anymore.

Any help shedding light on this would be greatly appreciated.

Thank you

 

[R413]

@Roadrunner40 does this wok

 

[Road Grabber]

@Roadrunner40 does this wok

thank you

 

[topside]

36 without vacuum is about right; with vacuum, cruise advance can be as high as 55.
Every stock or mostly stock BB I've owned really liked 10-12 initial, and the ones with big cams like 18-20; but in either case you have to limit the mech advance.
That's almost always part of re-curving a distributor.

 

[Road Grabber]

36 without vacuum is about right; with vacuum, cruise advance can be as high as 55.
Every stock or mostly stock BB I've owned really liked 10-12 initial, and the ones with big cams like 18-20; but in either case you have to limit the mech advance.
That's almost always part of re-curving a distributor.

I had no idea that cruise advance at 50 can be "normal"?

I had my 6 pack distributor set to 14 initial and 36 total. I got that intel from an old Direct Connect Engine Manual years ago and it worked out great. I did bring it to a guy who set up distributors though to do it for me.

So how is mechanical advance restricted? Reshaping/filling in the distributor counter weight slots? Not that I would attempt to do that without a machine....

I tried putting more washers in the vacuum advance but now I realize that doesn't make sense.

Thanks for the reply

 

[Geoff 2]

Yes, quite normal. Low compression engines that have a long[er] duration cam fitted might need 60* for cruise.

 

[turbine68rt]

Been using this for years with good results. 16° initial 18° mechanical.
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races

 

[HALIFAXHOPS]

Which port did you hook the vac advance to it should not have any vacuum at idle to it.

 

[Dragon Slayer]

Put a vacuum gauge on the ported fitting on your carb. As Ray said vacuum should be zero (off) unless you have opened throttle blades too much to get an idle. Unloaded in neutral raising rpm should bring vacuum in, but as you go high rpm for measuring total, engine vacuum goes back down to a much lower value and it should not be adding any vacuum advance.

So not sure how you're getting a total in the 50deg range with vacuum hooked up, but reving engine in neutral. You should see that on the road under load but cruising.

 

[Road Grabber]

Been using this for years with good results. 16° initial 18° mechanical.
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races

That was some great reading. I think I will look into this. Thank you

 

[AR67GTX]

The FBO advance plate is a great device.

 

[68 Sport Satellite]

This has been discussed in many other threads. 50-55 total with vacuum advance hooked up is not going to hurt the motor because it only sees this high timing at zero or light engine load. Once you put it under load and the RPM's climb, that added vacuum advance goes away due to the change in engine vacuum and you're left with your initial plus mechanical mid-30's total.

The problem for some (including my motor) is fine tuning how much vacuum advance is added by using a wire shim inside the vac advance can and separately fine tuning how early the added vac advance comes into play by means of the adjustable vac can spring by using an allen wrench through that vac advance can port. Without doing this, often times the motor will ping or detonate under load. It used to be for most of us the stock vac advance cans didn't add too much advance, but then with the onset of higher ethanol and lower octane fuels, (and for me, my wrongly chosen pistons which resulted in 10.2:1 compression instead of 9.5:1), this made it so some restriction of max allowable advance by the vac can was needed by means of a shim (wire stop).

 

[Road Grabber]

Put a vacuum gauge on the ported fitting on your carb. As Ray said vacuum should be zero (off) unless you have opened throttle blades too much to get an idle. Unloaded in neutral raising rpm should bring vacuum in, but as you go high rpm for measuring total, engine vacuum goes back down to a much lower value and it should not be adding any vacuum advance.

So not sure how you're getting a total in the 50deg range with vacuum hooked up, but reving engine in neutral. You should see that on the road under load but cruising.

I will do that thanks. When I disconnect the vacuum revving at neutral the total goes back down on the timing tape.
I like the idea of the plate because I could always remove it with no permanent change to the distributor.
I’d like to get the most from the timing and if a total of 36 gives me that I will have set the distributor up because right now total is 50. When I read the material link above this is exactly the situation they describe happens without the use of their plate or by filling in the slots to a custom size/location on the distributor post.

I do not get vacuum advance when attached at idle.

 

[Road Grabber]

The FBO advance plate is a great device.

Does sound like it. Not a big investment either.

 

[68 Sport Satellite]

The FBO advance plate is a great device.

is FBO Don still around? I haven't seen him post on here since 2018 and messaged him recently to purchase from his site and no response.

 

[Road Grabber]

This has been discussed in many other threads. 50-55 total with vacuum advance hooked up is not going to hurt the motor because it only sees this high timing at zero or light engine load. Once you put it under load and the RPM's climb, that added vacuum advance goes away due to the change in engine vacuum and you're left with your initial plus mechanical mid-30's total.

The problem for some (including my motor) is fine tuning how much vacuum advance is added by using a wire shim inside the vac advance can and separately fine tuning how early the added vac advance comes into play by means of the adjustable vac can spring by using an allen wrench through that vac advance can port. Without doing this, often times the motor will ping or detonate under load. It used to be for most of us the stock vac advance cans didn't add too much advance, but then with the onset of higher ethanol and lower octane fuels, (and for me, my wrongly chosen pistons which resulted in 10.2:1 compression instead of 9.5:1), this made it so some restriction of max allowable advance by the vac can was needed by means of a shim (wire stop).

Sounds interesting. Trying to visualize this wire stop location….

My 440 had a pinging problem when fuel changed. The motor is being freshened up now with aluminum heads. It’s a six pack motor with 10.5 to 1.

This is good to know in case I still have the problem with aluminum heads.

 

[68 Sport Satellite]

I will do that thanks. When I disconnect the vacuum revving at neutral the total goes back down on the timing tape.
I like the idea of the plate because I could always remove it with no permanent change to the distributor.
I’d like to get the most from the timing and if a total of 36 gives me that I will have set the distributor up because right now total is 50. When I read the material link above this is exactly the situation they describe happens without the use of their plate or by filling in the slots to a custom size/location on the distributor post.

I do not get vacuum advance when attached at idle.

your total of 36 should be with vac advance disconnected and plugged at carb.
Then with vac advance hooked up to manifold vacuum, your initial should increase to 30-50. After that's dialed in, you can leave it hooked up to manifold vacuum or reconnect to ported vacuum depending on your motor and what it likes best under load.

 

[Road Grabber]

your total of 36 should be with vac advance disconnected and plugged at carb.
Then with vac advance hooked up to manifold vacuum, your initial should increase to 30-50. After that's dialed in, you can leave it hooked up to manifold vacuum or reconnect to ported vacuum depending on your motor and what it likes best under load.

I didn’t realize load had an effect. Thanks.

 

[68 Sport Satellite]

Sounds interesting. Trying to visualize this wire stop location….

My 440 had a pinging problem when fuel changed. The motor is being freshened up now with aluminum heads. It’s a six pack motor with 10.5 to 1.

This is good to know in case I still have the problem with aluminum heads.

see this post with photos from an earlier thread:
From the A-bodies site:
How To Limit and Adjust Chrysler Vacuum Advance Cans

Here's one of the other many advance threads on this:
Setting Timing and adjusting vacuum advance canister?

By the way, not trying to bust your balls, but it's sometimes easier to find an older thread and read through it and revive the thread with a new post rather than starting a brand new thread on a topic that's been discussed to death in many other threads...I used to be guilty of this, but now read more existing threads first. Actually, this FBBO site has a new feature I love that, when I try to start a new thread, when I type in the title, it self-populates existing similar threads to review before posting a new thread (to avoid too much redundant info).

 

[68 Sport Satellite]

I didn’t realize load had an effect. Thanks.

The load doesn't effect setting things at idle, but it does effect if pinging or detonation will occur, which is why you won't know for sure if it's tuned the best it can be until you go out and drive it and hammer on it.

When hooked up to manifold vacuum at idle you will see the timing and rpm's increase at idle. It it's hooked up to ported vacuum there will be no change at idle. At off-idle they're almost identical and after high enough RPM (2500-3000 depending on your distributor springs) the engine vacuum makes it so neither manifold or ported vac advance has any effect.