Ignition timing with/without vac advance....sticking?

swisherred

Senior Member
Joined
Jan 19, 2016
Messages
1,222
Reaction score
891
Location
winston salem, nc
I'm running a stock 67-383 non hp with a Mopar Performance electronic dizzy and a GM ignition module conversion with 50kv coil and .042 spark gap.

I'm tuning everything again after rebuilding my 1806 carb and I've run into an issue with timing. With the vacuum advance hooked up and the initial timing set to 12.5ºBTC my max timing is upwards of 50ºBTC...way too much (checking in park in the driveway). If I disconnect the vac advance my max settles in about 36º. Is my VA bad? Sticking? Can it be adjusted down? It's my understanding that it should not add to the total timing because it will drop out once manifold vacuum drops at high RPM. Someone suggested that it may be sticking and still adding to the timing. Checking it with a vac pump it adds a total of 15º.....36º+15º= 51º...which is what I'm getting with it connected.


With the VA disconnected and the line plugged the car runs awesome...even at part throttle. Good response and power. Throw it back in and its not nearly as good.
 
I think your dizz is working just fine.

It looks to me that you're hooking the VA hose to a direct manifold source, instead of the intended "ported vacuum" nipple. At idle, with the throttle blades closed, that ported nipple will provide zero vacuum and your initial timing will be 12.5. As you're cruising down the road under light throttle/light load, your timing will go to 50+, which is just exactly what you want. When you go WOT, the vac goes to (near) zero and your max will be 36 due to the centrifugal weights in the dizz.

So if you're hooked to a nipple that shows vac at idle, that's the wrong one.

I have 14 initial, the dizz is limited to 18 by this thing FBO Mopar Distributor Limiter Plate which gives me 32...staying on the safe side of detonation. With my dizz's vac hooked to a ported nipple, I get something like 52 when I open the throttle just a bit and hold it at say....2400 rpm.
 
Last edited:
I'm definitely hooked to the ported...no vac at idle.....your saying its normal to hit 50deg? Will it still do that driving it or will the load keep it at 36?
 
When I rev it without the vac it stops at 36...reving with the vac it goes to 51.....all just in the driveway not driving.
 
When I rev it without the vac it stops at 36...reving with the vac it goes to 51.....all just in the driveway not driving.

Ahhhh...that explanation makes more sense. I assume that when you quit the reving it goes back to 12.5? If so, you're perfectly fine. That's exactly what it's supposed to do.

Now stop worrying and go enjoy Thanksgiving. :thumbsup:
 
Ahhhh...that explanation makes more sense. I assume that when you quit the reving it goes back to 12.5? If so, you're perfectly fine. That's exactly what it's supposed to do.

Now stop worrying and go enjoy Thanksgiving. :thumbsup:
Thank you....just didnt want to blow it up with advance
 
Recurving the distributor will slow down the advance which is what you want.
With today's corn fed gas timing is everything. The 50 year old specs no longer apply.
I do not run vacuum advance. Sure my MPG may drop but rather spend more at the pump than repairing damage from detonation.
12.5 to 14 initial with 34 to 38 degrees all in at 3000 rpm is good for a 383 stock or light mods like mild cam and duals. Vacuum should be between 17. ( slight mods)and 20 inches ( totally stock)
Every engine is different so test and tune. Drive between adjustments to see what makes her happy.
The motor should start first click every time without long crank time with little or no throttle.
Hope this helps.
 
It does take a couple more revs to start than I want...two or three pumps and it starts good though... Still have to adjust the new electric choke and some fine tuning. Adjusting the secondary air valve helped eliminate my mid pedal bog...rebuilding it eliminated the part throttle bog...pulls good now....will fiddle with it a bit more.
 
Thank you....just didnt want to blow it up with advance

Nope, you're fine. And if you're not getting detonation (ping) at 36 total, that's even finer. When I go above 34* I get some rattle, but my engine has 11:1 slugs in it so I keep it at 32/33. With octane boost I can go to 36, but why bother? And that's why I love that timing limiter plate. You could choose to go with 18 initial if you wanted for a great idle, and install the plate in the 16 degree position. 16+18=34*, etc. It's a great tuning tool that allows you to get far away from the FSM 2.5* initial setting.
 
It does take a couple more revs to start than I want...two or three pumps and it starts good though... Still have to adjust the new electric choke and some fine tuning. Adjusting the secondary air valve helped eliminate my mid pedal bog...rebuilding it eliminated the part throttle bog...pulls good now....will fiddle with it a bit more.

If my engine has been run in the past ~10 hours, it starts so fast with a tiny tap of gas that you don't even hear the famous Chrysler starter whine. Instantaneous light-off, and that's really gratifying. That's what 14-16 initial gets ya, but you sure don't want it to go to 42 max with the stock-spec centrifugal advance. Boom!
 
Most of the performance distributors have a small Allen screw inside of the vacuum diaphragm (Inside the hose bib.) that can be used to limit/adjust total vacuum advance as well.

Dave
 
Most of the performance distributors have a small Allen screw inside of the vacuum diaphragm (Inside the hose bib.) that can be used to limit/adjust total vacuum advance as well.

Dave
Yes. 3/16th. That Allen adjusts spring tension, and does not limit arm/plunger travel. Given enough vacuum (30 inches?), the diaphragm will still pull the timing (against the spring tension) to the maximum advance that was designed into that particular vac unit.

There are various vac units available with different amounts of vac advance but I've never used them. I fiddle with the "travel slot" in the arm if need be, but that's rarely needed for a mild motor.
 
Recurving the distributor will slow down the advance which is what you want.
With today's corn fed gas timing is everything. The 50 year old specs no longer apply.
I do not run vacuum advance. Sure my MPG may drop but rather spend more at the pump than repairing damage from detonation.
12.5 to 14 initial with 34 to 38 degrees all in at 3000 rpm is good for a 383 stock or light mods like mild cam and duals. Vacuum should be between 17. ( slight mods)and 20 inches ( totally stock)
Every engine is different so test and tune. Drive between adjustments to see what makes her happy.
The motor should start first click every time without long crank time with little or no throttle.
Hope this helps.

I was going to chime in on my Poly that is 10' at idle ported vacuum off the carb then 30' at 3000rpm
 
I think your dizz is working just fine.

It looks to me that you're hooking the VA hose to a direct manifold source, instead of the intended "ported vacuum" nipple. At idle, with the throttle blades closed, that ported nipple will provide zero vacuum and your initial timing will be 12.5. As you're cruising down the road under light throttle/light load, your timing will go to 50+, which is just exactly what you want. When you go WOT, the vac goes to (near) zero and your max will be 36 due to the centrifugal weights in the dizz.

So if you're hooked to a nipple that shows vac at idle, that's the wrong one.

I have 14 initial, the dizz is limited to 18 by this thing FBO Mopar Distributor Limiter Plate which gives me 32...staying on the safe side of detonation. With my dizz's vac hooked to a ported nipple, I get something like 52 when I open the throttle just a bit and hold it at say....2400 rpm.
That limiting plate looks great, Thank You. I have broke down distributors and brazed up the slots to limit advance but this will be way easier.

One other option would be to send it to @halifaxhops for a custom curve.
 
The FBO plate is absolutely genius. Can't believe someone (like me!) didn't think of it 40 years ago when I was brazing and filing the slots, too. It makes tuning SO simple.....even a cave man can do it.
UTEW0684.JPG
 
The FBO plate is absolutely genius. Can't believe someone (like me!) didn't think of it 40 years ago when I was brazing and filing the slots, too. It makes tuning SO simple.....even a cave man can do it.
View attachment 333339

Yes the FBO plate is a tuner's masterpiece. I have it in the Waygun's 318 dizzy.
 
Back
Top