Timing fluctuates with vacuum advance connected

[RobsRR]

I just noticed my timing is more steady without the vacuum advance connected. I am using an older Mopar electronic ignition. It was sent to FBO Don a few years ago and set up to use with full manifold vacuum. It's 383 10.5 compression comp cam nostalgia cam. Holley 3310. Should it be as steady ( the timing) as with no vacuum advance? This is at idle.

 

[65-440]

Yes I should be steady. Why are you using full manifold vacuum?

 

[turbine68rt]

Have you tried running it using ported vac? When you say "set up" to use manifold vacuum, what would be the reason to add advance at idle? If you use a limiter plate you can dial in as much initial as you need while restricting the all-in amount. Just a personal opinion.

 

[RobsRR]

Have you tried running it using ported vac? When you say "set up" to use manifold vacuum, what would be the reason to add advance at idle? If you use a limiter plate you can dial in as much initial as you need while restricting the all-in amount. Just a personal opinion.

I have not tried ported. I was not sure if the advance curve would change going that route? Would a bad leaking vacuum canister cause it to be irratic.

 

[RobsRR]

I have not tried ported. I was not sure if the advance curve would change going that route? Would a bad leaking vacuum canister cause it to be irratic.

I read that the vacuum canister has to be fully engaged at idle or this can happen. How do you check this and or change this.

 

[Bee1971]

Don @ FBO - From his website


Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.

Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.

This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.

 

[turbine68rt]

"So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job" Not sure where he got this from but since when does ported vacuum only start kicking in at 3k? It's designed to advance it under light load/cruise when the mixture is lean and needs the extra timing for complete combustion. Under hard acceleration the vac signal drops off and takes the vac advance out of the equation.

 

[turbine68rt]

https://www.moparaction.com/2020/08/16/advance-to-go/

 

[Don Frelier]

Never blown a crankshaft out the oil pan yet...
And 50 degrees is a good number for my stuff when just driving normally.

 

[RussT Plymouths]

I've always worked under the idea that the vacuum source for the ported vacuum port was ultimately manifold vacuum. That said, (and if incorrect please advise), port vacuum can never be greater than manifold vacuum. Further, if manifold vacuum was at zero, then port vacuum also had to be. Leading me to believe that if the engine was at WOT, and manifold vacuum was at zero, port vacuum would also be zero.

I guess I'm trying to ask; How will this "blow your crankshaft through the oil pan"??? Not sure I'm buying it.

 

[moparedtn]

The factory hooked them up to ported. Done deal to me, at least with close to stock engines.

 

[RobsRR]

You guys are getting off topic. This is not a ported vs manifold debate. There are plenty of threads on that. I am looking to find out about how to test my vacuum can. I guess I can use the hand vacuum pump to see if it holds vacuum. What about it not being engaged fully at idle. Would that cause fluttering and can that be adjusted?

 

[AR67GTX]

I would say yes, it could cause the fluttering. If you have an adjustable vacuum advance can you can back off the adjustment some and allow it to come in at lower vacuum to ensure it’s solid at idle vacuum.

 

[turbine68rt]

You guys are getting off topic. This is not a ported vs manifold debate. There are plenty of threads on that. I am looking to find out about how to test my vacuum can. I guess I can use the hand vacuum pump to see if it holds vacuum. What about it not being engaged fully at idle. Would that cause fluttering and can that be adjusted?

See post #8

 

[Don Frelier]

The PCV valve would make the vacuum unsteady at idle. So does each cylinder essentially.

 

[RobsRR]

I would say yes, it could cause the fluttering. If you have an adjustable vacuum advance can you can back off the adjustment some and allow it to come in at lower vacuum to ensure it’s solid at idle vacuum.

Thank you I will try that.

 

[65-440]

Yes test the vac can using a hand held pump. Remove dist cap and apply vacuum. You'll see the arm pull in, you can also take note at how much vacuum it takes before the arm moves. This is a good part of vacuum advance tuning, and driving with a vacuum gauge so you can where your engine vacuum is a various loads.
I'm confused, why would it flutter if the cannister wasn't fully engaged? I don't see that, maybe if the vacuum source was bouncing around.
Have you ran the car with the advance disconnected and see what happens?

 

[RobsRR]

See post #8

Thank you great article.

 

[RobsRR]

Yes test the vac can using a hand held pump. Remove dist cap and apply vacuum. You'll see the arm pull in, you can also take note at how much vacuum it takes before the arm moves. This is a good part of vacuum advance tuning, and driving with a vacuum gauge so you can where your engine vacuum is a various loads.
I'm confused, why would it flutter if the cannister wasn't fully engaged? I don't see that, maybe if the vacuum source was bouncing around.
Have you ran the car with the advance disconnected and see what happens?

The question came up because I noticed the timing at idle fluctuates more with vacuum adv. (Manifold).
When unplugged it smooth it out.

 

[65-440]

The question came up because I noticed the timing at idle fluctuates more with vacuum adv. (Manifold).
When unplugged it smooth it out.

So what's your initial at with and without the advance hooked up?