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Vacuum Advance Dilemma SB 340

Moparfiend

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Currently running a new MSD6AL with a billet MSD Dizzy on my 340 with a 4150 mechanical advance. Car is at least 10:1 steel heads and cammed aggressively and I don’t have the cam specs. Its a TF904 auto.

Needless to say I regret not running a vacuum advance set up as that would have been the right choice for a more street-able set up.

That being said I don’t think its possible to convert the dizzy and add a vacuum to the carb. I really don’t have the funds to buy a new carb and dizzy and just wondering what others have done to make a mechanical secondary in a set up like this more street-able?

Thanks for your comments.
MF
 
What kind of carb? They all have a vacuum port.

You would just need a vacuum advance dizzy.
 
Hi Frank. Its got a Holley 4150 double pumper 600cfm mechanical secondary manual choke
 
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The basic thing vac adv does is potentially allow for better fuel economy at cruise. More advance to fire a much leaner. mixture.

Some guys use it to cheat on initial timing settings as well, that's a bit different.
 
Hi Crackedback (hurts just saying that).

Right a lean mix takes longer to burn at idle and cruise and requires more advance. This is why I am asking.

I still have not verified my AF mix but I have the ability to do this when I get better.

I probably need to decam the engine and other things but its borderline wrt idle. So I am not far off where I want to be. I am looking to see the best approach to dialing in a mechanic advance on an automatic set up better.

I am pulling 12in vacuum. Would like to see what I can do with existing MSD and Holley hardware I have as I know the tune totally affects the the drivability. I have been able to make good progress as I put a 14 degree bushing limiter in the dizzy and slowed the advance down with the heavy springs to stop the detonation running 17/31 degrees of timing all in around 3500RPM. But without vacuum advance at idle she is rough and needs more timing? She idles at 1100 and not much lower. So there is room for improvement. Was thinking of going with a 10 degree bushing limiter and advancing base timing to 20 which I could do now and have 34 all in but thats a bit high with this set up wrt detonation.

Just trying to educate and get some ideas on improving drivability with tuning and not ripping and tearing up what I have if possible.
 
All i found i could do with my mech secondary's was to install and AFR meter and see what mixture reading you get at idle, cruise and WOT.
After re-jet accordingly to get the mixture in the ball park.
My carb was already tinkered with and air bleeds were probably drilled/chocked by a PO and re-jetting only did a small improvement so i moved on and got a vacuum secondary carb.
Mech secondary's are not the best combo with an auto tranny, getting a vacuum advance setup would help increase milage on cruise.
 
All i found i could do with my mech secondary's was to install and AFR meter and see what mixture reading you get at idle, cruise and WOT.
After re-jet accordingly to get the mixture in the ball park.
My carb was already tinkered with and air bleeds were probably drilled/chocked by a PO and re-jetting only did a small improvement so i moved on and got a vacuum secondary carb.
Mech secondary's are not the best combo with an auto tranny, getting a vacuum advance setup would help increase milage on cruise.
I think my main issue are actuially drive line related more than a vacuum vs mechanical secondary set up. I am going to replace the 2:71 peg leg with a 3:55 Suregrip that I have already acquired. Also will put in a ~3500 stall after that.
I started this thread because others have said you can use a mechanical secondary set up with an automatic but the approach is different. This is what I want to know. I am not in a position to ditch my MSD and Holley set up. I feel I am getting rather close. I have a lot to verify still and report back like compression data, vacuum readings, and AFR measurements.
 
It is definitely workable, my car was more setup for a drag race and therefore not really street friendly.
Mine has a 4.10 rear axle, your 2.71 is definitely a long haul, specially for a 340 i would say.
What are the specifics you want to change on it's current behaviour?
 
Aggressive cam? Do you know the specs and where it's set?
 
It is definitely workable, my car was more setup for a drag race and therefore not really street friendly.
Mine has a 4.10 rear axle, your 2.71 is definitely a long haul, specially for a 340 i would say.
What are the specifics you want to change on it's current behaviour?
Well thats a good question Wietse, a better idle transition from park to drive would be great. I would say improved vacuum in gear but I need to verify where I am at with that as I can’t find the measurements I had recorded previously. She needs more idle RPM in gear as not to teater and stall and I need to shut it down in gear to prevent dieseling.
Overall performance (the change gears and stall converter is going to help significantly then a tube).
 
I don't run a vacuum advance on my 440/493 stroker eng. I got a good deal on this Mallory all mech advance race type dist. But my car is a street/strip car and I run a very fast mech advance. I run around 22 initial timing and 36 total all in by 1800 rpm. I run on 92 pump gas and have nice .045 quence built in my eng. It has no ping at all. My cam is a 264 & 270 @ .050 with .585 & .592 lift and a 110 LSA. Now my car runs great with this setup and I use an 850 DP Holley carb. Since my car is a street/strip car I went with this dist with no vacuum advance because the price was great and I knew I could make it run good on all mech advance. And it runs awesome with great throttle response and awesome drivability. As for fuel mileage I could not care less since my car is basically my hotod and if I am worried about fuel mileage on it I am in the wrong hobby. You should be able to set up your dist and make your car run fine without the vacuum advance but I agree it will run better with a vacuum advance on it. If you have a double pumper carb it should have a vacuum advance port. But you will of course have to get a dist with vacuum advance on it if you want to run a vacuum advance. Good luck , Ron

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