vaccum advance question...

[oldmachdriver]

I have a MSD ready to run distributer. Is this type of unit a full time vaccum advance or is it off at idle?...makes the diff when trying to figure out wich vacum port to use on the edlebrock carb....

2nd question....are mortorsport plug wires (blue) ok to use with a MSD?...something about how i shouldnt use solid core wires....not sure if they are or not.....

car still blows black smoke when reving from idle for a second but definately runs smoother than before.....im going to use a dif set of jets...to lean it out just a smidge....


are anyone using anything other than MSD?...and with what luck...i want simplicity and just didnt know if there was anything out there that is simpler...

scott

[Mike Simard]

The goal is to have advanced timing when the engine has a light charge entering the cylinder. You could also look at as a non-vacuum retard, with full charge enerting the cylinder it will burn faster and you want less timing. The lighter charge burns slower, thats why if you locked your distributor advance you would have pinging at full throttle but not at part throttle.

[mr0077]

Most vacuum advance (particularly on warmed over motors) is what is called "ported" vac advance, that is, it goes to a port that is effectively shut off at idle, and only receives a vacuum signal above idle.
This is useful with a "lumpy" cam to help stabilize the idle, since the vacuum signal would fall and rise due to the cam, and thereby increase and decrease the amount of advance given to the ignition...the idle slows, vacuum drops, advance decreases, makes engine run slower, then engine surges, vacuum rises, advance increases, etc.

[Excaliber]

Direct intake manifold vacuum is a good call, in fact "back in the day" it was an old drag racing trick to get the vacuum signal respone time just a little quicker (didn't really work). But be advised your "base" timing will include base + vacuum advance + mechanical advance when using this method.

Ken and Mikes's posts are right on the money!

Manufuacturers (old school stuff) used a vacuum port placed just ABOVE the lower throttle plates. That way the vacuum advance would not come into play until the throttle was opened just slightly. Avoiding potential "to fast an idle" condition due to vacuum signal advancing base timing.

At WOT (wide open throttle) there is low or no vacuum present so your running on base + mechanical advance only. You should set your base timing and FULL mechanical advance with the vacuum port plugged and the vac advance NOT active at the time.

So WHY do some engines use vacuum advance anyway? Better "cruise rpm" fuel mileage is the MAIN reason. Say your total advance (base + mechanical) doesn't come fully in until 3200 rpm, but your cruise rpm is only 2800. Under partial throttle and load conditions (such as cruise) engines GENERALLY respond better to more advance. The vacuum signal is fairly strong at cruise conditions, thus advancing the timing using the vac signal is a "good thing" in that case.

Stepping on the gas to pass or accellerate lowers the vac signal and your back to base + mechanical advance. That keeps the timing from potentially causing "ping" or "knock" under those conditions.

Many people simply ignore the vacuum advance and set up their engine for base + mechanical and let it go at that. That works fine. But again, MANY engines respond favorably to a crusie rpm vacuum signal approach to timing control.

You might want to drop the jets a hair, but better to run a little on the rich side than risk being to lean!

[oldmachdriver]

when its running at cruising speeds it seems to run fine....but when at a stop sign it looks like it loads up on fuel...black smoke...hesitation...couple blips and it clears......my fuel PSI is 5psi, Im thinking its all in the MSD....but i want to make sure my carb isnt the prob....it did this same prob with the old carb also..but it was sooo much worse....now i can drive it....but i want it to be reliable....

this the equip im running

Boss351 /72 HO heads,
edelbrock performer 3514V intake and 750 CFM carb
holly (blue elct fuel pump) with regulator set at 6 psi no choke
4sp top loader
MSD ready to run distributer/ blaster coil.
ford DIAZ-6250A Cam....not sure if thats good or not....

[Excaliber]

Sounds way rich all right. The hotter spark from the MSD may be "masking" the problem by keeping the fire lit a little better. You need to address the cause, possible power valve, jets, float. Look to the carb for the next step, (but don't forget to verify over all timing parameters, especially idle timing).

[Frank Messina]

If you have black smoke at idle it's not the ignition or main jets, you've got other issues to deal with first. Black smoke when you punch the throttle could be that the accelerator pump shot is too rich, but that's just one thing to look at. If the motor runs good otherwise I'd leave the jets alone for the time being and go through the other circuits first. If you have a Holley, it's pretty easy to dial it in.
Frank

[Mike Simard]

It might be worth it to check out the float level too.

[oldmachdriver]

was just outside with it and fired it up....i turned on the fuel pump and it squirted through the meter rod area....but regulator guage still says 5.5 psi to the carb....

The carb is a edelbrock 750 its new....it did the same issue with the last carb too but not as bad....


i should have to be recalibrating already on the edelbrock...this is driving me nutz.....

[oldmachdriver]

i should mention the black smoke only pumps out when i throttle up in neutral and when i am cruising and floor it...seems to be loading up.....i have tried to play with the timming...doesnt change...what am i missing....bare with me im still learning how too do stuff....used to be just replacing the carb was weird science for me...now im getting ready to play with metering rods and jets....god help us all......now im super motivated to get this car running...someone passed me in a cobra on rt 204 outside ellabell ga today.....now i want mine running...!!

[sharpe 1]

Your problem sounds like a float issue.

[sharpe 1]

Thank you for the job you are doing. I am retired army also,30 years.

[oldmachdriver]

i appreciate that....3 more to go andi can walk away with 20 active.......but we will see.....illmake master sergeant this year and then it depends on what they want to do with me....

[trularin]

Popped power valve in the carb.

How can you have 1972 Boss 351 heads?

Anyway, if you backfired while you were playing with the timing, you might have blew a hole in the power valve. That would dump a big load of fuel into the intake while idling.

Just a thought.

[oldmachdriver]

its a 71 boss 351 block with 72 HO heads....it never back fired through the carb....it just wants to flood at idle all time.....at cruising its ok...still rich...but as soon as i stop at a redlight it loads up.....