My 79 vette has has a MSD digital 6 ignition, and a MSD Pro-Billet distributor. I have read Lar's paper on how to set timing; however, my distributor doesn't have a vacuum advance. I set the timing to 36* at 3000 rpm, and after doing that it's 15* at idle. I have read that after people reconnect their vacuum advance they get 52-54*, since I don't have that on my distributor should I set my timing to be 52*?

When I rev the car in idle it revs great, but when I drive it on the road and I hit the gas hard it sputters and wants to stall on me. In order to be able to go full throttle I have to slowly ease into it in order to not get this effect. I am unaware if this is a timing problem or a carb problem (or both). The engine is a 355ci and the carb is a Holley DP 700. I very much appreciate all the help/advice people have to offer, I'm still very new at this.

 

I had the same problem... first off, do not set your total advance to 52! Set it to 36!
The problem is that you only have centrifugal advance, which is a direct relationship to RPM. This means that your distributor can not sense the load of the car. That is what vacuum advance does, it senses the load according to vacuum and then advances the timing based on that. The only way for you to advance timing is to increase the RPM, which means you have to give it gas and rev it up.

To tell you the truth, I never got my car to run right with only centrifugal advance, but I was able to getting running well by shortening the timing curve (installing the largest bushing), which increased the initial timing and slamming the timing in real early. See, the trick I experienced was, you need to slam the timing in right off idle in order to eliminate the lag associated with engine waiting for the advance curve to start.

For example if your advance starts at 1500 RPM and ends at 3000, then it is going to have the same initial timing up to 1500, which in your case is 15 degrees. That is why I think I got a stumble and why I had to give it gas to get it going from a dead stop.

I tinkered with the SOB for about a year and never got it right. In the end, I felt that I was really close with this setup. By the way, I had a 6AL box, and a MSD distributor, just like yours. (I bet it is the same part.)

This is what I did:
I put in the largest bushing in to shorten the total centrifugal advance to 18. It also increased the initial timing to 18, while keeping the total at 36.
I set the idle speed to 800.
I installed the lightest springs, which started at 800 to 850 and was fully advanced at 1750.

As you can see, I was trying to eliminate the lag between the idle RPM and the starting of the centrifugal advance.

I WAS ABLE TO DO THIS BECAUSE MY ENGINE ALLOWED ME TO. THE LIGHTEST SPRINGS MAY CAUSE DETANATION ON YOUR CAR.

It made the drivability better, but not like vacuum advance. The thing was a rocket when it was fully advanced, but it was really hard to drive. That is why vacuum advance is used on street cars, for the drivability and throttle response. Your setup is more of a drag strip setup where the engine spends most of its time at WOT.

If you want my opinion, which you really didn't ask for, I would say that you can get it to work, but your gas mileage will suck really bad, it will be really jerky (really bad throttle response) and you will spend a lot of time down shifting to keep RPMs up. Basically, my gas pedal was an on/off switch. Also, you will most likely have to tune the carb to compensate.

In the end, I switched to a vacuum advance because the drivability just started to drive me nuts, but once again, give the setup above a shot and see if it helps.

If you need more info on my old setup, shoot me a PM or email at joecalo@gmail.com because I have already cluttered this post with a lengthy response.
Sorry guys!
Thanks,
Joe

 

Joe's got it mostly right! Most cars need vacuum advance, think about what it does. At light throttle and light load you have high vacuum which advances the timing and allows your engine to run very economically. Some systems have the vacuum hooked up to manifold vacuum, others to port vacuum (as on my small block).The advance curve hits earlier, great under light loads, but may cause pinging under heavy loads at low rpm. A lot depends on your compression ratio, octane of fuel, and temperature. When we used to soup up air cooled VW's, the first thing we'd do was toss the vacuum distributor in favor of a centrifugal advance only. YOu ought to be able to run your small block on centrfugal only, setting total advance to 35-37 degrees. You'd get consistent power and a small trade off in fuel economy.

 

My distributor has a vacuum advance but I have never been able to hook it up. The engine just does not like any more timing than 36 degrees. My engine does have a rather high compression ratio and yes, the gas milege does suck!

 

Joe, thanks for taking the time to offer such a solid and long response. Thank you lowbuck72 and 1979toy as well. It sounds like I should get a new distributor with vacuum advance, hopefully that will solve my problems.

 

I wouldn't jump into that just yet.
What is the MSD part number. I had part number 8571, which you can not add a vacuum advance unit to, but maybe you can add one to yours. post the part number.

Also, if that does not work, I bet you can get it to run well enough with the right settings.

Start by posting the part number and I will check to see if you can add a unit to it. I just hope it is not 8571.

Thanks,
Joe

 

Did you buy the dist new? If not maybe someone removed the vac adv can. For the sputter problem check to see if gas comes out the shooters as soon as you move the throttle, any lag what so ever and you car will sputter.

 

Joe, the part number is 85551. I appreciate you looking the info up for me.

Fevre, the distributor was on the car when I got it. I'll check out the shooters tonight to see if there is any lag with the gas.

 

I posted a question on the MSD tech forum.

http://www.msdignition.com/forums/in...t=0#entry17991

Thanks,
Joe

 

Looks like there isn't a way to add vacuum to that distributor. Not a huge deal, looks like I can sell my current 85551 on eBay and end up spending less than a Benjamin getting one with vacuum (after selling the 85551). I was looking at the MSD 8361, do you think this would be a good choice? Thanks again