351c timing issue
 

The 351 cleveland in my shop truck is coming together finally but I have an issue... I put a bracket master II cam in it with big valve closed chamber heads, full mallory ignition, mighty demon carb and switched from a c6 to a top loader 4speed. The problem is... with the c6 and the 3000-ish stall It didn't matter that I didn't have the vacuum advance cause of the stall, but, now I have the 4 speed and it pings pretty bad at low rpm's when I have the timing at 36-38 where it like to be. Is there a box that will retard the timing before the rpm's get up that will sort of simulate what the vacuum advance does when the vacuum drops? in other words... what do you do when you run a 4speed with only the springs in the dist.? the springs in the dist kick in at about 3k rpm's now

Some vacuum advance distributors were designed to run via a spark port timed vacuum off the carb, while others worked off of manifold vacuum. The advance strategies between the two of these is polar opposite of each other.

You need to put a hand vacuum pump on your distributor diaphragm and watch your timing marks with a timing light to see what mode distributor that you have. Then you can hook up your vacuum line according to what you are looking to accomplish.

Smms.

I thought I was clear. No vac on distributor

Whoops! Scratch that vacuum advance theory!

Now you have to find out how much centrifugal advance your distributor has in it. You may have to change some springs on the weights and/or flyweight travel stop points so that you can run around 10 degrees BTC at idle and still get your 36 total around 3000-3500 rpm.

I have seen where a lot of out the box centrifugal distributors have only 15 to 20 degrees of advance in them. Then you are forced to run a high initial amount of timing so that the total would be where you wanted it.

Anybody here old enough to remember those old timey distributor test stands at the old skool auto electric shops?

I need to figure out which bushings i have in the distributor and see how much advance I'm getting at all. My thoughts are get the big daddy bushing that does 28 and run stiffer springs to delay the advance till I'm pretty far into the rpm's. I can run 8deg initial to get the hole shot then hold on for the big end to kick in. Sound like a plan? Let me know

Sounds like a good plan. Be aware that most performance timing curves have max timing in by 3000-3500 rpms. That way on the big end of the tach in high gear, you have a simulated timing retard as the cylinders and cylinder heads are getting pretty hot by then and need a little less timing than what they would at a cooler temperature at the same rpm.

I don't really understand that. Isn't all the timing in by then anyway? I have been sold on using manifold vacuum and when the vac drops the timing would retard for pulling out of a bog. Does ported vacuum drop at high rpm's?

Standalone.......MSD, or EFI...done..

Take the weights and springs completely out of the distributor, plug the vacuum off, if you haven't already, and set it at your desired degree all the time. Thats we do in bracket racing, cause the springs are a constant thing to deal with, as the get older they get weaker, blah, blah, blah. Pull that crap out and if you run it at 32 degrees, set it there, and ride..:bigthumb:

That doesn't really address his problem.

A programable ignition box will give you the best flexibility, otherwise, lighter weights, stiffer springs and some testing. I'm sure someone in town has a distributor re-curve bench. The question is, do they know how to use it?

Ported vacuum starts to come on when the edge of throttle butterfly starts to rise above its port just above idle position. You will have to hook a vacuum gauge up to your ported port and go for a ride to see just how the vacuum signal from your port acts at various rpms and throttle openings. Generally, ported vacuum drops off the more you have the throttle opened, but has a tendency to increase slightly if at all as rpms go up. Vacuum advance was originally implemented to add some advance during light throttle openings just above idle for increased fuel mileage.

Richard give me a call. Shawn gave you my number.

Will do. Thx