[67chevelle509]

We're running a crate 502HO/TH400 combination in a Chevelle, It's got a slight off-idle hesitation issue that I'm trying to eliminate. I've heard that too much initial advance could cause this hesitation. Currently we're set at 16 degrees initial and 36 degrees total. Anyone have comments on this as a possible cause for the off-idle hesitation ?

The distributor we're running is an MSD Pro Billet with mechanical advance. Can the initial advance be adjusted independant of the total advance ? If so, how ?

Thanks in advance

 

[FRYNTYR]

I think your timing is fine. I helped a friend that has a 502 in his 68 eliminate the same type of problem. On his, we ended up rejetting the 850 holley DP we had on it 4 sizes front and rear. It had been jetted for a milder lower compression motor and the 502 ended up wanting more fuel. . We also increased the primary excellerator squirter to a 35. Now it's is way more responsive and no hesitation what so ever,instant tire smoke!

 

[BillK]

Mark,
If this is a street driven car, my bet is you do not have enough initial timing. You can verify this by starting the engine, bumping the timing up to about 24 and driving the car. Don't get into it too hard because you will have too much total advance, but you will know if your hesitation is gone.
I personally think it is next to impossible to get the correct timing curve on a street driven engine without vacuum advance on the distributor. Just for example .... the vacuum can in a 1971 402 distributor put 20 degrees of advance in at full vacuum. That combined with the 8 deg initial timing meant that with the vacuum hooked up, at idle (max vacuum) the timing was at 28 degrees !
I could very well be wrong but try it and let me know, almost every single off idle hesitation problem I have encountered on street engines has been due to not enough timing at idle.
Hope this helps,

 

[cjlandry]

So we're back to the ported vs. manifold thing again.

I recently went to ported vacuum, but I'm going back to manifold. Since the vacuum advance isn't active during cranking, it won't wear on the starter.

I've had as much as 45 degrees of advance, including vacuum advance, at idle. No troubles. Smoothest idle I've ever had, and the vacuum advance goes away during hard acceleration. Of course, I have to limit my vacuum advance now to avoid detonation, but it still runs better with more advance at idle and as little opening of the throttle plates as possible to achieve the correct idle speed.

I honestly think it depends on your application whether it's better to run ported or manifold vacuum on the advance. Ported was better when I had no control over the mechanical advance and had to severely limit the initial advance.

 

[ddeennis]

you can get some of those vacuum canister with verying degrees........i just so happen to have an extra hei unit on the shelf that was marked with a "10" and i new right away i needed that for my bbc. i run about 22 degrees for initial and 36 total on the mechanical side.

i tried the can i had with the hei to start with but it was marked "20" and that was giving me about 42 degrees of timming at idle and 56 cruising down the road...........way to much it was causing a highspeed miss..........

i changed the can to the "10" and that worked perfect.........at idle i have 32 degrees and 46 total while cruising...........it worked best on the full port vacuum side since the extra timming goes away once you floor it........

however i tried the adjustable unit before i saw my extra hei vacuum canister and that didnt work to well because my motor didnt pull enough vacuum at idle in gear. so when i was at idle(in gear) i had about 24 degrees of timming at about 600 rpms and when i put it in park the vacuum signal would pick up and really lay the timming into the motor and the rpms would jump from 600 to about 1100 rpms with something around 38 degrees........so the adjustable unit really didnt work with a 5.5 inches of vacuum at 600 rpms .

the stock vacuum canisters seem to be alittle less stiff and work ok with low vacuum readings like i had.

 

[cjlandry]

Good point. The Mr. Gasket HEI advance kit's instruction sheet has the GM part number for the 10-degree vacuum advance. I'll look it up when I get done unpacking all my boxes from the move.

 

[onovakind67]

If you use vacuum advance, you need to tailor the VA curve to your engine just like you tailor the mechanical advance curve. I would use a Crane adjustable VA can and put the hard limiter on the top end, the end closest to the can. The inner adjustment varies the low end of the curve, so you can adjust the starting point and the amount. You want to have full vacuum advance at idle to keep the idle steady.

 

[mr 4 speed]

My 454 runs great with 20* base/38* total by 3000 rpms with NO vacuum advance..very,very responsive and this is with an out of the box 3310 Holley