I have a 351m in my 89 f-150, No matter what i do and can't get the total spark advance below 55 degrees advance, at 2500 rpm. have already tried switching weights in dist. for less mech. advance and a adjustable vaccum advance, problem got better but i would like to see less, any suggestions?

Bill edge
Ohio

 

did you check the dizzy itself?

 

Where is the initial (static) timing set? (Without the vac advance hooked up)

-Brents

 

i wanted to run about 10 degrees adv. but i had to much advance, so along with everthing i replaced i dropped down to about 6 degrees advanced for my base timing.

bill
ohio

 

Hi Bill,

That sounds about right to me for the initial timing.

Is there still vacuum present on the vacuum advance at 2500 RPM?

I believe that there should be no vacuum there at 2500 RPM, if vacuum is present, that is the source of your problem, and you should move the vacuum source to a ported connection on the carb.

If there is little or no vacuum there at 2500 RPM, then perhaps someone has "reworked" the distributer to allow more total advance then nessarsary; perhaps a new "rebuilt" distributor from a parts store is in order.

Lastly, not to sound stupid, but are you certian it is at 55 degrees?

-Brents

 

when i rebuilt the engine i installed a new dist. i talked to a mechanic at ford and he said to use a venture port on the carb for advance. i did try ported and got the same result. it is 55 degrees advance right now, the mechanic said i should be able to get away with 45 to 50 degrees advance. he also said that at 2500 rpm is should have vaccum to give me my advance, i was not sure on every thing he had said so i check out my haynes manual along with a mitchel book and they all said the same thing as he did. not really sure what to do now? hope some one can help

thanks
bill
Ohio

 

Hi Bill,


Hmmmm maybe I am wrong. I was pretty sure that the vacuum advance should be connected to the ported source. From what I can remember (which isn't much) the vacuum sources behave like this:

Manifold: Starts out around 20" of mercury, and falls off to zero at wide open throttle.

Ported: Located just above/below the throttle plates, starts off low (1-2") and builds to 12" around 2000 RPM, then falls back off to 1-2" at wide open throttle.

Venturi: Weakest of the three, starts off very low and builds to around 8" at wide open throttle.

So, now I am confused, I told you that there should be no vacuum there at 2500 RPM, and now I am starting to re-think that.

It seems to me, the vaccum advance is supposed to advance the spark before the weights kick in and do their thing.

I am thinking there are two springs inside the dist. One of the springs is light and the second is much heaver. I think the first one is supposed to give a good quick advance at low RPM, and the second stronger one is supposed to kick in and resist/limit the total advance, but I'm not sure.

This is all I know (or think I know) I'm sure one of the experts will chime in here soon and straighten us both out.

If I get time tonight I will check my truck (79 w/351M) it runs awesome and has great throttle response.

By the way, I do think 50 degrees is way too much advance.

Edit: Oh yeah, I forgot to ask, does it ping or do anything else bad with the advance at "50"?

-Brents

 

Well not that I know a whole lot about advance (I posted about that too a few days ago) but using the "timed spark port" on a holley truck avenger carb i could turn the distributor to its stop point at idle without much pinging. I dont know how much advance that is but its got to be alot. My engine apparently doesnt have anything to indicate the advacne so I can really tell. But right now I am using full manifold vacuum advance with the distributor pointing just left of center at you, when you are standing infront of it. I do have some hesitation and poping between around 2000-3000 rpm when your at half throttle or more, but I think its the holley tunning because I had an edelbrock 750 on it and it was just fine at 2000 rpm and up. Just my $0.02