Vintage Mustang Forums banner

1 - 12 of 12 Posts

·
Registered
Joined
·
38 Posts
Discussion Starter #1
Quick Question, but no simple answer......

70 Mach, 351c, dual port distributor with Petronix, stock vacumn line setup with vacumn ported tree and etc.

I set the timing and test drove, it ran fine..........

When I got back home I put the light back on it and did not disconnect any of the vacumn lines.....I just wanted to see exactly where the total avance would max out at.

As I reved the engine up, the mark advances properly BUT it appears to never stop advancing! The higher you rev the engine the more it advances! I am talking about well beyond 38 degrees total.

Being an old chevy guy, I expected to see the timing total out at some point like the old chevys do!

Anyone got any suggestions???

Thanks in advance!!....lol
 

·
Registered
Joined
·
5,240 Posts
what dizzzy.
what rpm dod you max out at.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #3
Stock dual port distributor with a Petronix module.

It never did appear to stop advancing, just kept rolling on up the higher I reved the motor.....I reved the motor up alot higher than I wanted to and it kept on advancing.

I assume a stock distributor should stop advancing at some point right (like a chevy does)???
 

·
Registered
Joined
·
38 Posts
Discussion Starter #5
OK, I kinda figured that may be the case....not up to speed on these vacumn advance distributors and how they operate!
 

·
Registered
Joined
·
5,240 Posts
reved up lot means what. we need a number. did you use a tach to see the rpms?
 

·
Registered
Joined
·
38 Posts
Discussion Starter #7
lol, OK....here are the rpm's.

I figured the timing would max around 2500 to 3000 rpms, and at 3000 it was right at 36 degrees advanced.

I reved it on up to 3500 rpms and the timing advanced farther and I reved it up to 4000 rpms and it kept advancing.....just seemed curious to me.
 

·
Registered
Joined
·
121 Posts
Maximum advance will be limited by the weight stops inside the distributor. The vacuum advance does NOT add anything beyond where these stops will allow. So, although it's advisable to do your measuring with the vacuum line disconnected, if you're simply wanting to set your total advance it won't make a difference. That said, I've read many whose opinion is that the stock Ford distributor permits too much mechanical advance IF IF IF you're wanting to set initial advance at anything over 10-degrees. Sounds like you've just discovered this yourself. I remember reading an online article that indicated that SOME autolite distributors had two different slots for limiting the mechanical advance (sorry my memory is vague). Others physically modify (weld?) the advance mechanism so that it is more limited. They're then able to set initial timing up around 15-degrees and then add nice quick (light) springs. I'm sorry I can't be more specific on how the modifications are made. I'm sure someone more knowledgeable than me will chime in.
 

·
Registered
Joined
·
21,022 Posts
Mechanical advance is one thing and vacuum advance is another. Vacuum advance is there for fuel economy only. It does not hurt performance in any way. It just adds parts. Any way, vacuum advance diaphragm can pull in as much as 60* timing on Fords. It's completely independent of the mechanical advance mechanism. So, revving your engine with no load is going to pull in a ton of advance since it (thinks it) sees a very light load due to high manifold vacuum.

As the load on the engine increases, manifold vacuum drops as well does the total vacuum advance. If there is no vacuum advance, total timing is going to be what ever the mechanical advance is at that point.
 

·
Registered
Joined
·
38 Posts
Discussion Starter #12
EXCELLENT GUYS...this forum is fantastic!!!
Thanks alot for the GREAT INPUT!
Time to learn about vacumn advanced distibutors!
Thanks again,
Johnny
 
1 - 12 of 12 Posts
Top