Is this Vacuum reading Ok?

[rae61]

Just finished my 200 rebuild and have it running. The best vacuum I can get is 15" of vacuum, I have checked for leaks using carb cleaner, but did not find any.

Is 15" to low or is it reasonable, and satisfactory?

The motor has a Comp Cams 252H, bored .030, larger valves and ported, and a CR of ~9.2:1, Carter YF from a 240ci

Thanks!
Rick

 

[wsa111]

Rick, i would try another vacuum gauge
I would think you would be in the 17" range because your camshaft is very mild.
However if the engine runs great & all your distributor functions are ok so be it. What are you running for initial advance? Bill

 

[rae61]

Bill, I used my Mityvac to test the vacuum gauge, with the mityvac pulling 15" the vac gauge was reading 13.5". So, if this is a valid way to test the gauge, then the 15" I get when checking the engine
is really 16.5" ??

I'm running a DuraSpark-I from a 75 Maverick, I have a new (rebuilt) Cardone distributor. The initial advance is set @ 18 deg.

Thanks!

 

[wsa111]

Then your vacuum is fine.
Running 18 degrees initial you need to check your total advance. My gut feeling is you probably have way too much total centrifugal advance. Bill

 

[rae61]

Bill is there a good way to test the total advance? Is there to find out the centrifugal advance for this distributor?
I don't hear any pining, but the Bronco is not a quiet ride, so maybe I'm not hearing it over the vehicle noise. Rick.

 

[rbohm]

rick you will hear detonation trust me, unless you are running an unmuffled engine. detonation will sound like a can of rocks being rattled.

one way to test your vacuum advance is to disconnect it from the carb, and plug your mityvac in. then with the engine idling, pump up the vacuum and watch the timing mark move. if it does, then the vacuum advance can is fine. another test is to draw a vacuum and hold it. if the vacuum remains, then all is good, if not then you need to replace the vacuum can.

to find the total mechanical advance, you need a dial back timing light. you run the engine up to where the centrifugal advance is at its maximum, and then dial back the timing light until you get back to the original initial timing. the difference between the two is your total centrifugal advance, as long as the vacuum advance is disconnected.

 

[rae61]

I will check the vacuum advance with the Mityvac. I have ordered a timing light with dial back, should have it in 10 days.

I will post my findings once I get the new timing light.

Thanks!

 

[mustang6]

I used to get about 17" with a 252H in my 200 with a large 1 bbl (Autolite 1101), went up slightly to about 18" when I put the Weber 32/36 on. So with your gauge correction readings on a large 1 bbl you are probably just about the same.

 

[Asa]

Bill, I used my Mityvac to test the vacuum gauge, with the mityvac pulling 15" the vac gauge was reading 13.5". So, if this is a valid way to test the gauge, then the 15" I get when checking the engine is really 16.5" ??

Are you assuming that it's off by 1.5" across the board? It might be off by a factor of something that shows up as 13.5" at 15"

A nit-picky detail, but worth testing so you know what your gauge is actually reading.

 

[rae61]

Are you assuming that it's off by 1.5" across the board? It might be off by a factor of something that shows up as 13.5" at 15"

Good point...I checked and when the Mityvac indicated 5" - 10" - 15" my Vacuum gauge was 1.5" low for all three readings.

Rick.

 

[Asa]

Good
There might be an adjustment in the gauge that allows you to correct for that.

 

[rae61]

I got my new timing light today, and this is what I found.

The mechanical advance is 16 deg @ 2500 rpm with vacuum advance disconnected and plugged. Tested the vacuum advance with the Mityvac and I got 12 deg advance @ 15" of vacuum.

The initial advance is 18 deg.

What should the total timing be?

 

[tom954x4]

By the way, Rae61, if your Bronco is in Calgary your vacuum readings will be lower than they would be if taken at sea level due to your altitude. Vacuum in my truck at idle goes down several numbers between sea level and say 4-5000 feet, roughly from 20 to 18 or so.

 

[MustangSix]

With the vacuum disconnected your timing should be about 30* @ ~3000 rpm.

 

[rae61]

I rechecked the mechanical advance up to 3000 RPM and it stayed @ 16deg from 2500 through 3000 RPM. I backed off the initial timing to 15 deg and adjusted the idle/fuel and idle.

Runs very good, I will pull the plugs in about 500 miles to see how everything is doing.

Tom, yes the Bronco is in Calgary ~ 3400 ft, so that is good to know about the lower reading at increased altitude.

Thanks everyone!