Top Dead Center

After looking on the web, at Autozone, O'Reilly's and Napa I picked up a vacuum gauge for about $15 at Napa. The model there had the connectors, the hose and the gauge all together for about the same price as the other places. I had a Sears timing light from when I used to tune my MG ...

First I checked all the spark plug gaps and set them all to 035. We were using Auto-lite BF142 plugs (which run cooler than the BF42's specified by ford). Then I let Old Blue idle for about 10 minutes to warm her up.

My plan was to:

1. Start by first warming the engine and note the idle vacuum reading. Normal vacuum at idle should be 19-21 inches for a six cylinder, or 15-18 inches on a low compression engine. 
2. Pre-set the ignition timing so that it is close to the manufactures recommended setting of 8 degrees BTDC, before making any carb adjustments.
3. Lean out one of the mixture screws (turn in) until the gauge as well as the engine begins to shudder. At that point, bring the screw back towards rich (turn out), while watching the gauge.Stop adjusting when the gauge reaches it's highest reading. Then do the same process for the other mixture screw. 
4. After each adjustment is made, reset the idle speed.

I hooked up the vacuum gauge and with the idle set as low as I could set it, (I looked at the Tachometer in the cab and it was at about 700 rpm). I checked the idle mixture screws. Starting with both screws about 1 1/2 turns out, I read this will be on the lean side so all I need to do is open them a little more. As I opened the screws the vacuum gauge read higher and higher vacuum until it stops (about 19). Old Blues hits that point at 2 1/2 turns out. With that optimized I check idle speed again as a higher vacuum also means a higher idle speed so I set for 700 one last time.

I hooked up the timing light and pointed it at the harmonic balancer - I read 23 degrees Before Top Dead Center on the Balancer, I was shocked and checked again just to be sure. Nothing I had read indicated I would expect anything more than 10 or 12 degrees. I loosened the hold down bolt on the distributor and turned it a little counter clockwise and watched as the indicator dropped to about 12 degrees before top dead center and I left it here, cranking down on the distributor hold down bolt.

I went back to the web and read with a Mallory Comp 9000 distributor with NO vacuum advance I should have seen something like
1. Initial Idle advance of 10 degrees
2. When the RPM's increase to 3,200 the Mallory kicks in and adds about 24 degrees mechanical advance.
3. This would give me a total advance at ~3,200 RPM of about 34 degrees.

From what I had read on the web about FORD 352's this would be just about right with 36-38 total advance optimal. When I start checking the vacuum readings - if I need to advance the initial timing 2 degrees I can.

I took her out for a test run and wow she pulled from stops with hella torque and took off when I stepped on the gas while cruising - OHHHHH V8 engines!

A couple of observations on driving Old Blue after the carburetor change and the timing adjustment. First, I no longer have to step down HARD on the brakes at stop signs. It actually takes very little pressure to stop and hold old blue at a stop. I think decreasing her idle speed from 1200+ to 700 has something to do with this. Second, no choke. Old blue fires right up, idles smooth until warm. Third, she has significant power under the hood and I take care not to jump on her too much!!! We now cruise at 70mph at 3000 RPM which for this old truck is very different than the previous 55mph@3000rpm. More importantly, when I do cruise at 55mph Ole Blue is turning about 2300 RPM's which should give us a bit better gas mileage.


[This document is how to tune a Ford F250 352 V8 Truck]