Flathead timing - rpm's??

Mart

Mart

Well-known member
I have one of those (pretty good I think) Snap on Timing lights that also has a rev counter. I was setting the timing on my French motor. I had previously set up a pointer at TDC.
I knew I wanted 4 deg BTDC at idle. so I set the timing at 4 deg.
I then checked the idle speed and it was pretty fast - about 800.

I then checked online and found the idle speed stated as 475 rpm (or thereabout) I struggled to get the motor to run that slow (dual 94's), but did do it, and found the timing was way off.

I reset it and as far as I can tell, it seems pretty good.

I just read the chevy distributor post which states ideally you want 4 deg static plus 24 deg mech to make a total of 28 deg, and for it to be all in by 2000 rpm.

If I had set my timing at 4 deg at 800 rpm I'm pretty sure it would have been retarded, because if it's at 4 at 475 and 28 at 2000 then surely it should be at approx 9 degrees at 800rpm.

I mention this because I have never heard idle speed mentioned when quoting timing figures.

I'm wondering if there may be a few other people who are running retarded due to setting the timing "right" but having the idle too fast..

Mart.
 
Centrifugal advance is rpm controlled, usually doesn’t start until about 500-600 engine rpm, and ends when the flyweights hit their stops. Most shop manuals tell you to slow the idle and disconnect the vacuum advance, if applicable, before you adjust the timing. You can easily find what the advance start rpm is with your light.
I would suggest you start out with 20 degrees total (4 initial/16 mech.) and slowly move up to about 24 total (20 mech) once you are sure there are no problems, as 28 might be way too much for a street flathead. JMO
 
I agree to start at around 20 degrees total advance. I have NEVER tested any Flathead running pump gasoline that could use 28 degrees at WOT(wide open throttle) Most make maximum power at between 16 and 18 degrees when at WOT.
 
Idle speed and advance is a issue with the flatheads as most will idle very low rpm.
475 -500rpm etc. Makes set up a little tricky with springs and weights .
I would suggest setting the timing at 24 degrees at 3000 rpm with the dial up light and then checking the intial at idle ( where ever it may be).
I have found once the engine is modified somewhat , actual idle speeds no longer apply with specs found in manuals.
 
Or, you can set the timing with the engine off. Put the timing mark where you want it. Loosen the dizzy and turn on the power. Turn the dizzy backwards, then advance it very slowly. When you hear the spark jump, tie it down. With practice you can be spot on every time when you check it with a light.
 
Ole Don.. so alls you have to do is get the timing pointer lined up with the *** at tdc on #1 compression stroke and then do as you say and she will be all set?
 
On original '32-48 distributors the service bulletins there are curve specs...most call for zero to 1/2 degree at 400 engine RPM, several degrees already in at 800.
Naturally, tension on the vac brake affects this and adds in a confusion factor. The brakes are usually backed off to nealry nothing on modern gas.
 
Brake retards at low vac, but what constitutes low depends on the adjustment of the bolt. Many people have that adjusted pretty much at random...often way tight.
The retard effect is often unnecessary now...modern gas is higher octane than what was once out there. I back off the bolt by hand gently to end of travel (turn it hard and you pop off the spring seat) then back in maybe two turns to put slight drag on brake disc. You only turn it in if the thing pings. Not relevant here with late flathead of course.
 
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