My engine was professionally tuned last year, but the engine was cutting out badly under full throttle at around 4,500 RPM. The dyno result after tuning was also disappointing. I pulled out my 30+ year old dwell meter and timing light and a number of things do not look right. I've done a lot of reading on the forum this week. However I am a novice at this, so please let me know if my thinking below is on track.

When I checked dwell it was 21 degrees; way too low. I raised it to 28 and will bring it up to 30 next time I make adjustments.

I reset initial timing (w/ vacuum advance unhooked) to 12 degrees BTDC after I corrected the dwell. This is conservative for now. Target should be around 16-18 degrees.

Next I checked mechanical advance. This was a little difficult w/o a dial back timing light. However it seemed like there was not any mechanical advance coming in until very high RPM. The advance did not change as I raised RPM to the 2,000 RPM tachometer limit on my dwell meter. However if I rev the engine higher, at some point the initial + mechanical jumped off the scale (to ~ 60 degrees I think). The tuner had said I was getting too much advance, so he put heavier springs in. That may be why I'm not getting much at lower RPM (or maybe the weights are sticking).

Given there was too much advance at the higher RPM, I looked to see if a mechanical advance limit bushing is in place, and found out it is missing. When I manually turn the weights, the distributor cam turns approximately 22.5 degrees (which would be ~45 degree advance at the crank). So this could explain why I am getting too much mechanical advance - when it finally kicks in there is way too much.

Finally, I looked at my vacuum advance. It is a B1 canister, which kicks in at 8-11" Hg and maxes out at 16 degrees advance at crank with 16-18" Hg. My engine is a 406 with a big cam. I checked the vacuum from the advance line at the base of the carburetor (presumably manifold vacuum). It showed no vacuum at 750 RPM idle, 4" at 1,000 RPM, and about 13" at 1,500 RPM. I don't really understand these results.

So here is what I think I need to do.
1) Install a mechanical advance limit bushing. (I'm a little nervous about taking the distributor out to install this)
2) Set initial timing so initial + mechanical is about 36 degrees
3) See if different springs will bring the full mechanical in by around 2,500 RPM
4) Figure out my vacuum situation and probably get an advance canister that kicks in at lower RPM.

That was kind of long but would appreciate your thoughts. Thanks

 

Total advance at max advance is where the timing should be set. If the dist stops advancing at 3000 rpm set the timing at 38 deg BTDC at that RPM. If your octane rating is high enough to handle that adv your good to go if not retart about 2 deg at a time until the detonation stops under a load. Good luck

 

Before you take out your distributor make sure you set the engine up on #1 TDC on the compression stroke.
This should have the rotor pointing at the #1 position on the dist cap or very close and the timing mark on your balancer aligned with the pointer.
Mark the rotor position on the dist base with a sharpie and also the base's position relative to the block with the sharpie.
Then when you put the dist back in get everything to line up the same (Do NOT rotate the engine while it is out) and it'll be back where you left it before you took it out.

If you have a big cam you need to find out what kind of manifold vacuum you have at idle.
The vacuum you measured sounds like ported vacuum. Ported does not read anything until the throttle valve starts to open.

Manifold vacuum will be present even with throttle blades closed.
Then have a vacuum can that comes in 2" to 4" below your idle vacuum reading.
Use manifold vacuum for advance and you will have vacuum timing advance even at idle which will smooth out your idle and you'll be able to lean your idle mixture as well.

Your mechanical advance sounds like it may be sticking. Once you got it off check for freedom of movement.

 

Says who?

 

From what I have read on the forum. Lars' timing paper suggests 18 initial.

 

I think you need to read and understand all of the paper, not just one line. Initial timing is the product of what the mechanical advance will give you 'all in' minus the desired setting of 36-38 degrees.

Go back to square one, get the mechanical advance operating correctly, then start timing the engine.

 

You are right. You probably will end up with 18* or better for good idle.
I'm at 19* for best idle with my 350 with a cam with 54* of overlap.
Had to modify the mechanical and vacuum advance to prevent more than 51* at cruise and more than 36* all in.
I would highly recommend a digital timing light. It has not only a dial back feature but also a digital RPM indication.

Getting the timing dialed in on a big cam can take time and patience until you get what the motor wants. The numbers Lars gives are a good guide but not set in stone. Give the engine what it wants.
Using a digital timing light makes this process waaaay easier.

 

I'm discovering what you said about a dial-back timing light. It's hard to play with the throttle and try and read the timing with an old style timing light. I ordered a spring and bushing kit. Hopefully I can get the mechanical + initial set up reasonably close until I get a better timing light. Thanks

 

You don't necessary need a dial back timing light, but it helps. Measure the circumference of your balancer, and take 10% of it and put a mark (or piece of tape) on that spot.

That will give you 36 degrees, which is the recommended all in with the vacuum advance disconnected.

I had somebody sit in the car and give me RPMs.

I hope this helps.