High initial timing

[Texasdoc]

I got the motor put back together after the heads were rebuilt. Started right up. While the heads we off I verified the cam timing and crank timing pointer were accurate. Went to set the timing... Using the vacuum gauge I turned the distributor until I got best vacuum. RPM around 1000. Locked down the distributor them checked it with a timing light and it was at 55* BTDC!

Best vacuum was 16.5 inHg. If I drop the timing down to 15* or so, vacuum falls to 14 and it runs rougher. AFR around 14. Vacuum disconnected and plugged.

Any ideas what's happening? If it was a tooth off, i would just rotate it back and it would run better at ~15*. I have an old cap with a hole that i can verify the rotor is phased correctly. I just ran out of time today. Thinking thru possible fixes.

[DanEC]

I don't believe I've ever heard of setting initial timing by shooting for the highest vacuum gage reading. I use it to set the idle mixture setting on the carb - but not timing. A lot of timing at idle will smooth an engine out and make it sound happy but as soon as you start driving it and mechanical advance starts coming in - it will get unhappy real quick. I know a lot of old timers set timing by ear - I'm sure not that good - but a timing light is a much better solution.

[Texasdoc]

I was reading "somewhere" that you can do both with a vacuum gauge - to find out what the engine wants. I read it in the internet so it must be true! If not, great! I have three timing lights. I know ideally you put it on a dyno to get the advance curve just right. I just want to get it running where it doesn't studder.

[DanEC]

Is one of your timing lights a dial back unit? Those are the handiest to have. I'm not familiar with the 351 series engine family but probably somewhere between 35 and 37 degrees combined initial and mechanical timing - all in at somewhere between 2500 to 3000 rpm. I would set the dial back light at 36 degrees and rev the motor until the timing is all advanced and steady and turn the distributor until you are reading 0 deg on the timing marks. That will leave you at 36 degrees total (no vacuum advance in this) and should get you relatively close.

Not sure what distributor you have or how adjustable the centrifugal curve is. If adjustable I would try setting it for somewhere around 20 crank degrees mechanical advance. That will leave you with initial timing around 15 to 17 degrees. Of course if your engine has a pretty big cam it may still be pretty lumpy. That's why I like to use vacuum advance to give it another 8 or 9 degrees of timing at idle to smooth it out a little (usually takes an adjustable vacuum can). Others like to go without vacuum advance, limit the mechanical advance and use more initial advance which is just another way to go.

[moore_rb]

Originally Posted by DanEC

I don't believe I've ever heard of setting initial timing by shooting for the highest vacuum gage reading. I use it to set the idle mixture setting on the carb - but not timing.

X2..

Now, there is a method I am aware of, that sounds somewhat similar to what TexasDoc was doing, where you set your HIGHEST vacuum-assisted spark advance, based on your highest vacuum reading, but I've never heard of that process being done at idle level RPM's ....The procedure I'm aware of is done with the engine up in the cruising RPM range.

The procedure is to hook your distributor's vacuum advance to pure manifold vacuum source, and also have your vacuum gauge connected straight to manifold vacuum as well.

Run your engine up to your vehicle's highest (or most common) cruise RPM (say 2800rpm or so- It has to be an rpm level that is above your centrifugal advance upper limit), and set your timing to the lowest number of degrees which will produce the highest vacuum reading possible (so if your max vaccum is 16Hg, then set it at the lowest number of degrees that will produce 16) -

Again, this procedure REQUIRES your distributor's base acceleration curve, and your vacuum advance, both to be "all in"- What you are essentially doing is setting the highest amount of spark advance that your engine can possibly produce, under any driving conditions.

Then lock everything back down, reduce your rpm's back to idle, and do a final carb idle mixture set... Double check your idle timing with the vacuum advance disconnected and plugged (it should be down in the happy zone- 20* BTDC, or lower). Depending on your idle quality, you can then either put the vacuum advance back on a delayed port, or hook it back up to the manifold vacuum port (your choice). Set your idle RPM's where you want them (typically to the lowest RPM which still produces steady oil pressure), and you're done...

Now, all of that being said- This is a mega old-school method that guys would use to dial in their best vacuum assisted spark advance for fuel economy when running down the highway in old trucks.... I've ever heard of anyone using it as part of a performance tune.... (Not saying it woudn't work on Cobra - The principles are the same, and it should maximize your highway MPG, if that matters... )

[RET_COP]

When you bring the initial timing down to 15* the idle is dropping and your vacuum falls. Adjust the idle back to 1000 clean it up with the idle mixture screws and see what your vacuum is. I'll bet it's back to 16.5
Optimum timing is based on the characteristics of the engine. Dyno it and find out

[Texasdoc]

Thanks for all the replies. I'll set the timing back to what it was. For some reason, I interpreted what I read to say to set the timing to best vacuum as well.

I do have a fancy timing light with the dial-back feature, but the MSD 6ALs multiple spark thing makes it not work. I have a cheap light that works fine but doesn't have the dial back feature. I use a combination of them both to check idle and total (initial plus mechanical - vacuum advance disconnected). I have a 360* timing tape on the balancer and use a vacuum pump to test the vacuum advance.