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Timing question

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lbcfan

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I'm setting the timing on my new 1275 with a mild cam. The seller had recommended 28 degrees BTDC. I've set that at 2000 rpm. At 1000 rpm idle the advance drops to 10-12 degrees. I have a Petronix distributor with vacuum advance- these numbers are with the vacuum disconnected and sealed.

Should the advance drop so much with the reduction in rpm? This is my first time...

Thanks.
 
Yes.
You should probably check your all in advance at a little higher than 2000rpm. I'm not sure your distributor would have all the mechanical advance in at that rpm. Also is the advance recomendation total? If it is then I would think that vacuum advance should be hooked up. A recomendation of 28 without vacuum sounds about right though. Many more knowledgeable folks on this forum than I though--maybe they can add insight!

Kurt.
 
It will drop at lower rpm because while the vacuum advance is disconnected the mechanical advance is still operating at higher rpm. If you are going to run the vacuum advance you should hook it up and check your total advance once more. Even if you are going to run it without you might want to recheck total and run the rpm's up a little higher to make sure the mechanical advance is all in.
 
Thanks all. I do plan to run with the vacuum advance connected to the base of the DGV carb. My ignorance is such that I did not realize the dizzy had both a mechanical and a vacuum advance- that's why I was puzzled. If I understand correctly I'll reset to 28 degree advance at 2500 instead of 2000, with the vacuum connected.

I've also seen Bill's method of checking with a vacuum gauge. I'll try that as a check. I have a tap on the intake manifold.
 
Why did you decide to use 2000 or 2500 rpm? Is that simply where you feel comfortable revving the engine to?

The distirbutor operates on an advacne curve determined by the springs and weights installed. Generally you want to set the timing before advance starts or after advance ends, or you'll have a timing setting that can't be replicated without a whole lot of luck. Set the timing at idle to 10-14 degrees, and recheck it over 3000 rpms. Anything that happens in between will be very difficult to replicate.

I'd try 12 degrees at idle, and recheck for 30-32 degrees at full advance, vacuum disconnected.
 
Thanks. I spoke to Pertronix- the mechanical advance should top out around 3000-3200 rpm, so you are spot on. I appreciate your recommendations.
 
If you had a "dial-back" timing light you could plot your mechanical advance curve and if not to your liking (or factory spec) you could diddle with the weights and springs to adjust it. Likewise it's a good way to check the vac advance . Bob
 
I did purchase a better light so I could set the advance. I need to learn how to take advantage of what it can do. Instructions were lousy, and my background is non-existant...

Part of my problem may be that the engine passed through a couple of hands, and I do not know the specs on the reground cam. I don't know if that matters. If anyone knows of a comprehensive article please post a link

BTW, is it worth taking the car to a shop with a chassis dyno?
 
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