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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: dwg30 on March 18, 2008, 02:06:15 pm

Title: Timing ?'s
Post by: dwg30 on March 18, 2008, 02:06:15 pm

The 86 k-20 I recently bought has hard to start once the engine warms up and clatters when I got on it. So I check the timing last night forgetting to unplug the vac. advance. I found it way advanced 36 at idle and around 57 at 3000. So I back it down to 10 at idle and it ran about 36 at 3000. I then realize that I didn't remove the vac line so I check it again with it removed and plugged. I still had 10 at idle but around 26 at 3000. I am going to bump it up anther 10 tonight but I thought that vac. advance would advance it at an idle also. As far as I can tell it is a stock motor the only thing I didn't check is if the vac. line was hooked to a full vac. source or not. It is connected to the port on the front passenger side that is pointing at an angle toward the battery.

So my questions are, with the vac. line removed and plugged do I start off with 36 at 3000?

When the vac advance connected should it advance the timing at idle if it is connected to a full vac source?

When the vac advance connected should it advance the timing at idle if it is connected to a ported vac source?

Title: Re: Timing ?'s
Post by: VileZambonie on March 18, 2008, 02:16:32 pm

If your vacuum advance is hooked to ported vacuum you want have vac advance at idle. I personally like the stronger idle quality from vac advance @ idle. For emissions purposes it was hooked to ported vacuum.

Forget about your timing light for a minute and just try setting your timing with the vacuum advance connected to full vacuum and advance it til it pings. Retard it until it doesn't and record your total timing advance and base timing with your timing light. Using this method usually gives you the best results believe it or not and then knowing your settings gives you a good base line to dial in your advance curve and to further fine tune your timing. With Vacuum adv at idle your idle speed will be higher so you will need to adjust it accordingly. But don't be afraid to play around with it. You'll be glad you did because the results will be quite noticeable. Just keep in mind, it should start easy hot, and absolutely no pinging.

Title: Re: Timing ?'s
Post by: dwg30 on March 18, 2008, 03:01:12 pm

Thanks for the info Vile.

I found a good writeup explaining vacuum advance on another site. Heres the link if anyone is interested.

http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html?highlight=DISTRIBUTOR+TUNING+theory

Title: Re: Timing ?'s
Post by: HIGHPLAINSDRIFTER on October 28, 2010, 10:37:12 am

That really cleared things up for me.  Thanks for the link.