diz vacuum leak?
 

2 yrs ago I rebuilt a 390 for my '72. dont know what yr this 390 block was so am not sure about timing mark on it. The Chilton book says 10 degrees BTC for a '70, and 6 degrees BTC for a '71. So I turned the diz til it said 8 with the vacuum hose off and plugged. Seems to run fine, but when I hook the vac line back up to the advance diaghram, it changes the timing mark about 4 degrees, and the idle goes up. Do I have a vac leak on it?

the reason you plug it when you set the timing is to set base timing. plugged will be constant unless you have a bad timing set. when you hook it backj up it will draw vacume against the diaphram and will advance the timing. be sure you have the line hooked up to the right port on the carb. on the wrong port you could be pulling too much vacume and the truck will fall on its face when you gas it. its like it runs out of power too soon. if the diaphram is bad it will not advance when you increase the rpms

higher idle OK?
 

The truck seems to run fine, revs thru all 4 gears, but in all of the vehicles I've ever timed, I've never noticed the idle go up when I rehooked the vac line to distributor. Vac line is hooked right according to Holley 4 bbl pic. Wondering if I could do a vac pump check on vac advance diaghram?

that is perfectly normal. but what is your idle set at?

idle setting
 

My idle is set at about 800 when warm, ( 4 speed manual). when it is cold it will idle a bit slower. With it running and set at about 8 degrees before, it will idle there. But when I pull the vac line and plug it, the idle slows to about 600. Maybe its just me, but when its idling at 600 with the vac line off and plugged, the motor sounds like its idling better. With the vac line on, it sounds like a motor with a vacuum leak. Maybe I'm having a senior moment?

well if it drops 200 rpm then there is a problem. it should drop about 50-75 rpm when unhooked. take the line from the vacuum advance and see if you can pull a vacuum either with a pump or your mouth if not there is a leak. but that ported vacuum is only active after about 1,000 rpm .

success...maybe?
 

Thanks for all your ideas, but I think I've found the prob. Stopped at the parts store thinking about replacing distributor (or vacuum advance diaghram), and asked a few questions about it. Luckily one of the guys behind the counter was more than just a checker. Even tho I had hooked the vac line according to the pic that came with new Holley, it was hooked to the wrong vacuum port on carb. The right one is a METERED port. you can learn something every day I guess.

I have the same problem with my 300 six. I upgraded my carb to a Carter YF from an Autolite 1100, and ever since I have had the vacuum advance unhooked. When the advance is hooked up the engine lopes when you let off the gas on flat ground, and in high gear. Also at idle their is a noticable puffing in the exhaust. Unhook the vacuum hose and all this goes away. There are two ports on the Carter carb. One is manifold vacuum and the other is ported vacuum. I am correctly using the later. I have never been 100% sure that I don't have a Loadomatic distributor. Anybody know if this would be the symptom of the crappy Loadomatic. I already know some of the ways they say you can tell the difference but mine is still a bit iffy. Reason being is evidently the Loadomatic has a rigid shaft due to the vaccum only advance. My shaft is not rigid (no punn intended), you can manipulate it several degrees as would be charicteristic of the more normal/common dual advance distributor. The difference with mine is my rotor does not spring back into place if I turn it by hand and release. So, I guess I could possibly still have a Loadomatic with slack in the gear or something. Thus causing the timing troubles. I guess it would be easy enough to simply swap out the distributor for something I am sure of. It would probably pay for its self if it corrected the problem. I am sure with no vacuum advance I am loosing a few MPG. Very important.... Any ideas welcome