quote:

Originally posted by Jim and Tere:
I feel guilty taking up so much room on the "tech forum", what with four different subjects.

The more you post the more the rest of us learn. I find the exchanges of information extremely valuable.

Carl Feren

Carb is back from Oscar and Tere put it in, fired it up and she and "Old Blue" took off for a spin. She said it ran fine but we have a little roughness at idle. Need two pieces of info
1. What should the timing be set at? It's supposed to be on sticker on the radiator support but it isn't there. Looked all through my Chevy P30 manual and it just says to look on the radiator support.
2. The "vacuum modulator" which we installed seems to be working ok. But what should the vacuum be? Again, looked all through the manual and couldn't find out
My Knoxville Chevy truck dealer won't work on anything older than a '90 and claims they can't even look up the information.
Thanks folks
Jim

quote:

Originally posted by Jim and Tere:

1. What should the timing be set at? It's supposed to be on sticker on the radiator support but it isn't there. Looked all through my Chevy P30 manual and it just says to look on the radiator support.

Jim

Really short answer:

Probably 4 degrees at idle with vacuum hose removed and plugged. 8 or 10 is better if you don't ping.

Short answer:

Ignition timing is a big secret. The best official source is a smog inspector. They have charts that match the letter code or your emissions package. That code should be on the sticker on your air cleaner.

Medium answer:

It should almost ping on a full-throttle, hot uphill. The only way to get this right is to advance two degrees at a time until it pings, then back off a degree at a time. Use the same grade of gas for all testing and driving.

Medium-long answer:

It should almost ping on a hot full-throttle uphill with the vacuum disconnected and plugged. Once you get it set, reconnect the vacuum hose and climb that same hot hill again. If it pings, you have too much vacuum advance. You can buy a 10 degree advance can, Delco part # 1973577 or a Crane adjustable vacuum can or limit the travel of the diaphragm on yours by welding the slot shorter and reprofiling with a rat tail file. The ideal situation is to limit the travel and adjust the spring tension.

Long answer:

I hate to say this, but most P30s run too retarded to get the best mileage and performance. I think my 454 specified 4 degrees at idle and the max (initial plus centrifugal) ended up around 20 degrees all in at way too high an RPM. In addition, an older vehicle has wear and who knows what changed on it.

If you can find an old fashioned distributor guy who knows 454s, he can recurve the distributor and give you a lot of performance and mileage improvement. Things like limiting the vacuum can, limiting the centrifugal advance, different weights and springs, etc. If he is amenable, I can recommend some part numbers, etc.

My 454 liked 10 degrees advance at idle (tested with vac disconnected and hose plugged). 32-34 degrees total all in at 2800 RPM. My vacuum can was limited to 10 or 12 degrees, giving ~44 degrees total at a light throttle cruise. I connected my dist vacuum to full manifold vacuum rather than ported vacuum. I would not recommend full manifold vacuum unless the diaphragm travel has been limited. There is disagreement on that, but direct vacuum has served me well.

My current (more detailed) notes are for my 502, so they would be of little help.

Bill, that is a great answer and explanation of how to set the timing for peak performance and mileage. Shows there is more to it than looking at a chart and aiming a timing gun.

Nick

One other thing to check is the bushing wobble on the distributor. Try shaking the rotor, if there is any movement two problems arise. First idle will get erratic at low rpm, 550 or so. Second you may need to advance just to keep a decent idle.
Problem is finding anyone that remembers how to replace bushings. Used to be done in the back room of the local parts store. Now you just get "that look"