Timing?
Thread Starter
Drifting
Joined: Dec 2002
Posts: 1,946
Likes: 28
From: Long Beach CA
Timing?
I just finished installing a new intake manifold, rebuilding the carb, etc. on my car. I set the timing to 36 deg total advance, which comes out to 10 deg initial. Unfortunately, I didn't map the vacuum advance, so I don't know how much more that's adding to the equation. The car starts doing some bucking starting about 2,800-3,000 rpm, so I figure it's too far advanced. What sort of timing specs are people running, especially you California folk who have to cope with that crap they pass off as gasoline?
Le Mans Master
Joined: Jan 2002
Posts: 8,913
Likes: 283
Re: Timing? (LB66383)
Sounds like you are on the right track.
I am not sure the specs on you car (hi comp or not) so detonation is a function of gas available and your set-up.
I would start with 8-10 degrees initial.
What is you vacuum reading?
You may need to get a dew-diffearnt vac can. You should get a can that is fully deployed by 2-3 inches below your idle vacuum reading. So if your car idles at 12", you want a can that would be fully deployed by 9"-10". This will help with idle stability.
The second thing, the mechanical advance should be all in by 2300 RPM. YOu will need a timing light to check this versus RPM.
Mark
I am not sure the specs on you car (hi comp or not) so detonation is a function of gas available and your set-up.
I would start with 8-10 degrees initial.
What is you vacuum reading?
You may need to get a dew-diffearnt vac can. You should get a can that is fully deployed by 2-3 inches below your idle vacuum reading. So if your car idles at 12", you want a can that would be fully deployed by 9"-10". This will help with idle stability.
The second thing, the mechanical advance should be all in by 2300 RPM. YOu will need a timing light to check this versus RPM.
Mark
Tech Contributor
Joined: Aug 1999
Posts: 14,366
Likes: 6,348
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Timing? (LB66383)
LB66
You're doing the right thing with your timing setup by getting it up to 36 degrees on the total mechanical/centrifugal advance. However, many of the stock vacuum advance control units will add too much vac advance on top of this, giving you the jerking you're experiencing. You need a vac unit that pulls in a maximum of 16 degrees. Try installing NAPA part number VC680 for a mild-cammed engine. Use part number VC1810 if you have a bit of a cam. This will compliment your 36-degree total timing very nicely.
You're doing the right thing with your timing setup by getting it up to 36 degrees on the total mechanical/centrifugal advance. However, many of the stock vacuum advance control units will add too much vac advance on top of this, giving you the jerking you're experiencing. You need a vac unit that pulls in a maximum of 16 degrees. Try installing NAPA part number VC680 for a mild-cammed engine. Use part number VC1810 if you have a bit of a cam. This will compliment your 36-degree total timing very nicely.
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,264
Re: Timing? (lars)
Ditto what lars said. If you are working with a seventies vintage distributor, it was set up for emissions, and it's typical for these calibrations to have lazy centrifugal curves without much total and somewhere in the area of 24 degrees of vacuum advance.
If you are building or retuning an engine, the distributor calibrations from the mid-sixties are the place to start. The VC 680 is suitable for a 300 HP or equivalent cam, and the VC1810 for a OEM SHP cam or aftermarket high performance cam.
If your vacuum can doesn't have more than about 16 degrees the bucking could be an excessively lean mixture.
Duke
[Modified by SWCDuke, 3:26 PM 1/10/2003]
If you are building or retuning an engine, the distributor calibrations from the mid-sixties are the place to start. The VC 680 is suitable for a 300 HP or equivalent cam, and the VC1810 for a OEM SHP cam or aftermarket high performance cam.
If your vacuum can doesn't have more than about 16 degrees the bucking could be an excessively lean mixture.
Duke
[Modified by SWCDuke, 3:26 PM 1/10/2003]
Le Mans Master
Joined: Feb 1999
Posts: 5,824
Likes: 369
From: Mobile AL
Re: Timing? (LB66383)
I would check the location of the timing mark on the harmonic balancer in relation to the timing tab before playing with anything else.
At least half that I check are off by more than 4 degrees. Many of these are off by 8 - 10 degrees. My record so far is about 30 degrees off.
This isn't due to poor GM manufacturing tolerances. I don't think it's due to the proverbial "outer ring slippage" either. It is more likely caused by mis-matched components from different year models or aftermarket suppliers.
At least half that I check are off by more than 4 degrees. Many of these are off by 8 - 10 degrees. My record so far is about 30 degrees off.
This isn't due to poor GM manufacturing tolerances. I don't think it's due to the proverbial "outer ring slippage" either. It is more likely caused by mis-matched components from different year models or aftermarket suppliers.
Safety Car
Joined: Feb 1999
Posts: 3,631
Likes: 14
From: Fountain Hills AZ
Re: Timing? (LB66383)
I just finished installing a new intake manifold, rebuilding the carb, etc. on my car. I set the timing to 36 deg total advance, which comes out to 10 deg initial. Unfortunately, I didn't map the vacuum advance, so I don't know how much more that's adding to the equation. The car starts doing some bucking starting about 2,800-3,000 rpm, so I figure it's too far advanced. What sort of timing specs are people running, especially you California folk who have to cope with that crap they pass off as gasoline?
Thread Starter
Drifting
Joined: Dec 2002
Posts: 1,946
Likes: 28
From: Long Beach CA
Re: Timing? (LB66383)
Thanks for all your input. I'm working with a recurved factory tach-drive distributor with a Crane electronic ignition and Crane adjustable vacuum advance. I'll have to play with it this weekend to see how much advance the vacuum unit throws in. It's adjustable for advance rate, but not for total advance. I'm getting my timing information off of a Fluiddamper vibration damper, but you're right, I haven't indexed the timing marks to make sure TDC is really TDC.
