Timing problem
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Timing problem
I've read just about everything on this forum concerning SBC timing. Still, I have a real head scratcher. My 1980 with a 290 HP crate motor and RV cam is giving me fits. I set the total advance at 36 deg at 2500 RPM, with the vac line disconnected and plugged. Then when I drop back to idle, the advance is almost 30 deg.!
My understanding was that the mechanical advance gave 20 deg from idle to WOT, or thereabout. If I set initial timing at 16, I can't get to 36 at 2500. Any ideas? I've tried a variety of springs and weights. Real light springs seem to bring in the full centrifugal advance at idle (750 RPM).
When I woke the car up this spring, I found the mechanical advance stuck. I pulled the dizzy and loosened it up. Could something have been messed up then?
My understanding was that the mechanical advance gave 20 deg from idle to WOT, or thereabout. If I set initial timing at 16, I can't get to 36 at 2500. Any ideas? I've tried a variety of springs and weights. Real light springs seem to bring in the full centrifugal advance at idle (750 RPM).
When I woke the car up this spring, I found the mechanical advance stuck. I pulled the dizzy and loosened it up. Could something have been messed up then?
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
yes your still not getting full mechanical advance. you need to see if the mechanical advance is actuating fully. it sounds like it is not and it also sounds like your advance springs are not returning at idle. if you set it at 12 for example it should stay at 12.
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Problem found
Having spent a day away from the problem, I went back to the dizzy armed with fresh forum advice. One of our members, Lars I believe, mentioned the wires to the 4 pin module. That was the problem! I had taken my Mallory 6AL box off-line and replaced it with the original GM module. In the process I switched the pick-up coil wires. The spark was retarded, and mechanical advance was almost exactly making up for retarded spark so it looked like no advance was occurring as RPM increased. I asked how I could have been so dumb, and my wife gave me about 50 other examples.
So here's what I have now:
2800 RPM, vacuum adv. plugged: 37 deg. BTDC.
At idle (650 RPM): 22 deg. BTDC.
With vac adv. connected at 2800 RPM: 50 deg. BTDC.
At idle with all advance connected: 34 deg. BTDC
The initial timing concerns me a little. I have and RV cam in the motor and the idle rolls a little but not too bad. I believe at 2800 the mechanical is all-in.
So here's what I have now:
2800 RPM, vacuum adv. plugged: 37 deg. BTDC.
At idle (650 RPM): 22 deg. BTDC.
With vac adv. connected at 2800 RPM: 50 deg. BTDC.
At idle with all advance connected: 34 deg. BTDC
The initial timing concerns me a little. I have and RV cam in the motor and the idle rolls a little but not too bad. I believe at 2800 the mechanical is all-in.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
You provide the mechanical advance at 2800 rpm. What happens when you go to 3000? It should not increase. You want the FULL (max limit) mechanical advance at 2800 (or somewhere between 2600-3000 rpm). If you have not reached max mech advance by 2800 [or so], you need to put stronger springs on the weights.
Is it possible that the mech. mechanism is sticking?
Is it possible that the mech. mechanism is sticking?
Racer
Joined: Jan 2010
Posts: 348
Likes: 2
From: Lowell MA
7T1Vete, Thats very interesting. When I set my total timing with springs off and Vac disconnected to 36* at 3000 rpm and then reinstalled the factory springs, I had to install lighter springs to get it back down so all in by 3000 rpm. I must have a reverse distributor.
Last edited by Lemans Blue 69; Jul 20, 2010 at 05:28 PM.
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