Timing question

I was thinking the timing was somewhat off on my recent purchase, its a '76 1500cc with a weber/pierce manifold, with pacesetter header, resonator, no muffler. The dissy is a 45de4

I noticed last week tha the PO had plugged the vacuum at the carb port, so I want to get it set right with the advance.

Here is what I did:

1) read the current setting of 26 deg BTDC w/o vacuum line

2) reset to 10 deg BTDC, at idle,(900) w/o vac

3) read 15 deg BTDC @ 3000 rpm so the centrifgul seems to work

4) reattached vacuum line, and heres where it gets strange, as I was expecting to see total advance to go to somewhere in the 20-22 deg range, it went down to around 2 deg BTDC.

WHAT MIGHT THIS MEAN? Any Ideas, guys?

thanks for the help

Sam

Sam, are you saying you lost 2 degrees with vacuum advnce kicking in, or are you saying it drop to 2 BTDC. maybe something is up with the vacuum advnce unit on the distributor, this would be a damn good question for Jeff @ Advanced Distributor. Also when this is all sorted out, I'd shoot for a total advance with your car of atleast 32 BTDC. Let me see if I can't steer Jeff this way

Is there any chance you're reading the timing marks backwards?

You can test the vac unit by suckingon the hose and watching the breaker plate rotate with the distributor cap removed. Once it advances, you should be able to put your tongue over the end of the hose and it shoudl stay put, not rotate back to where it came from (hold vacuum with zero leaks).

You should also see roughly 20 degrees of advance between idle and 3000 rpms (not including vacuum advance). If you get much less than that, the advance mechanism in the distributor likely isn't working properly.

Your step #3 tells me you have a problem with the mechanical/centrifugal advance.

The vac unit will have numbers on it (very hard to find and read) that are somethign like 3-10-12. The last number gets doubled to tell you how much advacne it offers. 12 would mean 24 degrees of crank advance. My guess is that yours is damaged, allowing for a vacuum leak, a drop in rpms, and a loss of timing. Either that or its attached to ported vacuum so there is no vacuum at idle I think its time for a rebuild.

Are the various specs for distributors charted out somewhere? In Vizard perhaps? I wonder what is expected for my 25D on a 1098.
8 initial advance, then what the vac advance should give and then what to expect from mechanical and over all total. Since I have a timing light with advance knob, I I should be able to check it at various point of the rpm range. Sorry to hijack!

As I recall from my spitfire days, this distributer is a vacuum retard unit. So it is functioning properly but perhaps ported wrong.

Jim

That's retarded!

Dave

Your right, it is retarded (or to be PC, performance challenged). Apparently its doing exactly what they wanted it to do in the smog pump days, which was to kill rapid acceleration by forcing incremental acceleration. It seems like you could plug the retarding port on the vacuum unit, and drill a hole on the advance side, JB weld an 1/8" tube over it, and change this thing to make it advance instead of retard, anyone done anything like this?

Why bother? I have new units for cheap. The internal design of the retard-only units prevents them from working as an advance anyway. The spring is on the wrong side of the diaphragm, so it would always be pushed into the full advance position.

Just kidding:drinking:

Hey, since I rebuild these things, all of those options have crossed my mind at one time or another. Unfortunately the retard units are a really bad band-aid and they're not worth using. The good news is that they're pretty easy to replace with something that will actually work and provide decent performance, for very little investment.

I was thinking about melting down a bunch of old vac retard units to make a nice alloy Lucas belt buckle, but I don't want my pants to fall down... :D