I have been searching for a miss in the car for two weeks now. First logical place to start was plugs, so in went new AC R45TS gaped at .05" (because of the MSD 6A Box). Still had a miss!!! So new wires, spiral core Accel, cap and rotor. Still had a miss!!! Also I noted my timing was erratic when trying to time. Sounds like it may be the magnetic pickup in the distributor not sending a good signal to the MSD box. Put in a new magnetic pickup. Still had a miss and erratic timing. Well it has to be the MSD box. Replace the MSD box. Still had a miss!!! Well it has to be the distributor (only thing left). Put in new Accel distributor with everything new. Wala no miss (thank god)!!! Now for the problem I have with everything new. The timing is rock solid without the vacuum advance hooked up at 14 degrees initial timing just like suggests from the cam manf. When I hook up the vacuum advance to manifold vacuum my timing is still erratic with everything new. I checked the vacuum to the vacuum advance with a gage and it is rock solid at 15 in. Hg vacuum at idle. Hook it back up and timing is erratic at about 32 degrees (14 initial degrees and 18 degrees vacuum advance). Sorry to be so long winded but this one really had me stumped I can't replace anything else, everything is new! All voltages to the MSD are a solid +12 volts and no drop out while running and the grounds are all good. I just don't understand the erratic condition I'm getting with the vacuum advance hooked up and everything new. The car seems to run great without the vacuum hooked up. It has been suggested to change the strings that came with the distributor (to the black springs) to set the mechanical advance to start at 600 and total out at 4000 rpm and run without any vacuum advance. Has anyone tried this before? I'm stumped, any suggestions, recomendation or things to check would be appreciated, I have exhausted all my avenues and at a dead end.

Thanks,
YBnormal...drive a vette

 

Others might have a better idea than me. My first thought is the vacuum advance on your distributor. My second thought is that you've got the vacuum line connected to the wrong manifold port. Vacuum to the dist should be from the carb and not straight to the manifold.

 

Thanks for the suggestion, it is hooked to the carb. Edlebrock 1406. It is hooked to the manifold port on the carb and not the ported vacuum port. When hooked to the ported vacuum port on the carb it runs good until accelerated then the timing gets erratic.

 

I'll be interested in seeing what you've found for the solution. I purchased the 1405 and will be going through this in a few days. I was thinking the same thing re hooking to the wrong port on the carb until you mentioned this was checked. By the way I looked at you profile to see which year your vette is and noticed today is your birthday. Happy Birthday and good luck with the timing adjustments.

 

Thanks Cruzer, will let everyone know if I ever figure this one out!!!

YBnormal...drive a vette

 

YBnormal-

What kind of timing light are you using? I got a bit of a surprise after I put in an MSD box when I went to set the timing and found out that my dial-back timing light doesn't work with an MSD box. The timing mark jumps around like crazy because the multiple pulses throw the light off. I don't know why it does it even at 3000 rpm (since I thought the MSD gave off multiple spark only at low RPM) but it does. A regular old-school non dial-back timing light cures the jumping around problem. If you're already using one, then someone else will hopefully have an answer for you.


-the Bruce

 

Simple, just don't use the vacuum advance.

Or, use ported since it at least idles good there....ignore any readings you get by opening the carb without a load on the engine. Take it for a drive and see how it feels.

Or, use manifold, but you will have to lower your idle timing after you connect your vacuum up cause 32º of advance is waaayyyy too much at idle.

 

 

All in at 4000 is way to late. I set mt Big block t6o have all 37 degrees in my 2500.

 

I can't help you with the miss but I know exactly what your problem is with the erratic timming -
You have your advance can connected to the manifold vacuum but my guess is you are pulling enough vacuum at idle to pull in timing advance at idle. However their is not enough vacuum to max out the advance can, so your timing is bouncing back and forth.

The solution - connect to ported or get a vacuum advance can that brings in all your timing before your idle vacuum at the manifold port.

 

Spyder is right. You need a different vacuum advance can. Adjustable would be good but you need to find out what fits that Accell distributor.

What you want is a can that will pull ALL of the vacuum advance in at a vacuum signal that is about 2" less than your actual idle vacuum. That way all the vacuum advance is fully "in" at idle. If the can is not fully "in" at idle, the timing will move around and the idle speed will bounce around with it causing further variation in the idle vacuum and further variation in the timing and so on and so on and so on....

-Mark.

 

Well somehow I got it fixed by adjusting the accel adjustable vacuum advance. I forgot to mention that both the old and new distributors had the accel adjustable vacuum advances on them. I put the old stock dist. on the bench and started using a vacuum source on it. It would pull in stop to stop at 9-10in Hg with the screw all the way clockwise, all the way counterclockwise it would move off the stop at all with 25in Hg. So I turned the one on the car all the way clockwise and tried it again. I had 14 initial and with vacuum advance getting full travel >40 degrees initial + manifold vacuum at the carb. Then rotated the screw in the vacuum advance clockwise until it was down to 32 degrees (initial + vacuum) and it was steady. I have no idea why the change but it must have something to do with the vacuum advance. Both vacuum advances have the same numbers on them (AS-20), so I would take they both have the same characteristics and both will pull in at 9-10 in Hg (I have 15 in Hg at idle). I'm going to check it again today with another timing light and see what happens (fingers crossed). The final test will be on the road but won't be able to drive it for a while. Thanks for all the suggestions. I'm going out to check it again to make sure there are no gremlins changing thinks on me and causing it to be erratic again.

YBnormal...drive a vette