Motor is Non-CC Q-Jet 383. Heads are World S/R Torquers giving ~9.2:1. Weiand Action Plus manifold. Comp Cams CS 260HR-11 Grind (210/214 .474/.474) which is small I know. No EGR, Very Low stall Converter (1200) and an automatic trans.

I have a stock replacment summit HEI but It seems to only give 11 degrees of mechanical advance. I put a new moroso weight kit on it and tried all kinds of springs but it never gives more than 11 degrees. I think I read somewhere that there are limit slots that could be elongated but I havent tried to look for them just yet.

Anyway I would like some reccomendations for how much and how quickly I should bring in my mechanical curve. I plan on getting a standalone knock detector from MSD but would like to save time by getting some good advice here

Thanks for any help.

 

There is a pin between the upper lower part of the advance mechanism. You can see it with the rotor off. Often has a bushing on it. If it has a bushing, try removing it and seeing how much advance you get. Easiest done with the distributor removed.

 

Ok, The distributor is out right now and I am looking at it and I think I see the two posts you are talking about that pass through the base plate. Only thing is Im not sure how I would get the bushings off. They look pressed on there and Im no sure how to take the shaft apart either. Hmm I think I am in over my head here but I guess thats true most of the time.

 

To take it apart, drive out the roll pin in the drive gear and remove from the shaft. The shaft will then pull out of the housing and the lower part will slide off of the shaft.

 

ahh ok. so then I should be able to take the bushings off with needle nose pliers or something? I dont want to damage the pins as they look pressed in.

 

yeah... those bad boys are not coming off at all. Would it be acceptable to elongate the slots they fit into?

 

Just out of curiosity.. How much end play should the shaft have in the distributor? when its assembled I can stick about .045" worth of feeler gauges between the gear and the shim thats in there between the stop and the gear. Is that within spec?

 

.045" is fairly normal. You'd want to shim that down to about .010" or so, though.

 

very cool -- thanks. Any idea where I would get the shims? Summit hopefully?

 

Summit, or anywhere else. Moroso 26140 or Mr. Gasket 2822, or similar.

 

Awesome, thanks. Now all I have to do is figure out the mechanical advance limit thing and Ill be good.

 

TTT --- Can I elongate the mechanical advance limit slots or what?

 

You could if you're sure they're the reason you're not getting enough advance.

 

Ok I see. I will measure the max advance with the vacuum can attached. I am assuming I can do this with a protractor or something similar.

Thanks again for the help.

 

Not sure what vacuum advance has to do with it, but the amount the plate moves is 1/2 of the advance you'll get at the crank.

 

the vac advance moves the pickup thereby taking up some of the total advance that the slot and pins will allow. So if I figure the amount of vac advance the can is providing in terms of the amount of slot used, then the rest is available for the mechanical... I think. In any case I think the Dissy should be capable of in excess of 55 degrees total advance. So then I will be looking for 110 degrees + in terms of slot arc angle.

 

The vacuum and centrifugal advance are independent of each other. The centrifugal advance moves the magnet, the vacuum advance moves the reluctor.

You've got it backwards... 55 degrees of advance at the crank would be 27.5 degrees of advance at the distributor. You'll find probably that the total timing available is actually quite a bit less.

 

Just to clarify... This is the centrifugal advance mechanism removed from the distributor, minus the weights and springs. The toothed wheel can be advanced or retarded relative to the position of the shaft, limited by the pins through the slots in the plate on top of the shaft.




Now here's the vacuum advance mechanism. The plunger on the vacuum can pulls the lever attached to the reluctor which causes it to rotate slightly, allowing it to be advanced or retarded relative to the base of the distributor.



Either advance mechanism can be tuned or modified without affecting the other.

 

Oops. You are right. So I guess I am looking for ~40 degrees of slot lenght then huh? And right now I am guessing its more like 22.

<Sigh> just thinking about the slot lenght in my head though I can already tell that the slots would not be limiting. They seemed to allow the rotor to turn well over 45 degrees relative to the base. So I guess I am back to square one except I have eliminated the limit slots as the cause of my low mech advance.

Trying a different theory, it seems like the weights (both stock and replacement Moroso) have very long arms that contact the center rotor. seems like they cant turn the rotor very much before the contact point between the weights and rotor is in line with the shaft. If I shortened the levers on the weights would that help the weights move the rotor more by moving their contact point with the rotor farther from the shaft?

If you have no idea what Im talking about its ok. I am working on a picture.

 

In some cases, the configuration of weights will limit the centrifugal advance.

You'd be looking for about 10 degrees of rotation in the distributor, to give you 20 degrees of variability in the advance at the crankshaft.

Working on stock distributors by trial and error can be a pain in the ***, especially when they only way you've got to check the advance is to install it in the car and check it with a timing light.

Maybe have a look around some of the older shops in your area to see if anyone has a distributor machine in the back gathering dust. Sun was probably the most common/most popular, but there were others.

 

WHOA! what a monster. I guess I could ask my machine shop.

So back to the original question -- what should I be looking for for an advance curve? If I do manage to get 20 degrees out of the weights, I also have an adjustable vac can (both limit and rate) and an assortment of springs.

 

Typically you'd want the centrifugal advance rate set up so that all the advance is in before 3000 RPM. You can do a little experimentation to see if your car will tolerate a quicker advance curve without pinging at WOT. The actual amount of centrifugal advance isn't quite as important, you can just install it with whatever base timing you need to give you about 34 degrees (give or take a degree or two) of base+full centrifugal advance.

The vacuum advance is more of a part-throttle tuning thing, you can vary the amount and rate of advance so that you're not getting pinging under part-throttle acceleration. Something around 10 or 12 degrees of advance fully in by about 10 inHg might work.