My car is an '85 12a with an MSD direct fire.

Having a bit of free time today, I decided to play with my timing a little. I used a colored pencil to make a quick 20 degree BTDC mark.

With the light on a leading wire at idle it was right at TDC. I tried pulling the throttle to total advance. Here's what gets me: it went well past the 20 degree mark; I'd eyeball it at 25 at least, 30 at most. I wasn't sure of the RPM (I really need a helper) but I think it could have gone even further.

I decided that this had to have something to do with my odd vacuum advance setup (remember that thread?), so I disconnected it and got about the same result. So, what the heck? How far should the total advance be?

The leading/trailing difference was about 20 degrees at idle and at max advance. So what the heck? I've never heard or seen any detonation. Do you think I would if I decreased the split between leading and trailing? What should I do?

 

25º of mechanical advance sounds normal. Maybe your odd vacuum thing isn't really working? You could decrease the split down to 8º according to Yaw. I have my four port 13Bs set to 10º split.

You thing about the MSD is the amount of built-in compensation compared to a J-109. As RPM rises, the amount of split will vary. It's not very much, but I remember seeing a difference on my marked pulley.

 

Hmm, well, maybe not. I haven't a clue how high the engine is revving as I don't have an external tach. Or any decent friends apparently. I do have a rev limiter on the MSD (8k rpm) but I don't feel like testing it. I just don't feel confident pulling on the throttle cable. The total advance should be more with the vacuum advance hooked up, though, right?

Do you know how much the vacuum advance is supposed to do at any specific RPM so I can test it? I know that the vacuum advance setup from the tutorial does not work. If I can't figure this out I'm going to start re-adding the rats nest until I get some vacuum advance. But that'll have to wait as all those parts are at my folks' house.

 

I treat vacuum advance like 7UP; never had, never will.

 

So the total advance should be higher with the vacuum advance hooked up? I'll have to try it later when I can get someone to help.

 

By the way, so when folks say that they're advancing the timing, it's the trailing timing that they're talking about?

 

"advancing the timing" usually means rotating the dizzy. Adjusting the split means leaving leading alone and moving the vacuum thing in or out to adjust trailing.

At high RPM, there is less vacuum affecting the little hoses and stuff in the rats nest. Therefore the vacuum advance is less, but mechanical advance starts to take over.

 

Okay, I looked at the shop manual and here's what I've found.

The vacuum advance specs are only given for vacuum.

At 25 cm/Hg, it should be +4.5 degrees on leading, and about 5 degrees on trailing.

At ~36.5 cm/Hg, it should be +4.5 degrees leading, +15 degrees trailing.

I'm going to go check and see if the vacuum from the nipple that the purge valve is normally hooked up to ever gets that high.

 

Ok I've done a quick test and here's what I've found.

It gets to 50 cm/Hg real fast, and never falls off, at least, not up until total advance is achieved (I still don't know what the RPMs are). It also falls off faster than anything (it seems to be controlled by the position of the throttle).

Also, in order to get 10 degrees of seperation, I had to retard the trailing timing by almost that amount. It looks like, to get 26/16 or whatever yaw recommends, I'd have to retard my overall timing too?

 

Honestly at this point you're now out of my league. I could venture a guess but I wouldn't want you to crack the porcelain of your trailing plugs from too much advance (it's happend using Yaw's recommended timing settings). I try to keep my total advance lower than 24-25º on all my 13Bs.

 

Sounds like you really need some one to just hold the rpms @ 3k for you while you reset both your leading and trailing timming.

If you really cant find anyone at all take off the panel where the choke is, unplug the connector going to the choke, now yank it out until you hit 3k and it should stay out, if you dont hit 3k try adjust the choke cable on the carb but remember to adjust it back when ur all done.
It only takes a second to adjust, like a brake cable on a bicycle.


good luck

 

Better yet, rev the engine and look at the tach. Try to memorize what it sounds like at 3k. Then rev it to what sounds like 3k and set your timing.

 

Vacuum advance is used to quickly advance your timing just off idle then falls off as the throttle is opened up. This is why the shop manual gives you vacuum reading when checking the vacuum advance. The mechanical advance is RPM depended and is slow. At 4000rpm's you total advance should be around 24-25deg.
If everything is working as new, idle timing is on the marks, as you rev the motor slowly to 4000rpm's your timing should jump to around 20-30deg of advance ( depending on rpm and vacuum numbers ) and go no more then 25 at 4000rpm's. If the vacuum is not hook up the timing is gradual to 25 deg at 4000rpm's. If I remember right.
chuck

 

Yes, I do really need someone to help. But I can't do anything today 'cause it's raining.

As an anecdote, once, a spark plug wire popped off one of my coils and was sitting about an inch away and the engine was still idling perfectly. The only thing out of place was a loud popping noise. Knock it all you like, but that MSD sure is hot.

 

BTW, thanks everybody. I'll let you guys know how it works out.