I have been working on a friend's car and I am having trouble getting the timing correct.

With the vacuum advance unhooked, I set the initial timing at 14 degrees. After starting the car a couple of times the initial timing was now at 12 degrees (i am assuming there is play in the dist). With the vacuum hooked up the initial timing was at 33 degrees! Total timing was at 45 degrees at 3500 RPM! Way too high!

What could be causing the timing to go from 12 to 35 degrees with the vacuum advance hooked up at idle? I was under the assumption that the timming would only got up 10 degrees or so.

This is a points type non-HEI dist on a 70 camaro

 

i have the same problem but in a corvette. my dad took the gun to it and it read out like 40+ degrees at like 2000 rpm...yikes
could it be that the distributor is in wrong? it just started so im wondering if the distibutor is the problem and hopefully not the timing chain. it also wont run below about 2000 rpm either

 

Cory......... what port is the line from the distributor hooked into? (and even at idle it shouldnt rise much at all)............ or better yet... At idle if you hook the distributor line into a vac gauge what kind of reading are you getting? (and lets say idle is 650-800max)

 

sounds correct.
'70 Camaro's used a vacuum advance that pulled in a lot of advance. The replacements are a VC1605 which would be stamped "B9" on it. That particular vacuum can pulls around 20.6* of advance into the timing.
you said the car went from 12* to 35* so that would be 23* of advance from the vacuum can but originally you said the initial was 14*.
35* - 14* initial = 21* which is pretty much exactly what the VC1605 vacuum can adds in so it sounds correct to me.

Now, if your timing is flucuating like that, did you check the dwell? If the dwell flucuates as the rpms increase than the timing itself will vary. Check dewll to be around 28-30 and it should be nice and steady and not vary as you increase revs on the motor. If it does, it's time to rebuild and shim up tolerances in the distributor.

Now, if your total timing w/o vacuum advance is at 45* than you are way too high and you need to back that down so it doesn't exceed 36* w/o vacuum advance.

 

I just starting learning about vacuum advance and timing the last few days.

See Lar's excellent writeup on vacuum advance units:
http://69.253.166.197/page1/page65/f...Adv%20Spec.pdf

I think I learned more about timing reading this pdf than any others.

What is the vacuum at idle? Is the vacuum advance connected to the intake manifold or a vacuum port on the base of the carb? Depending on the amount of vacuum at idle, and which vacuum canister you have-- you could already be getting close to full vacuum advance at idle.

There is no set rule-- you have to see what canister you have and what your vacuum is at idle to see what advance you can expect at idle. If you move the vacuum supply to a ported vacuum on the carb-- that will minimize the vacuum advance at idle and possibly resolve your problem. Although many on this forum will inform you that your engine will run cooler and better when properly setup with manifold vacuum advance.

If it is a Chevy small block engine-- shoot for 36* total advance with the vacuum advance disconnected and plugged off at the hose-- with full mechanical advance coming in around 2500 to 3000 rpm. With the vacuum advance connected at the same rpm-- make sure it is no more than 52* total advance (i.e. make sure your vacuum canister doesn't add more than 16* advance as measured at the crankshaft).

I'm sure others have more details to add-- this is what I've learned from others on this forum.

 

Well crap-- I was on the phone before I sent it, and now I see BarryK already beat me to the punch. And I was so excited to show off what he has taught me!

 

actually *I* didn't teach you anything and believe me, I'm still in the "learning mode" myself and have a LONG way to go. When I post on timing issues I'm really just "passing along" the information that is easily available from experts like JohnZ and Lars.

 

This week I've learned the basics of the ignition and vacuum advance system and also a pretty thorough understanding of the headlight/wiper vacuum system. Can't wait to see what I'll learn next week!

 

want to know how to learn a lot...........??
do it the same way I did and how I learned a couple things about C3's.
1. Buy a C3
2. 3 days later pull the motor out - especially when you have never pulled the motor out of a car before like me
3. remove all parts off the motor down to the short block, glass bead and repaint all of them.
4. strip the frame & suspension parts and repaint them
5. clean and repaint the engine bay.
6. reassemble the motor and install back into the car
7. PRAY the car runs again. I was lucky - it ran perfect when I was done and when I bought it it wouldn't run at all and had to be flatbedded to my house.

Now, THAT'S how to learn about cars - know NOTHING when you start and learn as you go with the help of the forum.

i'm always earger to learn more new stuff anymore. I discovers I actually enjoy working on these cars and learning how things work.

 

That's my attitude. I actually enjoy working on the car and fixing things as much or more than driving it. I intentionally bought a car that needed some work-- because then I'm learning every step of the way. I haven't broke apart the engine yet (besides removing and cleaning out the intake). But I'm starting to think what I'll do next winter. I think I'll start with running a new emergency brake cable and try to get that working. I can see this turning into a "while I'm at it...offset trailing arms...new hubs..new rear fiberglass monoleaf spring...new rear bushings..."

 

I will check the vacuum from the dist and i will also check to see if the dwell changes with rpm.

The timing does go from 14* to 12* at idle.... sounds like a bad dist

 

Make sure that there is a stop bushing on the distributor shaft. If the bushing is missing you will get too much advance at idle (assuming manifold vacuum to advance unit).




Jay