mark79,80

I have GM Distributor no. 1104067 in a 1980 L82. The distributor came
with a number of GM crate engines and is considered curved for
performance engines.
Problem: engine will knock if accelerator is floored.
Regular gas with up to 10% ethanol is being used.
The base timing is set at 12 degrees @ 800 rpm. I have read Lar's distributor
paper and it indicates that vacume will drop when first accelerating
so I believe the knock is being caused by too much mechanical
advance.
What springs and or weights should be used to reduce the
mechanical advance?
I used an advance timing light and measured the following:
Mechanical advance:
1600 rpm = 12 degrees
2400 rpm = 16 degrees
Vacume advance:
3.0" = 0 degrees
7.5" = 20 degrees
I could lower the base timing, but the car runs extremely well
so am reluctant to do that.

cardo0

So is your mechanical advance measured with the vac line disconnected and plugged on the intake side? I read u have the same 12* mechanical at idle 800 rpm and at 1600 rpm (vacuum advance would only add advance if connected). Well that's not much of a curve - more like a flat constant advance.
I think u need to sort things out and get an increasing amount of mech advance from idle to 32* or more at 3000 rpm - this is with the vac can disconnected.
That vac can is for a very hot cam with 20* at only 7.5"Hg. Mine starts to move at 5-7"Hg (WOT) and is like full advance in at 15"Hg (idle). So i read your vac can could be adding too much advance at WOT if u still have over 3"Hg vac at WOT.

My recco is get dist weights and springs for a real timing curve and get a new vac can. U have to research, sort out, and choose your own vac can for your application.

Hope this helps ya,
cardo0

myk7

Sounds like too much vac advance to me.. is yours adjustable? Disconnect the vac advance and see if it still pings. I like my vac advance to pull 14-16 degrees.

scottyp99

OK, first of all, at what rpm does the mechanical advance stop advancing, and how many degrees does it stop at? It kinda sounds to me like you should take Lars' timing paper and read it a few more times. It's not rocket surgery, I had no trouble following Lars' instructions, and I'm not even all that bright.

Let's make sure we are on the same sheet of music. Do you have in your possession a paper called "How to Set Timing" by Lars Grimsrud?


Keep the shiny side up!
Scott

Tim H

I would get a Mr. Gasket advance spring kit and use your factory weights and change 1 spring out for a heavier one, if that dont do it change the other out for a heavier 1.
Without knowing 20 different details about your car thats about as much as I can help right now.

Garys 68

If you want to maintain your initial timing and decrease total, you'll want to change the bushings.

Mike Ward

'Regular' as in 87 octane? What compression ratio do you have?

drwet

You didn't mention what you have for an engine, so I'm going to assume its mostly stock. If that's the case your problem isn't so much ignition as carburetion. The factory Q-jet was very lean. There are a number of mods that were part of a service bulletin that were required to get these things to run right. Read Lars' paper on setting up ignition and do what he says. You should have about 12 deg. initial and 36 deg. total mechanical advance all in by about 3000 rpm. Your vacuum advance should add another 14-16 degrees. (Guys will debate the exact numbers, but these are close.) Then you're going to have to address your carburetion issues. And try some premium fuel. It won't solve your problem by itself, but it will help. The crap they sell us as gas these days is disgusting. Ethanol is great for drinking, but its not good for cars!

scottyp99

Mmmmmmm...........ethanol..........:smil iedrool:


Keep the shiny side up!
Scott

7t9l82

never ever take a " performance " distributor and just install it they will not perform. get Lars paper and have at it. you have too much vacuum canister, i think.

Jeff_Keryk

I would invest $5 on a timing tape; then measure and plot your curve. Shoot for 34 to 36* all in by 2800 to 3000 RPM. Springs and weights change the curve; they do not lower it unless they over restrict total timing. I would disconnect and plug vacuum advance and test run car for pinging. If it pings under load back off 2*, or perhaps slow curve somewhat with 1 heavier spring. Once mechanical is dialed in, add back in vacuum advance. And most of all, have fun with it. Just my 2 cents...

MotorHead

If you have a ping then try a higher grade gas. Never did like vaccuum cans they can create this problem too, take the line off and plug it at the carb.

69427

scottyp99

Hmmmmm.........an HEI in good shape should allow at least 20 degrees of centrifugal advance, and a good curve will let that come all the way in by 2800-3000 rpm. Your timing at idle is 12, and by 2400 rpm, it's only advanced 4 degrees. The curve on that distributor sucks. it needs a re-curve kit. Costs less than 20 bucks.

It pings when you floor it. The engine produces virtually no manifold vacuum when you floor it. The vacuum advance advances the timing when the vacuum is high. I doubt the OP's problem has anything to do with vacuum advance. Cars with really big cams that don't generate a lot of vacuum at idle can have some weird problems, but a normal street engine should be fine. If his vacuum can is pulling 20 degrees, and his centrifugal advance is pulling 20 degrees, and his initial timing is 12, then his timing at full centrifugal advance will be 32, and another 20 on top of that from the vac can, for a total of 52 at cruise, should not be a problem. A vac can that only pulls 16 will allow centrifugal advance to be advanced up to 36. It's all pretty simple math, I can't understand how some people manage to make it complicated. Just read Lars' timing paper and follow the simple instruction. here is a link to thread that does awesome job of explaining the whole deal.

http://forums.corvetteforum.com/c3-t...long-post.html

Keep the shiny side up!
Scott

Tim H

I did, I got the package in the mail, opened it dropped it in the engine, timed it and I was done.
Ebay $54 HEI distributar.

mark79,80

Thanks for everyones replies.
When I measured the mechanical advance I did have the vacume
line to the canister plugged.
The carburetor is the stock q jet. Engine is all stock.
I do have both of Lar's papers and from reading them it seems like
the mechanical advance is the problem as the vacume should drop
very low when you floor the accelerator, hence the vacume canister
will not add more advance. I will go for a test drive with the
vacume canister line plugged to see if there is still pinging.
I do have several different sets of primary jets and rods and could
try to richen the carburetor.

69427

Have you put a WBO2 sensor on your exhaust to determine that your fueling is presently lean? If not, it seems wasteful to pour in extra fuel to mask what may be a simple timing curve issue.

mark79,80

I don't have an O2 sensor. Car is due for state inspection in May and will see if the shop has one.

63mako

A performance recurve for your particular build specs will make any engine run noticably stronger. You don't know what your missing until you do it.

63mako

Recurve the distributor first. Install new plugs. Read them. If you need to go richer you can tell by reading the plugs. There is also a paper in the links in the tread scotty posted to tune the carb but your ignition needs to be right first.