I have a 77 CJ5 w/304V8. Timing was very off when I purchased it. I was having difficulty getting it set and keeping it set. I eventually decided to check the timing chain and gears. I changed gears and chain(minor wear)I did find the distributor drive gear and dist. cam gear badly worn. I changed both gears and put in a new distributor. I verified TDC and adjusted to 8 degrees.(book calls for between 8-12 degrees). I removed the vacuum advance hose and pluged it and hooked up the timing light. It started and ran poorly, back firing into the carb. I then adjusted to about 10. Still ran poorly. I eventually got to around 15 degrees. It runs the best at this. When taching high, it seems to miss slightly and lightly backfire through the tailpipe. Beyond 15 degrees and it does this worse. Any suggestions?

Timing not 100% single choice
	Timing help
	Timing not 100%
Votes accepted starting: 08/20/08 05:00 AM
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is your mechanical advance working properly?

Does it run pretty good at two grand and higher? Since you SEEM to know how to time the thing and MOST LIKELY have the plug wires on the proper plugs AND you have a new dist... I've got a guess.
You indicated the cam gear and dist gear were worn visably.
I'm betting the lobes are also worn and your problem is the valve openings..
Have you put a vac meter on there and checked vacuume? With the meter installed change the timing to the highest vac and see what the timing is..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Thanks for the advice guys. I am still learning so this may so this may seem silly to you all, but if I have a vaccum advance, would I still have a mechanical advance? I have not put a vacuum gauge on the cylinders yet, I will give that a try. Might be a week or so, but thanks again guys.

Ok Ralfy I'll give you timing 101.. Timing is just THAT! TIME IS WHAT IT IS ALL ABOUT.. The TIME it takes for all (or most anyway) of the fuel to burn. Since the burn TIME is static and NEVER changes as the engine speed builds the TIME the fire is started needs to be earlier and earlier.. This in order to keep the burn while the exhaust valve is still closed.. You must understand that as the engine speed increases it takes LESS time for the piston to make the trip DOWN under the power of the burning fuel.. Now once that is fully understood it is a simple matter to get timing ADVANCE into ones noodle.

To start the timing at IDLE needs to be ..whatever the factory sez.. Most engines at idle take from 4* to 8* advance.. OF COURSE this advance depends entirely on the rpm at idle and the length of the stroke.. All of which contribute to the TIME of which the burning will take place.

NEXT as the engine needs more advance the MECHANICAL timing comes into play which makes the timing increase at certain rpm depending on which length of stroke that particular engine has.. THIS timing is static and usually shouldn't be messed with..

THEN we have the vacuum advance.. This advance depends on how wide open the throttle is.. IF the throttle is WIDE OPEN the spark will start the fire and the fire will JUMP OUT of the carburetor if this timing occurs.. SO this amount of timing is connected to the vacuum which drops off as the throttle is opened. As the vacuum increases the timing begins to advance with the increased rpm's. So at certain rpm's ALL the timing is working as the vac is sufficient to keep the timing increased.

The neatest thing for a newbie to do is to make some EXTRA marks on the damper and put a light on and then increase the engine speed and WATCH all the timing come into play.
As the engine speed is slowly increased the mechanical will slowly open and the newbie will be able to watch it happen.
Then put the vac hose on the dist and watch as the engine is quickly revved..the timing will DROP and then get back to full advance..
But one needs some extra chalk marks on the damper to see this well. I advise marks at 10* 15* 20* 25* and 30* I even paint my dampers with different colors of paint to make the timing more evident.

I hope this helps.. Try to remember the timing in question here is really TIME! The time it takes to burn the fuel.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

<img src="/forums/images/graemlins/confused.gif" alt="" /> Okay, Don't know 'bout RAL, but I haven't got my noodle around it. I get the mechanical advance. But if throttle is wide open, rpms go up, would have thought you'd need even more advance. Or is that what lower vac does? So if vac didn't lower, timing wouldn't be fast enough, valve wouldn't be closed, and it would backfire through the carb? Have I kinda sorta understood? <img src="/forums/images/graemlins/baby.gif" alt="" />

No worries man, the only silly question is the one not asked, if you donÆt know the answer. Vacuum and mechanical advance are 2 different, independent systems that both affect the over all timing. Vacuum advance will come into play when you have high vacuum, and is there to advance the timing when there is a low load on the engine (ie highway cruise) to improve fuel economy. Mechanical advance comes into play constantly while the engine is running to adjust the timing based on engine speed. At idle there should be no mechanical advance, and as the engine speed increases the counter weights pull out and the timing is advanced. This is to allow for proper flame propagation across the cylinder. If it advances too far you will hear a ping because your engine is now trying to compress an explosion. If it is not advanced enough then you donÆt take full advantage of the combustion because the piston will already be on its way back down before the explosion reaches its way across the combustion chamber. The mechanical advance does its best to compensate for the flame propagation time at different engine speeds. So really you have 4 things that will come into play when setting your timing. The initial distributor set position, the mechanical advance, the vacuum advance, and any play from worn parts.

If you pull your distributor cap off and look down inside the distributor you should see some counter weights and springs on a plate, this should be just below the pick up for the electronic ignition, all of these parts should move if you grab onto the rotor and give it a little twist (say 20-30 degrees or so). The vacuum advance will attach to the plate the pick up for the electronic ignition sits on, and will move if you suck on the vacuum advance hose.

Man getting on this site at work sucks, I started this at 9:00 am and I am just getting back to it to finish it at 4:50 pm. Go figure.

Sean

Good point Jim, I remember on my 66 Chevelle with a 283 in it I set the timing at max mechanical advance, instead of at idle, and I want to say it was at 36 degrees BTDC, but I had a timing light with a dial because I didn't have the timing tape or the extra marks installed, but it was pretty neat to see the timing change with engine speed and vacuum.

Sean

I'll try to help with the noodle problem..
Ok so we are driving at say 1900 rpm..steadily along..we have most mechanical and full vac advance of perhaps 30* or so..
We punch it! Opening the throttle to WIDE.. if the timing stays where it is the advance will cause fire to jump out the carb top.. making backfire.. BUT HERE is what happens.. as the throttle suddenly opens the vac suddenly DROPS and the vac advance goes away..QUICKLY!! Now we have an engine that wants to gain RPM and the advance is prolly around 16/18* and at that advance the fire is contained inside the cylinder...
Now as the engine gains rpm the velocity inside the venturi (carb or TB) gets greater and greater causing MORE vacuum which in turn causes the vac advance to start opening and getting more and more advanced timing.. At around say...3,000 the vac is high enough and the velocity is high enough that the vac advance is full again..
The above numbers change from camshaft to camshaft but are representative of what happens with the advance..
If the engine will be run..say on a track and most of it's time will be spent at 5,000 and above it is not uncommon to have 42* of timing in the thing..
But our little engines can only use 35* or thereabouts and then only at 4,000..
And yes the timing changes from any movement of the skinny pedal at normal driving rpm..
When the throttle is suddenly OPENED the mixture try's to LEAN out from the low velocity.. It is this lean condition that the FLAME likes as the flame can travel thru it faster than it can the proper mixture.. So if the timing doesn't get retarded by the vac advance bad things happen..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Now I get it! <img src="/forums/images/graemlins/kewl.gif" alt="" /> <img src="/forums/images/graemlins/cheers.gif" alt="" /> Thanks BJ, and Sean.