I'll start by admitting I haven't played with points ignition timing in 20 years, but I figured I'd get my head back in the game...
On my '66 NY 440, I have the base timing set around 12 BTDC. (I think... vacuum disconnected, engine warmed up, timing light set to zero and adjust the crank marker to 1/2 way past the 10 degree "before" marker, right?)
It idles smoothly and I can rev it right up and it remains pretty smooth up to pretty much WOT. But, when I connect up the vacuum line again it still idles smoothly but at above about 1/4 throttle (or even a bit earlier) the engine starts to shake and by 1/2 throttle I get the occasional small backfire.
Looking for advice on where to go next... this may become a small saga as time to attack the problem is limited
I honestly only use the timing marks as a baseline. I have a better time setting by ear, and then test drives. I just loosen the distb bolt enough that I can turn but with some effort and where it will hold itself on a rev up. I think you need to set at 0 and then go back very slightly from there. If its a rubber balance they can move, things wear, including the chain etc... So don't just use the marks as set in stone.
I should add that the motor is fairly recently rebuilt but now broken it (~1000 miles or so on it), new timing chain, etc. at the time. Originally it had a miss at idle, but ran fairly well up through the revs. I had my mechanic (who worked at a Mopar dealership in the '70s so he knows these engines) look into it, and he did some timing and carb work to get it very smooth at idle but now it's crappy beyond 1/4 throttle. It's going back to him when he has time but I thought I would see what I could try myself before hand.
Somewhere I thought there was a thread on setting timing using a vacuum gauge, but I can't seem to find it... anyone bookmark it?
My LA block 318 was a complete rebuild too, unfortunately not by me so I don't know its complete history, but my timing marks are way out to lunch because either the chain is a tooth out or distb is out a tooth, but its still possible to set. If you have a buddy to help just pull the vacuum line and plug it, one guy hold about 1/4 throttle while to gently turn the distb and then listen when it either chugs more or cleans itself up. Don't rev past that point really to set, you need to drive from that point, but if it gets better, then you know which way to add more or bring back if its too advanced and pings.
Do you have your hot air tube hooked up or do you run a open air cleaner? Or do you have the manifold style heat riser? That can be a big issue in cooler weather, even on a warm engine
Joined: Fri Jan 18 2008, 10:06PM
Location: Richmond, VA
Posts: 383
john66ny wrote ...
I'll start by admitting I haven't played with points ignition timing in 20 years, but I figured I'd get my head back in the game...
On my '66 NY 440, I have the base timing set around 12 BTDC. (I think... vacuum disconnected, engine warmed up, timing light set to zero and adjust the crank marker to 1/2 way past the 10 degree "before" marker, right?)
It idles smoothly and I can rev it right up and it remains pretty smooth up to pretty much WOT. But, when I connect up the vacuum line again it still idles smoothly but at above about 1/4 throttle (or even a bit earlier) the engine starts to shake and by 1/2 throttle I get the occasional small backfire.
Looking for advice on where to go next... this may become a small saga as time to attack the problem is limited
I had a problem once when I connected distributor vacuum advance to intake manifold vacuum instead of ported vacuum. The car ran better at idle, but acceleration and cruise weren't right. After changing to ported vacuum, performance improved. I then disconnected vacuum advance, while I tuned mixture and timing for max intake vacuum on a vacuum gage. I had a Holley 750 and came up with max vacuum around 15 degrees BTDC with both mixture screws one turn out. If none of this helps, you can use an allen wrench through the vacuum advance hose fitting to decrease vacuum advance by turning counter clockwise.
Joined: Fri Oct 28 2011, 04:05PM
Location: UK
Posts: 103
Hi John....there is a lot of stuff out there concerning timing and lots of theories that can help. It may be that as you are running 12 deg btdc initial timing, when you are up the rev range and all your advance is in at around 3000rpm, your total is too high. A lot of guys over here run a lot of initial (18 deg !!) And use a limiter plate in the dizzy to restrict it to 34 deg total, all in at 3000rpm. They are easy to fit and are available through FBO. The secret is to work out how much advance you have built into the dist, set your initial timing, then add the two numbers together. Then use the relevant slots on the limiter plate to restrict the total all in to 34 deg. So, for example, you find that the dizzy has 30 deg of advance in it and your car runs, starts, best at 14 deg initial. Add 14 to 30 and you get 44 deg at WOT. Thats too much so use the 10 slots in the plate to restrict it to 34 deg total. You would then be running 14 deg initial, 34 deg total, and play with springs and aim for all your advance to be in by 3000 or so rpm.
Im no professional at all but this works for a lot of guys this side of the pond. If in doubt with what your initial should be, try warming engine up, plug advance hose, and loosen dist hold down bolt. Gently rotate the dizzy and listen to the revs increasing. Just where the increase starts to shalow out is around where the engine wants to be. Use a dial in timing light to read your timing. Tighten clamp. Now add your built in timing figure and limit it with a plate to 34 deg.
Forgot to say, before you start, set warm idle to 750 rpm and I know its horrible but with an auto, she should be set in drive. Get a friend to stomp on the brakes!!! When you are happy with timing, set idle mix and should be ok.
Hope this helps a bit. I take a spanner, cheap digital tach, and my friends Snap On light out with me and just try her on various settings. If you pull away from a standstill on a hill you will soon know if your initial is out!
Good luck and keep us updated please...very interesting subject
The easist way to time your engine is with a vacuum gauge-forget the timing mark on the crank pulley, most of them aren't even close (mine shows over 25 BTDC at 900 rpm idle, when it is actually closer to 18 BTDC). Hook the vacuum gauge up to the port that always has vacuum (usually drivers side) and advance the timing while adjusting the fuel mixture/idle speed until you get the highest possible vacuum reading. Next, retard the timing slowly until the vaccum reading drops 1/2'' to 1", lock it down and go for a test drive. If it pings on acceleration, retard it in 1/2" increments until it stops.
A lot of overheating issue in our engines can be traced back to engines that are timed way too retarded....