Timing Advance Issues

H

Heston 79

Member
I have a 79 Scout II with a 345 & automatic. It currently has the thermoquad on it, but I am getting ready to swap it out for an edelbrock 1406. I've had it a couple years and never quite had it running right. I'm trying to get the timing dialed in, and I am having a hell of a time... I've looked through lots of other threads, but haven't found my problem. I have tried to apply all the trouble shooting advice I've read, but haven't nailed down the problem yet, and I am nowhere near an expert when it comes to this.

I've got the idle set at 600-650. I am timing off the #8 cylinder (rear passenger side), the truck is running pretty well, but it stumbles off idle.

I checked the timing with a timing light, and it is at 20 deg... I've tried to back it down to between 0-5 deg as a baseline, but it will cause me to made huge adjustments to the idle screw if it's even possible... It really starts running rough as I've tried to make that adjustment.

Is that what I should do? Is it possible the distributer is off a tooth or two, or would that have completely different symptoms?

I appreciate any and all suggestions. Thank you for the help in advance.
 
That was with the vacuum line disconnected, but it was the same when it was connected.

I believe because its a later model Scout where there is no vacuum advance at idle? (ported vacuum advance off the carb?). I believe I read that on one of the threads
 
When using timing light, you are looking at the actual time of the spark in relation to the crank. All other factors are removed, unless you have an adjustable timing light. Are you sure you're timing off the correct Mark. What I do on my IH cranks is to paint the Mark so it can easily be seen. Then there is no question.
 
I sure thought so. It is an actual line cut in the metal piece. When the timing light hits it, I am looking at that line which appears centered on a 4" metal strip. Its about the same length as the piece the timing marks are on. I'm not looking at the truck right now, but does that sound way off?
 
As soon as I have a chance I will Mark the line with white out so there is no question but I'm 90% sure I am reading it correctly.
 
Something's fishy. If your initial advance was truly at 20 degrees btdc at idle, I would expect that the engine would be noticeably hard to start and you'd have a tough time maintaining an idle speed below 1k rpms. What happens to the Mark when you apply throttle? Maximum advance, be it mechanical or vacuum should be somewhere in the mid to high 30's at full throttle. 10 or 12 degrees of initial advance is generally about all a 345 will tolerate before you start getting some detonation (pinging) under load.
 
It starts easy, it runs well, no detonation that I've noticed. The only symptom I have is that it stumbles a little off idle. At idle with the vacuum line off and capped, I've turned the dizzy both directions and it seems to be in its sweet spot. And I know I'm timing off number 8. I even verified with my manual that the wires are run correctly...

So it runs great except it stumbles and is showing 20 deg of timing at idle.. I was trying to fix the stumble, I decided to replace the carb with an edelbrock and one of the install instructions is to make sure the timing is correct. That's what set me off down the road.

Just doesn't make sense to me
 
Sorry. I didn't answer the original question... When I hit the throttle the line advances well off the scale. If I'm remembering right the scale only goes to 20 deg, but it seems to advance normally. Probably 30-40, but I'm just guessing based on the part of the scale I can see
 
Me either. How about posting a pic that clearly shows how the vacuum advance canister on the distributor is oriented in relation to the water pump on one side and the even bank valve cover on the other. It should be roughly in the center of those two points. A circle has 360 degrees. I don't have an sv8 distributor out handy to look at, but going by images online, it looks like there are 16 or 17 teeth on the drive gear. 16 sounds more logical, so lets go with that. 360 divided by 16 is 22.5. So if the distributor had been stabbed one tooth off, your initial timing at what would otherwise seem to be top dead center would actually be either 22.5 degrees above or below tdc depending on direction of rotation. By then rotating the seated distributor one direction or another, it should be possible to change that figure by ten or more degrees one way or the other. That still doesn't explain the 20 degrees that you're seeing. Perhaps two teeth off? I dunno. Just spitballing. This speaks directly to why I want to see how the vacuum advance canister is sitting. It could shed some light.
 
I am just now leaving for a couple days. I will post a pic as soon as I get back.

But if your math is right that might make sense. I was trying to get 0 deg timing, but have 20 deg, indicating it might be off a tooth... (if that's how it works, I'm not sure but I'm trusting you there)
 
Its only conjecture or theory at this point. We don't yet know enough to reach any conclusions about being x number of distributor teeth off. But taking this conjecture a step further, 22.5 times two is 45. There are eight spark tower positions on the distributor. 360 divided by 8 is 45. If the distributor was one tooth off in the direction of being too far advanced, I would think by rotating the seated distributor clockwise until the advance can hit the valve cover, you could possibly go from 22.5 degrees btdc down to maybe 5 degrees btdc. 5 isn't 20. If it was one tooth off retarded, I would think you could rotate the distributor counter-clockwise to go from 22.5 atdc to about 5 degrees atdc before the vac can would hit the water pump, which also isn't where you're at. So I think we could be looking at two teeth or one spark tower position off. Either way would be 45 degrees. 45 degrees too far advanced could possibly be dialed down to 20 degrees through clockwise distributor rotation. 25 degrees subtracted via rotation seems like a lot to me, but maybe. I doubt you could physically get much lower than that before the vac canister smacked against the valve cover.
 
I was thinking about what you said Scout boy74.

The firing order is 8-4-6-2-3-7-5-1. If I time off #4 and if its tdc, then the distributer is in the right place, but the rotor would simply need to be turned ccw 1 post (2 teeth-- if you are right about 16 teeth, 2 per cylinder)

I'm thinking if I put little marks around the dizzy so I know where each tower is. Remove the cap, remove the hold down, pull the dizzy out 3". (so the gear is clear of the motor), then rotate the rotor ccw one tower, and slide it back into into the motor... It sounds really simple, so it probably won't work or doesn't work that way.

Does that make any sense? Does it even work like that? Does the rotor turn both ways or only clockwise? Sorry for all the dumb questions.
 
The rotor only turns clockwise. It matters not which tooth you're on as long as the plug wires are the corresponding holes and you have enough room to adjust the dizzy for the correct advance. The timing light shows the direct relation between the spark and the position on the crank (an on our IH motors it is in effect a crank lobe and not a harmonic dampener that can move). I suggested painting the timing Mark because I have miss timed IH engines by miss reading the timing Mark. Yes it seems like it would be easy to read, but after you see some white or yellow paint on a red background there will be no question. I have also shifted a dizzy by a tooth just because I didn't like where the vacuum canister was positioned.

Most likely, almost guaranteed, the bog is caused by the thermobog carb installed. The edlebrock should run fine.
 
Ok. I will paint the marks when I get home and try it again before I do anything else.

By that logic, if it still times at 20 deg, do you then just rotate all the plug wires one position cw? Or do you rotate the rotor 340 deg cw to reset it... Just curious.

I don't have much faith in the guy that built the Scout up, so the dizzy being installed incorrectly wouldn't surprise me one bit.
 
scoutboy74 said:
so if the distributor had been stabbed one tooth off, your initial timing at what would otherwise seem to be top dead center would actually be either 22.5 degrees above or below tdc depending on direction of rotation.
Click to expand...

Op, even if the distributor is one tooth off you still will see the real timing of the spark in relation to the crank like chappie said. The timing light senses the spark at the wire and flashes the light onto the pointer showing the position of the crank at that point in time. In this case 20 degrees. So unless the balancer has slipped (not very common) or the pointer is off then it is firing at 20 degrees btdc, assuming you're reading it correctly.

heston 79 said:
I was thinking about what you said Scout boy74.

The firing order is 8-4-6-2-3-7-5-1. If I time off #4 and if its tdc, then the distributer is in the right place, but the rotor would simply need to be turned ccw 1 post (2 teeth-- if you are right about 16 teeth, 2 per cylinder)

I'm thinking if I put little marks around the dizzy so I know where each tower is. Remove the cap, remove the hold down, pull the dizzy out 3". (so the gear is clear of the motor), then rotate the rotor ccw one tower, and slide it back into into the motor... It sounds really simple, so it probably won't work or doesn't work that way.

Does that make any sense? Does it even work like that? Does the rotor turn both ways or only clockwise? Sorry for all the dumb questions.
Click to expand...

It would be better to first check where the rotor is pointing with the #8 piston at tdc on the compression stroke. While you're in there check the end and side play, advance mechanism especially bushings, and the module and wiring for heat damage.

heston 79 said:
I've got the idle set at 600-650. I am timing off the #8 cylinder (rear passenger side), the truck is running pretty well, but it stumbles off idle.

I checked the timing with a timing light, and it is at 20 deg... I've tried to back it down to between 0-5 deg as a baseline, but it will cause me to made huge adjustments to the idle screw if it's even possible... It really starts running rough as I've tried to make that adjustment.
Click to expand...

Stumble off idle is often if not almost always the carb going lean for a split second. Even if your distributor is a tooth off it should still run ok at 5 degrees. It's worth checking, but I'm thinking you have other issues. Do you have a vacuum gauge?

Edit: chappie nailed it.
 
heston 79 said:
By that logic, if it still times at 20 deg, do you then just rotate all the plug wires one position cw? Or do you rotate the rotor 340 deg cw to reset it... Just curious.
Click to expand...

No can do.

The dizzy should be on the correct tooth and the plug wires on the exact cap towers IH specified. There are pictures online showing which tower is #8. Some caps are also marked. If the wires or tooth is off one position you can theoretically spin the dizzy housing to get the correct timing, but usually the vacuum can runs into something.
 
I do have a vacuum gauge.

Assuming the carb is the culprit on the stumble.

I really don't have any symptoms except that the timing light shows cylinder 8 is 20 deg btdc. (which I will double check tomorrow by marking it) the motor runs great. When I turn the dizzy to tune it, it gallops one way, runs rough the other way so it seems like its in the sweet spot. Maybe I should just leave it alone, but I can't help but want to know why... And then of course I will want to fix it.
 
I do have the diagram of the plug wires. They are in the correct order, but I will double check that they are in the exact correct position on the cap. I feel like there would be some serious indications if they were not.
 
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