My '85 MGM, "Maisa"

[sly]

Usually the bucking at those speeds is related to misfires. Might need to check that the timing isn't too advanced and the wires are in the correct order (sanity check).
+1 checking for vacuum leaks - especially from the throttle plate down. Some leaks may not present until under throttle if the lines are hard or have a hairline crack.
If the lines rub off on your fingers, they're probably porous enough to cause problems under load and need to be replaced. I remember dealing with that on a chevy 305 with a quadrajet on it. Had to replace about 18 feet of vacuum lines. Then it ran great.

[Arquemann]

Base timing wasn't 10° BTDC, more around 15-17°...
Timing has moved, distributor hasn't. I adjusted the timing back to 10°, now the dizzy is all the way counterclockwise.

BUT
Giving throttle lowers the ignition advance at first, then the advance starts increasing with more and more throttle.
The advance bottoms out at around 2-3 degrees BTDC before it starts climbing back up with more throttle.
Is something fucked up or am I losing my sanity?
Should I be running manifold vacuum instead of ported vacuum?

I think my car is cursed, like Christine. Except mine refuses to run right instead of refusing to die and looks like a chest refrigerator instead of a stone-cold killer.

[matth825]

My '85 MGM, "Maisa"

You should be running ported vacuum. Connecting to manifold vacuum is what causes advance to drop when tipping into the throttle like you are seeing. Also, did you have the vacuum advance line disconnected when setting base timing?

I would check the ported vacuum port on the carb to make sure it’s zero with the throttle plate closed. When you set the carb up did you set the idle stop so the transition slots were squared off? If you did is the idle stop screw still that position or have you adjusted it?

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[Arquemann]

You should be running ported vacuum. Connecting to manifold vacuum is what causes advance to drop when tipping into the throttle off idle.

I would check the ported vacuum port on the carb to make sure it’s zero with the throttle plate closed. When you set the carb up did you set the idle stop so the transition slots were squared off? If you did is the idle stop screw still that position or have you adjusted it?


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If I were running manifold vacuum, I'd also have alot more advance on idle. In the ideal case, the vacuum advance would fall off as the mechanical advance increases.
I did set the transfer slots initially, but since I've screwed out the idle screw, then in a bit and out again and who knows how the transfer slots look now...
Ported vacuum port works correctly, seemingly no vacuum at idle, comes on a bit after cracking the throttle.

[sly]

Does the distributor spin freely? I had this issue with an EFI vehicle (truck) with a distributor that was pretty much roached. When I pulled it, it did not spin well at all and required quite a lot of force to turn it. Oil seals were pretty much doing nothing. It did have that issue at more speeds though and especially off-idle.

But yeah, I think you're on the final "tune" step. Just dialing it in so to speak. I don't like the fact that the dizzy is pegged one direction, which makes me think something is off in regards to the timing. I've never dealt with vacuum advance, so I'm not used to tuning that kind of stuff.

One last thought. Is the distributor cap all aluminum studs or brass studs? We seem to always have issues with the aluminum ones. They seem to wear out or just get loose fast. The brass ones seem to work a lot better. The rotor contactor may be fouled too.

I would check spark color too. Should be a nice strong white to blue spark and not yellow. Might need a better coil.

[gadget73]

put a vac gauge on the ported vacuum connection. That will tell you if the slots are open too far. Should have no vacuum at idle but it should increase as soon as the throttle opens.

Timing should be set with the vac line unplugged, and it shouldn't be adding anything once connected if the blades are set right.

[Arquemann]

I'd be inclined to believe that the decreasing timing advance that's happening on low throttle is the cause of my bucking issues.
If it's seriously having less than 5 degrees of advance at low RPM - high vacuum - low load cruise situation, from my reading it'd probably cause late burning and bucking.

When I checked the ported vacuum yesterday, I didn't feel any vacuum at idle, by hand. I wonder if the transfer slots are way too closed currently, since it takes quite some throttle movement to get anything from the ported vacuum port.

Since my engine has nothing to do with a CFI LoPo anymore, I wonder if I should try out the manifold vacuum advance... It'd probably smooth out the idle and get me better MPG cruising.

[Arquemann]

Can the harmonic balancer spin on these if it's worn?
Since readjusting the timing yesterday the idle RPM is relatively the same, but the tailpipe chuffs pretty often now.

[DerekTheGreat]

...Since readjusting the timing yesterday the idle RPM is relatively the same, but the tailpipe chuffs pretty often now.

Does it try and suck up a paper towel/napkin?

[Tiggie]

Yup it can spin.

I find TDC with my finger on the number 1 to confirm compression stroke, then put a screwdriver down it and see where it stops pushing up. Center the crank between when stops pushing up and where it pulls back down and you'll be real close.

[Arquemann]

Seems like my problem is twofold:

-The Duraspark distributor seems to be screwed up and retards the timing almost 10 degrees before starting to actually advance it. (mechanical advance)

-The ported vacuum source comes on late, which reduces the total amount of light throttle even more.

Basically the timing advance falls below the initial 10 degrees at cruise speeds, and where the timing is the lowest, the severe bucking appears.

Yesterday I bumped up the initial timing to around 15 degrees, and it reduced the severity of the bucking a bit. There's still alot of it, but it's not as violent, plus it is at a slightly different RPM. Which kinda goes according to my theory.
The lack of any meaningful timing advance at cruise speeds would also explain my poor MPG a bit. The harmonic balancer seems to be fine, no play at all on the outer ring, but I'll check with the TDC soon.
I might be able to borrow the HEI distributor off my dad's Montego to try it out.

Now, the ported vacuum issue:
-I believe my throttle blades are too shut, so it requires too much throttle actuation to activate the ported vacuum port to still be in light throttle cruising.
-The idle RPM is good, but if my throttle blades are too shut, I have vacuum leaks. That's why I just bought a couple cans of starting spray.

To be continued...

[gadget73]

unplug the vac advance and see if it does the same thing. If it does, it has to be a mechanical fault. Not much to that system but if the mechanism is worn or something it could be hanging at partial advance. HEI distributors were prone to that, the weights would get flat spots on them or just get sticky and the timing wouldn't reliably come back to base timing. Usually it wouldn't retard the timing with throttle, just your idle was always someplace different. Can't say I'm excessively familiar with Dspark distributors to know if they had the same issue, but it wouldn't surprise me too much.



out of the box factory timing curve tends to be for a whole lot of potential timing, but with heavy springs so its very lazy about advancing to the point where it usually never hits the advance limits. For more performance you want softer springs so you get all the advance in by 3000 or so, but total has to be limited so its not at some absurd point that detonates like crazy. Probably don't want more than 25-30 degrees of mechanical advance.

[Brown_Muscle]

That’s good you found your problem, one other thing to consider is maybe it is the electronic control unit itself. I know it retards timing during cranking, so maybe there’s something wrong internally. Not as likely as it being bad mechanically/vacuum, but just something to consider

[Arquemann]

-Didn't find any vacuum leaks outside, might just be worn carb butterflies and throttle shafts. Doesn't seem too loose though.
-Took apart the dizzy and didn't find anything wrong with the advance mechanisms.
-Timing gets retarded by the mechanical advance.
-I turned up the idle by about a turn, now the ported vacuum opens up a bit sooner.
-With the vacuum advance coming sooner, the total advance doesn't drop as low before the vacuum advance comes on. Drops to a minimum of 8-9° BTDC from about 14° initial. Enough for now, test drive will tell.

I'll get a new dizzy ordered, I think the current (new) unit is just a DOA piece of shit. If that won't fix it, I'll replace the HEI module. If that won't fix it I'm trying the HEI dizzy from dad's Montego. If that won't fix it I'll sell my car.

That’s good you found your problem, one other thing to consider is maybe it is the electronic control unit itself. I know it retards timing during cranking, so maybe there’s something wrong internally. Not as likely as it being bad mechanically/vacuum, but just something to consider

Don't have a Duraspark box, 4-pin HEI doesn't retard during cranking.

[Arquemann]

With more initial timing, the bucking is at a lower RPM and not quite as bad. Still sucks tho. New distributor is coming.