My '85 MGM, "Maisa"

[87GrandMarq]

I had the exact thing happen on my 87 but it was due to not enough wrap on my ac compressor. I added a smog pump and problem solved. Hopefully it’s just the idler pulley for you.


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[gadget73]

The other thing that makes the idler sit funny is the missing lower support from having no smog pump. Its usually not a problem but sometimes the brackets bend. The cheater fix is to use the really long-ass bolt that ran through the smog pump and a spacer into the block and a piece of pipe equal to the length of the original spacer and the smog pump. Pipe goes between the bracket and the block, bolt goes through it to hold it all together.

those 2 piece stamped idlers are fairly shit though, never been a fan of them. I like the smooth one piece stamped steel ones if I can get them, or the plastic if I have no choice. the one piece steel ones you can press a new bearing in when it wears out. The Continental had one of those two piece deals that you're supposed to replace the entire assembly on. Can't get it, so I modified it to take a rebuilt Chevy S10 part I had on the shelf. It was an old one off the truck that got squeaky, replaced the cheapie China bearing with a better grade SKF.

[Arquemann]

The other thing that makes the idler sit funny is the missing lower support from having no smog pump. Its usually not a problem but sometimes the brackets bend. The cheater fix is to use the really long-ass bolt that ran through the smog pump and a spacer into the block and a piece of pipe equal to the length of the original spacer and the smog pump. Pipe goes between the bracket and the block, bolt goes through it to hold it all together.

those 2 piece stamped idlers are fairly shit though, never been a fan of them. I like the smooth one piece stamped steel ones if I can get them, or the plastic if I have no choice. the one piece steel ones you can press a new bearing in when it wears out. The Continental had one of those two piece deals that you're supposed to replace the entire assembly on. Can't get it, so I modified it to take a rebuilt Chevy S10 part I had on the shelf. It was an old one off the truck that got squeaky, replaced the cheapie China bearing with a better grade SKF.

Doesn't seem like I can get me anything else than the 2 piece stamped unit. Oh well. If the cheapie lasts even a tenth of the original's lifespan of 37 years, I'll be content. The pulley face will need a bit of black paint though.
I'll check the smog pump spot, I still have all the bolts and bits, so figuring out a spacer in place of the pump shouldn't be too hard.

[Arquemann]

I have to admit that this carb swap has not meaningfully affected the jerky / bucking decel. What in the fuck could it be...

The bucking can still be distinguished from the exhaust note and feels way more obvious in 3rd & 4th. That's why I'd still believe it's an engine problem, not trans or else.
Literally the only things that haven't changed is the coil, spark plugs (NGK UR45) and plug wires. All of those are a couple years old, plug wires are bit mismatched though.
On decel the AFR stays consistently at around 13, but the gauge shows only the pass. side bank.

[friskyfrankie]

Sounds as if it could be a possible misfire. I'd go through the entire ignition system especially on the bank you believe has the issue.

[matth825]

Does it feel smooth with no misfires when accelerating hard? If it were an ignition issue heavy load is when it would show up.

It potentially could be the trans. I’m not an AOD (or trans in general) expert but I believe there is a one way clutch in there that prevents engine braking when decelerating when the gearshift is in the OD or D position. If it’s not working properly it could be intermittently allowing engine braking which could cause jerkiness.

I would try to decelerate in manual 1st and see how it feels. This bypasses that clutch and allows engine braking.


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[Arquemann]

Does the AOD have such one way clutches for 3rd and OD? From my learning of the AOD, those gears are direct drive. Would direct drive gears still have a freewheeling clutch? Or whatever you want to call it.
Third gear feels like I'm engine braking, but I cannot say for sure if the bucking is freeing up or locking up more momentarily.

[gadget73]

No freewheel function or one-way clutch that would do that.

Check your wire routing, if 7 and 8 lay next to one another for a long stretch it can cause cross-firing and do exactly this kind of stuff. Chevy engines have the same problem on 5 and 7, which are physically in the same location as Ford 7 and 8. Both are adjacent in the firing order. Bad wires will do it too. I remember working on a CFI car years ago that ran like hammered shit, turned out to be the brand new wires were just garbage. Stuck a used set of something that was laying around the shop on the car and it ran perfectly. That one didn't so much buck on decel but under high load at low RPM, so going up hills in 3rd gear at steady speed was garbage, foot to the floor and it ran fine.

[Arquemann]

No freewheel function or one-way clutch that would do that.

Check your wire routing, if 7 and 8 lay next to one another for a long stretch it can cause cross-firing and do exactly this kind of stuff. Chevy engines have the same problem on 5 and 7, which are physically in the same location as Ford 7 and 8. Both are adjacent in the firing order. Bad wires will do it too. I remember working on a CFI car years ago that ran like hammered shit, turned out to be the brand new wires were just garbage. Stuck a used set of something that was laying around the shop on the car and it ran perfectly. That one didn't so much buck on decel but under high load at low RPM, so going up hills in 3rd gear at steady speed was garbage, foot to the floor and it ran fine.

If 3rd and OD are locked, direct drive gears, the trans shouldn't be trying to do anything else while in gear? Right? Since there's no converter lockup or such to be misbehaving...
I prettied up my plug wire routing just yesterday, made sure they're not close. Being on the opposite ends of a plug wire separator is as close those are to another.

[Arquemann]

I think I can feel the bucking in 1st, and maybe even on very light throttle in some scenarios in 3rd.
Was going to check the spark plug wire resistance values, but I don't think I'll have time today where the engine is cold enough. Already tried, but exhaust manifolds are hot and also very close to plug boots lol.

I adjusted the idle mixture a bit leaner, it's pretty close to stoich now.
Also tightened up the TV cable adjustments and made marks with a sharpie so I can see if it has moved. I am quite happy how it's shifting now, firmer and better than it was with the CFI.

Another thing I've got to do it disable the automatic e-brake release, it has started getting sloppier and leaking since last fall and now it's just annoying, a tiny vacuum leak, but the whistling is just freaking annoying.

[sly]

Yeah... the valve for the brake release is at the base of the steering shaft at the firewall and the line coming from the firewall off to the right is the source. Pull that and plug it. Done.

[Arquemann]

Yesterday I took the car on a 70 mile round trip, cruise AFR is pretty good, bounces around between 13-14. I got jets one size smaller to put in next.
The car was acting a bit weird and idling rough when leaving back for home and when I got back home. Feels like a proper misfire at idle, even though the AFR was showing a pretty steady 14,5-15. Still, the lambda is only on the passenger bank.

I got a new set of plug wires to install and I'll check the plugs at the same time. For now I'm on a weekend excursion at a friends cottage, and I'll return to repairs on sunday or monday. I'm damn sure I'll drive the MGM to this cottage some day! Not last summer, not yesterday and probably not Midsummer this year either, but I have a shot in July.

[DerekTheGreat]

Have you checked compression recently? Sorry to ask if you already have, but perhaps you have a burnt valve or some aren't seating properly?

Oh, how does it feel acceleration wise with the carb & four barrels opposed to CFI and two barrels?

[Arquemann]

Have you checked compression recently? Sorry to ask if you already have, but perhaps you have a burnt valve or some aren't seating properly?

Oh, how does it feel acceleration wise with the carb & four barrels opposed to CFI and two barrels?

Not too recently, maybe two years ago? Nothing notable back then. Same issue was present then aswell.
Probably should check again, it's just my compression tester doesn't have the correct adapter and I've the rubber cone thing, which is hard to get to the passenger front and driver rear cylinders.
A crapped out valve would explain a lot of my problems, and it would also be an expensive pain in the ass to fix. Knowing my luck, it might just be that.

As for the carb setup, the car does actually feel a bit faster, even though WOT goes way too rich for optimal power. Top end feels more awake, even though it still sounds like it's gasping for air at 4000+ RPM. I'll have to time some pulls when I get the WOT mixtures set properly. I might've lost a tiny bit of throttle response, but it doesn't really matter in a boat like this. And in return I got the sounds of a 4 barrel carb, the sound of those secondaries opening is just bliss, even with the factory air cleaner.

[DerekTheGreat]

It could be. Another way to test would be to position each cylinder at TDC on the compression stroke and feed compressed air into the cylinder. If you hear/feel air coming out of the intake or exhaust, toast. Seems perhaps you've got gak on at least one intake valve, for it to buck on decel like that. Carb or CFI, they're both dumping fuel into the intake manifold and if the intake valves aren't seating properly, it's gonna pull fuel & air in when it shouldn't be.

Before I had the engine replaced in my K1500, it made a "chuff.. chuff.. chuff" noise from the exhaust and would try to suck up a napkin if you placed it near the exhaust. Turns out the exhaust valve for #1 had burnt up. It would run ok for 100 miles or so and then foul out the plug. It occasionally had a stutter when accelerating hard. Do you have any of that noise from the exhaust or does it try to suck up whatever you put at the exhaust tips?