Seff

Peter: Gotcha. That'll be my long-term goal. :P

Olds307and403: So I could isolate all my vacuum hoses, and my choke pull-off, one at a time and see if the idle improves? I can do that.

Trip: Wouldn't that mean that spark would be bad along the entire RPM band?


The weird thing about my problem is that I only feel it when I hold the car by the brake in Drive, and it's as much a jerking feeling in the seat of my pants as it's a thumping or klonking sound. Not constant, but a few times a second, like it's on one bank and not the other. Would it be a good idea to pull spark plugs and check for funny deposits, as well as doing a compression test? Would a sizable exhaust leak between header and exhaust pipe (under driver's seat) be able to cause this kind of jerking?

TripDeuces

Without being there it is hard to diagnose. It could be anything you mentioned. Low fluid level in the trans, dirty trans filter, loose cross member, loose torque converter bolts, loose transmission mount, etc.
Whenever I feel or hear clunking noises I think something is loose.
Whenever it runs rough I think ignition or fuel delivery.

To me an engine is just an air pump. It needs fuel, air and spark to run. When it doesn't run well it has to be one of those unless it's a mechanical failure.
Checking any of the things you mentioned can't hurt but the compression test points more to a mechanical failure which I like to think of last. I start with the simple things first. This is a stock engine right? No 4/7 swap cam or any other weird stuff? I assume there is no EGR valve or air pump/emission type things? I know Europe has different requirements for older cars, this is why I ask.

Seff

Denmark only demands that my car lives up to the factory emissions specs, so no funny business in that department.

The engine is completely stock apart from the HEI and the '77 carb (which still makes it stock, but I mention both to be clear).

Last time I was under the car, torque converter bolts and trans+crossmember bolts were secure. I've never changed the trans filter, and the fluid was checked a thousand miles ago. Should have thought of that. Could too little trans fluid make the TQ slip?

TripDeuces

Yes, check the fluid when warm and on a flat surface.

Seff

Will do. If it's low, I'll check for drips and whether the engine's burning ATF.

olds 307 and 403

Just make sure the hoses are in good shape and tight on the connections. Should have said spray the carb cleaner around the carb and base gasket with the engine running. If the rpm goes up and smoothes out, you found your leak.

Seff

I figured that was what you were referring to.

MDchanic

This makes me think of a vacuum leak.

You can test with any flammable material that you can spray: Carb cleaner, propane, MAPP gas, startin' ether, whatever.
Just use a very directed tip if possible, and don't spray too much, or you'll just hit everything at once.

- Eric

Seff

Dad has diesel start/ether, I'll try that. All day today was spent in the forest splitting logs, so it'll have to wait.

Seff

Topping off the ATF drastically reduced the effect. Changing the filter might do the trick completely.

There were no vacuum leaks around the carb, and the choke pull-off was good too.

Seff

The current tuning certainly seems to be good, because an 88 mile half freeway, half backroads trip netted a measured 21 MPG. Not too shabby with a TH350, is it? :P

oldcutlass

Me thinks you've erred with your math somehow.

Seff

https://maps.google.dk/maps?saddr=As...3&via=1,2&z=13 is the exact route from gas station to gas station, the first at which I filled her completely up, the second where I topped her off in the same manner. 15.73 liters used.

Unless I used British gallons instead of US gallons, it should check out.

EDIT: Did the math manually, 21.08 MPG.

MDchanic

You're doing something right. Congratulations - that's an unusually good mileage.

What rear is in there again?

- Eric

oldcutlass

Ok, if thats what it comes to, just not used to a Cutlass with a 350/4V and T350 trans get that kind of mileage.

Seff

I was very surprised too. :P

Eric: Thank you - it's a 2.56 posi.