Vacuum issue.... maybe?

[xs-style]

Ok guys, here's my situation and I need ideas.

About a month ago I put a transmission in the truck. The guy was local, seemed reasonable, and had a couple nice chevys in his stable. The tranny was supposed to have about 25k on it, but had sat for a year or so. I installed it, new stock torque converter, vacuum modulator, and fresh fluids and filter. The only difference from my original being that this one was for 80 and up, and I have a 78. As far as I know the only difference is leaving the electronic plug open. Anyway, my problemo is that I lack acceleration and im having to manually change gears. It also stalls in reverse and when your riding the brake like in slow traffic. All in all, its still no more use to me than it was before the transmission, even though I've probably spent a grand in parts plus my time doing the work.

Im looking for some ideas here. All my buddy's local are 50/50 split. On one side you have the guys who say I got a bad transmission. On the other, there's the notion that its vacuum related.

I would say bad transmission, if not for a few things. One, when I first put it in, it shifted fine. It was stalling at low speed (less than 2 mph), and in reverse. At that point I figured vacuum issue. So I went through and did all new vacuum lines. I also changed from having my transmission line and brake booster sharing a vacuum on the manifold, to putting them each on a dedicated port. The brake booster got the manifold port, and ive got the transmission line on the back of the carb. I also changed my booster check valve and pcv valve.

The results of those things which should have been improvements is that not only does it still stall, but now it won't change gears on its own.

I had a buddy swear is gotta be the edelbrock 1406 im using, but swapping to a holley 600 he had didn't change anything. Same symptoms, so I put the 1406 back on. That right there makes me think bad transmission, but it's really coincidental that my shifting issue didn't start till I cleaned up those lines.

On top of all that, now I can't get the dang thing to idle below 1200 rpm, where before it was dead on 700 rpm. That's driving me nuts since I didn't change any setting on my 1406, I just unbolted, sat it to the side, trested the holley and put it back on an hour or so latter.

Sorry for being long winded, but I gotta do something here. I was in a wreck a few months back that totaled out my main transportation. Im constantly borrowing a vehicle from family and I hate it. I want my truck lol it's 90% there to being what I want it to be after having it 11 years. I don't want to have to sell this thing now just to get reliable transportation, not when im this close, but im kinda at that point. So yea, anybody got any ideas?

[cadillac_al]

"electronic plug open"? That doesn't sound good but we would have to guess at what trans you have.

[xs-style]

Its a th350. I believe the ones after 1980 were called the 350C. They had an electronic connection on the side for computer controlled shift points. However if your not running a computer, it will still work as normal. At least they're supposed to from everything im told and have read

[Big Port Jimmy 6]

Plug the modulator line and see if the idle issues abate. Rob the modulator off the old trans and flush it and put on the new one. Is your truck cammed up? Did the old trans have a bigger stall convertor than stock? Jay

[Dead Parrot]

The C is also a lock up converter.

On a stock motor, the brake goes to the carb throttle plate port and the transmission goes to the manifold port.

Was the new converter for a 350 or 350C? Not sure they are interchangeable.

[xs-style]

Jimmy: no cam and stock converters both ways. And ill try that idea tomorrow.

Parrot: ill switch those lines out and see what I get. The converters were different, so im running the one appropriate for the transmission.

[xs-style]

Jimmy: no cam and stock converters both ways. And ill try that idea tomorrow.

Parrot: ill switch those lines out and see what I get. The converters were different, so im running the one appropriate for the transmission.

[xs-style]

Well I guess ill update. I tried flushing and reusing the old modulator. No change. I switched vacuum lines so that the transmission has the manifold vacuum port. I changed the dipstick and tube to be sure I was getting an accurate read from the fluid level (check its good.). I changed the oil (not that that should have anything to do with my issues or so you would think). The power connection on my carbs electric choke was making intermittent connection so I fixed that.

The end result is no change whatsoever. Still not shifting right, and still stalling at low speed and in reverse.

The idle came back where it was though. Funny part of that is that I didn't do anything. The idle came back to where it was after I changed the oil. So now it idles at about 800rpm and sounds great. I mean I still can't drive it, and there's no good reason why changing the oil effected the idle speed, but its the possessed truck, so I don't really question to much anymore.

Were all stumped here, so anyone got anymore ideas? This is how bad its become....One of the guys I go to when ive hit a wall is my bossman at the parts store. He was a service tech for 27 years at the Chevy dealership before selling parts. Even he's saying find another and use this as the parts truck. So I dunno which way to do this. Still open to suggestions lol

[xs-style]

This may be important, may not. The tailshaft has developed a leak in the last week or so. Only if its parked on an incline, but it's still there. And every now and then while I've been working on this thing ill hear a thunk in the rear while rolling. I did the universal joints while the transmission was out. I also checked the u bolts and shock mounts while I had the transmission and bed floor out of the way.

[wilkin250r]

While I can't point you to a specific solution, I would suggest a change in philosophy at this point. A different way of thinking.

It sounds like you've checked all the likely scenarios, all the "one-stop-shop" possibilities. It might be time to start separating your issues, and tackle them one by one rather than looking for a singular cause.

Your stalling issue can only be related to your transmission by either a vacuum leak, or by something terribly wrong with your transmission/torque converter placing too much drag on your engine, which is an unlikely scenario. I would do some more diagnostics on vacuum leaks (vacuum guage, maybe plug EVERY vacuum port and diagnose from there). It's entirely possible it's not related to the transmission at all, and it's purely an engine problem, and you need to check timing/carb settings/compression/valves/ect.

[Dead Parrot]

Mine had a similar 'on an incline' leak. Didn't impact operations until enough fluid leaked out to cause the transmission to be too low. The fix on mine was to replace the interior real seal. It wasn't the one on the end of the tailshaft.

Mine also sometimes has a clunk with starting out from a stop. More felt then heard. Clunk survived all new u joints and carrier bearing. Figure a drive train with 200k+ miles has a right to clunk once in a while.