Technical Turbo 350 help

Good morning, I recently had the trans rebuilt for my 1941 ford coupe. I’m having a problem when driving at about 35 mph. It continuously shifts from 3rd to 2nd and back. It only does it just above idle not when pulling. I also changed to a pretty big cam and the result is 6 in. vacuum. Trans builder, Abruzzi Racing Transmissions suggested adjusting the vacuum modulator but I can’t get to it without major work due to the x frame and crossmember.Does anyone think this adjustment would solve my problem? I would hate to go through Almost dropping the trans with no results. Thanks for any advice. Tom

 

that is what I would do. can you make an access opening in the frame? a small hole is all that is needed.

 

What he ^^^^^^ said

 

I thank you for your reply one way or another I will get to it. Was just verifying what builder said before I dig in. Have a great day

 

Some modulators were adjustable and some were not, make sure yours is before making access holes.

 

I think you have not been able to buy a new, non adjustable modulator for a couple decades.

 

Have you tried disconnecting the vacuum line to the modulator, plug it on the engine side, and see it makes any difference? Just a suggestion before tearing into the harder job of adjusting it.

 

It is attached to manifold vacuum. Ported vac is less than 6” at idle

 

Is your kick down cable adjusted right.(The one for passing gear) If not it will downshift early. It is easy to adjust if needed.Pop the clip at the mounting bracket,hold the carb wide open and lock it back in.

 

I have not see a non-adjustable modulator in a very long time, either.

Obsolete product support logic would tend to dictate making a single, adjustable part, that fits all applications, rather that a multitude, to cover all potential applications.

 

Agree that the problem is likely the modulator is getting confused by the low vacuum. Thinking heavier throttle than actual engine load.

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Might be tough to get it right with that little vacuum. Usually something like that would have a manual valve body.


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Well if I can’t get it dialed in I can always drop it in 2nd when erratic shifting occurs.

 

Vacuum sounds really low, all my trannys took vacuum from intake manifold 3/16/-1/4 Id line. are you sure you do not have a vacuum leak , have run hipo cams, I never had that low on vacuum. Also I had one that the rebuilder changed the governor springs that changed rpm shift points, not related to vacuum.

 

3/16 hard line with 2 short hoses no external leaks. Engine just freshened by reputable builder, .Cam specs mech roller .660 .666 lift 254 [email protected] duration 110 lsa no problem with engine I even have marginal power brakes. I’m sure vacuum gauges vary a few inches.

 

IMO, Pretty radical cam for street, recognize lift from a BBC I owned, mine did not have that much duration, yours BBC ? suggest you make frame/crossmember access hole to the modulator big enough so you can change it if necessary, I have a few other suggestions, hopefully a real tranny builder will comment as I like most here buy rebuilds and install them, not a tranny expert, tranny shop in your area ?, might be worth the money for a pro opinion, thats how I found out about governor springs, he knew instantly the problem I described and fixed it, reasonable $ charge, curious, try a different vacuum gauge, 6 is really low at idle. The last 2 trannys I purchased were from a shop that has a trans dyno, No problems, a few more bucks involved.

 

Yes it’s a big chevy
The builder I used came highly recommend. Abruzzi in Warren Ohio. He does Dyno his builds. Motor runs great and idles fine at 600 rpm. No problems other than vacuum. Just need to get under car and get things adjusted.

 

Are you running a distributor with a vacuum advance? Is it connected to manifold or off-idle vacuum? Do you know what the initial timing is set at?

 

No vac advance modulator on manifold vac initial timing 18 total 38 all in about 2800

 

Vacuum operated transmissions will not shift properly with 6 inches of vacuum.Usually you would have to lift your foot off the throttle to get it to shift to high gear,but when you try to accelerate it will shift back down to 2nd.Vacuum should be checked at the modulator.You can try to back the screw out,I would take it right out.They used to make a mechanical modulator operated by a cable hooked to the carb or a manual shift kit or valvebody may be your only options.Hope this helps.

 

Marty Thank you,
once I get access I was going to Back it out one turn at a time Are you telling me it can be removed? I never had the pleasure to mess with one

 

With the vacuum line off the modulator, it would shift at the maximum shift points. Might have to go 70 or so just to get it in high.

 

Guys who do diesel conversions also use a mechanical modulator.

 

Reread your post it shifts into high properly problem is when putting along at 35 mph it continuously downshifts to 2nd and back to 3rd if it is pulling a little it stays in high gear

 

If access to the modulator screw is a real pain take it out.This would give you the earliest shifts possible.If there is a noticeable difference you can put it back in and adjust from there.If there is easy access go 2 turns at a time, 1 turn out may not make much difference at 6 inches vacuum

 

Thank you sir good information

 

Did you try it with the cable disconnected?

 

Don’t think kick down cable comes into play at 1/8 throttle or less it has a ton of slack and seems to work as it should I’m banking on modulator adjustment and if that doesn’t help I will manually pull it into second before this condition occurs thanks again, Tom

 

Sometimes a vacuum advance distributor using full manifold vacuum can be a big help as far as increasing idle vacuum. But with your current 18° initial it may be about as good as it gets.

Does the pipe from the intake to the modulator appear to fabricated or is it possibly a GM production, factory, original part? From the factory this pipe often had an intentional restriction to dampen the vacuum signal to the modulator. It looked like a short area of the tubing that was crimped down in size to create the restriction. The restricted area can get plugged but the restriction can usually be easily cut out and replaced with a section of vacuum hose.

On a TH350 the trans kick-down cable, I believe, only controls the kick-down at near wide-open throttle. The vacuum modulator and the governor are responsible for controlling shift timing and firmness.

 

Well I managed to get to adjustment screw, turned an alen wrench Into a screwdriver. Backed it out 2.5 turns. Unfortunately could not test it they tar and chipped the 4 mile road out of here. Thanks to all who chimed in on my dilemma and will post results when I can.Tom