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Old 07-30-2006, 10:34 PM
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th350 bad downshift

I know zip about tranny's, so here goes.
I've got a 41 pontiac with a strong 350 and a chevy th350 trans. It has great torque at any speed. spins the oversize radials off the line nicely. It seems to upshift pretty early, but if you are cruising at about 30 mph and kick it, it will downshift and go. However, if its at 45 or 50 and you jump it, all hell breaks loose. near as I can tell, without a tach, The engine just revs hard with nothing happening for a couple of seconds, by the time it downshifts the engine is screaming and I let up on it, and it upshifts again.
Since it pulls great under any other circumstances, I suspect it's a pure shift problem, not a slipping tranny. Any suggestions?

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Old 07-31-2006, 08:59 PM
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I had a similar problem with my setup in my delivery. It took some trial and error with adjusting the kickdown cable. I believe mine was too tight and my thottle cable was binding. Go over the obvious with the cables, and try an adjustment. My setup is w/383 & th350. good luck.
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Old 08-01-2006, 02:24 PM
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350 bad downshift

Fatty50. Maybe you could give me a shove in the right direction. There appears to be about 1/4 throttle opening before the kickdown cable even engages. Does that sound right? Do you tighten the cable so it kicks down earlier, to cure my problem?
Also, since I think it upshifts a little early, I was going to try to adjust the vacuum module. Any chance the early upshift is part of the problem? Shouldn't I adjust that part first? thanks, don
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Old 08-02-2006, 08:42 PM
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By no means am I an expert, but I made sure that I had good vacuum from the intake manifold to the modulator and kept tightening the TV cable a little at a time. When I'm driving normal the car upshifts fine. When I'm cruising and nail it, it kicks down a gear and were off the races. I hope this helps you out.
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Old 08-03-2006, 09:48 AM
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bad 350 downshift

Spent the evening adjusting and testing. First off, the trans upshifts so smooth, even my buddy had trouble telling where it was shifting under normal driving. we adjusted the modulator a little and did some WOT tests. It goes to second about 25 and to 3rd at 55/60. Even WOT it is not a slam shift, fairly smooth. What is a reasonable/normal speed/RPM to shoot for on the upshift to 3rd?
AT 30/35 I can hit it and get a smooth downshift.
Trying to downshift above 45 it feels the same, but I'm beginning to think it just has to get the RPM's up high enough before it can down shift.

Maybe I don't really have a problem. Hooked up a temporary tach and got a surprise, assuming it's right. With the exhausts, the carb sucking air, and old girl trembling a bit under full power, I thought my RPMs were much higher.
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Old 08-03-2006, 08:16 PM
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What kind of gears are you running? Is there a shift kit or anything else done to it? Tire size. My set-up is probably different than yours. I did have my th350 redone and set-up for some abuse. I had built a 383 sm chevy for my delivery and had put in a 2800rpm stall,3:89:1 gears . Is your convertor stock? I'm just taking shots at the problem right now and might be wrong . But I'm going of what I have done and picked up form others by asking 1 million questions. Hope that I have helped some. Maybe you have it correct. If your in high gear that will put the motor at 1:1 ratio and if you hit it the tranny should drop a gear.
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Old 08-06-2006, 10:46 PM
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bad350 downshift

Fatty50. I just did the rear end gear ratio thingy. Both wheels off the ground (non posi), 2 turns of the wheel = 2.5..on the driveshaft. Are we talking 256 gears here? It's a Nova rear end I think.
whats a good 1st to 2nd and 2nd to 3rd shift point, in RPMs for a th350. ?
I think it is shifting at about 2500 RPMs.
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Old 08-07-2006, 07:52 PM
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I'm not sure on the shift points for a nova. But if you have an older chilton's manual I bet you could get a better idea for the mph/shift points. I hope that some of this has helped. I checked out your project. pretty cool.
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Old 08-07-2006, 10:29 PM
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th350 downshift

I don't remember posting anything on the 41 pontiac. Got quite a few pics and will try to get around to it soon. You must be looking at the 28 ford we are doing for my son? We put that on hold for a couple of months, cause the 41 is driveable now, and we are only tweaking it.
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Old 08-08-2006, 05:58 PM
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It was the 28'.
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