Hey everyone,

I'm not new to Pontiacs, but I am new to this forum, so I'll start with hello, and I'm glad to be part of what looks to be an outstanding community! My question:

The TH400 in my 73 Formula 400 has been experiencing problems with the shift points for a while. I rebuilt it about 25,000 miles ago and installed a B&M street/strip shift kit. Everything ran great until one day coming off the highway when the shift points permanently changed. The 1-2 shift came in at a slightly higher rpm/mph, but the biggest difference was the 2-3 shift. Instead of shifting at approx 30 mph under light acceleration, it now shifts at approx 43 mph under the same conditions. When cruising on the highway in 3rd, I can let off the gas and the tranny acts like someone pushed in a "clutch" (engine rpm drops to 1K, no engine backpressure felt). If I give it gas after that, the engine will quickly winds up to normal rpm where I will feel the tranny "engage" and allow acceleration.

The clutches/steels still feel good during shifts (crisp & firm), the modulator has been replaced - no luck. Any ideas? I didn't want to easter-egg with $$$ stuff like new converter, etc. I had a friend say maybe the governor, but he wasn't sure.

Many thanx for any help!!

"...ridge reamer and ring compressor? Do they have tools like that?"

Hey everyone,

I'm not new to Pontiacs, but I am new to this forum, so I'll start with hello, and I'm glad to be part of what looks to be an outstanding community! My question:

The TH400 in my 73 Formula 400 has been experiencing problems with the shift points for a while. I rebuilt it about 25,000 miles ago and installed a B&M street/strip shift kit. Everything ran great until one day coming off the highway when the shift points permanently changed. The 1-2 shift came in at a slightly higher rpm/mph, but the biggest difference was the 2-3 shift. Instead of shifting at approx 30 mph under light acceleration, it now shifts at approx 43 mph under the same conditions. When cruising on the highway in 3rd, I can let off the gas and the tranny acts like someone pushed in a "clutch" (engine rpm drops to 1K, no engine backpressure felt). If I give it gas after that, the engine will quickly winds up to normal rpm where I will feel the tranny "engage" and allow acceleration.

The clutches/steels still feel good during shifts (crisp & firm), the modulator has been replaced - no luck. Any ideas? I didn't want to easter-egg with $$$ stuff like new converter, etc. I had a friend say maybe the governor, but he wasn't sure.

Many thanx for any help!!

"...ridge reamer and ring compressor? Do they have tools like that?"

One our two things may have change first most new modulator's are adjustable, or the vacuum line is leaking to the modulator. or where it comes off the intake. Most likely that's your problem.
Try adjusting the modulator, if the new one is brass in color you can adjust it with a screw driver go inside the hole where the vacuum line hooks up.

I made the mistake of hooking the vacuum modulator line to a ported source(during a manifld switch) instead of full time manifold vacuum and it acted similar to what you are seeing.

"Why that's impossible...and yet it happened!"

Check the governor to be sure it hasn't come apart.

If he was having this problem at WOT the governor might be suspect but he seems to be describing partial throttle problems. It's tied to the vacuum modulator. I'd suspect a line got plugged into ported source at this point. My 2 cents.

Those were the symptoms on my Olds TH400 right before it stopped shifting at all. Cause turned out to be the Vacuum Modulator. As others have said it makes sense that these part throttle troubles are tied to vacuum.

John from Apache Jct.

"Nothing Beats an Ol' Goat..."
http://www.wacomquartz.com/images/li...on2orqcopy.jpg

I agree. But before you replace it, check the vacuum line to be sure there is no crack and that it is in the correct port on the manifold. If it is old and is cracked or come off it usually will do the 1-2 shift at higher rpm's and then if and when it goes into 3rd it downshifts back to second easily or if you let off the throttle it will downshift as you apply the throttle again. Cracks usually occur on the ends of the hose and not in the middle unless it has been pinched or rubbed.

JLP

Thanks, all. I think you may be on to something. I cleaned out the tube going from the carb to the vacuum modulator, and checked that the vac hoses were ok. I think I found a leak. Anyways, when reinstalling the tube I think I scooped up a heap of gunk into the end of it, because now it won't get out of first gear (symptoms got worse!). I'm gonna replace all my vacuum hoses just to be sure and because they probably need it! I curently have the line connected to the carb vac port on the front drivers side down at the base near the intake. Is this what you mean by "ported" vacuum?

"...ridge reamer and ring compressor? Do they have tools like that?"

Usually the vacuum connection on the drivers side front-of-carb is a ported source.This is not where you want your connection for the trans to be.Find a full-time vacuum source on the manifold or rear of carb and make your connection there.If in doubt, use a vacuum gauge to check the tap.

"Why that's impossible...and yet it happened!"

I think I read something about this in Car Craft a few months back, but I can't find the issue. On a a stock 73 q-jet, which ones are which?

"...ridge reamer and ring compressor? Do they have tools like that?"

the adjustment on the tv cable didnt slip did it??? that can cause what your describing too...really assed up shift points...."mean"

Fixed it. It was the governor. A pivot pin on one of the governor flyweights had partially come off, causing only one flyweight to pivot and thus no governor valve movement at all. As a result, my tranny was shifting only on the vacuum modulator input. $20 bucks and 1 hour later, everything is just fine. http://forums.performanceyears.com/g...on_biggrin.gif http://forums.performanceyears.com/g...on_biggrin.gif http://forums.performanceyears.com/g...on_biggrin.gif Now I just need to fix the leaks... http://forums.performanceyears.com/g...n_rolleyes.gif

Thanks to everyone who gave input!

"...ridge reamer and ring compressor? Do they have tools like that?"