C4 shifting into second too early?

When I leave a stop light (under moderate acceleration) the car wants to shift into second gear almost immediately. It bugs me as the car doesn't accelerate well in second with the tall gear.

The car is a stock 1971 200/C4 with 2.79 gears and slightly shorter (then stock) rubber.

Thanks!

 

what about at WOT? and what about light pressure aka grandma drivin

 

Sounds like the vacuum modulator may be bad, if it does what I think it does. I believe it holds your shifts out longer under moderate to heavy acceleration. I may be wrong.

 

we need PaulS

 

OK taking off @ WOT, it upshifts at 2200rpm for a second and then downshifts back to first for a second or two then upshifts again.

Taking off granny... Upshifts at 1400rpm for a second and back to first just like at WOT.

Now in moderate acceleration it doesn't kick back down to first but the shift is still coming too early.

 

Yes, the shift "hangs" specifically going from 1st to second. Vacuum modulator huh? I'll look into that.

Thanks guys.

 

make sure the vacum line didnt come off the modulator either , could be that simple

 

If the vacuum line comes off your C4 won't shift untill it gets close to 4000 rpm.
I just built a STREET transmission for my wife and you can't feel it shift. It goes from 1-2 and 2-3 before you cross the intersection under light to moderate acceleration. That is how it is supposed to shift with a stock engine. The early shifts are easier on the tranny and engine. If you HAVE to have it shift later then get a green strip modulator and install a pin between the modulator and the valve that is 1.6" (1 5/8") long. That should get you decent acceleration unless you haven't got the down-shift linkage connected. If you don't have it connected or adjusted properly then that explains the back shift to low. Get that linkage on and adjust it so there is .060" clearance with the throttle wide open and the down-shift rod pushed all the way to its stop. You can make the pin by cutting an 8d (8 penny) nail to the proper length and rounding the ends with sandpaper or a grinder.
DO NOT ADJUST the modulator to make it shift later. If it doesn't shift where you want it to with a green stripe modulator and a 1.6" pin then you want a modified transmission. You can get one from Broader Performance for about $900 plus freight. Then you can install a big auxillary cooler to help keep it cool.
If you are in the Seattle, WA. area give me a call at 206.725.0881 and I will talk to you and let you know what your options are from me.

 

Thanks everyone for the responses. I have replaced the modulator with the standard Fram FM2305 that the local autoparts recommended for my '71 200/c4. Is there a difference between this part and a "green stripe"? I reused the pin that was in the car and I'm unsure of it's length.
Having driven the car only once since I made that change, it did not seem to help. As before, under moderate acceleration the car "hangs" on the 1-2 shift. I have replaced all of the vacuum lines, except the 90degree piece that actually connects to the modulator. I plan to replace this shortly as it is fairly stiff and could be creating a small amount of loss.

Paul, Regarding the down shift linkage. I'm not sure I know what that is for a mav. Can you have a look at these pics and let me know if I'm missing parts. I did replace the carb on this car shortly after I bought it, maybe I didn't transfer all of the linkages?

Thanks!

 

Is there more linkage connected to the bellcrank mounted to manifold?
If you look at the transmission - where the lever connects to the shift linkage - there should be another lever and linkage that goes to the carb linkage. If it is connected then follow it to the throttle linkage and check for clearance. (.060 at wide open throttle with the kick-down rod fully depressed)

 

I have heard but never tried about using a regulater like on an aquaium

 

I'm still having the shifting issue with the Mav. Can anyone with a early I6 shoot a pic of the carb linkage (like the ones' that I posted) so I can compare and see if I'm missing something? I'm wondering if I have bigger trans issues or just missing a linkage?

Thanks!

 

Did you look to see if you had any linkage on the bellcrank? (the adjuster screw that the carb linkage hits when you go to wide open throttle)
Look at the transmission where the shifter linkage is connected - there should be a second lever with a rod or cable that connects to that bellcrank. It should be adjusted so that there is only .060" when the throttle is held wide open and the linkage is pushed completely toward the transmission.

From what you are telling us your transmission is shifting 1-2 before it gets half way through the intersection - right? Does it shift into third before or after you clear the intersection with light to moderate throttle?

What you can safely do to increase the shift timing:
put a new pin in between the modulator and the valve. the pin should be 1.6" long (just shy of 1 5/8"). You can make it out of an 8d nail. Cut it to length and round the ends as much as possible. Remove the modulator and pin and install the modulator with the new pin.
If you aren't happy with the results of that you can turn the adjustment screw that is inside the modulator hose nipple no more than two turns clockwise. Be sure to mark the modulator with a paint pen "+2" so the next guy knows it has been adjusted.
If that doesn't make you happy then you will have to shift it manually because anything you do from that point will slowly cause the failure of the transmission. Shifting manually will not hurt the C4.

 

I do have the complete linkage from carb to trans. I have it adjusted such that there is 0.060" between the throttle linkage on the carb with the kickdown linkage fully depressed. Sorry about the confusion Paul, I assumed that there was another one that I was missing.

What I have learned is that when the car flares in the 1-2 shift, I lift off the gas slightly and the car shifts straight to third. This is not a big, foot-off-the-gas lift either, I merely backed out a bit to get it to upshift.

I've read a lot about adjusting the bands on this forum and did adjust the intermediate band (drivers side) today to 1 1/2 turns back from the 10lb/ft tension. No change.

I did not alter the setting of the one on the pass side.

As for the vac modulator and pin. I did a little test & unplugged the modulator at the intake manifold & plugged the intake. Took her for a spin and no difference, the 1-2...er 3 shift happens about 2 grand max. Does that indicate that there are other issues here and not just a "refinement" to the modulator/pin? I don't want another ATF bath to change out the pin if it's not the root cause of my problems.

I also tried shifting the car manually. With moderate throttle applied and shifting at 2500, the trans flares at both the 1-2 & 2-3 shifts. However, in steady state there is no sign of slippage even @ WOT.

Any guidance is appreciated as I was really hoping to get through the summer on this trans.

Thanks, Todd

 

while we're on the subject, what's the purpose of the dual line modular valve? my '73 4dr has one (factory 302, P/S car)

and i just checked with Advance and they keep the green stripe mod. valve in stock for $12.95 (prices and availability may varry)