Folks: What happens inside the TH350 vacuum modulator on close-to or full open throttle? I am dialing my GS in under a number of driving conditions. When I floor it while all ready underway, it has seemingly a delayed shift from 2-3. If I lift slightly it shifts. Otherwise if I don't lift at WOT the car will pop/backfire, which tells me to lift, allowing the 2-3 shift. The modulator is new, unadjusted from the box. Could the modulator be creating a vacuum leak and causing the pop? If so, could poor modulator adjustment be the culprit? I have checked and rechecked and noted no vacuum leaks when the car is shut off and on the lift. 1) I'd like to understand what can delay the shift from 2-3 (detent at the pedal to loose or tight?) 2) What could cause a pop/backfire when the shift is delayed? 3) Are the trinity of adjustment points different among stock TH350/vehicles? (Trinty = modulator, governor, & detent) The trans is from my Apollo and is stock with a fresh gasket kit. no mods. Ideas? This is nearly a stock drivetrain, although transplanted into a larger vehicle (from Apollo to Skylark GS.) Thanks
The pop is because the mixture is lean. At WOT, the modulator is out of the picture, the governor controls WOT shift RPM. When you lift, vacuum returns and that is why it shifts.
Larry: I have not tuned the secondary's yet. Could that help eliminate the pop. Thanks for the advice on the modulator.
Yes, that is the first place I would look. Are we talking about a Q-jet? If so there are a few places to look. First, make sure the front vacuum break (choke pull off) is holding the secondary air valves closed at idle. Check the spring tension on the air valves. Make sure the plastic cam is raising the secondary rods as you rotate the air valves open. Finally check to see what secondary rods are in the carburetor you are using. The biggest mistake people make with Q-jets is assuming they are all the same and expecting that they can just transplant them to another engine. They were calibrated for the year engine/make/model they were meant for.
Board Member CARMANTX built the carb (QJET) to my application last season (great person to work with) The carb came with build details. I'll dig them up I am not certain the secondary's are opening. No "wooshing" under WOT from a dead stop. The car doesn't seem to wake up until 35MPH or so. I am aware of the tensioning spring and set screw need to be backed off to aid the blades to open. (but not too much)
Vacuum leak would be another cause. Does the engine have a cam? How much vacuum does it produce in Park, fully warmed up?
Below is cam suggested by TA we installed last Fall. If I remember correctly, it had 14 lbs of vacuum with car in D with wheels chocked. I don't remember the amount in P as I am to believe tuning the carb with load is preferred. I can check Product ID: TA_212-350 350H.470"-218'/.475"-230',110' Details 350 Hydraulic Cam, Gives 25 HP increase over stock cam.
14" is good. Popping back through the carburetor means the mixture is going lean. Sounds like the carburetor unless your fuel pressure is low.
It feels like the pop is under mys eat. It only happened 2x and unexpectedly so I'm not certain. Not something you enjoy intentionally repeating. Right or wrong I run a clear plastic fuel filter which normally is full. Not sure if that is worthwhile info. The fuel tank is brand new as is the pick up and pump. It only pops/backfired when the car is well warmed and the 2-3 shift seems to hang. I'm bound to get more data as I continue to drive the car. It drives great cruising at all speeds.
The 2-3 shift isn't hanging, it just isn't happening before the engine goes lean. At WOT, the 1-2 and 2-3 shift points should happen around 5000 RPM for that cam. Your engine isn't running well enough to get there. What ignition are you running?
Well it sounds like a lean misfire to me. The question is, what is causing it? The modulator would cause shifting problems or no shift if it wasn't holding vacuum. It might also cause ATF to be drawn into the intake. How are your minimum and part throttle shift points?
The shift points under light or moderate throttle are quite smooth. I run 87 Octane. Fresh batch the day of testing. (If that is of note)
