Popping/backfiring/shifting issue

[Angelo Voltura]

I, for the life of me cannot figure out what the heck is going on, and the only thing I can figure out is I need a torque converter or a governor. I dont think I have posted this here before.

So, here is the deal. 351M built up. Here's a low down on what's under the truck.

1976 351M car block.

400 rotating assembly

.030 over Silv-o-Lite pistons

1974/1976 74cc OC Cleveland/M 2V heads w/ springs

9.5:1 comp

160 stat

Cloyes double roller straight up

Cam is (dont have a cam card but have rough idea on specs) somewhere in the range of .494/.512 @ .050, 236/246 duration (I could be way wrong on this, cold it chops like hell almost wont idle, warmed up sounds smoother, never had a vac gauge on it could be really more like 274 or better) 110 or 112 lsa

Valley pan

Stock length sealed power pushrods and rockers

1 5/8 full length long tubes

Performer 400 intake

1" spacer

1406 Edelbrock with the electric choke removed

MSD 6AL

Blaster 2 coil with stock DS2 dizzy

I would say the engine is making a SOLID 300 horsepower, 1/8 mile ET is in the 8s with a 80+ mph trap speed in a 4700lb pickup.

C6 trans, from a car as far as I know.

B&M Stage 2 street strip shift kit

External trans filter

B&M super cooler

Floor mounted cable operated B&M ratchet shifter that curiously wont shift into first but I beleive has a stretched cable. Never crawled under it to check

9" rear from a Bronco

3.89 Motive Gear rear ratio

28" tall rear tire

Here is the low down on what's going on.

In drive, thing drives MINT. Absolutely beautifully. Nice firm shifts, never skips a beat. Could not ask for anything better.

Wide open is another story. Down shifts are beautiful. Upshifts...not so much. I have to usually lay off a hair for it to shift before I can load back into it, which is accompanied by a loss of power, backfiring and spitting popping for a short time before if seems to transition past it and return to full power and take off like a rocket. It's almost like its loading up. I personally feel that its starving for air on a whole when it's running in top shop and could a slightly larger carburetor. Otherwise, this thing takes off like a snot rocket and blows the tires off it.

However there have been a few instances where I will not have to back off and that is usually when that transmission is HOT. I mean scorching from beating on it.

I have changed the vacuum modulator to no avail. I am getting vacuum at the modulator and the hose is stainless braided. What I'm coming up with is either the governor sets the shift points differently being out of a car or the torque converter is loading up from being overloaded by the torque when the torque comes down as the rpms increase it works better. I did not build the transmission but I beleive it is otherwise stock from what I listed and I beleive it has a 1600 or 1800 stall. I would eventually like to put in a 2500 stall next year as I race it anyways.

Any chance the kick down rod is out of adjustment? Seems to shift a little quick in drive, usually by 35-40 I'm in 3rd. But that could be just me. Down shifts or feels great, no problem blasting into second from a 60mph roll or even lower at lower speeds.

Like I said thing thing takes off like a rocket but I have to let up just for a split second for it to shift and then if I lay right into it in pops and spits a little then takes right back off. This is more often then not. If i dont lay right back into it but ease into it this does not happen. I don't beleive it is flooding the engine, I run open headers with about 4 feet of pipe off the flanges and I do not smell unburnt fuel or foul smelling exhaust. I almost forgot to mention this only happens on 1-2 upshift and never on 2-3.

What's the dealio guys? Never been a big trans guy. The motor is healthy. I would like to get this rectified before I proceed with my 429 swap.

Thanks all.

[Gary Lewis]

Wow! That's a lot to get our heads around.

But, it appears to me that the transmission is shifting too late on the 1-2 upshift and that then allows the R's to go too high and the valves to float and cause the engine to pop and backfire.

So, maybe the kickdown lever is maladjusted? The General Automatic Transmission Service page says:

Kickdown Linkage (Except AOD)

Check for wide-open carburetor and linkage travel at full throttle. The carburetor full-throttle stop must be contacted by the throttle linkage and there must be a slight amount of movement left in the downshift linkage. Be sure the downshift linkage return spring is connected and the downshift lever returns to a closed position

But I don't know what happens if there isn't the "slight amount of movement left".

[Angelo Voltura]

Wow! That's a lot to get our heads around.

But, it appears to me that the transmission is shifting too late on the 1-2 upshift and that then allows the R's to go too high and the valves to float and cause the engine to pop and backfire.

So, maybe the kickdown lever is maladjusted? The General Automatic Transmission Service page says:

Kickdown Linkage (Except AOD)

Check for wide-open carburetor and linkage travel at full throttle. The carburetor full-throttle stop must be contacted by the throttle linkage and there must be a slight amount of movement left in the downshift linkage. Be sure the downshift linkage return spring is connected and the downshift lever returns to a closed position

But I don't know what happens if there isn't the "slight amount of movement left".

Gary, it seems to happen after the shift has occurred rather than before. I did check the kickdown linkage and it is appearing to move properly.

It doesnt sound like valve float to me, its happening at much lower rpm than what the engine will rev to, I usually don't take it over 5200 though.

Edit: By the way, this is in my 79, I know it's not a bull but I know a lot of guys that run this very setup in the bull body.

Also I lied, I have video of it doing it on a 1-2 and 2-3.

I can provide video of it doing it if anyone would like to see it.

[Gary Lewis]

Gary, it seems to happen after the shift has occurred rather than before. I did check the kickdown linkage and it is appearing to move properly.

It doesnt sound like valve float to me, its happening at much lower rpm than what the engine will rev to, I usually don't take it over 5200 though.

Edit: By the way, this is in my 79, I know it's not a bull but I know a lot of guys that run this very setup in the bull body.

Also I lied, I have video of it doing it on a 1-2 and 2-3.

I can provide video of it doing it if anyone would like to see it.

I'm stumped. Let's see what others say.

[Angelo Voltura]

I'm stumped. Let's see what others say.

Sorry for the dark video, it was nighttime when I recorded this. Here is a quick example of it doing it on a 2-3 shift. Notice the excellent downshift, and the delay on the upshift due to backing off a bit and then I put it to the wood and you can hear the popping...as soon as it works past that it feels like it picks up 20 horsepower and takes off.

Like I said it doesnt always do it. Usually has to be hot but sometimes how much I'm on the throttle affects it.

[Gary Lewis]

Sorry for the dark video, it was nighttime when I recorded this. Here is a quick example of it doing it on a 2-3 shift. Notice the excellent downshift, and the delay on the upshift due to backing off a bit and then I put it to the wood and you can hear the popping...as soon as it works past that it feels like it picks up 20 horsepower and takes off.Like I said it doesnt always do it. Usually has to be hot but sometimes how much I'm on the throttle affects it.https://youtu.be/NWqQ6rkrpmY

So, it does it right after the shift? It sounds like it is flooding out. What fuel pump are you using? Do you have a pressure regulator? I'm wondering if the sudden backoff is causing the bowl to overfill and flood.

 

Edelbrock's don't like more than 6 psi, and it wouldn't take much to cause it to flood.

 

[Angelo Voltura]

So, it does it right after the shift? It sounds like it is flooding out. What fuel pump are you using? Do you have a pressure regulator? I'm wondering if the sudden backoff is causing the bowl to overfill and flood.

 

Edelbrock's don't like more than 6 psi, and it wouldn't take much to cause it to flood.

 

That's what I initially thought except if i dont back off it will not shift at all and will rev until the valves float. Unless as I mention on the rare occasion that the trans is very very hot. Then it will shift. Otherwise it will not shift at all without backing off.This only occurs at wot. No other time in any throttle position.I never had a pressure gauge or regulator but I dont beleive it has anymore than 6-7 psi. I am using a factory mechanical fuel pump.

[Gary Lewis]

That's what I initially thought except if i dont back off it will not shift at all and will rev until the valves float. Unless as I mention on the rare occasion that the trans is very very hot. Then it will shift. Otherwise it will not shift at all without backing off.

This only occurs at wot. No other time in any throttle position.

I never had a pressure gauge or regulator but I dont beleive it has anymore than 6-7 psi. I am using a factory mechanical fuel pump.

Edelbrock's literature says not to give it more than 6 psi. If you are running close to that and suddenly back off the rushing fuel could overpower the needle and flood the bowl. I'd put a regulator on it and dial it down to the 5.5 psi Edelbrock recommends.

[Angelo Voltura]

Edelbrock's literature says not to give it more than 6 psi. If you are running close to that and suddenly back off the rushing fuel could overpower the needle and flood the bowl. I'd put a regulator on it and dial it down to the 5.5 psi Edelbrock recommends.

I'll have to check that out.

That wouldnt explain why it wont shift wide open though.

[Gary Lewis]

I'll have to check that out.

That wouldnt explain why it wont shift wide open though.

You may have two problems. Solve one and then address the other.