Popping/backfiring/shifting issue

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.

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.

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.

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.

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.

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.

That could be possible. I'm going to get a regulator here shortly then for when I get the top end back together so I will have it.

That's where my thought with the governor comes in.

But it would make sense if it was flooding out.

I've used this one from Summit and it worked well.  They also sell this gauge that works nicely with it.

I was thinking maybe there isn’t enough vacuum at WOT to get it to shift.

Don’t think that would explain the popping and back firing though.

Gary I have that one on dads 85 and it works amazing. I have yet to have a problem with it.

As far as not pulling enough vacuum at wot...that could be true. I never put a vacuum gauge on it to see what its pulling. I always had good shift at lower throttle positions and good power brakes so I never put any thought to it. But with that monster cam in it, it may already be at minimum operating psi before wot... you may be onto something...

Where are the rpms when you have to back out to get it to shift?
Where are they when it does shift, and you get back into it?

Is your distributor locked?

Sounds to me like the weights are bouncing, or you just have a bad spark box.
Maybe the limiter's just doing its job.
You shouldn't be able to float valves if it's set right.

Angelo, I ran into the same thing with my 1977 F150, I built the 390 FE engine from a Camper Special short block, .030 over stock pistons but used a set of heads from a 1965 300 hp engine along with the intake. I had the actual correct Holley and Motorcraft DS-I distributor. I used my existing DS-II box and the DS-II cap, rotor and wires. The C6 I traded a Mercedes-Benz transmission overhaul at a AAMCO center a friend owned for. He built it using a car governor. First time I really ran it up, it floated the valves on the 390, backfired in both directions, fortunately being at WOT, no power valve damage, I actually think that carb was pretty immune to damage due to the interesting power enrichment system. Bottom line, the car governor was probably for a 390 GT or 428 CJ as it was not providing enough pressure to override the downshift pressure (line pressure applied to the throttle end of the shift valves). The solution was to exchange the governor for a low performance or truck one, never had another problem. Unfortunately I did not have a tach in that truck.

BTW, 16-17 mpg highway running unloaded and was a damn hot rod, real fun to mess with people.

I have my limiter set to 5500 and it usually shifts out at 5000 or 5200. I turned it down 500, it was at 6000. However it will wind until I do that probably hit the limiter. When it does shift it drops to 3000-3500 or so...its been a while and I took the tach out so I cant really remember all that well.

The vacuum advance being connected or not never made a difference. It always ran better with it off and in full advance due to having a both broken springs. I have since fixed that.

As far as the governor goes that what another thought since the trans is definitely from a car although I do not know what. I never tested line pressure but I could see it not being high enough to override. I've never changed one on one of these before...how much of a PITA is it? I heard the tail shaft has to come off.

This is partially the reason why I belive the ratchet shifter got put in other than the ease of installation.

Best I ever did was 12mpg driving it super nice. Otherwise its smiles per mile :)

3k is where the MSD goes from multiple sparks to a single one, right?

Not the vacuum advance, a bolt in the works to lock it tight.
The truck should be bouncing off the limiter when you let off the line lock and launch.

You say that you replaced the vacume modulator, Did you adjust it after you put it In?  There is an adjusting screw inside the spout where the vacume hose hooks up and can be adjusted with a small screw driver like they give away at the parts store.

Tail shaft has to come off, the governor is bolted to a counterweight on the output shaft. The valve itself has a specific weight, but the initial pressure is from a heavier weight with a spring. You might try a small washer inside the valve where the spring goes to add some weight. If this brings your WOT shift points down where you want them, then you can tweak the part throttle with the modulator.

Here are a couple of pictures for you: