transmission problem?

I have a sbc 400 with a 700r4 transmission in my Model a. during idle and when I rev the engine in park the engine sounds great and runs fine. when I put it in gear and just sitting there it runs like it should. when I begin driving, it begins popping and acting all kinds of weird. almost no power and lots of backfires. any idea what could be the problem? thanks for all the input

 

Appearently electrical.

 

I had a bad HEI module act like that in a GM car I had once.

 

Start at the distributor, after you check the grounds.

 

What is the idle speed in neural and in drive? Timing will/may change a bit as the r's drop. Is the coughing when you try to accelerate in drive but not as evident when in neutral? Wonder if you are lacking the initial accelerator pump worth of fuel on tip in.

 

i've checked all of my grounds and vaccum lines. distributor is acting fine (went through it less than a month ago). only happens when i try to accelerate in gear. does not act up in neutral or park. idle around 1200 in park and 650 in drive. i replaced the fuel filter thinking it was starving for fuel but thats not the case

 

Seems like way to much difference in RPM between idle and drive. Is the vacuum advance working?

 

So steady throttle in gear (moving) the engine runs ok, but to accelerate you get the cough and belch? What kind of carb? Can you see the discharge of fuel when you open the throttle (with the engine off)?

 

Maybe a bad connection on the spark plug wires?

Or a weak fuel pump?

Or something wacky with the carb?

 

it has always dropped that much as long as i have owned the car. just an old quadrajet on it. yes i see the fuel in carb. any kind of load while in gear seems to make it cough. whether it is acceleration or up a hill

 

new fuel pump, sparkplug wires are all tight. i would think carb but it acts fine when not in gear

 

Don't know Q-jets that well. Can you check fuel bowl level, is it set right?

 

What kind of distributor?

 

im not exactly sure how to check the fuel bowl level. would it all of a sudden change like this? it just has an HEI distributor. would the distributor cause a problem like this? acts fine when not in gear

 

I've also had an old coil act like that...yeah, yeah, I know...everybody says "it's the coil"..well, sometimes it is the coil.
I had a 215 Buick once and the coil was slowly going bad on my way home...ran worse and worse but idled ok. It was dark out and when I opened the hood there were sparks across the top of the coil...didn't make it home and the next day I swapped the coil...ran like a top after that.
Had a 389 Pontiac that gave out with a backfire and quit running...my buddy says "try the coil...I have an extra if you want it." Checked timing, bought new distributor cap and rotor, coil wire and rebuilt the carb..all in the Portland rain with walking trips to the parts store for the parts...wouldn't start.
Swapped in the used coil and viola! Lit right off and ran better than it ever had with the new ign-carb parts in it.

 

I kind of agree with the RPM drop is kind of extreme. To me a car in gear that idles about 650 should only climb to 700-750. Timing can be huge change. Doesn't take much fuel in neutral, and much more in gear with load.

 

I was thinking of something else on a different brand of distributor.

However, GM does have an issue with the distributor leads. They have beed known to break inside the insulation and causing anything from what you are experiencing to not running at all. Check your leads with an ohm meter and wiggle them around while you're testing.

 

okay i will change the coil out and see if that does something. I will also check to see if the vacuum advance is acting as it should

 

I had a similar problem with an hei ignition and found a problem with the electrode that contacts the coil and the rotor being burned and arcing

 

I replaced the module in my HEI, problem solved.

 

Sounds lean, or vacuum advance not working correctly. Easy to check timing and advance. Then pull plugs and see if they are bright white. Go from there.

 

I was thinking the module, but these symptoms usually show up when they get to full (hot) temp. The O.P.'s problems seem to happen immediately.

 

Trash in carb cross wire a couple wires and pull full throttle it should pop back through carb this has worked countless times on a Qjet for me

 

okay i replaced the coil and checked the vacuum advance. drove it down the road and the same problem still. timing has been fine on it so im pretty sure that not it. is there something else i am missing here?

 

Try a set of plugs.

 

just tested timing. that is all good. i did a tune up (including plugs) within the last 1500 miles. i guess its off to the parts store to get a module. seems to be the only thing left in the distributor system that hasn't been checked.

 

just replaced the module. the parts store said it was bad when they tested it. put a new one on...still having the same problem! any other suggestions?

 

Do you still have a choke plate on the carb? Try closing it off some to restrict air (fatten up the mix) and see what that does for the cough. It it improves drivability, I'd guess a carb problem is leaning you out.

 

Is the pickup coil in the distributor good? The wires can break inside the insulation and cause this problem. If you pull on them fairly hard they should stay together. Does it start coughing or chugging as soon as it is put into gear? Possibly the TCC is staying on, although it's rare to see that on a 700r4.

 

I would really look into that quadrajet... My brother had a "rebuilt" one on a 327 in his '65 C-20. Under ANY load it would backfire very badly. We checked points, dwell, timing, voltage. All were fine, so out of frustration we simply slapped on another Q-jet we found in a friends shed. And it accelerated perfectly.