Backfires when put in gear? - 1994 Buick Roadmaster

Is there a metallic noise (clanking) when you put it into gear?? You may have a crankshaft sensor or crankshaft wiring that is causing problems. Check to see if there is any wires on the exhaust manifold or the exhaust system anywhere.

Backfiring through the carb is a timing problem. I would bring it back up to top dead center and reinstall the distributer. Also double check your firing order on your plug wires.

check compression is it more then 10% difference between cylinders you have a leak also check for fouled plugs. wires fuel pressure, catalytic converter. i have the same problem in my lincoln its a leaking valve seat

It could be 101 things! but, try changing fuel filter first, sometimes they are
so clogged only enough fuel gets through to let it idle.

hi :) 1) Have the valve check sound like they are burned abit and check to see if the exhaust is in good shape and no holes. 2) The back pressure helps close the valves and prevents the backfiring. 3) The timing chain could be loose and when you change gears this would cause it to backfire too.

Check your fuel pressure, and your spark. If you have good fuel pressure and spark, then it's probably something to do with either your ECM, ICM, or timing gear set. Autozone and O'Reilly's have a fuel pressure gauge that you can put a deposit on to use it. You can ask either if they can test your specific ICM, ECM.

Backfiring can be caused if the timing has changed.
Judging from the age of your truck, it's quite possible that the timing gears have slipped. This will throw the timing of the spark off even if you set the timing according to the marks. The way to tell is to remove the #1 spark plug and crank the engine over in taps... until you get it up on compression in the #1 cylinder...you can tell if you put your finger over the spark plug hole as you tap it thru..
Then check the timing marks...if the marks aren't near the proper lcoation, it's probably the timing gear/chain...
This is a common occurence when after tuning up an older motor... the timing gets adjusted, and it's running good for a bit, then the timing gears jump and it'll backfire, spit, cough and make life miserable...

engine can be out of time
low fuel pressure
or can be a burn valve
need to check ignition timing and fuel pressure to see if they are ok first

you did replace computer also, the engine you put in will need the computer for it,also get all connections.

'till you do some in depth testing of all systems, I'd place equal weight on every possibility. Make sure you cover the basics like fuel filter etc before getting crazy with it though. Also check cap. rotor, coil and wires. Idle cylinder pressure is different than that encountered when driving. This additional pressure will cause ignition spark to seek a better easier ground,(not the spark plugs where it should be going) causing missing under load.After checking , if you haven't found anything, its time to begin testing fuel pump pressures, modules and controls.
good luck