Backfiring under load

[silverscottsk10]

Vacuum advance tube from the distributor should connect to the driver's side port (manifold vacuum) on the Edelbrock 1406 unless the engine is equipped with a working AIR pump. Connecting it on the other side will make it idle poorly.

Note legends on those ports.

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It's always been set to other side and just started having problems

 

[rich weyand]

Combination of factors. Change the vacuum advance port, re-time it to 14* BTDC at idle with vacuum disconnected, and see if the problem persists.

Then you can cross timing off the list of possibilities.

 

[silverscottsk10]

What about timing chain?

 

[rich weyand]

Could be. The pistons/valves/ignition definitely do not agree on what should be happening when, but why not start by eliminating the easy stuff first? Your vacuum advance connection is incorrect. Symptoms are identical to timing being way retarded. Description "under load" is same as "at low vacuum", a.k.a when there is no no vacuum advance, giving further indication it may be way retarded on base timing. Total time to move vacuum line and re-time engine: 15 minutes. Total cost: $0. Start there.

 

[silverscottsk10]

Will do got new cap wires and rotor for it

 

[silverscottsk10]

Switching vacuum ports helped a little
Still diesels bad when it's shut off and seems a bit harder to start
It's at 14° btdc without vacuum connected

 

[rich weyand]

Which distributor is it exactly?

 

[silverscottsk10]

Ac delco from carquest

 

[rich weyand]

It will want to start very rich. Push the pedal to the floor and hold one second, four times. Turn the key and it should wick right off.

 

[rich weyand]

Next question is mixture. What jets and rods are in the carb? Which step-up spring?

Have you re-adjusted the idle mixture screws since you changed the advance and timing?

Idle mixture procedure. With the engine off, turn both screws in until they stop. (Do not tighten!) Now back each screw out 2-1/4 turns. Start the engine. Adjust idle to 700 rpm. Back both screws out 1/4 turn. Did the engine speed up? If so, back both screws out another 1/4 turn. Continue until the engine slows down, then turn both screws back in 1/2 turn. Adjust idle to 700 rpm (automatic) or 600 rpm (stick).

 

[rich weyand]

Another thought here. Your step-up pistons could be stuck. When you get on it hard, the vacuum which is holding the pistons down goes away, and they should pop up, lifting the rods in the jets and richening the mixture. You could be going lean enough for some of the piston strokes to not fire, and pop off in the exhaust.

Pull the air cleaner off. There are two Torx screws on either side of the carb between the front and rear barrels. Loosen them a couple turns and swing the cover plates out of the way. The pistons should pop up. bob them up and down to make sure they aren't sticky. Better is to pull them out, clean them off, and spray down the bores with some WD-40 to clean them, then put the pistons back in. You should be able to "bounce" the pistons on the springs when you put them back in. Push them down, swing the cover plates into place, and tighten (lightly; don't strip the holes!) the Torx screws.

 

[silverscottsk10]

It's definitly got more power
It'll spin out with the little rain we had
I'll do this after the storms pass
Suppose to get bad

 

[silverscottsk10]

So now the truck is missin out under load after switching the vacuum lines and new wires

 

[rich weyand]

Did you clean the step-up pistons? Without them popping up, you will run lean, which can cause intermittent miss.

 

[silverscottsk10]

At 2200-3200 it's fine
It's just getting up to speed when it does it