Backfiring Options

[BfloRandy]

Happy Spring, aka fix your car so you can drive it all summer!

Having an issue tuning my Weber 40's.

The car does run, and idles quite well, sounds smooth and even. If I rev beyond 3k, I get backfires. Starts fine too. Ordinarily I would assume it's a simple matter of resetting and resyncing.

Passenger side carb is as it always has been, fine. Mix adjustment screws work normally, idle adjustment also. I see the squirt from the accelerator jets.

Driver side, not so much. It's getting gas. For some reason, the mix screws have little to no effect. This is on BOTH cylinders. The only time I notice any change in the engine's sound is if I back them way way out, then I get backfiring and carb throat sneezing. Otherwise, I can have them set just about anywhere, including fully seated, and there is no difference. I've pulled the carb off, (three times now) torn it down and blown everything out a couple of times already. I did see some blockage in the idle jets and mains before I blew them out.

I also noticed that I didn't see any squirt from the accelerator jets. I tore it down again, and fiddled with the pump (new last spring) and reassembled it. Accel jets now appear to work normally.

It doesn't at all help that my kids decided to reorganize a ton of things, and I now can't find my Weber manual. (IMG:style_emoticons/default/chair.gif)
Can anyone help? Thanks!

[IronHillRestorations]

From behind my computer, I'm going to guess that your main jets are too small, or the main circuit is plugged or gunked up.

Is it at all possible that you've swapped the air bypass screws and mixture screws?

I've referenced this post several times over the years, here's a cut and paste, this was written for 3 barrels, but the tuning principals are the same:

Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.

Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns.
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest idle, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way, you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.

That's a rough, five minute draft of my carb tuning proceedure, hope it helps!

If it goes good it should take about 45 minutes, if not about three years.

PK (IMG:style_emoticons/default/cool.gif)