Backfire on downhill

I had to opportunity to drive my B in the hill country of Texas this last week for a few days. It was beautiful to be out and driving the hills and curves. For anyone who knows the drive from Medina to Vanderpool on 337 knows it has a couple good hills to climb and then on the backside you either ride your brakes or use the engine to slow you. I noticed that I was getting a lot of backfire when doing this downhill and engine braking in 3rd etc. Can anyone tell me is this normal or is my timing off or something. Do others encounter this problem..........first time I have taken a big hill with my b

thanks,

Greg

You're either running rich or your timing is retarded, or both. Its a simple tuning issue in most cases.

an over-rich idle mixture can cause that too.

I have the Weber DVG carb and have tuned it according to the instructions...if I go any further clockwise with the jet adjustment she stumbles...rpm set to aprox 900 rpm at idle.....am I missing something? I was thinking I may be running a bit rich also. Valve lash was just set before the trip so that should not be a factor.......any ideas?

There also could be a slight exhaust leak and the back pressure is sucking fresh air into the system and causing combustion of unburned gases in the system.

With the DGV you're only adjusting the idle mixture. The fuel mixture while driving (throttle open) is determined by a fixed jet in the float bowl. There's actually a pair of jets - the primary (main) and secondary (2nd progressive throttle butterfly).
If you have the stock 25D with the manifold vacuum avance, your advance rate on deceleration may be too low for the ported fitting on the carb. How is yours attached? What's your timing set at? Try 12-14 BTDC at idle.

Jeff.....as for the vacum advance line it is routed directly to the carb and the manifold fitting has been plugged. I am not sure exactly where my timing is as I have never adjusted it and am not real experienced with doing so............have left well enough alone until now.

If you have the original distributor, there should be a small aluminum know on the bottom side with an A/R in the distributor casting next to it. Turn the knob about 2 fulls turns in the A direction to advance the timing a few degrees. That may cure your problem and pep up the car a bit.

I found the knurled knob that Jeff was reffering to and tried to move it and it wont budge. I noticed that the whole assembly of the vacuum advance moved with it slightly when I tried to really force it and felt sort of loose in its grip.....now have I opened a new can? I wonder if the advance is working properly..........I think I was sleeping the day timing and vacum advance was taught :)

Hopefully you didn't twist off the steel rod in the middle if the knurled nut. You can find out by pulling on the main (round) body of the vacuum advance. If it pulls out (more than 1/16") its broken. You'd need to remove the distirbutor and replace it.

so how much backfire is expected/appropriate when using the engine to decelerate? is none the ideal?

Yes, none is ideal. Backfiring is fuel in the exhaust system igniting, and it SHOULD all be burned before it leaves the combustion chamber. Otherwise your mileage is probably poor or performance is lacking. It can just be late ignition timing too - the spark is still burning after the exhaust valve starts to open.

I'm having the same issues, down hill backfiring ditto with the DGV. Tried timing as well as adjusting DGV per factory, still remains. mileage is poor.