Back to the Backfiring Problem

I got my re-jetting kit today. Now, if I can get some time, I will use the information in the Weber tech manual I got to see what I can do. As I said earlier, I noticed that if I pushed down on the interconnecting linkage between the primary and secondary bores, (the secondary bore is not vacuum operated, rather it is operated by mechanical linkage) so that they were both coming into play during acceleration, there was no backfire and I could accelerate quickly with no issues. Then I tried it with the choke part-way on, and the same thing happened. Once I back off the manual choke and tried to do it, as it made the transition from idle to part-throttle, backfiring started. I will let you know as soon as I get a chance to play with it. As soon as I get off of here, I am going to read the section of the tech manual that deals with this issue and specifically with the carb I have. Have a Merry Christmas!!!

 

I already went back to the original distributor, after I cleaned it up and rebuilt it. I set it up on the bench and then pulled the electronic unit out and re-installed it. I timed it at 5 degrees BTDC and it did exactly the same thing it did when the electronic distributor was in the engine. Thanks for the idea. I was hoping for a change, but no luck. As I said above, after having talked with the Weber rep for about a half hour on the phone, he gave me some good ideas which I can't wait to put into place, once I get some time. I will keep you informed. Thanks for your suggestion. By the way, it doesn't matter whether the engine is warmed up or cold. Same thing...

 

Check the throttle plates very carefully. I believe you may have one of them bent just enough to un-ballance the fuel to air ratio.

 

Check your spark plugs. Clean and re-gap them.
Re-check your timing. Advance the timing a couple of degrees. This will result in the vacuum increasing a little.
Pop the air cleaner off, and with the key removed from the ignition, Press the accelerator all the way, one time. Observe down the carb throat. Did you get a steady stream of gas? or did it just sort a dribble down the side? Steady stream, good power piston/diaphram. Dribble down the side, damaged power piston/diaphram.
If you have to pull on the choke a tad to have it accelerate smoothly, your vacuum is not high enough. Increase the timing a tad.
What do you have your idle speed set to? My engine is not happy unless I set the speed to 1,000 rpm on my tach. Some are like that.
And you might want to pick up that Holly 12-804 fuel pressure regulator and install it. Set the fuel pressure to 3 1/2 psi. The 32/36 DGV Weber is sensitive to pressure.

Do you let your engine warm up a minute or two before driving? What is your Temp where you are? What is the humidity percentage? You just might be a victim of carburetor icing. You will have to let your motor warm up a couple of minutes more.

Check the secondary butterfly valve. Is it closing all the way? Any air gap around the edge of this valve will cause the backfire on acceleration.

 

Might give this a read : http://www.classicinlines.com/WeberTune.asp

 

Thanks for the information. I should have some time to look at this thing as I am off for the next 5 days. Later, and Merry Xmas

 

The carb is brand new.
Timing is at 5 degrees BTDC
The accelerator pump is working as it should and I do get a steady stream of gas as you said.
I checked the power piston as I was worried that a backfire might have damaged the diaphragm. It is fine.
Vacuum is within spec.
I can idle my engine down to 500 rpm and it still likes it. I have it set at 900 rpm.
The fuel pressure is the only thing I have not checked yet. I will see if I can find a gauge to do that today. The fuel pump is brand new.
I live in Calif. No icing here.
Secondary is fine. Thanks for the checklist. have a very Merry Xmas.

 

Carb icing can happen in 80* weather. Seen it happen. It's typically caused by humidity level being high and the velocity of the air going through the venturi dropping the air temp down to a point the moisture in the air freezes. Not saying that's an issue for you but it can happen in warm weather.

 

You're right. I had the same problem with air start motor diesels in the state of Virginia. The venturi would ice up, even in warm weather, if the humidity was high, which it usually was in Virginia. But in the Central Valley where I live, during the summer, we almost never have humidity to deal with. I have seen it into single digits where I live. Being originally from Northern Utah, I love it. I will get to the bottom of this and when I do, I will let everyone know. Merry Christmas to you and yours.

 

Any luck with the jetting?

 

Yes sir, and also with the brake lines. I have them all installed and ready to hook up to the master cylinder. I had to replace all wheel cylinders, as they were toast and found a few places that made me think that putting new brake lines in was a smart move. There were a couple of spots where the old copper lines had been rubbing, and they were very weak at those points. The funny thing is, I put the copper lines out on the front lawn with some of the crud scraped off of them so people could see they were copper, and one of the neighborhood tweakers snatched them up right away. Got him on the security system cameras. I laughed when I played back the footage. Tweakers. you gotta love 'em....

I only had to change one jet, the idle jet. From what I have read, this is one of the problems in that I guess even though it is called an idle jet, it plays into there being a smooth transition between it and all the rest of the "circuits". I will see how it runs once I get it on the ground. With any luck, that could happen this weekend. We are supposed to get a ton of rain tomorrow, but Sunday looks good. I sometimes start the Jeep just to warm it up and circ the fluids a bit and it sounds so good now. It fires so evenly on all cylinders and never misses a lick at idle. The valve train has settled in and is barely audible. The engine sounds better than any other Jeep engine for its era has ever sounded to me. The proof will be when I get it on the road. I am sure I will be doing some "tweaking" but not of the kind spoken of earlier in here...