Back to the Backfiring Problem

Earlier I posted in here that I was having a problem with backfiring on my '62 CJ-5 with an F-134 engine. I had installed a Weber K-533 2-bbl in place of the original single throat Solex. It was running fine and then all of a sudden, I started having problems with backfiring. I found out that one of the intake valves was out of adjustment and was not closing all the way. The jam nut had backed off. I discovered this by doing a compression/bleed down check where the number 1 cylinder was reading about 87, when all others were in the mid-120's. So I adjusted all the valves to the proper lash, rechecked the compression which was fine, but still had an issue with backfiring.

I found that if I push on the progressive linkage between the primary and secondary, so that the secondary opens along with the primary instead of progressively, I can accelerate to high rpm's with no backfiring. If I leave the choke on just a bit, I can accelerate with no backfiring, too. So I am thinking that this points to an excessively lean fuel situation in the primary side of the carb. I finally found a breakdown of the carb on line. Here's what I am wondering, before I tear into the carb. There is a power piston assembly in the carb. If, when I had the backfiring issue due to the intake valve, it were to have ruptured the diaphragm on this assembly, what would the net result be? Could this be causing it? I am not nearly as familiar with carbs as I am fuel injection, so I am looking for help.

Could jetting in the primary be a problem? If this were the case, I would think that the backfiring would have been a problem from the start. I am trying to piece all this info together in some fashion that leads to a cause and effect reasoning, but it isn't adding up. By the way, I have checked for intake manifold/carb base leaks, and there are none. I even disconnected the hose going to the vacuum boost unit on my brake system and plugged the port to make sure it was not part of the problem and it wasn't. I haven't had time to work on the Jeep, but this weekend looks promising. I need a new viewpoint to start with. Thanks...

 

Two words.
Carter YF.

 

Take another look at the timing... most carb problems tend to be exaggerated by ignition problems.

 

Is it backfiring through the tail pipe or is it spitting back up the carb?

 

Might try running some Sea Foam through the carb.

And I agree with Scoutpilot. Through the carb or out the tailpipe?

You might spray some carb cleaner around the base of the carb and the butterfly pivot points to see if you have a vacuum leak.

 

+1 on ignition issues, or valve issues. It is really hard to get a properly igniting engine to backfire, fore or aft, with just fuel issues. The valves should be sealed during ignition. Either your valves aren't closing all the way, or you're igniting at the wrong time. Unless you are really rich and getting so much raw gas spilling through to your exhaust that it is igniting. But that is more "gunshot" -style backfiring than just popping.

 

Through the carb.

 

Through the carb, as I said above. I have done the leak check stuff, just to make sure, and things are tight. No vacuum leak. Once I got the valves adjusted, I put the vacuum gauge, and it was solid and reading where it should. I forgot the value, but it was not an issue.

 

Somebody told me that the advance might be too much, so I asked for a value. I checked it with my timing light, and it was within the range they gave me. You have to excuse me, but I haven't had time to work on the Jeep for almost a month now, so I don't remember those values offhand. I agree with all of you in here as to what the normal backfiring through the carb problems are, but this is just different somehow. Why would opening both the primary and secondary barrels at the same time by pushing down on the progressive linkage in a pivot point between them allow me to accelerate with no backfiring? Or why is it I can pull the choke part way out (after the engine has reached operating temp) and accelerate with no backfiring, too? The other thing that is difficult to understand is why was it that everything was operating normally until the jam nut in the valve adjustment backed out and then it started backfiring? I sure would like to be able to answer these questions. They are driving me nuts!!!

I will keep working with it. If it had been running funny right after I installed it, I would suspect the carb. But it ran for several weeks and ran good. Then all of a sudden, this started for no apparent reason. When I do get to the bottom of it, I will let you all know what happened. It's just kinda crazy...

 

"The other thing that is difficult to understand is why was it that everything was operating normally until the jam nut in the valve adjustment backed out and then it started backfiring?"

The initial backfiring could have opened up an intake gasket leak, or messed up something inside the carb, or etc,etc. (Maybe.)

 

When a motor experiences a “Spit-back” through the carb caused by an intake valve that is out of adjustment, it is possible, and likely, if not corrected immediately, the power piston diaphragm will rupture.

 

The old saying that 90% of carb problems are ignition has a lot of validity, but it sounds more like a lean carb on the first throat. I battled this (and still am to some extent) on my 240z. It has a set of Del'Lorto triples on it that were jetted for a 2.4l motor and now reside on a 2.9l engine.

On the Weber/Del'Lorto side drafts, the "idle" jet controls the mixture up until the transition point of around 2500 rpm or so, much higher than you would think. Increasing the idle jet size will help if you are having a lean backfire from the carb. Depending on how the rest of it is setup, an emulsion tube change could benefit it as well without increasing jet size.

On my DHLA carbs, those aren't available new anylonger so idle jets is what I work with.

 

You're right, Glenn. There has to be a reason that things have changed. I just haven't found it. I am a patient man and will get to the bottom of it. My brother knows a lot about them, having owned, and still owning vehicles with carbs. I sent him an exploded view of the carburetor and he has told me some things to investigate

 

That's what I was wondering. Looking at the exploded view, it seemed that this could be a possibility. I will be looking at that today. I just didn't know if it would cause it to run rich or lean. I couldn't figure that out. Thanks for the affirmation...

 

Mike,
Thanks for the information. I had a 280Z, but it was fuel injected. I would liked to have ridden in a 240 with the triple Webers. I can't imagine that set-up and trying to keep it tuned. I will check the power piston diaphragm and see if it has ruptured, and if it hasn't, I am going to buy the book on Webers and the re-jetting kit. I will get this thing dialed in. Later...

 

I just pulled the top off the Weber and checked the power piston diaphragm. It is fine. The inside of the carb, being new, is clean -- no dirt, not a sign of anything out of the ordinary. I am going to get the book and the re-jet kit and play around with it. I told my wife she is buying it for part of my Xmas. I owe her a bunch of bling anyway. She just hasn't gotten around to picking it out. This will just cost me more. I am sure this will prompt her. I will keep you and everyone else in here (thanks for all the help -- gave me a list of things to check and I appreciate it) posted as to what I find out. By the way, the backfiring doesn't occur until I hit the rpm's mentioned in here, about 2100 rpm.

 

It may be that the transition to the secondary side is where the lean issue is occurring. This is about the point where handover from the idle circuit to the primary circuit would occur. Will be interesting to see how you finally resolve it.

 

Is this the jeep that also got the electronic ignition? (Sorry if not - it's hard to keep all the builds on here straight in my head.) Before you totally commit to tearing into this carb, if you can, I'd swap back to the standard ignition just to rule that out.

Does it make I difference if the engine is warm vs. cold?