CJ - backfiring problem help (a little long)

I have converted my 86 to a manually tuned engine with about 9 deg static timing and a ported vacuum advance. Mine pops and burbles when I decelerate down a steep hill.

If it actually 'backfiring', (firing on the back side of a valve) that can mean your exhaust and/or intake manifold is loose. This isn't uncommon. You can test for it by spraying a mist of water along the seam or carb cleaner or I think even WD40. If any of these make the engine stumble, then you need to re-torque the intakes back down.

You also could have a broken mechanical timing advance spring or a seized up advance plate. The advance plate inside the distributor needs to be lubricated regularly. To do this you pull off the rotor and put oil on the sponge or felt pad you should find in the center of the distributor shaft. You then can hand work the advance plate to be sure it moves freely and springs back hard and fast when released.

Mike

The most common cause for backfiring on deceleration is idle mixture too lean. That could be a vacuum leak with this vintage. They really do have too many vacuum lines, don't they? If that is not it, then short of a new or rebuilt carburetor, I don't know how you get a proper idle mixture. With older models you simply back off the idle mixture screw, until the engine is getting a rich enough mixture. With the smog controlled models, there is usually something to prevent that.

Mike and Bill have some good ideas too. Just don't take either of those guys too seriously. They are pretty old and grizzled, both of them. ;^)

Earle

Mike is not all that old......

Hey there old timer, who you callin' old?

One sign of a vacuum leak is oil puking into the air filter. If that is also happening along with a crappy idle, the OP should post back.

The carb 'can' be adjusted for mix, but it is supposed to stay factory set so the computer does the mix. It 'can' be set back factory if needed. That is another fix.

If the engine won't idle under 1100 or so rpm, then there is another fix for that. (plugged idle tubes)

Mike

Plastic caps that are easily removed with a pocket/exacto knife or the tip of a soldering iron (both are standard shop practices).

Dan

Quite. If you are going to do that, it is a good idea to record the number of turns out for the factory setting. Just in case you later decide, that the problem was something else.

Earle

Nope - no puking of oil into the air cleaner. I'll do the various leak checks that several of you have suggested. Not sure if there's any "plastic caps" left anywhere on the carb that may be hiding mixture adjusting screws. Approx where on the carb will they be located?

Lots of great advice - even from the couple old guys (says Lynn with the full head of white hair!!)

Thanks!

Earle Hort> > > >

Try looking somewhere low, about the level of the throttle plate. You will see a plastic cover, over an idle mixture screw. Like Hootowl says, you can pop, cut or melt it off, depending on tools available and what works best. In the late seventies, early eighties we used to take these off all the time. But we found out, that if this was necessary, there was usually another cause. Sometimes, we just didn't like the factory setting. I guess changing it qualifies as "emissions control tampering".

Although you can tune the idle mixture "by ear" or with a tach., it is helpful to have some kind of exhaust gas analyzer for the proceedings. Since you probably don't have one, I would recommend, that if adjusting the idle mixture helps, do it just enough to eliminate the backfire on deceleration.

Earle

If the idle mix screws haven't been messed with recently, they are not the issue. I would touch them only as a last resort. There are two of them in the front of the base plate.

You are far more likely to have a loose manifold or a carboned up EGR valve. The carb itself is known to come loose at the base plate too!

Mike

On the "main body", just behind the primary bowl on a 4-barrerl carb (I don't remember what engine you said you have-if a V8, It will almost certainly be a 4-bbl). About the same place on a 2-barrel, IIRC. A dealer of parts house can tell you.

Dan