Backfiring and Stalling?

[jonnyc]

Just got my SerIII back on the road in September (after 5 years in the garage) and so far it's been running pretty well. It is a 2.25 gas with a 1950s Rochester, K&N filter, and a Crane ignition. About 2 weeks ago I started getting a few backfires out the exhaust and an intermitent hesitation in the engine (I now think these might be little backfires), mainly when I let off on the gas. It has gotten a bit worse and a few times the truck has stalled while sitting at a light/sign. I had some new bits around so I replaced the in-line gas filter, cap, rotor, and wires...no change. I have been thinking I need to rebuild the carb or sort-out the fuel pump in some way.
Last night I had a thought...perhaps the intake manifold bolt and/or carb-fixing bolts could be loose? I will check that issue tomorrow.

Any thoughts on whether I'm on the right track or missed something important?
TIA

[jac04]

Yes, you are starting on the right track, but you need to go further. Check your timing and carb mixture.

[SafeAirOne]

My vote is for a leak in the exhaust manifold or manifold-to-downpipe joint. That's a common cause of backfiring on deceleration.

[jonnyc]

Thanks, gents. I'll tighten everything around the manifold tomorrow and post any changes. I'm pretty sure timing is where it should be, but the mixture and idle adjustments on the carb are a bit "iffy". It was rebuilt by Recarbco and I haven't been happy with their product. It is the best of the 3 or 4 Rochesters I currently have, however.

[o2batsea]

A rebuilt carb is never as good as a new one. If it were me I'd spring for a carb if I were in doubt.

[jonnyc]

I like the size and power of the Rochester in comparison to the other available carbs. Got a suggestion?

[o2batsea]

Well, actually my suggestion would be to put a TBI and a Megasquirt on it and be done, but I'm sure that's way too scary a proposition.

[Billy5]

I have a Weber 32/36. Zero issues thus far, easy to tune and adjust.

[4flattires]

5 years sitting, hopefully that fuel is not in the system. In Sept, did it run flawlessly, or had it some issues back then, and now it got progressively worse. How old is that fuel?

[jonnyc]

I had a bit of Staybil in the tank, and it ran fine from the get-go. It has been my every-day driver since the first week of September and the tank has been filled at least 10 times since. I haven't taken the pick-up tube out of the tank for a cleaning, perhaps I should look at that also.
Tightened everything in the intake/exhaust line today, nothing really loose. Had to take a family trip, so no testing done yet.
I'll investigate the Weber.

[jonnyc]

Situation is better after everything tightened, new plugs/cap/rotor/wires, fuel filter and pick-up tube replaced/cleaned. Still getting a bit of back-firing and hesitation, mainly when moving at idle (foot off gas). Serious carb rebuild on the school break schedule, everything else seems to be fine.
Always open for suggestions, but no plan at this time to switch to a Weber, one or two bbl.

[Alk-3]

my money is on a bad exhaust as well. I had one for ages, and it did just as you describe. I finally replaced it with the stainless one our host sells, which is louder than the old one, but doesn't backfire or sputter etc.

[jonnyc]

Things got worse after the carb rebuild (see my "Rochester Guru" thread).
Short of replacing the exhaust system at this time, I'm focusing my efforts on vacuum leak issues. After today's search I think I have lots of little ones adding up to a big one. Engine rev'd when sprayed at:
-head-manifold joints (mainly at the exhaust ports?)
-engine side of the int/exh manifold joint
-front of block betw manifold and carb
-front of carb base
-ends of throttle rod

Manifold gasket set is on the way, so I will take the manifolds off and do everything I can do to tighten up all the seam. I will also look for any problems in the int/exh manifold joint.

I hope all this solves my issues.

[Kiloengineer]

Any updates to this thread Jonny? I am having similar issues. I rebuilt my Zenith 36IV and have had a hard time getting it running correctly since. I had leakage from the top cover so I flattened all of the surfaces and cured the leak, some improvement in running. I also found that my PVC valve was not working so I replaced that temporarily with a simple check valve and that fixed the smoking issue I had. Now it idles great but when I accelerate I get backfires and has no power. I have cleaned the carb very thoroughly (4 times), all new jets with a genuine rebuild kit, checked the float level, new genuine Lucas 45D distributor, timing done dynamically, new aftermarket coil (suspect?). I am now leaning toward manifold leaks and/or cracks.

Hope you were successful in fixing yours.
Jason.

[Terrys]

Replace your condenser.