Quick Fuel goes intermittently lean

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dartman59

dartman59

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Quick Fuel 780 SS on a 408 small block. The other day, the motor "bucked" when starting--not sure what happened; I may have released the key for a microsecond and then re-engaged the starter; maybe a tad to much initial timing--and ever since then my carb's been very lean but only intermittently. I have O2 sensors in both header collectors so I get an instantaneous display of how things are running. Normally, I run on the rich side, at both idle and cruise, 12.5-13.5. When it goes lean, it's 15.5-16.5 at cruise, and about the same or even leaner at idle (makes for a rough, uneven idle and sometimes stalls when coming to a stop sign).

It'll run lean for 2-3 outings (this my daily driver), then mysteriously go back to normal. I thought for awhile that if I put it in neutral and punched it, so it went up to the rev limiter and then back down, that doing such would clear it, but I don't think that really works because it doesn't consistently make a difference. The first time I did that, I did it because I was pissed off at the situation, and much to my surprise it went back to normal after that. Probably a coincidence.

I'm stumped. Oh-when it bucked, I saw a puff of smoke come from under the hood. It didn't backfire. Thanks for any ideas and suggestions.
 
what kind of fuel pump? Electric or mechanical? What's fuel pressure at? It might be time to pull carb apart and blow out the main body and metering blocks, could be some trash in it. Also Check fuel filter. Also, could your o2 set up be off? Bad ground, bad o2 sensor?
 
My fuel pressure is good (mechanical). When it's in the lean "mode", it also just generally runs poorly and rough and uneven. But not horrible, just noticeably not good.
 
Is it after the engine is to full temp? Could it be boiling the fuel in the carb? I ran into that once last summer, ran like crap and felt like it wanted to quit every time I let off the throttle.
 
Yes, I've read those two articles--they're both great, and I have them bookmarked. What is so unusual about my issue is that it started immediately and out of the blue. One minute, fine and dandy (albeit a bit rich), and the next minute (and I think it was immediately following the "bucking" incident), it runs like crap and super lean. When I come to a stop, the A/F ratio jumps up to about 18-20, it almost stalls, then settles in at about 15-16, while it used to ideal at about 12.5-13.5. And sometimes it seems to clear up and behave like normal, only to revert to "misbehaving" for no apparent reason.
 
How are your PVCR's tuned? This could also be something as simple as a clog.
 
I've never touched the power valve. Do you think my symptoms point to some kind of blockage in that circuit?? I've been thinking some sort of blockage somewhere in the carb but not sure where to start looking. It's interesting to me that it's lean at idle AND at cruise, and I think it's lean at WOT, but I'm not 100% on that.

A clog would make the most sense, because it started misbehaving so suddenly and maybe also because it can be intermittently better or worse.

Normally, I wouldn't even be driving this baby this time of year, but it's been a mild winter and my wife's car is on the fritz so she's driving my "winter" car lately, and I get to drive the Dart. Don't get me wrong--I'm loving driving it. But the salt on the roads, and the often slippery conditions, leave me gringing most of the time. I find that I wash the car about twice a week. :cheers:
 
Yeah, I had this problem last summer. I had a 'loose' power valve that opens pretty much whenever I touched the throttle above 2600, because it wasn't tuned right, it leaned out my cruise for some reason. Now I have a valve in there I need to be meaningful before it opens and the channel restrictions correct there..

The whole manifold vacuum divided by two thing worked by me.

On the other hand, your ride was fine, so I'd take off the metering block and clean it out with some carb spray.

Just a thought.
 
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