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Unread 03-01-2012, 01:28 PM   #1
aBoyAndHisCJ
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1978 CJ7 
 
Join Date: Aug 2011
Location: , GA & AL
Posts: 45
Can anyone explain throttle plate deposits?

Hey guys— can anyone explain to me just how throttle plate deposits interfere with carb functioning, in mechanical detail?

I'm about to rebuild my Carter YF, which has recently started to give me a stall while running, or a hard start from cold every now & then, which a few shots of Gumout down the throat and about 5 minutes to soak in always instantly fixes. It's for this reason that I've narrowed the problem down to a gummed-up throttle.

Is carb gum heat-sensitive, meaning that once the fuel really gets flowing and the carb gets hot, the gum is less 'gummy' than when it's cold and been sitting? The stalls I've had are always soon after a hard start. When the engine achieves highway temps, or city stop-&-go driving beyond the first 45 min or so, it will never stall— only on its way to being truly fully warmed-up.

I'm rebuilding the carb for this reason, and also because it's been running rich for a long while now, just DUMPING fuel into the intake to the point where when I first rev up, there's always a black spot on the ground from my tailpipe. But does anyone have any alternate explanation for the intermittent stalling/hard start symptoms I'm describing, & why carb cleaner down the carb will fix it every time? I just want to make sure I'm on the right track— sounds to me to be a gummed up, dirty carb with gunk down near the throttle plate and varnish probably in some critical places in the main circuit.

I'd appreciate any good descriptions of throttle plate deposit functioning, though- I'm not fully clear on the concept of how deposits interfere with the mechanical functioning of the thing.

Thanks!

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Unread 03-01-2012, 10:09 PM   #2
jnicewan
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1995 ZJ 
 
Join Date: Dec 2010
Location: Norman, OK
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Maybe the gumout is giving it a fuel to burn like ether/starting fluid. Deposits usually wouldnt let one dye when going down the road and also cleaning or spraying out the carb should not be something you need to do often. When does it die when going down the road, ( when hitting the throttle, coasting, cruising etc) Hard starts when cold are usually a problem with the accel pump or choke and possibly a vaccum leak, those conditions could also contribute to the stalling problem when running. I would check the idle mixture screws and choke, then verify accel pump and vaccum leaks too.

Carb deposits are usually not a big concern with carbureted engines because fuel flows through the carb and throttle plates, they kind of self clean the important parts with frequent driving. Deposits can do weird things though especially if they are inside the carburetor. They can restrict or partially restrict jets, fuel circuits, vacuum circuits and even cause linkages and metering rods to bind and stick. Right at the throttle plates there are ported vacuum circuits (small holes in bore where the plate closes) Ported vacuum is manily used for things like dist vaccum advance and Evap emissions purge line to charcoal canister.
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Unread 03-02-2012, 06:17 AM   #3
aBoyAndHisCJ
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1978 CJ7 
 
Join Date: Aug 2011
Location: , GA & AL
Posts: 45
The stall has always happened at moderate open throttle while cruising.

I'm led to believe the choke is working correctly by the fact that the choke plate closes the throat when you depress the pedal when starting the engine from cold, and the fact that after the engine has been running, the choke plate goes completely vertical.

I'm not quite sure what a metering rod does- figured I'd learn during the rebuild. Seems to me like whatever the problem is, it's with the main fuel circuit, but I have no carb experience.

The Gumout has worked to allow it to start even when giving it a shot of gas from the jerry can wouldn't do a thing and when a spray of starting fluid would render the plugs obviously wet-sounding for the next few start attempts...the problem was so mysterious to me until I bought a can of Gumout- the Jeep would randomly be unstartable. It's still a really strange carb behavior. It's like I've gotten closer now that I realize carb cleaner will help it start, but I still don't know the exact nature of the problem....
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258 inline 6 , carbon build up , carburetor , carter carb , cj7
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