won't idle -- carb bubbling gas -- SD40 (1 Viewer)

[rgentry]

Just driving along with my 1-BBL Aisin and it started idling really rough and cut off due to flooding. When I took the top off the air filter it appeared that gas was pouring into the carb from a port just above the throttle plate. It almost seem like the gas was boiling out onto the throttle plate. The gas seems to be at the right level in the carb window so I don't think the float is stuck.

If I crank it up with the throttle wide open it runs rough for a minute then clears up. If I let it idle, it runs smooth for a about 20 seconds and then the pattern starts all over again. Something is stuck or plugged --what do you think?

 

[grant5127]

Appears you have some bungle in the jungle going on there.... that is, contaminates may have entered the playing field... what does the fuel filter indicate?... ya know, the filter is the crystal ball that tells all..........

 

[rgentry]

Fuel filter seems OK. I spent the evening with the service manual. I think it's a stuck power valve in the carb. I'll pull the top off the carb later this week to check.

 

[Pin_Head]

Are you sure you are not confusing the choke plate with the throttle plate? The brass tubes at the top with angled ends are bowl vents and fuel will pour out of these if the needle valve and float are stuck.

There is nothing else that the fuel will pour out of at idle unless the casting is cracked. If you have your idle speed set too high fuel will also be pulled out of the transition slot that is right above the throttle plate, but this is not normal for idle.

 

[rgentry]

Hmmm. I don't think it's coming out at the top of the carb, I think it would be REALLY obvious if the float was stuck down, would it not? It would have to be tons of fuel coming out. I'll check this afternoon.

Rather, what I see is at the bottom of the carb, where the throttle plate is completely closed (because it's trying to idle) a puddle of gas that is sort of dancing around on the top of the throttle plate with nowhere to go. I assume that this gas is getting pulled out of the transision slot. Would gas be dumped here if the power valve was stuck open?

This is a great carb, fairly new rebuild, been running like a champ, fuel filter looks fine .... It's like something just went CLANK inside. The engine was fairly cool when it happened so I don't think it's heat related. Thanks all for your help.

 

[65swb45]

If you see a ' puddle of gas that is sort of dancing around on the top of the throttle plate', can you be sure it's not dribbling down from the emulsifier? That would be an indication that the idle circuit is not providing sufficient fuel, and that the idle has been turned up to the point where vacuum is drawing fuel [mind you, at a VERY inefficient rate] into the emulsifier.

 

[rgentry]

OK. I will check the float.

Mark, maybe I am interpreting this wrong, but you seem to be saying that if there is too much vacuum, the carb will pull gas through the emulsifier tubes (from the main jets I guess), even at idle. Is that true? Could that happen even if the float is operating properly?

 

[65swb45]

OK. I will check the float.

Mark, maybe I am interpreting this wrong, but you seem to be saying that if there is too much vacuum, the carb will pull gas through the emulsifier tubes (from the main jets I guess), even at idle. Is that true? Could that happen even if the float is operating properly?

Not saying that at all. If the idle circuit will not provide enough FUEL, or there is not enough vacuum to draw that fuel thru the circuit, the engine will not idle. At that point everyone's first instinct is to screw in the idle speed screw and bump up the idle. As you turn the screw in, you are opening the throttle plate, causing vacuum to reach the port that engages the primary emulsifier. Mind you, it's not ENOUGH vacuum for the given engine speed for the emulsifier to spray the gas in a good pattern, just release it as a dribble.

GL

Mark A.

 

[eddybear]

Mark A,....pm'd you. Thanks.

 

[rgentry]

Not much of a resolution for future cases, but here's how it went.

I took the top off the carb and it appears that the float, needle valve and seat were OK. I did notice some sediment in the carb which should not have been there as this is a fairly recent rebuild. The fuel filter (one of those clear Fram universal types) had some of the same muddy sediment in it too. Cleaned the carb and put it back together -- now it idles like a champ. Vacuum at idle right at 18 inches. Something must have gotten in one of the passages and blocked a jet or a vacuum passage.