620 carburetor problem

55 50 Ron

Well-known Member
Needs slight choke at full engine RPM to run smooth. With choke fully off, still full RPM, it "hunts" (varies RPMs some) and the hunting gets worse as the RPMs are lowered. Increasing the choke as the RPMs decrease, keeps it running smoothly. It was reported to not run smoothly prior to the carburetor being cleaned and it was thought the cleaning would clear the problem. Cleaning has helped but not totally eliminated the hunting. This is without pulling a load.

Carb was cleaned and new gaskets installed. Adjustments to idle needles and the fuel metering screw have been made per the Carburetor Service Manual SM-2024. No adjustments on the linkages have been made.

I'm not super good on solving carb problems. Any help appreciated. Thank you.
 

Well, there are 2 possibilities. First, check for adequate fuel flow to the carb. If you have good flow, check the intake manifold gasket. Spray a little ether around the intake to head joint. If the rpms pick up, the gasket is leaking air, which leans out the engine, thus the need for a little choke.
Good luck.

Adrian
 
Ron, quite possibly your idle circuit still has a small amount of debris in it. A piece of dirt or rust the size of a piece of pepper will mess up these carburetors. It's also possible the metering valve "umbrella" has stuck to it's seat and is limiting the fuel flow to the two sides. I have also seen the seat get turned which blocks off the holes in the seat. Mike
 
2ND POSSIBILITIES IS CHECK THE INTAKE MANIFOLD. i HAVE SEEN THEM GO TO PIECES INTERNALLY AND MIX AIR FROM THE HOT - COLD SIDE OF WARMER ,AND MAKE THEM RUN LIKE THAT . LOOK IN THE CARB SIDE AND THEY DECAY AT THE BEND .TOOK ME MONTHS AND A LOT OF HEAD SCRATCHING TO FIND IT .GOOD LUCK JD435
 
I agree with Mike on the idle circut as being the most likely problem if just choking a little makes it run alright, and to do a good job cleaning them out,you need to get the steel balls out for each circut. By trying to blow air through,you should be able to tell if it is just one side or two you need to clean, and which one. Good luck
 
Mike, You mentioned metering valve "umbrella". I'm not familiar with that description. Would you explain that a little more in terms of the parts breakdown in the parts book? Thanks.
 
Thanks to all for all the good information. Lots more things to check. I may become a carb "expert" yet. Ron
 
Ron, the center metering valve on a Duplex carburetor has a riveted-on tapered stem I call an umbrella. This fits down into a tapered seat in the carburetor body. This seat is commonly made of brass and has two holes that are "square" to the carburetor. These holes feed additional fuel to the main nozzles in addition to the fixed jets. The book suggests to bottom the tapered metering valve in it's seat and then back it off an appropriate number of turns (commonly in the range of 2 to 2.5 turns out). The "umbrella" is made of steel and as mentioned is riveted to the metering valve. It is not uncommon for the seat to be turned in too hard and the umbrella sticks to the valve. It then comes apart from the metering valve and then you have a mess. I have seen this happen many times and have also noticed the seats turning in the carburetor bodies. The seat has two marks on it that look like a screwdriver slot (they aren't). If the seat is indexed correctly in the body these slots will be "square" to the carburetor as well as they line up with the holes. If the seat has moved the carburetor will not allow the tractor to run or pull like it should. This is a very often overlooked item on a Duplex carburetor. Hope this helps. Mike
 

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