742B will not run properly

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Am second owner of this machine (2006) w/ 3200 hrs. In the last 8-10 yrs it has never run absolutely correctly. It starts fine, but seems to always need partial choke. I have rebuilt the carb twice, and added a second fuel filter --clear plastic so I can see the fuel. It was running adequately in Dec and Jan so I could plow snow, but then would not stay running. Vigorous pumping of the throttle and full choke would get it started again, and with various and sundry choke and throttle dancing it would act OK, but it wouldn't last. My latest repair project has involved another carb rebuild by the town's best rebuilder, all new ignition components, (except the distributor itself) using all Bobcat parts, new coil and resistor, plugs and wires. Every time I replace a part, it seems to run better, but the improvement doesn't last. After enough restarts to wear the battery down, it will run OK for a max of 5 minutes, lose rpms, backfire,, and quit. I have checked the timing, and it is right on (5 deg BTDC w/o vacuum connected). The only thing I haven't replaced is the fuel pump, but I don't really think it is the problem. Every restart involves a lot of throttle pumping and choking. Today when it was running long enough to work on, I backed out the idle mixture another 3/4 turn. That seemed to help a lot--for a few minutes. One frustrating issue is that the Bobcat service manual and the carb rebuild kits tell how to make adjustments, but none of them indicate the correct settings of adjustable screws to begin with. Any suggestions?

 

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PS--the gas has ethanol treatment and dry gas added.

 

[craigb93]

PS--the gas has ethanol treatment and dry gas added.

Think I would start with using a temporary fuel can w/ a hose to the fuel pump instead of the fuel tank. If that is successful dig into the tank. There is a screen over the tank pickup that can get plugged. You can also remove the pickup assembly to check that. The screen is welded on and should not be discarded. A good tank cleaning may be needed for a long time fix. -Dick

 

[flyerdan]

Think I would start with using a temporary fuel can w/ a hose to the fuel pump instead of the fuel tank. If that is successful dig into the tank. There is a screen over the tank pickup that can get plugged. You can also remove the pickup assembly to check that. The screen is welded on and should not be discarded. A good tank cleaning may be needed for a long time fix. -Dick

It does sound like a fuel delivery issue, also check to make sure that the tank is properly vented; if it's pulling a vacuum it can act like its running out of fuel.

 

[[email protected]]

It does sound like a fuel delivery issue, also check to make sure that the tank is properly vented; if it's pulling a vacuum it can act like its running out of fuel.

I hate to declare victory too soon, but debris in the gas tank seems to have been the problem. I tried the hose in a gas can procedure, and it ran well enough to merit removal of the tank. There was a thin-wall 1/4" tube connecting the heavy fuel line with the metal intake screen , and that thin tubing had broken into many pieces. I can't see how it was ever connected to the thick 5/16" fuel line, but after cleaning all the pieces out of the tank, and reconnecting the intake screen with some new fuel line, the engine seems to run just fine for a prolonged time. I will start it again tomorrow to see if I actually have a pattern. Regarding the vent, there was a 1/4" bolt screwed into the rubber insert. The insert does not fit snuggly, so the tank was not actually air tight, but I removed that bolt so I have a clear 1/4" vent passage. When I refilled the tank, the gas flowed in better than it ever has.

 

[Tazza]

I hate to declare victory too soon, but debris in the gas tank seems to have been the problem. I tried the hose in a gas can procedure, and it ran well enough to merit removal of the tank. There was a thin-wall 1/4" tube connecting the heavy fuel line with the metal intake screen , and that thin tubing had broken into many pieces. I can't see how it was ever connected to the thick 5/16" fuel line, but after cleaning all the pieces out of the tank, and reconnecting the intake screen with some new fuel line, the engine seems to run just fine for a prolonged time. I will start it again tomorrow to see if I actually have a pattern. Regarding the vent, there was a 1/4" bolt screwed into the rubber insert. The insert does not fit snuggly, so the tank was not actually air tight, but I removed that bolt so I have a clear 1/4" vent passage. When I refilled the tank, the gas flowed in better than it ever has.

That is not uncommon, hopefully it was indeed the issue.