Low Speed Roughness

Bernie W · 07-02-2008, 01:41 PM

I have a Craftsman riding lawn tractor w/17-1/2 HP B/S engine. When running at full throttle with the mower blade engaged, it runs fine. But when I disengage the blade and keep the throttle open, the mower snorts, farts, misses and ultimately dies. After that, it starts right up and does the same until I engage the blade.

A friend told me that the problem lies in the low speed idle throat and that the carb needs to be cleaned. I've cleaned it 5 times already with no improvement.

My issue though, is that this motor has an idle mixture screw that has a limiter cap on it. In effect, it prevents the operator from setting the idle mixture way too rich or way too lean. The limiter cap is not removable. The manual states: "This engine is equipped with an idle fuel adjusting needle with a limiter cap, which allows some adjustment within the limits allowed by the cap. Do not attempt to remove the limiter cap as it cannot be removed without breaking the asdjusting needle."

I believe my stalling problems lie in here because I've cleaned everythuing else so many times. So, how do I get the adjusting needle out to clean this fuel passageway too?

Thnx.

loupoin · 03-19-2009, 02:47 PM

I was hoping to see a more replies as I have the same problem and model (but it's my neighbor's mower- I'm just trying to help the mechanically declined). He's had it in storage 2 years (he did run out the gas before storing). I charged the battery, removed and cleaned the carb and air filter, replaced fuel line (it was cracking), and put in fresh fuel. After re-assembly, it started on the first crank but was loping, or surging. It runs fine at full throttle under load but surges at anything less (but does not die out- like yours). After reading a lot of posts here, I going to now check 1) fuel cap vent, 2) plug and gap (again), and 3), governor and carb linkage (again). My neighbor thinks he remembers it doing this just before he stored it (ahh haa, maybe not storage related).

My thoughts... FWIW! My first thought was the limited fuel/air mixture screw. When I cleaned the carb, I noticed most of it is plastic including the jets. I guess 5 years of use could wear away plastic, so it could be running a little rich. The limited range of the low speed fuel/air mixture screw did nothing for the surging and the low idle adjust dscrew did nothing when I adjusted it from fully closed to fully open, huh?! And the tension spring on the governor linkage connects to nothing but itself. The end toward the carb is mounted in the linkage hole and the other end mounts on the governor arm coming out of the engine less than an 1/8" from the linkage. It effectively has NO tension on the spring. When I use a finger to buffer the governor, the mower runs smoother, but still surges. To me it seems it should be mounted to a fixed location. If anything gives, I'll let you know. If anyone has words of wisdow or advice, please let them flow.

loupoin · 03-20-2009, 08:43 AM

If you're still around Bernie, the problem is solved. After trying all those other solutions I mentioned yesterday, it still was surging. All that was left was the idle mixture so I removed the plastic limiter with a dremel. I adjusted it all the way in then out (even took it out completely) with no results. So no fuel was being delivered via the low-speed jet. When I cleaned the carb all I did was clean out the gunked-up pickup tubes, not the passage in the top metal part that leads up to the idle mixture screw. So I removed the carb (again) and thoroughly cleaned the top passages. She purrs like a sewing machine now.

I imagine the surging is the governor's attempt to keep it running at low speed by momentarily kicking up the speed until the hi-speed jet delivers fuel then idle back. When no fuel comes from the low-speed jets, it continues the cycle. My theory anyway...

07-02-2008, 01:41 PM	#1 (permalink)
Bernie W Registered User Join Date: Jul 2008 Posts: 6 OS: VISTA	Low Speed Roughness I have a Craftsman riding lawn tractor w/17-1/2 HP B/S engine. When running at full throttle with the mower blade engaged, it runs fine. But when I disengage the blade and keep the throttle open, the mower snorts, farts, misses and ultimately dies. After that, it starts right up and does the same until I engage the blade. A friend told me that the problem lies in the low speed idle throat and that the carb needs to be cleaned. I've cleaned it 5 times already with no improvement. My issue though, is that this motor has an idle mixture screw that has a limiter cap on it. In effect, it prevents the operator from setting the idle mixture way too rich or way too lean. The limiter cap is not removable. The manual states: "This engine is equipped with an idle fuel adjusting needle with a limiter cap, which allows some adjustment within the limits allowed by the cap. Do not attempt to remove the limiter cap as it cannot be removed without breaking the asdjusting needle." I believe my stalling problems lie in here because I've cleaned everythuing else so many times. So, how do I get the adjusting needle out to clean this fuel passageway too? Thnx.

03-19-2009, 02:47 PM	#2 (permalink)
loupoin Registered User Join Date: Mar 2009 Posts: 2 OS: Vista ultimate	Re: Low Speed Roughness I was hoping to see a more replies as I have the same problem and model (but it's my neighbor's mower- I'm just trying to help the mechanically declined). He's had it in storage 2 years (he did run out the gas before storing). I charged the battery, removed and cleaned the carb and air filter, replaced fuel line (it was cracking), and put in fresh fuel. After re-assembly, it started on the first crank but was loping, or surging. It runs fine at full throttle under load but surges at anything less (but does not die out- like yours). After reading a lot of posts here, I going to now check 1) fuel cap vent, 2) plug and gap (again), and 3), governor and carb linkage (again). My neighbor thinks he remembers it doing this just before he stored it (ahh haa, maybe not storage related). My thoughts... FWIW! My first thought was the limited fuel/air mixture screw. When I cleaned the carb, I noticed most of it is plastic including the jets. I guess 5 years of use could wear away plastic, so it could be running a little rich. The limited range of the low speed fuel/air mixture screw did nothing for the surging and the low idle adjust dscrew did nothing when I adjusted it from fully closed to fully open, huh?! And the tension spring on the governor linkage connects to nothing but itself. The end toward the carb is mounted in the linkage hole and the other end mounts on the governor arm coming out of the engine less than an 1/8" from the linkage. It effectively has NO tension on the spring. When I use a finger to buffer the governor, the mower runs smoother, but still surges. To me it seems it should be mounted to a fixed location. If anything gives, I'll let you know. If anyone has words of wisdow or advice, please let them flow.

03-20-2009, 08:43 AM	#3 (permalink)
loupoin Registered User Join Date: Mar 2009 Posts: 2 OS: Vista ultimate	Re: Low Speed Roughness If you're still around Bernie, the problem is solved. After trying all those other solutions I mentioned yesterday, it still was surging. All that was left was the idle mixture so I removed the plastic limiter with a dremel. I adjusted it all the way in then out (even took it out completely) with no results. So no fuel was being delivered via the low-speed jet. When I cleaned the carb all I did was clean out the gunked-up pickup tubes, not the passage in the top metal part that leads up to the idle mixture screw. So I removed the carb (again) and thoroughly cleaned the top passages. She purrs like a sewing machine now. I imagine the surging is the governor's attempt to keep it running at low speed by momentarily kicking up the speed until the hi-speed jet delivers fuel then idle back. When no fuel comes from the low-speed jets, it continues the cycle. My theory anyway...