Carb icing? Strange carb problem CR500

[n16ht5]

Yesterday we rode near Whistler, and my CR500AF (38mm keihin airstryker pulsed) ran great, except if I held the throttle pinned for more than 1-2 minutes straight.. it would run out of gas and sputter/stall very abruptly, as if the floats were not letting in enough gas to keep the bowl filled. If I only got on it for a minute or less it ran fine, and half throttle I could run indefinitely. I checked the gas cap / fuel tank flow, and checked all the vent lines which seemed to be clear. Also changed the plug with a new one, no difference. Could this be the carb icing up? It was 40deg in 2-3ft fresh mashed potato snow. I have not had this problem before. I'm going to pull the carb apart and hope it's a sticky float, but that would be strange as it ran 100% last ride. I've only had a carb ice up on my yz450f, and it just ran poorly the whole time, unlike this.

 

[byeatts]

Yesterday we rode near Whistler, and my CR500AF (38mm keihin airstryker pulsed) ran great, except if I held the throttle pinned for more than 1-2 minutes straight.. it would run out of gas and sputter/stall very abruptly, as if the floats were not letting in enough gas to keep the bowl filled. If I only got on it for a minute or less it ran fine, and half throttle I could run indefinitely. I checked the gas cap / fuel tank flow, and checked all the vent lines which seemed to be clear. Also changed the plug with a new one, no difference. Could this be the carb icing up? It was 40deg in 2-3ft fresh mashed potato snow. I have not had this problem before. I'm going to pull the carb apart and hope it's a sticky float, but that would be strange as it ran 100% last ride. I've only had a carb ice up on my yz450f, and it just ran poorly the whole time, unlike this.

sounds more like a dirty strainer in the tank or obstruction in the float needle

 

[M5-]

Depending on air flow, which in your scenario would be quite a bit for a 2 minute pull, carb icing can start at 6 degrees C which is about 43 F so its possible. I'm not sure that the other symptoms are saying carb ice. When my 500 occasionally ices it wont rev out until the ice melts, it runs super fat.

M5

 

[wwillf01]

Yesterday we rode near Whistler, and my CR500AF (38mm keihin airstryker pulsed) ran great, except if I held the throttle pinned for more than 1-2 minutes straight.. it would run out of gas and sputter/stall very abruptly, as if the floats were not letting in enough gas to keep the bowl filled. If I only got on it for a minute or less it ran fine, and half throttle I could run indefinitely. I checked the gas cap / fuel tank flow, and checked all the vent lines which seemed to be clear. Also changed the plug with a new one, no difference. Could this be the carb icing up? It was 40deg in 2-3ft fresh mashed potato snow. I have not had this problem before. I'm going to pull the carb apart and hope it's a sticky float, but that would be strange as it ran 100% last ride. I've only had a carb ice up on my yz450f, and it just ran poorly the whole time, unlike this.

I had the same issue but it was a faulty venting gas cap.... It would get worse and worse though as the ride progressed ... Ice in the check valve cause this also for me.. I went with a ventless cap.. I know you said you checked this ...worth running with the fuel low and without the cap next time.. I would leave my drain open on the carb and my fuel would not drain though with this issue...


Sent from my iPad using Tapatalk HD

 

[partypete]

With that pulsed carb it's hard to tell but your lean. I thought I was draining the fuel bowl faster than it would fill on the wfo big load scenario, but I progressively went up on the pilot jet until it went away. Then just fine tune your jetting. Also with that carb the pilot jet affects the whole rpm range.

 

[n16ht5]

what pilot did you end up with???

 

[kell8071]

Some of the worst carb icing I have had has been in the mid to upper 30's but with high humidity. It got to the point of running super fat and stumbling but also the slide freezing open, not cool...


Kell

 

[summitboy]

Chris tell us what your jetting is on your Pulse ? Also which PWK 38mm is it. The short carb off the YZ250 or is it a regular round cap PWK ? What is your float height ?

I highly doubt its ice especially if it runs fine for a minute at WOT. I bet your float height is wrong

 

[n16ht5]

55p 200m jets.straight avgas. Ill go ahead and check the floats. it starts first kick in just about any weather. Im thinking the gas cap isnt flowing fasy enough.

 

[summitboy]

Which carb ? There are two different airstyker PWK 38mm carbs amd they both have a different float height. The short carb was used on the yz,kx and suz bikes. Has a tps top on it and the other carb had the regular round screw on cap.

 

[n16ht5]

regular, straight from Ron at pulse

 

[mxracer299]

The carb on my girlfriends 300 was doing the same kind of thing most of this year and I couldn't figure out for the life of me what was going on, narrowed it down to the ktm gas cap... stupid. It sounds like you have something similar going on.

 

[summitboy]

So the round cap. I believe the stock PWK 38 Airstryker carb float height should be 16mm. I would ask Ron what he thinks when running the big main such as the 195+. That carb comes with a 170 main usually from the factory. I never had a fuel running low issue on either of my YZ or Regular Pulse carbs that were on my snow or sand 500. Mine were set to factory spec float height.

If memory serves me right the short YZ250 carb float height is 5.5mm-7.5mm

 

[Kieselguhr Kid]

Carb icing is more likely at partial throttle, not full throttle as there's added cooling effect from the air expanding on the back side of the throttle.

If it's not icing at partial throttle it's likely not happening at full throttle. As others have said, it sounds more like a fuel starvation issue.