anyone have a guess? sand in the carb.

ok, my yz400f has been running a bit poor when you nail the gas from nil to WOT. as part of my winter tear down i decided to investigate. everything in the carb checked out as perfect except for this..


that's not cool! how'd this happen? there are no leaks anywhere. i clean my filter after every ride. i believe i'm currently using bel ray (maybe maxima, i can't remember right now and don't feel like walking downstairs) filter oil, but it doesn't feel as tacky as the twin air stuff i used to use. the twin air stuff could actually be substituted for glue in a pinch. i cleaned everything up and the diaphragm and all associated rubber appears to be ok. i know for a fact no sand is getting into my tank while fueling. the diaphragm is intact and isn't rotten yet. this is also the only place i could find sand in the carb. everything else was immaculately clean inside as i took it apart. anyone seen anything like this before? i want to know if there's anything i should be fixing before i put the carb back together.

It's either fuel, the air element or the seals to the AP need replacement. I would replace the AP Diaphram and Orings. Even check the AP Rod. Dump your fuel and clean the tank and I GUARANTEE that's CRAPS in the engine.

where would i expect to see it in the engine? my guess would be around the plug and valves, correct?

Thats normal on the old carbs. that sand comes in along the sides of the rod. The parts you are showing is not in contact with the fuel :bonk:

The plug would give and indication. Sand gets into EVERYTHING over time. If it's just isolated to the AP then your lucky, but since your doing a winter tear down take a further look. It sure as hell can't hurt.....DEFINITELY DO AN AP OVERHAUL...

That's from water getting on top of the AP diaphram. This is a common problem on bikes equipped with the FCR carb with the plastic cover over the belcrank/cable assembly. Later FCR carbs have a metal cover with a rubber seal to keep water out.

The trick for reducing the likelyhood of this happening again is to put a small amount of waterproof grease on the AP shaft when it's assembled.

thanks all for the help! :bonk:

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