Cavitation After Fuel System Work....?

[RocketFox650]

I've been chasing a slight miss all season and the last time I took it out I pulled the seat to find the entire engine shaking, (Misfire on one cylinder) so I just pulled out since it was the end of day anyways. Figured enough was enough, pulled the carbs off and stripped them. Sure enough, both internal filters were dirty, one musta had a qtip sized amount of green gum once I cleaned it out. Reassembled everything, new gaskets, install. Adjusted a little slack out of my oil cable, I'm smoking a touch so probably a bit high. Decided to get that thumb lever slop out and crisped up the throttle. Little too much, idle is ~1800 with no screw contact, I'll back it out a thread or 2 and tighten the retainer nuts back up in the morning. I am running without a silencer right now so I can tune my low speed especially. Otherwise, everything is normal. I STILL have a infinitesimal miss at zero throttle. With the seat off I can see it better than feel it. I'm gonna pull the plugs in a minute to check them, maybe close the gap .001-.003. engine sounds really tinny if you know what I mean. 2 strokes sound metallic tho, and I'm listening to it echo in a hull with my head upside-down in it... Probably nothing.

So here's where it got weird- out of the no wake zone I started slowly throttling up. I wasn't gonna take it past a ⅓ throttle since I'm tuning anyways, but I'm already at 3k well before even a ¼ throttle, so I'm not sure that's exactly correct. Anyways, I decided to push a little past, maybe 3,500, and I heard a "pop", nothing serious mind you, literally sounded like exactly what it felt like, the pump broke free and began cavitating. Sounded just like it lost suction, super mild pop. Well I'm easily hitting my RPM limit, but it will not hook up. Trim up, down, center, roving- no change. Hard lean, straight shot, standing, sitting, leaning fwd/bkwd, etc-no real change. I think I was able to hit 10mph. I power loaded this ski in waist deep water, pulled out, and this is the first time back in the water. Launched in deep water, seemed great until I opened it up and it broke free. I have no idea what caused this. I mean it's usually look for what changed, simple stuff. Just nothing changed...

I might pop the pump off tomorrow anyways, skimming various parts catalogs I realized 1 specific GTX has an impeller I think will improve performance and I wanna try it out.

This sounds stupid, but is it possible the increased power of the engine is ripping the impeller free and breaking that seal? Like the wear ring is worn and the impeller isn't perfect, but the engine wasn't running strong enough before to break it free, but now it's running a bit stronger so....? Yeah, just looking at what changed and trying to find a common thread, stupid or not.

Thanks guys

 

[RocketFox650]

+1 for stupid theory... I got it to hook up, I don't think it's running full speed but I can break 40-45, haven't pushed for more yet. But any condition that strains that jet seal- hard acceleration, sharp turns, etc- and it breaks free and limits out until I back down and smoothly transition in to accelerating. I bet a new impeller and ring would make a massive difference, but this will work for now. If I have to, I'll rebuild the pump at the launch tomorrow or the next day. Only takes an hour now unless I start dropping o-rings.

It might be something else, but that's what changed, and it sounds good and logical in my head so far.

 

[Lothsahn]

Sounds like a gap between wear ring and impeller to me, or a damaged impeller, or both.

The clearances are really important, and it doesn't take much to make the pump cavitate.

Also I've heard the wear ring can slip and start rotating in the housing. I've never experienced it, but if you heard a pop, maybe somehow the wear ring came loose and got a little chewed up.

I was told to put a VERY thin bead of silicon on the wear ring when installing it to prevent slippage, but I don't know if that's recommended by most people. I have no idea if I can ever get my wear ring out of the housing now.

 

[POPPS]

Just had a spnning wear ring,,,i put 2 circular beads of rtv on the wear ring.