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  1. #1

    Mikuni Carburetor Corrosion Question

    Hi, I've been lurking around these yamaha forums for nearly a year now as I was trying to decide which "jetski" to buy. Essentially I didn't want to pay a lot, didn't mind a project ski, "low" hours, 3 seater, and something that would be pretty fast and reliable. I finally decided upon the ~2000 yamaha skis w/ the 1200cc motor in it. I preferred non-power valve, but it's not a big deal really. I did end up finding what I wanted, and got a 80 hour 2001 XL1200 limited. It's in nice shape, but a compression test and poor running of the engine revealed that there were issues in at least cylinders 2 and 3. I got it pretty cheap....but now that it's all apart, I wish I could have gotten it cheaper lol.

    Anyway, I tore the whole thing down and besides needing to rebuild the entire engine, I'm pretty disappointed to see that the inside of the carbs (especially carb w/ the accel pump on it) are quite corroded inside w/ the whiteish powder (aluminum oxide I think?) from old nasty gas sitting in the carbs for too long. So I'm wondering if anyone has encountered carbs this bad before and whether or not they're worth trying to clean out and rebuild. The goal is to keep this ski and use it for a long time, probably at least 5+ years. I want it to be reliable, and if I'm going to dump like $1500 into rebuilding the engine, I don't want the carbs to kill a new engine. I'm fairly certain plugged up carbs ruined the current engine; everything looks to have been way lean and got toasty. All 3 pistons are trashed, all 3 jugs have light scoring but also have a couple small gouges, piston wrist pins were jammed in 2 of the pistons, wrist pin bearings are bad on 2 pistons, middle connecting rod/crank bearing is very sloppy, drive coupler end of crankshaft has bad bearings and middle piston crank bearings feel rough as well. Luckily the crankcase is in nice shape.

    Back to carbs, it looks like I can get new, genuine mikuni's for about $160 a piece at a few sites on the net. Good, used carbs are options too, but I don't want to pay nearly $150 for used carbs that would each need a $50 rebuild kit. So it really boils down to do I risk a newly rebuilt engine on a set of corroded carbs?

    The plan with this thing is to do osidebill's carb rejetting, osidebill's fuel balancing, probably keep stock intake box but drill it out like crazy like one guy did, keep oil injection but oetiker clamp the lines, waveeater clips + the better PV sync thingies, replace all fuel + oil lines, clean out the tanks, and just get the thing running. This does have a riva s3 stinger and the catalytic sensor resistor already done (not ideal, came w/ the ski and original parts are long gone). Then after I can get on the water and have some fun w/ it, I'll deal w/ the ride plate, intake grate, pump sealing, and if I still haven't spent enough, maybe the upgraded solas prop.

    Here's the carb pics of the worst carb:
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    Thanks for your time!
    - Andy


  2. #2

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    Those are not the worst I have seen. Seems most of the corrosion is where the gaskets are. You can use a brass wire brush and clean them up, just be sure you get all the passages clean of any corrosion. I suggest upgrading the jetting to the O-Side Bills recommendations and adding flame arrestors for better performance, mileage, and longevity. Longevity due to the fact the original jetting is so lean any tiny disruption in fuel will score a piston. Also, I have been using Stabil Marine in all the gas lately to reduce the above corrosion potential. Seems to work, also the boats I have had apart that are premix seem to not have the corrosion, but maybe that's because those boats use SO MUCH gas it doesn't have time to corrode.

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  4. #3
    Quote Originally Posted by LT1GMC View Post
    Those are not the worst I have seen. Seems most of the corrosion is where the gaskets are. You can use a brass wire brush and clean them up, just be sure you get all the passages clean of any corrosion. I suggest upgrading the jetting to the O-Side Bills recommendations and adding flame arrestors for better performance, mileage, and longevity. Longevity due to the fact the original jetting is so lean any tiny disruption in fuel will score a piston. Also, I have been using Stabil Marine in all the gas lately to reduce the above corrosion potential. Seems to work, also the boats I have had apart that are premix seem to not have the corrosion, but maybe that's because those boats use SO MUCH gas it doesn't have time to corrode.
    Oh ok cool, I'll give that a shot. To me, these are the worst looking carbs I've seen in all the small engine carbs I've rebuilt/cleaned/whatever with the exception of one carb whose bowl was completely full of rust. I've been running stabil in everything too, so I'll try that w/ every fillup once I get this ski all set. I'll do oside bill's rejetting and fuel balancing too.

    I was planning on going the cheapo way like Chris03HD did and drill out the airbox and use his carb settings. I think you commented on that much later on in that thread, but do you think it's better to do the aftermarket FA's instead even though it costs a couple hundred for whatever performance gain? It does seem like the aftermarket FA recipe is much more proven, but I also wouldn't really mind changing out $3 jets. Also, what FAs would you recommend? All I could really find were prok FAs from island racing.

    Anyway I'll clean the carbs up and post new pics. I'm getting genuine mikuni rebuild kits from a local guy from craigslist. 3x sealed kits for $100. He sold his XLT before rebuilding the carbs, so no need for the rebuild kits.

    - Andy

  5. #4

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    There were some F/A's for sale in the parts for sale section. I have used the Riva kits, and also WFO has had some alternative brand ones. All are pricey though. I went with stock setup on my XLT for many years, but after seeing how much better it runs with the carb mods and F/A's, highly recommend them. I cannot comment on opening up the stock box, but O'side Bill did say that was ok to do. Not sure if the same jetting works however.

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  7. #5
    Finally got around to cleaning up the carb in the pictures above, it really cleaned up nice w/ the brass brush and a little 800 grit sanding on the gasket mating surfaces where some thicker corrosion had developed. Luckily it was mostly all surface corrosion. The metal is hardly pitted at all where gaskets need to make good seals. I'll see if I can get some pics tonight if I can remember.

    On a side note, the outside of the carbs have a bunch of old dried up nasty looking gas on them that has eaten through the paint and corroded the outer aluminum. The brush cleans that up nicely too, but should this all be blasted away (bead blaster, sand blaster...whatever) and repainted? I honestly don't care what the carbs and engine look like, but if there really is a benefit to painting, I suppose I could go that route. I'm gonna have 3 silvery jugs anyway unless I paint those too =). And this ski will never see saltwater or be stored outside in the winter if that matters....

    - Andy

  8. #6
    bikermikeyblack's Avatar
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    that's wat that dam ethonal does to the carbs ...ive seen it in some lawn mower engine so bad I had to scrap the carb ...I was going to add somewhere I read that if u put on flame arresters u have to put in a bilge pump but I wast sure why is that anyone ?

  9. #7

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    I would not go to the trouble to paint the carbs unless you are just that kind of guy. I would however use a good preservative/lubricant on them line Boeshield or something thicker than WD40. Do that a time or two a year and you will be fine. (I used to paint the jugs and such but it takes alot of extra time and there really isn't a payback unless you ride in saltwater.)
    Quote Originally Posted by awh4992 View Post
    Finally got around to cleaning up the carb in the pictures above, it really cleaned up nice w/ the brass brush and a little 800 grit sanding on the gasket mating surfaces where some thicker corrosion had developed. Luckily it was mostly all surface corrosion. The metal is hardly pitted at all where gaskets need to make good seals. I'll see if I can get some pics tonight if I can remember.

    On a side note, the outside of the carbs have a bunch of old dried up nasty looking gas on them that has eaten through the paint and corroded the outer aluminum. The brush cleans that up nicely too, but should this all be blasted away (bead blaster, sand blaster...whatever) and repainted? I honestly don't care what the carbs and engine look like, but if there really is a benefit to painting, I suppose I could go that route. I'm gonna have 3 silvery jugs anyway unless I paint those too =). And this ski will never see saltwater or be stored outside in the winter if that matters....

    - Andy

  10. #8
    Quote Originally Posted by bikermikeyblack View Post
    that's wat that dam ethonal does to the carbs ...ive seen it in some lawn mower engine so bad I had to scrap the carb ...I was going to add somewhere I read that if u put on flame arresters u have to put in a bilge pump but I wast sure why is that anyone ?
    From what I've read, the extra bilge pump is only necessary if you get a lot of water in the bilge (or you want that bit of extra insurance. Personally I think for the XL/XLT skis, the extra pump won't be necessary, but hey it's not a bad thing to have. I guess if your bilge is always got a ton of water in it, then maybe it's a good idea.

    - Andy

  11. #9
    Here's the pics from the cleaned up carb

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  12. #10

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    If you're going to paint them, paint carefully around the bushings that hold the butterfly shafts. A little bit of overspray or paint may cause the shafts to get sticky, which adversely affects your throttle. michael

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