2001 LS2000 Mikuni Needles and Seats

[kuwona]

Any carburetor experts out there? I am rebuilding my carburetors as preventative maintenance on my new-to-me 2001 LS2000. So far I have one carb apart and a disappointment is that the needle tip is visibly worn. The seat is stamped 2.0, but I think OEM should be 1.2. (2001 Yamaha LS2000 (LST1200Z) Carburetor | Babbitts Yamaha Partshouse)(Need a Needle Valve cross reference... (pwctoday.com)) Can anyone confirm?

I can say that I only heard the engines run briefly in the driveway - I can't say if they were running "well" or not. Also, from looking it seems one set of three carbs may have ever received maintenance as the hose clamps on the fuel hoses were a different type. Another thought, might one set of carbs be a different model? They look the same to me but maybe there is a marking somewhere? I plan to get the seats out of the other 5 carbs soon too and will report what I find.

So knowing not much else, is it better to use a needle the same size as what came out (2.0), or the OEM specified size of 1.2 (if that's even correct)?

 

[kuwona]

Okay folks. Looks like not a real common problem I guess. I have the needles out of the 3 carbs on that side - all 2.0, and one from the other engine: 2.0. Of course 3 out of 4 needles are not worn - which makes it even harder to make the decision to go to 1.2 on all carbs. (Had none been worn, I would have reused and hoped for the best.) I am assuming that a previous owner or mechanic who knows more than I do made a conscious choice to put in 2.0 seats and that that person knew how to set up the carbs appropriately. I don't, so I will do my best to get them back to factory specs with 1.2mm needles and seats. (I will cross my fingers the jets are original size.) I guess I will also have to actually figure out how to test the popoff pressure with a bicycle pump. Following the thread @the MfM posted, I am planning for 95 psi springs and 50-55 psi popoff.

 

[itsdgm]

Hey @kuwona i just saw your post.

Just to be straight forward, I have zero experience with a Yamaha 2 stroke engine. But loads of experience with all sorts of 2 strokes. GoKarts, paramotors, etc.

Previously, I did have a 2 stroke sea doo that had Mikuni super BN carbs in it and I did have issues When I bought It. My specific issue was that one engine ran perfect, the other would lay flat in the upper mid range. After tearing into both carbs (which obviously weren’t original) I found them to be jetted differently. So I upped the jetting size on the one that was leaning out and the issue disappeared.

That being said, I’d be very cautious about changing any setting in those carbs until you know if there’s even a problem. If you happen to lean out a carb (or even all 6) you could easily burn holes in the pistons. It’s the most common mistake (other than not mixing oil in the fuel) that destroys a 2 stroke engine.

You probably already are aware, sorry if I’m stating the obvious, but Diagnosing carb issues can be a long process. It sounds like you’ve got the skills to get it done. But be cautious. whatever route you take. You’ll need to run the engine for a minute, look at the spark plugs, run some more, re check, etc. Get it right, happy boat owner. Get it wrong…….complete engine failure.

Anyway, good luck with your project. Hope it all works out.

 

[itsdgm]

Looking at one of the links you referenced.

 

[itsdgm]

Personally, I’d get rebuild kits. Tear everything down, (pay special attention to each jet setting), put everything in an ultrasonic cleaner, make notes of any internal jet numbers, and replace all the parts that come in the rebuild kit. I’d use the needles and seats that are in there for now and see how it runs. Then go from there.
Hopefully somebody with actual experience for that same situation will chime in too.

 

[kuwona]

Oh man. So much stress! So your advice would be to count the turns on the three still assembled carbs, and use whatever those settings are on the other side as well, and hope for the best? I feel like I have signed up to fix something that ain't broke.

 

[AZMark]

Replace all the needles and seats at the same time. Those tips wear out easily.
Carb rebuild kits include them so you could just buy that and keep the other parts (indoors) until you need them.

Find a name brand kit if possible, I’ve had a lot of issues with aftermarket on my jet skis.

Size on a needle really shouldn’t matter, any size on those should flow plenty of fuel for your jets unless the engine and carb are highly modified. All those are doing is regulating the fuel flow into the bowl where it’s available for the jets.

 

[kuwona]

I have 6 OEM Mikuni kits, but they do not include needles and seats. I plan to replace the o-rings, diaphragms and gaskets.

 

[AZMark]

I have 6 OEM Mikuni kits, but they do not include needles and seats. I plan to replace the o-rings, diaphragms and gaskets.

Bummer. I thought they were included.

I’ve not done custom work on those carbs, just rebuild like you’re doing, but I’d generally go with an original size vs assuming someone before you did the right thing by changing sizes. Maybe it was just what they had available.

 

[HawaiiBreeze]

I've tuned dual set ups for years but 6 is a beast. If I understand this thread correctly the boat hasn't hit the water yet? Easier to get a 2.0 for the 1.2, clean everything, replace with the kit parts and go for a spin. Engines running with out load (on the hose) is not much to go on as long as each engine idles nicely.

 

[itsdgm]

Oh man. So much stress! So your advice would be to count the turns on the three still assembled carbs, and use whatever those settings are on the other side as well, and hope for the best? I feel like I have signed up to fix something that ain't broke.

I’ve never synced a set of carbs that are tied into a single engine before. I’ve only rebuilt dual carbs that were operating a single engine each. I know that there’s a vacuum gauge that will help sync them.

I would simply count the turns on each carb and compare them. It’s possible that they’re all set the same. But not necessarily. It would give you a starting point though to set them to.