Yamaha 1200 Carb Catastrophe - PWC Forum: The best hang-out for Personal WaterCraft enthusiasts
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post #1 of 29 (permalink) Old 09-15-2019, 11:35 PM Thread Starter
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Question Yamaha 1200 Carb Catastrophe

Hello everyone!

Just finished my first engine rebuild on my 2001 SUV 1200 that I got for free with an almost blown motor. Actually went quite well. Finally blew about five weeks back, just finished rebuilding it last Thursday. Aside from engine I also rebuilt the carb although the parts I chose probably wasn't my best choice. It was $94 and the seller was 100% feedback on ebay. I would post a link but cant. I only found out that OEM kits are best just recently.

About carburetor: Mikuni SBN series.
At first it would never start, unless I dripped fuel down carb neck. Then after correcting the mistake I made, engine was receiving way to much oil premix (guide says 3oZ per gallon, but that seems like a lot and I didn't rebuild my crank so shouldn't really need premix), after draining gas and adding fresh non premix engine ran decent (more details below). Took carbs out and realized two things 1) One needle was missing the clip that goes around lift arm and 2) accidentally did pop off pressure according to lift arm color. (oops). The current carburetor config is using all stock jets, seat is 1.2 (probably my first issue eh?), H and L screws set to stock, and the correct pop off pressure it set at 47 psi.

About Engine:
The new rebuilt top-end has a 1.5 overbore (adjusting carb H and L screws didn't really fix issue), has new head as well as gaskets. M cylinder (front) has 100 psi, C has 98, and PTO has 100, seems pretty damn healthy to me Reeds are in good condition still, no light passing through and they snap back into place instantly.

Issue im facing more in depth:
Okay, so the jet ski idles great in and out of water. When out of water I can rev it pretty high, but am afraid of going full tilt with no cooling. When in the water I can build a tiny amount of speed around 10mph then anymore throttle and it drops RPM and cant rev past 2/8 throttle. When I had the premix in I tried my best to tune the high and low of carb with no success and when the time came to put new fuel in with no oil it was getting dark, and didn't have time to tune. Still, it struggled to accelerate. Looking at my spark plugs they seem to signify good engine operation. So yes, it still could need just a little tuning especially because of overbore, but I would appreciate any other ideas evolving around this issue. This is my first ever carbureted two stroke engine that ive owned and worked on so I'm a little lost. I guess while I wait for the next weekend to go to the lake I'd like to hear what y'all have to say. Thanks!
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2001 Yamaha SUV 1200

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post #2 of 29 (permalink) Old 09-16-2019, 04:47 AM
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Several mistakes.
Premix is not just for the crank on a new engine. Premix the first tank to achieve a TOTAL mix of 32:1, not 32:1 in the tank. Figure the system runs 50:1, so the difference is what you add to the tank. Around 1.5oz per gallon. If you added 3 per gallon, that's closer to 25:1. If you had the oil injection system apart, you would mix about a half tank at 32:1 on a new engine or 50:1 on a engine with more than 10 hours on it until the injection lines (pump to carbs) are bled.
Use the manual to set the oil pump cable.
Put the old springs back in the carbs and check pop off. Dave might have some good pop off number for that engine. Never use new springs unless they are true mikuni kits. Some after market kits are ok to use for just the gaskets, diaphragms, and seals, some shouldn't be used at all.
Compression should be closer to 120 on a fresh bore, but they are within a couple of psi of each other.
High speed screw has now relevance to anything other than 3/4-full throttle.
Low speed screw is used to set the highest smooth idle ONLY. That's all it it's used for. Idle quality, not idle speed. If it is set right, and pop off is right, starting, idle quality, and off idle acceleration will be good and crisp.
1.2 needle and seat was oem for alot.
You need to check for spark. Last plug looks like it's not firing at all, or dangerously lean.
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post #3 of 29 (permalink) Old 09-16-2019, 05:23 PM Thread Starter
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Wow! I really did mess up quite a bit. Rookie mistakes I guess. I feel like quitting at this point and just selling it as is and someone can figure it out. Sorry, at this point I have lost a lot of confidence because to me, it should be perfect after all the time I put in to it. Unfortunately I know no PWC mechanics, only car guys. Obviously they don't know everything and may not be completely knowledgeable on these things. I can do just about anything with my Yamaha Raptor, but for some reason this is like rocket science to me. lol

Enough about me, I'll try to follow everything you said starting with a Mikuni rebuild kit (old springs gone) and see where I'll go from there. I know I'm close, and regarding the low speed screw, machine idles, and starts great. About the compression, is this only affected by head gasket and piston rings? It's on good but I feel like the gasket on lower part of cylinder may not be perfect, RTV as recommended by SBT was starting to dry when I finally got cylinder walls on.

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post #4 of 29 (permalink) Old 09-16-2019, 09:58 PM Thread Starter
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Ok, so I have changed my mind a bit. As I have previously mentioned compression is about 20 psi low, thanks for that notice rod! Im gonna start by replacing head gasket again, because I think I didn't let the spray sealer sit long enough, unlike RTV it's supposed to dry before applying gaskets, was still just a little tacky. If that doesn't do the trick and comp is still low I will then move on to my cylinder base gasket, and practically rebuild the engine again without replacing pistons obviously. Ugh it's gonna be winter by the time I'm done. I noticed that earlier today I had sort of a fit, I'm much better now lol.

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post #5 of 29 (permalink) Old 09-17-2019, 03:39 AM
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Don't worry about it and just ride it. If you used wsm pistons, that could be the issue, or could be the base gasket was to thick. Just work on the carbs, get it running, and leave the other stuff for winter.
I don't use rtv on the base gaskets. If anything, use permatex high tack gasket. available at most parts stores.
https://www.summitracing.com/parts/p...kaAr-6EALw_wcB

If you set the mixture screws to factory specs, they should be very, very close. Make sure the regulator diaphragm is in correctly. The metal disk with holes faces the needle. The mikuni kits should come with new arms. I don't usually recommend using them, but in your case, do, and check pop off and arm height to what the manual states. Also, since your new to this, I would turn the low speed screws out a additional 1/8 turn to make them a tad richer.
Look in the general how to forums for carb stuff. There is a mikuni tuning manual there. I suggest you read it. It will give you some in site to how these carbs works, and what does what, and when, but use the carb settings in the repair manual for your ski, not the mikuni manual.

Last edited by rodsatheart; 09-17-2019 at 03:51 AM.
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post #6 of 29 (permalink) Old 09-17-2019, 06:58 AM
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Unfortunately I don't have any factory pop off info on the 1200. The 800 is around 75/80 psi. But I'm not sure what the 1200 is. There the same carb so I'd assume they'd be the same.
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post #7 of 29 (permalink) Old 09-17-2019, 08:40 PM Thread Starter
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Thanks Dave and Rod! Using SBT pistons. I believe the pop off is 47, but im not positive. Found a little datasheet on google. Even the service manual doesn't state that. I'll give the carbs another look, after putting in the new head gasket properly and trying without a spray or RTV I now get 114-118 psi compression. Thats a bit more exciting for me. Gonna do another premix tank of gas with 1.5OZ instead and continue after that to 0.5OZ all the time in case my oil pump ever goes bad. Also managed to put that in spec, it was only about a 1/16 off though. I'll update y'all on what happens, gonna put er back together and see what I do from there. Everything else but carbs seems perfect because I literally inspected almost everything, I'm almost positive. One of my guys however said replace fuel lines, can that actually help because they seem to be fine from what I can see.

Thanks!

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post #8 of 29 (permalink) Old 09-17-2019, 10:00 PM Thread Starter
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Woah!!!! It revved real high, had to kill engine before it hit rev limit. Seems like the gasket could be part of the problem. Time to screw around with L screw to make idle better, i think...

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post #9 of 29 (permalink) Old 09-17-2019, 10:05 PM Thread Starter
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Okay oops, didn't actually runaway just revved up by itself.

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post #10 of 29 (permalink) Old 09-17-2019, 10:19 PM
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"I believe the pop off is 47, but im not positive. Found a little datasheet on google"

Ya that's why I say I'm not sure on 1200 pop off.

Reving up on its own sounds like vacuum leak.

Fuel line should be ok. Yamaha used a very high quality rubber line. My 01 & 02 gpr still have original fuel lines. That is if it's still the original blue,green, gray colored stuff.
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