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YZ 125 Climbing/high idle

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I have a 2000 YZ 125 that won't run right. I don't get to ride much, so my bike sits in storage most of the time. The last time I put it in, I forgot to drain the carb beforehand. So when I pulled it out I wasn't surprised that it didn't run right. It ran fine on choke but would bog and die with the choke in. So I pulled the carb, and cleaned it out. While I'm in there, I pull the reeds and they seem fine. Still bogged and died. Also, the carb seems to be dumping fuel out the overflow pretty steadily except when it's running. I figured the pilot jet/circuit must be clogged so I blew the whole thing out with carb cleaner and compressed air. This is when things started getting strange.

After reinstalling, the bike immediately revved to WFO without me touching the throttle. The kill switch didn't work, had to yank the plug cap off which gave me a nice shock. It was getting dark so I called it quits. This seems to point to an air leak, so I figured I messed up putting it back together. While I reassemble everything, I replace all the carb vent hoses just to make sure they're not clogged and mucking things up. After reinstalling, it fires up and settles into a smooth albeit somewhat high idle. Still dumping fuel out the overflow so I figure I'll adjust the float. Bend the float tab a small amount and now the bike will not start. I put in a new plug, still won't start. Bend it back, still doesn't start. I spray some carb cleaner in cylinder to act as starting fluid and it revs out of control. I again pull the plug cap to kill it and give myself a nice electrical burn.

Thinking I may have messed up the gasket around the reed block when I pulled it, I pull all the old gasket material off and RTV it. This has no effect. The boot seems to still be pliant and in good shape, no cracks. So then I RTV the carb to reed boot junction. This time it idles a little bit and then quickly climbs to WFO. It does respond to throttle during this though and I am able to kill it with the killswitch. I think this is just chance based on how much fuel was in the float bowl at the time. So I build a tool with some tubing and a rubber stopper to try to set the float as per the manual specs, which is a giant pain. After an hour of fiddling, it seems to be in spec. While I'm doing this, I let the jets soak in some brake cleaner just in case. I also drain all the old fuel and put in brand new fuel. Again it idles a bit and then quickly starts climbing, it still responds to throttle. It also drained some fuel out of the overflow after it shut off.

I am at a complete loss. It seems like an air leak, but it's somewhat intermittent and I have no idea where it would be coming from. I think it has to be something with the carb, because it ran fine on choke until I messed with it, but I have no idea what.

Any help or insight would be greatly appreciated. Thanks.

TL;DR Bike ran fine, but only on choke. Attempted to fix, idles WFO. New plug, new gas, reed boot looks ok, cleaned jets and carb, set float (or tried to at least). No idea what to do next.

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Interesting...

A couple of really easy things to check first:

1. Adjust your throttle cable slack, maybe just back it right right off at both the handlebars and just above to carb, just in case somehow its changed and that is causing it to idle so high.

2. Adjust your idle screw on the side of the carb, again back it off.

See if that helps.

If you do figure this out please post your solution as I have a 00 125 too and just before I tore it down the carb kept leaking out of the overflow.

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Thanks, but I really don't think that's the issue. The throttle seems to move just fine, and will in fact rev the bike higher while the idle is climbing. But the idle also climbs without touching the throttle. The idle screw is just a throttle stop and that hasn't moved in years so there's no real reason it would be causing the problems.

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just adding two cents to the equasion....overflowing carbs can be an issue with the needle and seat that regulates the amount of fuel in the carby bowel....my cousin has the same bike and i just did his carb....i made sure to check that the needle and seat were clean and that the tip of the needle was not grooved or anything like that be careful to install the needle and float correctly they are sensitive....about the idling problem..........you know much about tuning?....well the air/fuel screw for the pilot jet could have a lot to do with that if that is out too far or screwed in too much it can cause high idle/no start(remebering that the pilot jet is responsible for starting and idle)....i would fix you kill switch and take the carb off remove everything and start from scratch set the air/fuel screw to 1 and a half turns out this is a good starting point. oh and just check that the o-ring and washer for the air fuel screw are there and proper installed.....you dont want air leaks there!Good luck.....oh check this out http://www.cmsnl.com/yamaha-yz125m1-2000_model10887/partslist/A-12.html the fuel air screw is number 6 on the diagram :thumbsup:

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Well as far as the carb. leaking, make sure ur jets and screws arent damged and that it doesnt have varnash on it. if the gastkets are dryed out or cracked replace it. remember ur float is what controls how much gas enters the carb bowl, makes sure it moves nices and feely with no hesitation. make sure the pin on the float also sets in right and that it doesnt have any damage. u will have to adjust the carb after u put it back together. Make sure ur cage is good and that ur reeds arent cracked. one facture to the reeds can cause the bike to run like shit. just remember this is a fuel to air raito problem. leading problems bad reeds/cage/gaskets on both carb and cage/carb dirts and out of tune/ along with bad gas. Process of elimantion man, its the best way to go. and alway reffer to the service manual.

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I may have found the problem. I spent so much time messing with the carb that I was always on the left side of the bike. A friend noticed that there was some exhaust spooge leaking from the pipe. There's a crack in the pipe which is where the spooge is coming from, so it's a good bet that's where the air is coming from as well. There's also some leaking from the exhaust port/pipe interface. So I'm going to clean that all up and try to seal everything in the hopes that that's the problem and will fix it. Does anyone know if JB weld will stick to the pipe? It's got an FMF Fatty which has a shiny finish.

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Glad to hear that it will stick. Once my work schedule allows, I'll take it around to the local welder and see what they say. I'd much rather have it welded but it'll depend on how much they want to charge.

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JB Weld can withstand 500 degrees which may not be enough if the crack is close to the cylinder. http://jbweld.net/faq.php

As for the leaking carb i just bought and rebuilt an 02 125 for my son and after taking the carb apart 3 times I determined that the overflow was not a float level problem and my leak was that the o-ring that seals the seat (needle and seat) to the carb body was bad once I replaced it it worked great

when I adjusted my air screw it seems really far out (like it could fall out) so I think some more fine tuning is in order.

I am just not sure I want to lean out the pilot jet any more. Anyone have a good needle and pilot recommendation mostly high desert in southern California 2000 to 5000 ft.

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like i said if there is a bad gasket it can mess everything up. and in one of my threads about my bike " compression and starting problem" a guy posted a link to a site where i could download a free sevicr manuel for my bike but u could also choose from a crap load of other yamaha bikes. check out the manuel and try setting the carb to factory spec and work from there.

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Well, I got the pipe welded and that hasn't solved the problem. The only thing I can think of now is to start replacing parts. I figure I'll start with the carb and a rebuild kit unless anyone has any other ideas.

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AFAIK ebay has carb rebuild kits for like $40, I think that would be a good place to start. Just make sure they include the float needle thing too.

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Sorry if I missed it and someone already said it, but it sounds like an air leak to me, possibly intake or a head or base gasket... not a carb or pipe issue.

Edited by ftball90

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The kits on ebay are more like $30, and they're supposed to include the float needle, seat, jets, gaskets, etc which rocky mountain doesn't offer.

I agree that it seems like an air leak, but I can't explain/locate it. I've applied new gasket in the form of RTV to every joint between the carb and the engine. The intake boot itself seems fine. It ran fine initially, albeit only on choke, which I would think would eliminate the head and base gasket. Logically, it worked ok and then I messed with the carb, and then it didn't work properly anymore.

I suppose I could replace the top end gaskets, just to be sure. They're not too expensive.

Edited by DoctorWack

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Check your main bearing seals, could be leaking air into the bottom end causing a lean condition

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Do you have to split the case to get at them? And do you need a puller for the clutch and rotor? I can't tell looking at the fiche. Will it be obvious that the seals have failed just by looking at them?

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