Home Forums Tech Talk YZ125 screaming idle

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    • #45329

      First post, but as the title suggest I’m having issues with my kart powered by a 98 yz125.

      It’s my first shifter kart and this is all new to me so looking for a little help please.

      Starts fine, but after about a minute the idle steadily increases to what would be the rev limit if I didn’t kill it.

      Been trying all the things I’ve researched online to include:

      Replacing the reed block and gasket with a boyesen along with the new boot for the Kehin 39mm.

      Checked the throttle cable, idle adjustment screw along with pulling the carb and cleaning it.

      No stuck float, clogged jets, etc.

      I noticed the bowl hardly had any fuel in it.

      I also noticed when running a solid stream of fuel to the pump from the tank, but a very bubble filled line from the pump feeding the carb. (Not sure if it’s supposed to be like that or not)

      Any ideas? It just started doing it.

      Thanks in advance.

    • #45361
      Alan Sheidler

      I’m trying to understand how an engine could get to the “screaming” mode in any case unless the throttle slide was being raised, at least a significant amount.  In order for an engine to wind up, it needs both fuel and air.

      Of course the things you have already done are a good start.  But an air leak alone won’t cause this effect.  As far as the bubbles in the line, that can happen, but a rebuild of the pump may be in order.  Your system uses just one pump, is that right?

    • #45376
      Steve O’Hara

      Actually, an air leak will cause the exact behavior described. It takes very little extra air to allow the engine to rev way up without load. First thing to do is pressure test the engine by sealing off the exhaust and intake and pressurizing the engine with modest pressure. No need to go over 6 psi. Ideally it will hold pressure indefinitely but in the real world if it loses 1 pound or less in 10 seconds you are in good shape as long as the leak is not the head gasket.

      If the engine will hold pressure then the problem is in the fuel system and those bubbles may be a clue. It’s pretty common that a two cycle engine will rev way up if the fuel shut off valve is closed while the engine is at idle. The condition can last from just a few seconds to as much as 10 seconds or more depending on the type of carb and the amount of residual fuel laying in the case. Eventually it will die from lack of fuel but in the 5 to 10 seconds it revs way up you can do nothing about it unless you have an electric kill switch.

      Your discovery that there was very little fuel in the bowl leads to the question… is the fuel system starving the engine making it act like the fuel shutoff has been closed?

      Make sure your lines are all good with no holes or cracks and pressure check the pump to make sure there are no leaks. Then  test the fuel flow by installing a “T” fitting between the pump and the carb and running a line to a container to collect fuel. When you run the engine at idle the pump should be able to deliver the fuel the engine needs to the carb and also keep a steady stream of excess fuel to the container. If it won’t deliver any excess to the container you should rebuild the pump.

      Good luck.

      Steve O’Hara

    • #45384

      Thank you for the response!

      I just started it again today after work. Started on the 4th pull. As usual idle crept up. I tried to let it run like that for longer than I usually do to see if things would calm down, but It did not and I ended up killing it.

      I tried spraying carb cleaner around the carb, boot, reed area and both sides of the engine in hopes of a change in RPM but there was not.

      I checked all the fuel and vacuum lines for any holes or chafing. There was some chafing on the vacuum line from the pump to the case due to it resting on the chain gaurd, but nothing was chaffed through.

      I also tried restricting the the return (just because) with no difference as well and the gas tank is well vented.

      If I blow in the vent tube from the gas tank and pressurize the system I can see fuel come out of the pump and feed into the carb in a MUCH better stream of fuel… It still crept up.

      On another run I pulled the choke up and it amplified the effect 10-fold. I talked to a buddy at work and he said I might need to adjust the float. (he’s a big MX guy)

      I do not see a fuel shut off valve anywhere, other than the choke lever on the carb. I definitely need to install a kill switch as I am just pulling the plug boot.

      I will tackle it more tomorrow with all the tips you pointed out. Always something. Haha!

      Like I said, this is all new to me and the only 2-strokes Ive ever had was a moped when I was a teenager and lawn equipment. All bikes, Atv’s etc. have all been 4-strokes.

      Thanks again!


    • #45385

      ^ Oh, I pulled the air filter off as well and the slide is staying down and not creeping up. Ive checked the throttle cable tension, needle and seat.

      I also only have one pump with 3 connections. Fuel in, fuel out and the vacuum line.

      Throughout my Google research I am coming to the conclusion it is leaning out obviously; whether It be related to lack of fuel or an air leak.

      Ive read about everything from as simple as a carb boot to more in-depth crank seals. I sure hope its something simple such as  a pump or float adjustment.

    • #45393

      I got to thinking, since there are bubbles in the fuel line feeding the carb from the pump, air has the be being introduced somewhere…

      Could these be a possibility:

      1) Pump bad; causing air to be introduced via the pump housing or allowing it to somehow suck from the carb.

      2) Vacuum port from the case; either not pulling enough vacuum to allow the pump to operate properly or actually intermittently allowing the opposite to happen  where air is being introduced from the port to the pump

      The last possibility would lead me to believe crank seals or something along those lines… Just spit balling here as this has me puzzled. Sorry if I am being a pain.

      I also thought about bypassing the fuel system all together and setting up a temporary gravity feed system and see how that plays out.

      Like I said, I will mess more with it tomorrow. Pick up a gauge and some T-fittings.



    • #45404
      Gary Smith

      I think you are on the right track trying a temporary gravity feed system, air bubbles feeding the carb is not good. If it idles and runs fine with the temporary gravity feed system, then your problem is the fuel pump. If that doesn’t solve the problem, check out your carb, make sure fuel flows when the floats drop and fuel stops when floats rise. Also check fuel filter for being clogged. Lack of fuel in the bowl is a concern, that should not be happening, sounds like fuel is not getting to the carb.

      Hope this helps,


    • #45405
      Steve O’Hara

      When you do your gravity feed test run the line coming out of the fuel pump to a container and watch the rate of flow to verify the pump is working. Air bubbles in the outgoing line indicate a leaky gasket or a small perforation in the pump membrane but a small amount of bubbles in the fuel flow should not have much effect on a float bowl carb. It is a problem on a pumper carb because the air has to go through the system but in a float carb the air just rises to the top of the float chamber and the engine should never see the air in the venturi of the carb.  Have you tried adjusting the shutter position to see if you can settle it down? One way to clearly identify an air leak is to back the idle adjustment screw completely off so the shutter is completely closed.

      Steve O’Hara

    • #45436

      Made a makeshift  gravity feed system out of the top of a 2 liter and drilled a hole in the cap a tad smaller than the line and wedged it in.

      Added some fuel and the carb took in quite a bit.  (assuming it was filling the bowl)

      Started it up as usual and noticed I actually had to burp the throttle periodically to keep it running, but I didn’t get to it quick enough due to having to add more fuel and it died.

      Haven’t been able to get it to start since… I’m taking a break. Haha!

      On a lighter note I did not see any indication of increased idle for the short time I did get to run it; so hopefully that’s a real good sign!

      I did not have enough fuel line to on hand to run to a container from the pump however.

      I capped the vacuum line that went tot the pump as well during this test and the carb was being fed a solid stream of fuel.

      Ill try again here in a few with my brother in law and see how long I can run it having him on refuel duty… That is if I can get it to start again. I’m running out to grab a box of spark plugs.






    • #45444

      No plugs anywhere around; so I had to order them… (NGK 3530 B9EG)

      I got it started again. It dies even while nursing the throttle after a while, but not more runaway idle!

      Pulled the plug after it dies and it was wet (reason why it wouldn’t start before I’d take it) , cleaned the plug and started it again, same outcome – wet plug… Rinse and repeat.

      So now at least now it looks to me its a fuel issue and not a bad gasket, seal, boot, crack, etc.

      Could it just be that the carb was jetted for the pump style fuel system and that is why the gravity feed is causing it to run rich? (I would think the carb would only pull what it needed…)

      Feel like I am on the right track though!

    • #45446
      Steve O’Hara

      OK, you are making progress. The fact that the idle control problem went away with the gravity feed settles the question of an air leak in the engine…. if there was an air leak the problem would persist with the gravity feed.

      You need a container of fuel big enough to run the engine until it gets warm… consider using your fuel tank and laying it on its side above the engine and run a line from the fuel tank vent to the inlet on the carb.

      Leave the normal fuel line plumbed to the fuel pump and run a line from the pump to a container and leave the pulse line (vacuum as you called it) connected so the pump will still work. When you run the engine and it is cold you should need to apply some throttle every few seconds to keep the engine running… that is a normal condition. In fact the engine may be set up so it will not idle and you have to keep cracking the throttle to keep it running. From what you have described so far I think your fuel pump is not working up to par so you have to test the output to see if it is delivering the required amount of fuel.

      Steve O’Hara

    • #45448
      Steve O’Hara

      Oh, one more thought on using the fuel tank… if you use the vent line for the gravity feed you will need to remove or at least loosen the cap so the tank will vent air in while pumping fuel out.

      Steve O’Hara

    • #45450
      Alan Sheidler

      A 2-Litre jug is plenty of capacity to feed a carb.  Taking off a tank and using that for gravity feed is so much work… On both of my karts you pretty much have to pull the floor pan to get the tank out.

      But some things have been found out.  Get a kit or two, and rebuild the fuel pump.  I don’t recall reading whether you have the round or the smaller rectangular pump.  Whichever, just be sure that all of the gaskets are properly aligned, and tighten the screws gradually in a crossing pattern.  The flapper valves in the round pumps can be finicky…  It occurs to me that if the main membrane between the fuel side and vacuum side of the pump is faulty, then that pulse line could have been scavenging fuel and air.  Not good, and it could cause the runaway condition you described.

      As far as the pulse or vacuum line, that is a critical connection.  It is OK to use fuel line for it, but I prefer a fiberglass reinforced line, as it is very resistant to distortion due to the negative pressure.

      Sounds like you have already checked out the carburetor well enough.  Just be sure that the float needle is not able to stick in the inlet and block fuel flow.

      Keep at it, you’ll get there.

    • #45882

      It was the pump. Installed new lines, filter, pump and plug. Runs like a champ! Thanks all

    • #45888
      Alan Sheidler


      So, for us curious-minded types… Did you pull the old pump apart, to see where the fault was?  I’m still betting on a split in the main membrane, which would allow fuel to be drawn directly into the pulse line, therefore bypassing the carburetor entirely.

      And FYI, don’t toss the old pump.  It is so very rare that anything could be wrong with the castings, maybe a fitting….  And the kits are easy to install.  Will do you good to rebuild it, then put caps on the fittings to keep junk out, and keep it as a spare.  I am aware that I probably carry too many “just in case” items in my trailer.  But if I was forced to whittle down the selection, a spare pump would be one of the very last to get cut.

      There was that one time when we were on the grid, ten minutes to go, and the motor would NOT start!  And we had just warmed it up/checked it out an hour earlier.  One side of our double-pump (AKA Pump-around) system had failed.  We got the spare and put it on, and made the scheduled start with a minute left to warm up.  Yea, the spare pump stays where I can get it on short notice!

    • #45890

      I kept the old pump and did exactly as you said by putting the new pumps caps on the old.

      There was a small tear on one of the rubber discs.

      Just finished running new coolant hoses and installed a new radiator and overflow. The guy who had it before had a Frankenstein job going on.

      Throwing in a Hinson clutch kit tomorrow as the old is locked up. After that, it should be ready to rip again.

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