YT-125 Motor Tuning

[Trimoto Mike]

New to 2 Strokes -- have a few questions on tuning the YT-125 motor.

It's never easy to start up - but when cold , it's about the three pull start while keeping the throttle open a bit.

When warm, I have have to give it full open throttle to get it to startup again, or cut the fuel and then turn the petcock back on immediately after it starts up.

In neutral it starts to surge high RPM if I give some throttle, but does not go back down to idle unless I put it in gear again.

No issues running flat out for long periods of time on my flat dirt roads outside my house -- I have a 1 mile private road. But, when I was running fast and hard on the freshly plowed field out back -- way too much fun -- after about 20 minutes it starts to bog down and I can barely get back to the barn unless I run it in first gear at higher RPM.

Any clue as to what's going on? I'm at 5600 feet elevation just outside of Denver on my ranch. It ran a but better when I turned the mixture 3/4 to the right. New motor, new carb -- Jetting? Mixture? Cooling?

Thanks!
Mike

[El Camexican]

That's a lot of symptoms.

Start with a compression test. Remove the air filter, connect the gauge, open the throttle full and pull till the needle stops going up. Should be at least 115# at high altitude (6,000' plus) and no less than 130# below 3,000'. If you have less than this , remove your exhaust pipe and look inside the port with a flashlight for smeared aluminum on the piston, or a dark band over 1/2" wide below the rings. Either one says "I need a rebuild".

If you want to reduce the risk of ruining your engine the next thing you should do is a pressure test on the engine to check for air leaks using the proper tools. Google it and make sure to use the words 2 stroke, or two stroke, you need to do a pressure test, not a leak down test.

If you want to skip the pressure test and go the half arsed way about this, get it running and idling and spray carburetor cleaner around the base gasket area and carburetor manifold connections. If the rpm's surge you found your leak. However, using this method bypasses the crankcase seals, so you aren't eliminating a possible problem, only a few areas where there could be one. On an engine this old anything could be leaking.

The starting problem could be due to incorrect float height and a loose, incorrect, or even missing, or broken pilot jet. Drop your float-blow, clean your jets, make sure the numbers on them match what the factory put in (Pilot should be a #20 and the main a 105, 110 or 115) set your float height (Google it) and you're probably going to want to replace your float-bowl gasket ($7.00 on RMATV) and needle seat assembly ($17 bucks, same place). Makes sure your choke is functioning properly as well.

If it all checks out, put a clean filter on it, open the air screw 2.5 turns out if you're above 6,000' (needle clip on the top ring), 2 turns out if you're between 3,000 and 6,000' (needle clip on the second ring from the top) and 1.5 turns out if your under 3,000' (needle clip on the middle ring) . Get it running what ever way you have to. Let it warm up, set the idle speed and then shut it off and see if it will start normally.

Let us know how it all goes. to 3WW!