2001 yz125 riddle

[mj4trax]

To be clear, this isn't a thread of me asking for advice, since I have figured out the issue (and I will tell. ..  after a few guesses for fun). ..   just curious if anyone would actually guess the problem with my 01 yz125.

When I went to test ride the bike, I had some trouble getting the bike to start.  The owner indicated that I shouldn't crack the throttle AT ALL during the kick stroke (I have a habit of blipping the throttle just a little bit near the bottom of the kick.)

With his advice, it started and blipping the throttle would keep it running.  I took it for the test ride and it SCREAMED down the street, but bogged and died when I let it get back near idle.  I figured that the jetting was off and I'd get it straightened out once I got the bike home.

At home, I'm adjusting the air fuel screw and the pilot jet. ..  I end up chainging the pilot jet from a 25 to a 27.5 to a 30 to a 35, and there's really no noticeable difference. ..  on any of these jets it would run the closest to correct with the air fuel screw turned almost all the way in.

I went through the electric system and checked everything out. .  all measured within spec.

Thinking that the pilot circuit could be plugged, I did a complete disassembly on the carb and cleaned that circuit out fully.  No change. The bike still requires the same starting procedure and acts very lean from idle to mid-throttle, but runs great above that.   

[ford832]

You left out one piece of info-how does start with the choke and without and what difference when pulling it on while running.
Properly set up,your bike shouldn't idle and you should never have to touch the throttle when starting.
Regardless,if it runs better with the air screw turned in you are richening it up though if it runs better that way it should have run better with a richer pilot as well.
If everything is set up correctly and properly jetted,I'd guess air leak or chipped reed.

[Coop]

Properly set up,your bike shouldn't idle and you should never have to touch the throttle when starting.

Well that's a common thought process with jetting, but really it is a matter of opinion. My bikes are all jetted properly and I set them up to idle since I ride some very technical trails and if my bikes didn't idle for a few seconds I would be kicking it all day from all the stalls  . For MX racing = no idle; tight, nasty trails = idle.

Your explanation confuses me because like Ford said turning the air screw all the way in was richening the circuit just like a bigger pilot would have accomplished. But a bog like you described is generally a lean condition so an air leak, torn up or no air filter, something along those lines is my guess.

[MyckMcClung]

reeds, richen the needle

[MyckMcClung]

I too set my bike to idle, I ride tight woods, and frankly hate sitting blipping the throttle during warm up to keep it running.

[SachsGS]

Carbs with a plunger style choke circuit are designed to start in a closed throttle position when the choke is used.Cracking the throttle when trying to start a bike with the choke on negates the venturi effect on the choke circuit.I've proven this many times to guys having problems coldstarting their bikes,I'd put my hands on the crossbar,flip the choke on and after a few kicks the bike was running.

 As for the YZ125 riddle,I'm still thinking.

[ford832]

Properly set up,your bike shouldn't idle and you should never have to touch the throttle when starting.

Well that's a common thought process with jetting, but really it is a matter of opinion.

Agreed.There's my opinion-and the wrong one

[ford832]

 As for the YZ125 riddle,I'm still thinking.

The wind is from the west here today.I wondered why there seemed to be a little smoke in the air-I guess that explains it

[mj4trax]

I jet my bikes to idle as I trail ride. ..  no MX.

Don't get too hung up on the starting thing.

A couple more things I can add to help with the riddle:

1.  I checked the reeds, no chips, but they weren't real tight, so I replaced them with new Boyesson Pro Series reeds.  This made ZERO change to how the bike runs.

2.  This is a very serious bog. ..  after running it at higher RPMs, if I let the speed down it bogs down and dies almost every time. ..  no method of blipping the throttle would keep it running.  

3.  The guy I bought it from stated that it had a very recent top end rebuild from the local shop in the area.  

Any more guesses, or do you guys want to hear "the rest of the story" yet?

[mj4trax]

I should add that it would ONLY start with the choke on and no throttle. ..    even after it warmed up a bit, when it died, I needed to do the same thing to get it to fire. 

[mj4trax]

reeds, richen the needle

Nope. ..   I did both of these and NO CHANGE. 

[ford832]

Don't give it away yet,out in Western Canada,the gears turn pretty slowly
New top end eh?Piston was in backwards.

[mj4trax]

Don't give it away yet,out in Western Canada,the gears turn pretty slowly
New top end eh?Piston was in backwards.

DING DING DING!!  You got it!!   

I had done everything else I could think of and just figured freshening the top end again to make sure couldn't hurt. ..   found the piston in backwards.

It appears the ring end still lined up away from any ports that way. ..   but the big gap in the intake side wreaked havoc on the plating over the exhaust bridge.

I have the cylinder out for replating now and can't wait to let 'er rip when it gets back.



Did everyone have fun with the riddle/guessing game?? 

[ford832]

I did.When's the next one?

[MyckMcClung]

I was just about to say that, when i scrolled down and saw Fords answer, that was fun, I was thinkin about it all day at work, and scrolled down to read the replies before I answered. I was also considering the powervalve being in backward.