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2 stroke help.

2K views 15 replies 10 participants last post by  BigMoneyGrip 
#1 ·
OK, I've got this bad bog when ripped to WOT. I have no idea what it could be. I've done evrything I can think of.

2003 KX125

To begin, it's all new top & bottom, PV's are clean, new governor assembly, Compression is at 200psi, very goos spark and no air leaks.
It uses a Phathead cylinder head with 1 size smaller dome but runs on 50/50 mix of 110/93, has a FMF pipe and PC shorty silencer. Other than that, bike is stock.

Now for the difference:
It has a brand new Kehien PWK 38 instead of the Mikuni. This is a well known swap for the Husky & Honda guys. But can't seem to get any replies from them.

Now for what I've done so far. Put stock reeds back in, went completely through the powervalve system including a new governor. It works properly when running.
As for jetting, I've been everywhere from 170 to 185 Mains and 38 to 55 pilots. Every needle position and I even purchased a JD jetting kit and using the Blue needle.
I do know the 45 PJ is correct right now since my AS is only out just under 2 turns.
But this has me at a loss. I've seen posts and vids where others had the same issue but nothing ever replied as to what fixed it. If you can hit it when it's on the pipe or roll the throttle, it'll run great. But this cost my son 2 podiums at arenacross

Here's a vid.

 
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#3 ·
The original carb was toast. Was leaking very bad, kept running lean and smoked a crank and piston and had multiple issues.

The bog happens twice. Almost killed the bike the first time and DID kill it the 2nd time. It even bogged then revved.

here's another. The bog is at the end of the vid.

 
#9 ·
Does it get any better if you turn the air screw in? What slide cutaway does it have? Do they offer others? Seems like its lean transitioning from the pilot to the main. Do these carbs have changable needle jets? (not the needle itself, the jet it rides in in the main body)

Unless the old carb body is broken in half, I doubt its"toast" You might have been money ahead to go through the stock carb at this point.
 
#10 ·
On my 03'kx125 it only had a FMF pipe . But I would adj the air fuel screw ..
If it had a bog I would turn the air screw in to richen it and it would take it away
Of course that was after I jetted the main jet etc.
but i always adj the af screw to adj for diff tracks and conditions ..
Hope this helps
Dave
 
#13 · (Edited)
I have an alky burning 1984 RM125 that hits over 125mph on dirt....I also have a 2002 KX250 w/302 stroker kit...Both have aftermarket carbs

I had a similar problem when switching over to a larger Sudco openclass racing carb in the RM and a larger flatslide carb in the KX.....

My fixes was changing out the emulsifier tube inside the carb from a Q6 to a Q4.....The 250/302 needed an atomizer with modified holes to run right

Hope this helps you out............
 
#14 · (Edited)
I left out 1 other thing on the KX carb I did to get it dialed-in too...On the backside of the carb inside the inlet there are circuits that pull air in to get the fuel to flow inside to be properly atomized inside the venturi and a circuit for the choke too....

My flatslide carb had these brass inserts that were pressed-in and they come in different sizes to customize the flow through the a/f venturi circuit aswell as the choke circuit.....I had to go 1 size larger on the brass insert for the a/f mix circuit too.
 
#15 ·
Olds, I think you may be on to something. I'm going to hate trying to figure all this out.
I'm gonna check the float height again.
It has a #7 slide, 45 PJ, 180 MJ, AS out just under 2 turns. JD blue needle on 3rd Pos. If I go go any more than a 180 main, it spooges bad. I got it to finally stop spooging.

After all the time and money I have in this, I should have bought a Lectron. But I'm gonna get back into this and start looking what internal parts I need to look at.
Definitely some good info here. Thanks for the insight.

BTW, yes, it's 50/50 fuel mix but using Amsoil Dominator @ 40:1
Back to the bench with this.
 
#16 ·
I wouldn't judge the jetting by the spooge. One oil will burn differently than another. Look at the plug color. With you using some race gas, it will be easier to tell by the plug. Another great way to jet a two stroke, is the wet line method.

Again, I wouldn't let the spooge dictate my jetting.
 
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