Part throttle bog, low end, on KX250

[Morvo]

Ok,

We have this very tiny problem with our 1995 KX250, when you lightly apply the throttle at low revs the engine will bog ever so slightly and then hit the powerband. It's a bit annoying because it is not as smooth as we would want it for riding on track, it's either off or on power if you get what I mean?

The bike has great power throughout the rev range once past this slight bog so I am guessing maybe the air screw needs adjusting or possibly the jets or needle needs fiddling with? I live at sea level so I am currently running the reccomended jetting as per FMF's website states, since we are running their FMF Fatty pipe and Powercore 2 silencer.

Any ideas?

Cheers!

 

[snowskater101]

does it only "bog" when you ever so slightly increase the throttle up till about a quarter throttle? would it still bog if you gave it gas faster from 0-1/4 throttle instead of lightly increasing.

sounds like a normal 2-stroke in low rpms, mine does the same thing, up until the needle jet kicks in.

you can check the jetting of your pilot circuit... thats the only jet that is affected at low rpms up to about a quarter or so throttle.


also by "bog" do you mean it doesnt combust or do you mean it wants to continue to rev higher but isnt unless you give it more gas, almost like its jumping forward then not then forward again and so on?

 

[Morvo]

I think it's more of a gentle roll-on of the throttle at low rpms, but 'jabbing' the throttle quickly probably returns the same results too, I can't quite remember now...

The "bog" does combust but it seems shakey and not very consistant coming upto the powerband, just makes the bike a bit jerky whilst manouvering slowly at low revs. I am pretty sure the pilot circuit and main jet are stock but I am unsure about the needle valve, I will have to pull the carb top off to inspect and put it back to normal.

Thanks for your reply.

 

[IndyMX]

How many turns on your air screw?

 

[Morvo]

How many turns on your air screw?

Without looking at the bike right now I think I set it at 1½ turns out, I am at sea level so I usually set the jetting to the standard positions to start off with. Do you think I should screw it in little by little?

 

[IndyMX]

Nope.. I'd drop the pilot jet one size first.

 

[Morvo]

So my pilot jet is a .45 at the moment and the main is .160 out of the factory, should I drop the .45 to a .44?

 

[IndyMX]

I was thinking more of dropping to .40, I don't think you can get a .44

 

[Morvo]

Yeah, just checked that now and realized :ugg:

I take it that bogging is caused by too much air in the air/fuel mixture especially at low revs before the powervalves kick in? Why wouldn't adjusting the mixture screw have any effect on the throttle or is this purely to do with the smooth idling of the bike?

 

[IndyMX]

no.. .backward, you would be leaning out the mix.. giving it more air in the mixture.

The mixture screw does have an effect on that bog, but if you are already 1 1/2 turns out, it's doubtful that you'll get much better response with it turning it in more.

So, drop your pilot to a .40 and retune your mixture screw.

It worked for me, but I also had to drop the pilot from a 45 to a 37.5

 

[Morvo]

Strange. I always though that the number on the jets corresponds to the size of the hole drilled in the jets, so a 45 would have a larger size opening than a 40 thus allowing more air into the engine. So if you have a small pilot jet/air corrector and a large main jet the mixture should be rich and vice versa for a leaner setup. Or is the pilot jet totally different to an air corrector in say a car's carburetor?

Sorry for all the questions, this was just my understanding before on all carbs.

 

[_JOE_]

The pilot jet controls fuel flow. You are correct about the numbering. The air screw meters the air that mixes with the fuel coming through the pilot jet. Hope that makes sense.

 

[snowskater101]

the higher the pilot jet number, the more fuel it lets in, it doesnt have anything to do with how much air it lets in, thats the air adj. screw. dont worry about your main jet either, that only has effect when the needle jet kicks in up to full throttle. try jetting your bike before installing a different pilot jet, your air/fuel mixture could be off, so if you put a different pilot jet in, it will still be off.

by the way pilot jets are incremented by 2.5, if you do get a new one, i would try one only 2.5 away from the stock jet.

really check your jetting for your pilot jet and remember your two stroke isnt going to run as smooth in low rpms like a four stroke, two strokes are meant to be ran around 3k rpms or so unlike a four stroke

 

[IndyMX]

try jetting your bike before installing a different pilot jet, your air/fuel mixture could be off, so if you put a different pilot jet in, it will still be off.

Excuse me, but isn't that exactly what we are suggesting he do? Changing a pilot jet is JETTING THE BIKE.

His air/fuel mixture is off, that's why he needs to change the pilot jet. It's just so happens, that the pilot jet is what affects the air/fuel mixture at 0 to 1/4 throttle opening.

 

[Morvo]

Ahhh right. On my old turbo'd car I had an air corrector jet which controlled the air coming into the engine, obviously different to what is on a 2-stroke bike.

I think my main is a 160 so if I whack in a 40 pilot and go from there I might improve things at 0 to 1/4 throttle? I take it then that bogging is caused by too much fuel entering the engine?

 

[vanja_cr250]

1. Try to clean air filter
2. Use 30w Two stroke sintetic oil for mixure not 40w or 50w, 20 - 25 ml on 1 Litar fuel (this is conected with wiscosity of mixure. High wiscosity not good on Low RPM and cold weather)
3. Worst situation is that you have little staked ball bearing on crank shaft(if you replacing bearing on crank replace and seals but this is last opinion)