EDIT: Whole 300 kit was done, not just head.

So I recently purchased a KLX250s that has the 300 head on it. Bike has about 19,000 miles, but new head was put on last year.(I have paperwork). Owner did not know too much about it because his son had it before him and he wasn't around for questions. Bike runs great (to me) but the bikes I rode when I was younger were YZ 125 and 250 two strokes. I am not familiar with working on 4 strokes and their particular maintenance needs. I've ridden around town on the bike, but never longer than about 20 miles. Got bike up to 75 before I chickened out, it seemed to have a good amount left so that is nice to know. I thought the speedometer must not be accurate, so used a GPS and it seems pretty close.

Questions

1. What should I be running through, or have mechanic run through, to just do a general tune on the bike and make sure everything is as it should be? More concerned about engine stuff (valves, timing, etc). Keeping in mind head was changed last year by a real mechanic and I don't want to spend money unless it is necessary. I have checked chain, fluid levels, plan on changing oil, cleaning air filter.

2. What should I specifically have, carburetor set up wise, that will make sure I am getting full potential out of the 300? It has a Q4 slip on and one of the needles where changed as part of the 300 head swap (don't recall which one, it's on paperwork at home if anyone needs to know to help answer questions), but everything else is stock.

3. Does a Powerbomb exhaust have better results on the 300 head than stock 250 head? Read some mixed reviews about how much the Powerbomb increases power on 250, was wondering if there is a consensus with 300 head.

4. Lastly, I have read plenty about taking lid off airbox, removing snorkel only, or putting in snorkel from a different bike. Also read that the bike runs lean from the factory and doing this will lean it up further, so carb adjustments required. Is messing with airbox more desirable with 300 head? or Less? Carb adjustments if I do (this ties into question 2)? An interesting note, when I looked at lid I have, someone had drilled holes in the lid but there was tape over them. It looks like someone modded airbox but then was told this was bad idea and tried to fix?? Current setup is taped up holes, snorkel on. Again, bike runs good to me, pulls pretty hard for 4 stroke 300.

Any input for a new, and now forever, dual sporter and forum member is much appreciated!!! Overall love ripping around on this thing.

P.S. If anyone knows a solid mechanic in Sacramento area that knows how to get these specific bikes to 100% potential, let me know.

 

I believe the heads are the same, no performance benefit. Carb adjustments will be the same for the 250 unless they put a 300 cylinder in it as well.

My only question is why did they put a 300 head on ? sounds like there might have been an issue with the valves or cams.

 

Sorry if I was not clear. The head and piston are 300. The whole 300 kit was done.

 

Only maintenance item is really check valve clearances. $10 feeler gauges and some hand tools and you can check it yourself. Oil, filter, air cleaner.

IMO, an aftermarket header is necessary on anything bigger than a 250. Pbomb was very noticeable on a 250, you are pumping more air through it with a 300 or bigger. The stock header is tiny as it is for a 250. May shift powerband up in the RPM but IMO very worth it...

 

Gotcha.

I would pull the carb and see what kind of fuel setup it has unless the previous owner provided all that info. If it has a non adjustable carb needle I would look into a dynojet kit for it (either the 250 or 300 kits will work, but the 300 needle is a bit better according to Klxster).

I tried running lidless with my 331 but its just too damn loud and I never personally felt a performance increase as much as I messed with the jetting. I do feel that klxster's advice about dropping the needle all the way and sizing up the main jet has merit and am doing some testing with that avenue while retaining the airbox lid.

I don't think the powerbomb would hurt, want to get one myself eventually but its low on the list of priorities (getting equipment for track days is expensive).

 

Eric, your best performance will be with a properly set up lidless airbox, carb and the full FMF exhaust system. Near sea level, use the DJ144/1N recipe but with a DJ146..

Wildcard, with the lid back on, the pressure gradients between airbox air and intake manifold vacuum are pretty much restored to "stock" - especially at higher RPM's. This alone will change how and what parts should be used. Point is, even with your BB , it is possible that not much of my lidless recipes will apply. However, applying theory certainly does- (E.G. - DJ spring quickens slide response/fueling - stock spring slows response and lengthens the fueling curve) ...

 

I don't want to derail the thread, but so far i'm having good luck with lid on, dropping the needle to first notch, stock spring and bumping the main jet up two sizes. Before I was running too rich even one size main jet larger. Dropping the needle and putting the stock spring back in fixed that definitely.

The dynojet spring worked well when my bike was still a 250 (seemed to anyway) but I have a feeling that increasing the bore made a big difference in the engines ability to draw air through the carb and was massively over powering the reduced stiffness of the DJ spring.

Since the bike isn't street legal anymore I can't simply take it out on the road to test but hopefully soon I can get it trailered up and to a test area to finalize jetting and see how the lightened flywheel performs. It definitely feels like it has more power and revs quicker, throws a bit of a rooster tail in the dirt even with supermoto tires.

 

Cool, this helps alot. So it sounds like I don't need to mess with the carb unless I pop the airbox lid?

Do you know where I can get the DJ144/1N parts?

 

Did the previous owner tell you what was in the carb ?

DJ144 refers to the main jet size (144 Dynojet) and 1N refers to having the needle adjusted to the first notch (so the needle sits lower in the carb. The stock needle isn't adjustable and has a terrible profile so any changes of the main jet really need to be accompanied by a new needle.

Jetsrus.com sells main jets but I don't know where you can buy a Dynojet needle by itself (maybe direct from DJ ?)

 

Eric, DJ won't sell anyone a needle separately - they will sell anything else separately..

You'll need the DJ2206 kit , and a separate order of a DJ144 main jet.. You'll be using only the needle and needle washers outta the kit - do not even touch the other stuff in there..

You have a used bike with an unknown carb setup. So there may be more work to do if your slide has been drilled or your stock slide spring has been shortened...