CVK carb tuning

[Dom_V8]

Hey guys,

I have a 1988 KLR 250 that is pretty much stock as a rock! The only modifications made have been to the float level, and this screw on the bottom of the carb which I assume is the pilot screw (can anybody clarify this?). There was a HUGE increase in throttle response when I adjusted it counterclockwise. There is also a rubber boot on top of the carb (see picture) which I have no idea what it's purpose is. I read somewhere it can be used to adjust the vacuum in the carb but haven't been successful finding a definitive answer.

I am basing a fair amount of my knowledge this off a forum:

https://www.mychinamoto.com/forums/showthread.php?344-CV-carburettors-They-re-different...

I am looking to further modify the bike for increased throttle response, as well as to gain power in the higher RPM ranges which, right now is virtually non existent . The bike just seems to slow up completely. Any suggestions for mods??

As far as throttle response is concerned is there anything that can be done at all with the stock carb?? Someone told me I could fit a bottom end off a Harley carb (CV40) that has an accelerator pump such as the one below:

https://cgi.ebay.com/ebaymotors/Harley-Davidson-Float-Chamber-Kits-NEW-/150511872300?pt=Motorcycles_Parts_Accessories&hash=item230b34e92c#ht_500wt_1182

That said, would just completely replacing the carb with a CV40 be any good?

I'm open to any suggestions.

Thanks in advance.

[Syris the Indomitable]

The KLR's carb has a pilot mixture screw which controls the amount of fuel that enters the engine while at idle. (Not idle adjuster!)
Set up and Reassembly instructions here, diagram too: https://klr250.blogspot.com/
https://img214.imageshack.us/img214/6668/klr250carb.jpg

[Robby]

You aren't going to gain much in the way of top end power on that bike, it's an off road (or at least off road oriented) bike, so the power is in the low and mid range. This is also common to most 4 stroke single.

By the sound of it you adjusted the micture screw back to where it was meant to be. Generally anti clockwise makes it richer, so it must have been set pretty lean before, likely from someone messing with it. Check the factory spec and see if you're close to it, will be printed in the workshop or haynes manual.

A pumper carb gives you better acceleration if the bike is set up for it. My CB250RS uses one, so it may be worth seeing if one of these will fit if you really want to mess with it. As I understand the main advantage of pumper carbs is that you can use a comparitively small carb, which will be more efficient, but still get good power when you open the throttle. The downside is that opening the throttle a lot really kill the fuel consumption.

Really, the best thing to do is set the carb to the stock settings and leave it there. You've not going to make any big gains by fiddling with it, the only reason to fiddle with the carb is if you have made a change elsewhere (air inlet, exhaust, or engine tuning) and the carb is no longer supplying the right fuel/air mix.

Don't start listening to ped boys either. You will not get more power by fitting a bigger main jet, the mixture screw does not alter the mixture anywhere but at idle, and making a change to one setting on the carb will require changes everywhere else to balance it out.

[Dom_V8]

Hey guys,

Thanks for all the responses!! I now feel a little smarter than I did 24 hours ago.

With regards to throttle response, I heard from a mate of mine that while riding, I should quickly flick the throttle, close it slightly and open it up again to fix my lag down low. I took the bike for a quick ride and this seemed to work very well. I do think it is running a tad on the rich side right now though.

The CVK30 or 32 carb sounds like a good idea (maybe even FCR although its a bit more pricey), but it would require a lot of work with regards to throttle linkages and all those other little nuances like that. Anybody got a clue on how to go about those sorts of things?

Thanks again!

[Kickstart]

[stevo as b4]

The only way you would gain any performance on that bike IMO or to gain the very last bit of top end power would be to go for a flatslide smooth bore carb, which would be very expensive.

Your's is an early non restricted/detuned KLR, and they go very well for a 250 thumper in std form if in decent condition. They had 28bhp@9000rpm, and were clocked at 90.2mph and a 16sec 1/4mile by MCN.

I'd say getting yours service and making sure the engine is in peak health is the best way to go. You could add a performance exhaust, a free flow air filter and modify the air box lid maybe. Then a bit of re-jetting could be needed to get the most from the std carb.

If you have some cash to spend, then porting the head and skimming if possible would add the most extra performance to your bike. But asking for much over 30bhp from a 250cc single is being a bit harsh id say really!

[Robby]

I don't have the sud bits any more, but have played with the CB250RS pumper carb. If you operate the throttle with a float bowl full of fuel you get a huge squirt of fuel out of it. Always tempted to how it works as a flamethrower.

[Dom_V8]

Thanks again guys for all the responses.

I've had a little bit of a surf round the net and found a couple of interesting things:

https://www.bikebandit.com/1988-kawasaki-kl250-exhaust-accessories/c/a643648a644075?mg=4611&t=1

https://www.bikebandit.com/1988-kawasaki-kl250-exhaust-accessories/c/a643648a644071?mg=4611&t=1

Would this stuff be any good? Particularly the FMF universal spark arrestor??

I'm not really looking for too much of an increase in power, just general responsiveness from the bike, which is why I thought swapping to a carb with an accelerator pump.

[stevo as b4]

In that case then something like a DEP silencer or similar straight through light weight alloy unit, and maybe a K&N type filter element, and possibly a drilled or modified airbox lid, combined with re-jetting or a dynojet kit and some setting up would perk up the motor and throttle response as much as is possible on stock parts. You could look at trying to get a 3-4deg ignition advancer, but you might find it won't help the top end and that you need 98-100RON fuel to get it to run better then.

The std KLR with 28bhp not the later 22bhp versions is actually quicker than a Honda XR250, and almost as fast as the brilliant in it's day XT350. A tested true 90mph from a 4stroke 250cc single is all you could ask for IMO. I doubt that a modern 250 like a CRF250F etc would be much faster at the top end with road gearing than the KLR was.

I dare say that with lots of money to spend, that a fully ported head with possibly a bigger inlet valve, 3angle seats, and a bigger flatslide smoothbore carb, along with bigger bore exhaust header pipes, could get you into the low-mid 30's bhp, which would be fast but probably a fair bit less reliable and not have the same bottom end and midrange that you get as std.