Here we go again, another carb tuning thread

[B0000M]

ive recently convinced the mrs to ride, so she now is the proud owner of an 04 CRF250x (i couldnt convince her to get a 2smoker )

the only problem im having with it which hasnt affected her yet is when you quickly open the throttle from low to mid revs the motor just bogs down unless you gently wind the throttle on.

does this sound like an air screw issue? or more like a accelerator pump issue? does it even have an accellerator pump?

bloody 4 strokes......

and if it was air screw, which way would i want to turn it to attempt to rectify it?

cheers

[Danger]

It could be worse. She could have got a Homo Chariot!

I believe I have heard of this being a bit of a problem with the Honda $ strokes.
$ strokes do not have air screws like 2 strokes. They have fuel screws, so winding the fuel screw out gives the carb more fuel (as opposed to a 2 stroke that gives it more air).
Whether or not it is bogging from being too rich or two lean only you can tell by riding and or listening to it. An air screw or fuel screw is a very sensitive adjustment. First check where it is at by closing it softly and counting the turns it took to close. Take note, go back to where it was set and only adjust about an 1/8 of a turn from there at a time and test would be my approach. In the cold weather it just might be running a little lean more likely than rich unless someone has been messing with it.
There is a fix for some of the Honda $ stroke carbs that have had similar problems if adjustment does not get it working better for you, but I have not really looked into it because I don't ride diesel Hondas.

[xwhatsit]

Four stroke singles are prone to this -- because you're only getting a vacuum pulse in the intake fairly infrequently, when you suddenly crack open the throttle at low revs it takes a little while (a couple of pulses maybe) for the fuel to make its way through the carb and match the new (large) volume of air that you've just let in. So you get a lean bog. At high rpm this doesn't matter of course -- intake pulse is happening frequently (like in a 2T).

You can get around this by using a CV automotive type carb -- most four strokes (road bikes especially) use one. Slide carbs for offroad use often have the accelerator pumper, as you've mentioned. However if they don't have it, then you have to ride old-fashioned-like and be gentle with the throttle. It doesn't make a huge difference whether you let the CV carb do the thinking for you or whether you're just smooth with the throttle -- the engine's not going to accelerate any faster.

If it's got a plain slide carb (no pumper), then the only real way around it is fit a pumper carb or a CV carb. You'd be wise, though, to fiddle around with the needle height and pilot mixture first, see if that can mitigate the symptoms. It's a fuel screw, not an air screw -- turn it out to richen up the pilot mixture. Turning up the idle a bit can help too.

At the heart of it, though, is that you just need to be smoother. Four-strokes don't like to be abused in the same way smokers do

[Reckless]

You've probably done this booom but I'd check everything is factory. Needle, jets and factory screw settings as someone may have shagged with it. Internet search might give you the info or Botany Honda are pretty good guys. Then start fiddling from there if there is still a problem.

[camchain]

Welcome to the world of modern 4 strokes Booom, where pushing the envelope means every ingredient of the engine recipe is in a very delicate balance! You'll find this 4 stroke caper (one power pulse in four) very different to that phat creamy 2 banger power. Bet you thought jetting was the last issue you'd strike with 4t.

This is a CRF niggle that still exists in some form even in latest models. Others will know more on it, but Blue Wing Honda (NZ) recently issued a service bulletin for easy DIY remedy on new ones. Costs nothing to have a look at it, bulletin should be easy to get from any dealer and '04 might be similar? (yeah, - it should be right from the bloody factory!) Hondas made in US so might even be a bit lean from factory?

www.thumpertalk.com will definitely get you headed in the right direction quickly. truckloads of interesting info on what different models are like to live with. (It even has 2 stroke sections but www.pingertalk.com is for 2t)

Check out CRF250 forum and also the specific jetting forum. They have a jetting guru who really seems to know his stuff - Eddie Sisneros (AKA 'burned')

When I was looking at CRF's I did an info-trawl on thumpertalk. I recall a popular mod to help bogging issue was to fit a JD jetting kit. Have fun getting the carb out (remove shock?) or maybe just loosen carb and rotate? I know there is also a (fairly expensive) carb gadget available to help 4t bogging but can't remember what it's called.
My KLX had a CV carb which has throttle connected to butterfly valve (not the slide) which means engine suck (via a diaphram) is required to lift the slide, which means - wait for it... sluggish throttle response, which is essentially the same problem wrapped in a different package. $100 Dynojet kit and breathing mods helped KLX a lot though.

There are a number of other tweaks to help a 4 stroke's inherent asthmatic tendencies as well. A good one is make sure crankcase is well vented (Piston stroke alters crank case pressure = resistance = sluggish performance. Fat rubber breather tubes that look OK can have plastic joiners etc. with small IDs)

Important: Keeping the oil clean and at correct level seems especially critical on CRFs with their seperate engine and gearbox chambers (and current 4t fashion small oil capacities, more stress on oil). I've heard it's not uncommon for even new crf's to use a bit of oil, so might pay to keep a close eye on it. Seen a few fairly new bikes with bottom end rebuilds.

Good on your other half for giving dirt bikes a go. Don't max her out too much, remember it takes a fair while to get the basics and the confidence happening!

[B0000M]

im fairly sure its just a jetting problem, ive got a mate with a 04 crf250R, also being ridden in the same paddock and not having the same problem,

ive not yet looked at anything on it as ive been busy with other things and its not yet a problem for her.

cheers guys ill have a dig around in the book and find the factory settings and go from there

[cheese]

I heard that there is a mod you can do to the bowl or buy a new bowl or something for the honda 4's tha fixes this.

[xwhatsit]

I heard that there is a mod you can do to the bowl or buy a new bowl or something for the honda 4's tha fixes this.

The only thing the bowl is going to change is fuel starvation when hitting really nasty rough stuff or going over jumps etc. Keihin FCRs, for instance, come in a special offroad version with baffles fitted in the float bowl to stop fuel from getting away from the jets. I think you can buy aftermarket (or maybe they're Keihin) bowls for them if you've got a stock one.

[B0000M]

i would doubt then that is the bowl starving too, i can make it die by giving it some throttle just sitting in the shed. still havnt pulled it apart yet tho. what a bastard of a thing to work on, 2 strokes are so much better

[clmintie]

Our CRF100 does this (cuts out if throttle opened too quickly) but only until it has completely warmed up. I richened it a titch (tech term) and it helped heaps......YMMV