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Old 04-07-2011, 10:15 AM
fsmith46's Avatar
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Default Keihin Carb Choke/Idle

New guy here on the forum. Truly you guys have a wealth of info. I hope that I can ask this question about a CR500 - as basically the issue at hand is really targeted at the carb which is a Keihin - as used on a CR250 and a CR500. I have a 1992 CR250, 1993 CR250, and a 1994 CR500. I bought a well used 1989 CR500 about a year ago. At the time - it cranked, would run, and bascially was decently sound. Over the last year - I have rebuilt it, refurbed it, etc. New top end, new bottom end, etc. New everything ... but the carb. Well replaced the slide-as it was worn somewhat. It's a Keihin PJ - standard for that year. Now all my other CRs - will crank easy, are jetted perfectly, will run excellent and idle until the cows come home. No fouled plugs, no problems. Now.. the 89 CR500. It will crank easy, typically 2 kick starter when cold, one kick when warm. It will run through the gears - no problems, jetted spot on, no flat spot, never fouled a plug, no blubbering, etc, I am very happy with the motor, almost even more than the 94 CR500 as it will instantly jump out of a turn when you crack the throttle. No lag. The 89 is a little more "snappy?" (if that is a word I could use) than my 94 CR500. However.... I cannot get this 89 CR500 to idle. I have cleaned the carb, many times, adjusted the air screw to 1 1/2 turns out, etc. It doesn't bog, even when letting off the throttle and the motor is about to shut off - just blip the throttle and the motor rips right back up. However, I need it to idle for my riding style. I pull the choke/idle plunger/valve out (many times by now) look at it... ponder what the heck is going on. I adjust the nut on the threaded part of the body manually, the plunger does what it is supposed to do, compress the spring as it changes the "depth" of the needle up and down as you simulate changing the idle if it were installed in the carb body. I don't see any issue. I have not tried a different one from one of my other bikes - I usually don't like to swap parts from bike to bike, especially if the donor bike is working. Sort of a "don't fix it if ain't broke".... So I am wondering... is there a set up spec for the nut on the threaded part - as in ..where should the nut be - in the middle? or on one end or the other? Is there a way to make sure the needle is aligning correctly in the hole in the carb body? In other words .... if everything looks correct...and there is no response in terms of the idle changing when turning the knob from full in to full out..... what the heck is going wrong? As is - the bike cranks easy .. runs like it should... but it won't idle. Close the throttle - and the motor shuts off. I have ordered a new choke\idle valve to replace this one - and if I have to, I will buy another carb. But it sure would be nice to know what is going wrong as to the idle. Thanks in advance ...
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Old 04-07-2011, 10:19 AM
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Welcome to the world of PJ carb, I have the same issue with my 96 CR250 runs great will not Idle to save it's life. The biggest recommendation I got was to switch it to a PWK airstriker.

I would seriously consider it down the road but not right now. I will be following this post closely to see if you get any better recommendations.

Thanks
Scott
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Old 04-07-2011, 11:01 AM
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I've thought about the PWK also - and if I do not resolve the idle issue this weekend - that is what I am going to do - buy a PWK. One thing that I read was to make sure that the reeds were in good shape - and these are a new set of Boyesen reeds - and that the reeds lay flat. Thanks for your reply.... I'll post as to the PWK carb if I have to go that route.
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Old 04-07-2011, 11:23 AM
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we had all three of the mx bikes that came with the PJ. NEVER worked very well at low speed or on trailrides with any of them. The pwk is the way to go, and I prefer the nonairstriker on the cr500. it is just more controllable and you have fewer issues if you want to size it out. best is the 39.5 that comes on the KX500's.
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Old 04-13-2011, 09:41 AM
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Default ok...update

As I noted in original post - My 1989 CR500 with a Keihin PJ carb would not idle. Worked great everywhere else - but no idle. I finally was able to work this out and get the bike to idle with the original carb. Took a lot of cussin' and about a case of beer, but success finally. Since the slide on the PJ - when the throttle is closed shuts off the throttle body completely, there is no way for air to flow into the carb except through the idle/choke circuit - of course the air screw too - so the problem had to be that the choke/idle plunger was not doing its job right by letting air flow though the passages on the side of the carb. Basically I think there were two issues, one .. the needle valve was binding and even though I was adjusting the choke knob to raise the needle to allow air flow - it was staying down shutting off the airflow. Also - before installing the plunger into the carb, it seems that the adjustment nut has to be midway in the threads. Right in the middle. I used a fine steel wool on the (for a better way to describe it) the brass plunger body and the needle on the valve to make sure it was clean and smooth. Look down in the carb body, and also make sure the hole where the brass body and needle go are smooth and clean also. I installed the standard jets in the carb.. I had a one step leaner main jet -which should not and would not make a bit of difference in the idle circuit, but just to be sure - and since I have had this bike for many years and it has lived in Texas (hot and dry and approx 800 feet above sea level) to NC - (wet and colder and 1200 to 2000 feet above sea level) and now in SC - (temps varies and at 360 feet above sea level) the carb had seen many jetting changes - so back to the OEM settings. Long story made short as possibly I can - over two days - many times starting this 500, and adjustments, I found that the air screw adjustment is pretty critical, as the motor would idle best...at 1 1/4 turns out (seems that the standard is 1 1/2 turns out) and with the plunger set into the carb with the nut set dead in the middle to start ---- key word - set in the middle.... I actually used a set of calipers to get the exact middle of the threads, used a light oil on the brass when installing the plunger, carefully snugged it down, just tight enough, and did not turn the choke knob at this time. Cranked the bike and then adjusted the air screw only a tiny 1/8 of a turn at a time. then make a small adjustment on the choke knob. Now this is when the motor is nice and hot - as in normal operating temps - and after approx 30 minutes I found the sweet spot. The motor would sit there and idle real nice -- no surges, no almost shutting off, it would just idle. Now granted I did not let it idle for longer than 5 minutes, but truthfully all I wanted was for it to idle for about 30 seconds - and it didn't need to idle that long as all I really need is a down to idle for a few seconds as in braking and entering a turn before turning the throttle back on. As most folks know .. unless your last name is Ward, Bailey, Johnson, Glover, Thorpe, Geboers, or Jobe, it is a rare 500 rider who holds a 500's throttle open all the time around a motocross track. Jeff Ward on his factory KX500 back in the late '80s - had his KX500 setup to not idle at all.... He used the throttle to steer the big KX around turns and he never shut off. It was his style. For me on the 89 CR500... as soon as I closed the throttle, the motor would shut off, and that was useless for me. Now... it works. On the downside, I still need to do a little tighter jetting, as before, this 89 500 would just jump up in revs as soon as the throttle was cracked open a hair, and rev out instantly, with the idle now working, and since this adjustment is affecting the pilot adjustments with different air flow, it is just not as sharp a response. Still good, but as I remember an article about jetting, there is a delicate balence in the transistion from off idle to the pilot when using the choke/idle knob as it also has an effect on the low end jetting. It is like changing the air screw mixture a bit. As a comparison, my 1994 CR500 is slow off the bottom compared to the 1989 CR500. Of course, that could be a number of things, the 94 is stone stock - cylinder, etc, and the 89 has a mild port job and polish. Coupled with that the 1985 CR500 was a brute and Honda (IMHO) really did not start to tame the CR500s until the 90s, the 89 model motor was still a helluva hitting motor. Both bikes - I have Pro Circuits pipes and silencers. Of course, truthfully, at 3rd gear, both bikes are making more HP than I can use, and it is a matter of hanging on for dear life at that point. I suppose that is why I short shift these bikes and avoid getting drilled into the ground. However, the nice thing about a CR500... you are only a blip of the throttle away from clearing any double at any speed. There is always instant HP on tap. So anyway - this how I managed to overcome the idle issue. I will be continuing to re-fine the jetting and if I run across any other issue/fixes I'll repost.
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