Keihin flatslide PWK carburetor, oppinions, issues?

[mrriggs]

So far I'm liking them better than the BS38's that were on the bike. I still haven't got them tuned in [too rich] but I'm zeroing in on it.

I was getting a little weepage from the left bowl overflow so I pulled the carbs off to check the float levels. The floats in both carbs were set to 19.5mm. I'm assuming that is right since these carbs cam with no instructions. The nearest I could find online was for the 28mm flatslides (same carb, smaller bore) which has a float level spec of 19mm.

Looking at the float bowls, it appears that the overflow tube in the left bowl has been cut shorter.

It looks like a hack job, much rougher cut than the tube in the right bowl. You can't tell in these pictures but it also come out at a different angle which makes getting the bowl on and off very tricky.

To complicate matters further, the rubber bowl gasket swelled up when it got gas on it and popped out of the groove.

I had to hit it with a heat gun to dry out the gas and shrink it back down to size. In the future I will pull the bowls before inverting the carbs to prevent the rubber from getting wet. The first time I pulled the bowls (to change pilot jets) the carbs were still on the bike and I did not have the same issue with the bowl gaskets.

That's the best thing about these carbs. You can pull the bowls, change jets and needles without having to pull the carbs off the bike. You don't even have to pull the tank or air filters or nothin', it's all right there.

So, who else is running the PWK's [PUKES]? What is your opinion of them? Any issues, tuning or otherwise?

 

[5twins]

Have you tried lightly tapping on the top of the overflow tube? On most carbs, they are just a press fit into the bowl so maybe the high one wasn't seated in far enough.

 

[Travis]

Only issues were throttle cable routing. Oh.. and I had the same problem with the gaskets.

 

[pamcopete]

You can actually change the mains just by removing the bowl drain plug. You can loosen the carb holder clamps and rotate the carbs outward to make it even easier. Mikes sell the bowl gaskets.

Mine ran a little rich as well, but instead of screwing with the jets, I installed a set of Mikes "Commando" mufflers Part #07-0065 on and its like a whole new bike. Mixture is perfect now.

 

[grizld1]

Re. bowl gaskets: Rubber bowl gaskets on Mikunis so equipped also swell. They shrink back to original dimensions as soon as they dry for awhile; I just keep a spare set around so I don't have to wait for that to happen. Re. uneven vent tubes and vent tube with ugly burr on end: That isn't typical of Japanese Keihins, which are produced to very high standards. Carbs made in Thailand, India and China have shown up in the marketplace bearing Keihin and Mikuni trademarks; don't know whether they're produced under license or are simply pirated, but all are cheap and none show Japanese workmanship. The 30 mm. Keihin PWK's offered by JRC Industries for Triumphs and BSA's fall into this category.

 

[Jack]

Mrriggs, are those carbs going on your rephased project?

 

[mrriggs]

Re. bowl gaskets: Rubber bowl gaskets on Mikunis so equipped also swell. They shrink back to original dimensions as soon as they dry for awhile; I just keep a spare set around so I don't have to wait for that to happen. Re. uneven vent tubes and vent tube with ugly burr on end: That isn't typical of Japanese Keihins, which are produced to very high standards. Carbs made in Thailand, India and China have shown up in the marketplace bearing Keihin and Mikuni trademarks; don't know whether they're produced under license or are simply pirated, but all are cheap and none show Japanese workmanship. The 30 mm. Keihin PWK's offered by JRC Industries for Triumphs and BSA's fall into this category.

I've had my suspicions ever since I took them out of the box. The quality of the castings and hardware were not what I was expecting. For example, I've only been running them a few weeks and all the screw heads are rusty. I also have my doubts as to whether they are custom made for the XS650. When researching jetting, I found that the jets and needles that come in these are all the same parts you get in an off-the-shelf PWK, with the exception of the slow jet which is one size smaller. These things are insanely simple, there is not much more you can do to "customize" them other than swap brass (of which there are fewer parts than a VM Mikuni). I could easily be wrong on this. Can anyone tell me WHAT they actually changed to classify them as custom carbs?

 

[mrriggs]

Mrriggs, are those carbs going on your rephased project?

Yep, the re-phase is done, been bombing to work on it for a few weeks now. It sounds pretty burley once you get past the plug fouling rich spot these carbs have off idle.

 

[mrriggs]

I just installed some JJK jet needles and the too-rich-off-idle thing is gone. Idles smooth, takes off cleanly, and pulls hard anywhere you hold the throttle. I could probably fine tune it a bit more but I would say that with just the needle swap it is 98% there. I had tried different pilots and clip positions before getting the new needles but now those are back to the out-of-the-box settings.

 

[Jack]

I've had my suspicions ever since I took them out of the box. The quality of the castings and hardware were not what I was expecting. For example, I've only been running them a few weeks and all the screw heads are rusty. I also have my doubts as to whether they are custom made for the XS650. When researching jetting, I found that the jets and needles that come in these are all the same parts you get in an off-the-shelf PWK, with the exception of the slow jet which is one size smaller. These things are insanely simple, there is not much more you can do to "customize" them other than swap brass (of which there are fewer parts than a VM Mikuni). I could easily be wrong on this. Can anyone tell me WHAT they actually changed to classify them as custom carbs?

Just contact Keihin directly and asked what's been change or customized over original production PWKs. Do you honestly think Keihin's going to stop production to re-tool for a few hundred PWKs that are exclusively made for the XS 650
Also PM me your mailing address MRRiggs.

 

[mrriggs]

I just installed some JJK jet needles and the too-rich-off-idle thing is gone. Idles smooth, takes off cleanly, and pulls hard anywhere you hold the throttle. I could probably fine tune it a bit more but I would say that with just the needle swap it is 98% there. I had tried different pilots and clip positions before getting the new needles but now those are back to the out-of-the-box settings.

Never mind... After riding this thing some more, I'm thinking it still needs larger diameter needles.

 

[weekendrider]

Opinion the fit and finish was alittle disappointing.
Issues one intake had the vacuum barb pulled out.
Mikes was good to respond and replace those no fuss.
Still haven't ridden it, but it is running so..............we'll see.
I think the "custom design" for the 650 may be nothing more than the fact the intake was designed like the XS's.
Thanks for tip on speeding up the drying of the rubber gasket.

 

[mrriggs]

I think the "custom design" for the 650 may be nothing more than the fact the intake was designed like the XS's.

Mike's XS clearly states, "Our PWK Carbs were Dyno developed by Heiden Tuning and specially built by Keihin for Mikes XS for use on four strokes."

The more I read about these, the more I'm convinced they are selling knock-offs. A genuine, not "specially built", Keihin carb starts around $200. I looked into getting different slides to tune out the rich spot but genuine Keihin slides are $65 a peice. Mikes sells them for $9.

 

[mrriggs]

I finally got these carbs dialed in. I kept stepping up the needle diameters until I got it to run lean, then went back to the last one that ran well. The final setup is;

JJM jet needles
Needle clip in #3 position
#40 slow jet
Air screws 1 turn out
#125 main jets

This is on a stock bore 650 with a 256 cam, Unipod filters, and stock pipes with mufflers cut off and homemade baffles welded in.

One other issue crept up as I was getting these dialed in. The slides were sticking when I let off. I've heard that this can be an issue when using "2-stroke" carbs on a four-stroke because of the manifold vacuum. I'm pretty sure it was the slides and not the cables because I could make it better or worse by turning the air screws. With the air screws 1/2 turn out the slides stuck really bad. When I turned them out to 1-1/2 turns then the slides stuck less. The manifold vacuum theory makes sense since I got 22" of vacuum at 1/2 turn, and 18" of vacuum at 1-1/2 turns.

The solution to the sticking slides was pretty easy. Pull the slide springs out, stretch them out an inch or more, then put them back in.

 

[Travis]

^ I had the same problem with the slides and the same solution with the springs.. I guess you can get stiffer springs for these carbs too. I might also polish the front inside of each carb body where it makes contact with the slide..

 

[mrriggs]

Do you know where to get the stiffer springs?

 

[hotdog]

Thats why all four strokes used to have the push-pull throttle casble system, just incase the slides stuck open you could close them manually by winding off the throttle.

Cheers.

 

[grizld1]

Right, throttles sticking due to engine vacuum can be a problem when cable-pull flat slide carbs are installed on 4-stroke motors (not an issue with round slides). There's no "used to" about it, hotdog--final generation 4-stroke flat slide carburetors like Keihin CRF and Mikuni RS (and TM-series 4-stroke single) pumper carbs all use push-pull throttles, as do current EFI throttle bodies. (The CRF slides also ride on needle bearing rollers--beautiful, but pricey!) There's no way to fit a push-pull throttle unless the slides are lifted by shaft and bell crank.

Mrriggs, congrats on getting that fat midrange issue resolved. My guess is that the Keihins (like generic-jetted VM's) would also have responded to smaller needle jets. Hope you find a source for stouter springs; a sticky throttle can make life interesting in a hurry.

 

[Punkskalar]

Im not sure if the Lectron Springs will fit, but I had the same "valve sticking" issue with my flatslide lectrons. I polished the slides and then added a heavier set of springs. You can buy them from www.lectronfuelsystems.com or fastbygast.com

I'm using the "medium" springs and they have a good feel at the throttle housing and return nicely.

 

[Travis]

mrriggs, A friend of mine that works at Cycle X told me about them so they probably carry them if you want to give them a call. www.cyclexchange.net

Also I quick search on google led me to these: http://www.tboltusa.com/store/keihi....html?osCsid=ec55f6c1fe6a7e1c129bf9d6c28599d0

hotdog, there are tons of old 4 strokes that don't have push-pull throttles.