Keihin flatslide PWK carburetor, oppinions, issues?

I DID IT! I DID IT! I finally got these things working. And when I say working, I mean working like a finely tuned fuel injection system. It is absolutely flawless. There isn't even a hint of hesitation or bogging anywhere.

The off-idle transition has plagued me from day one. No matter what I did it was too rich. I had to feather it off of idle and it would blubber and stumble for a split second before taking off. The slides in the newer carbs did not have this issue but they stuck so badly that it made the bike unrideable. Comparing them side-by-side with the older slides I can not see any difference between them. The only thing I can think is that one of the older slides has some sort of manufacturing defect in it. After many miles the bore of the right carb was all sooty looking so I suspect it was the right slide that was bad.

I've known for a while that I should use the newer slides but have been putting it off because they needed to be lapped. When I originally lapped the old slides it took me over four hours and killed my fingers. I recently read a post on the Home Shop Machinist board where a guy showed how he lapped some valve rods using a Dremel contour sander. I got online and found that Dremel has discontinued the contour sander, but lucked out and found a good used one for $20.

The contour sander has an adjustable speed reciprocating head. I think it only moves about 1/16". All I needed to do was make a quick, simple mandrel to mount the slide to the sander.





It's just a wooden dowel with a hole drilled in one end for a wood screw and the other end turned down to fit in the sanding head.

It worked like a charm. I got an excellent finish on both slides and it only took a half hour each. After thoroughly cleaning the carbs I reassembled them, dropped them on the bike and gave it a quick sync. The difference is like night and day. It is so crisp and clean EVERYWHERE. I am so used to the off-idle hesitation that it had, it surprises me now when I crack the throttle and it just goes. It seriously feels like a fuel injected bike. The throttle response is instant and perfect. After all the time and money sunk into these carbs, it is finally worth it.

I am new to the forum and coming in quite late to this thread, but I was wondering if you could summarize your set-up, settings, jets, adjustments, swaps, etc. now that you have the carbs dialed in. I see you have been posting them along the way, but wasn't sure if anything had changed since you last listed your set-up. I apologize if I've missed this somewhere.

Great thread by the way! Unfortunaetly, I purchased the XS Performance Carb Kit before discovering this thread and learning what I got myself into. Your progress gives me some hope that my money may not have been wasted.

Thanks

Right now I'm at #40 slow jet, #125 main, JJS needle with clip in #4 slot, air screws 1-3/4 turns out.

Other changes;
-Replace air filters
-Replace cables and adjusters
-Replace carb boots
-Stretch springs
-Lap slides
-Bend overflow tubes

Mrriggs- Where are you measuring the float levels at? I feel I am getting fuel starvation at sustained Freeway speeds. You said you found 19mm on the smaller bore carbs but where is the measurement taken from? I've got almost the same set up but with K&N pods and Emgo PeeShooter silencers. Ran very rich with the #40 Pilots, had to go back down to #35s at 1 1/2 turns out on mixtures. Anyone know if the PRI petcock setting flows the same as ON and RES?

Greenbush said:

Mrriggs- Where are you measuring the float levels at? I feel I am getting fuel starvation at sustained Freeway speeds. You said you found 19mm on the smaller bore carbs but where is the measurement taken from?

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Like most carburetors, from the bowl face to the bottom of the float. Well, the top of the float when you are holding upside-down.

Last week I was riding some back roads and found a LONG incline. I held it WFO and after a half mile or so it started cutting out. I'm going to try setting the floats to 18mm but I suspect that the needles just weren't made to flow that much fuel. After all, these carbs have been bored out 30% larger than the 28mm carbs they are based on. I've even read that some of the knockoffs have smaller float needle sets than even the genuine PWK28s, though I haven't measured these to see if they do.

When I find a solution to this problem I will post it here.

WOW, really, from the bowl seat to the top of the floats? WOW. Mine measured like that at around 22.5 mm. I got 19.5 when I measured from the bowl seat to the metal frame near the plunger tab. That is a huge difference. So you are saying to measure from the bowl seat(carb frame where bowl mounts) to the highest part of the floats or right in the center of the bowl seat rim to float top?
Have you experienced a slight intermittent hiccup in the power when accelerating? Also, did you try the crossing of the insulator vacuum tubes to each other, as PemcoPete did? Opinion
Thanks.

Greenbush said:

So you are saying to measure from the bowl seat(carb frame where bowl mounts) to the highest part of the floats or right in the center of the bowl seat rim to float top?

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Yes

Have you experienced a slight intermittent hiccup in the power when accelerating?

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Yes, it took me a year and a half of fucking with these to get rid of that hiccup.

Also, did you try the crossing of the insulator vacuum tubes to each other, as PemcoPete did?

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No, I fixed it so they could be synced properly which involved replacing the cables and slides.

What finally got rid of the hiccup?


Also, I dont understand your cables and slides replacements on the Insulator mount tubes. These cable assemblies they send are cheap and I know that splitter cylinder is shight!! I had one slide not wanting to fully return. Swapped sides and it seemed to go away. Fingers crossed.

MrRiggs, I just tried to measure my floats in my PWKs. The only way I can get 19mm on the float height is to hold the float nearly bottoming out, touching the body of the carb, top of the bowl cavity. This cannot be right. The plungers would never stop the gas. i

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Okay, so I adjusted to float levels to as far as I could, and still sure they would stop full flow, at 20.3mm.
Test rode it and seems to have solved the sustained speed problem.
Dont understand why the float levels would be so far off when coming from Mike's XS.

Thanks

Greenbush said:

What finally got rid of the hiccup?Also, I dont understand your cables and slides replacements on the Insulator mount tubes.

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If the carbs are properly synced then there is no need for Band-Aid® fixes like running a vacuum tube between the two ports.

The first issue with the carb sync was the cable. It's absolute junk, replace it.

The second issue was the sticking slides. To fix this I needed to stretch the springs and lap the slides.

The third issue was a defective slide. This was also the cause of the hiccup. The base of the slide was not square. With new [lapped] slides these things work like they should.

Dont understand why the float levels would be so far off when coming from Mike's XS.

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Inconsistent build quality has been the problem of these carbs from the start. The last time I talked to someone at Mike's, they said they had a guy in Tailand who was meeting with the factory to discuss the quality issues, and they are planning to send a better cable in future kits. When these things work, they do work quite well. But right now it's kind of a crap shoot whether you get a good set or not.

mrriggs,
Did you replace the slides with the ones sent in the test carbs or spring for new ones.
It seems you mentioned earlier the new ones were quite pricey?

weekendrider said:

mrriggs,
Did you replace the slides with the ones sent in the test carbs or spring for new ones.
It seems you mentioned earlier the new ones were quite pricey?

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I used the slides from the XS Performance carbs. Genuine Keihin slides are quite pricey and there is no guarantee that they will fit in a knockoff carb.

mrriggs said:

The second issue was the sticking slides. To fix this I needed to stretch the springs and lap the slides.

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What did you use for lapping compound? And how did the "chrome" finish hold up, or did you remove that?


With my hiccup, I believe I felt the same issue with the stock carbs. It's not my bike and I only road it once before pushing him to get this carb set up. Lapping these for someone else seems a bit much. If he was willing to pay properly then maybe. But I'm doing him a proper in getting the ghosts out so he can run around a bit.

I got some garnet lapping compound from Brownells (I think it was 600 grit). http://www.brownells.com/.aspx/pid=25658/Product/GARNET-LAPPING-COMPOUNDS

In spots, it wore clear through the chrome and copper base coat into the zinc casting.

The lapping isn't hard, it just takes some time and elbow grease. Tear the carbs down and hand him the slides and bodies. Say, "Put this stuff on the slide then move it up and down in the hole 20,000 times." When/if he ever comes back, clean everything up and put it back together.

The single biggest improvement I made to these carbs was the thicker jet needles. With the thin needles it was too rich off idle which required running the idle and cruise mixtures lean to compensate. With the thicker needles I was able to step up the pilot to improve the idle and raise the needle to improve the cruise mixture. I eventually bought every size needle available and the throttle response just got better and better with each step up. I am now runnning the JJS needles which are the thickest.

If you had the hessitation before swapping carbs then make sure the ignition system is up to the task before monkeying with other things. If you haven't done it yet, a good baselining should be first on the list; adjust cam chain, valves, timing, and sync.

Ok. Maybe I'm experiencing some of the same symptoms.
I got one cylinder running rich and other running pretty good. Although both carbs are the same set-up, jets, needles and screw turns. I think it's the idling when it's rich.

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Ok. Maybe I'm experiencing some of the same symptoms.
I got one cylinder running rich and other running pretty good. Although both carbs are the same set-up, jets, needles and screw turns. Is something you experienced?o

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Pull both slides and compare them, especially the cutout in the bottom. There was a guy here who was sent two different slides. Also, measure the needles if you have a micrometer or dial caliper.

If there is no obvious difference then stick them back in the carbs, hold them up to the light and look through the bores. Adjust the slide stop so there is just a sliver of light under the slide. On the slide I had issues with, the opening was kind of triangular-ish, one side was opening before the other because the bottom of the slide was not square. I suspect that's what was causing my problems.

Have you done a compression check to be sure you're not just fighting a weak cylinder? How are you syncing the carbs?

I'll take a look at my slides and needles.

As for cylinders, just did top end rebuild for a broke off from cam chain guide. So, new chain and new front guide. Valves lapped and adjusted twice. 151 and 150 psi on compression, with kick start.
Syncing carbs first by holding fingers on each slide to make sure they lift at same time with throttle pull. Have not got to vacuum guage style sync.
Whats your sync technique?

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These things are real particular about the sync. I use a mercury manometer to sync at idle, then hold the slides slightly open and do an inital sync of the cables. To fine tune the cables, I zip tie a couple of vacuum gauges to the handle bars so I can watch the vacuum at a steady cruise speed.