Carburetor Float Height Level Measurement & Adjustment- Keihin PWK 38 Air Striker Short with STIC 

That's funny. I grew up riding 2-strokes, but 5 years ago I was riding an EFI 4-stroke and now I'm back riding 2-strokes (one carb and one TPI). But as we all know, it really doesn't matter what you're riding so long as you're having fun...

I think you meant to say "TOO" bad. Yeh?

Where did you find the 6-7.5 number? I have 2 32mm PWK carbs with incorrectly set floats, and no idea what the correct setting should be. Mine are greatly different from the one you are using.

@@billll3022 look up your carbs online, or go to the manufacturers website, find the CONTACT icon and email them a letter asking for proper float height. Attach pics and numbers off carb also and they will identify it. Mikuni usually writes back the very next mornimg.

Pleased to hear that you like the video. I just added "Level" to the title of the video to help with searching. Thanks for the feedback

Pleased to hear that the video was useful.

Pleased that you enjoyed watching the video. It sounds like your float may be stuck, or there is some debris stopping the needle valve closing. Or the needle valve rubber might be damaged and need replacing. It should be easy to figure out what the cause is by taking off the float bowl and checking whether the float moves freely and if the needle valve is OK.

hans lefebvre good job,well said

Glad it was helpful!

Pleased to hear that you enjoyed it.

Deric Nichols - Thanks for the feedback and pleased that you found it useful.

@@TokyoOffroad I have a question, the brass hole that the needle valve seat in there are two small holes on the sides and one in the middle. is there supposed to be a small channel on the sidewall? can the brass needle valve orifice be replaced? I cant get it to stop leaking and i think the orifice is bad

Glad you enjoyed it!

If fuel is constantly overflowing then either the most likely causes are that the float valve is stuck, there is some dirt in the float valve, or the valve rubber is damaged and leaking. I recommend taking the carb off the bike, remove the float bowl and inspect the valve for damage, or debris.

Thanks! Far from perfect, but I always try hard to make clear and understandable videos. I use a Canon 80D (with various lenses) with exposure and focus manually controlled and a Sennheiser AVX wireless microphone system.

@@TokyoOffroad Nice! I remember you started with a more "cheaper" model of Canon camera. The current setup really gives you the option to bring all the details video and audio wise.

Nir Hason - Yes, I used to use a Canon G7X compact camera. Video quality was OK, but there is no audio input for an external microphone, battery life was very short and the small screen made it difficult to focus manually with accuracy. The Canon 80D setup I have now is nice to use and I have an external LCD monitor making manual focus easy.

@@TokyoOffroad Tnx for investing for us! :)

At a 60 degree angle the tang lightly touches the valve. There is actually quite a wide range of angle you can hold the carb at before the weight of the float starts to compress the spring. 60 degrees is about in the middle of this range and as such is a good angle to use as a reference. Moving the carb manually from 90 degrees to 0 degrees you'll easily be able to observe what happens. If you're aiming for a float height of 6.5mm then the float seam edge will be parallel with the float bowl surface. I prefer a float height of 7mm as it helps reduce spillage while riding and with this setting its a little off parallel.

Chylo Racing - Yes, you can safely use the same float height as shown in the video

@@TokyoOffroad Thank You for reply🙂👍

Interesting. I hadn't seen the Yamaha method before and just looked it up. Upside down will give a different float height to measuring when the carb is at 60 degrees. The Yamaha float height spec (when the carb is upside down) is listed as 5.5 to 7.5mm. I think the 60 degree measurement is a good method, being repeatable and continue to use it.

Both are acceptable but you will never get the float from the base to the top of the bottom of the float to measure 6.5-7mm, the float itself is taller than that.

What carb do you have?

@@TokyoOffroad KEIHIN PWK 36S AG on my 2016 ktm exc 125

@@TokyoOffroad I am asking that because my bike always show low fuel, and i want to make sure is it set correcty

The float height setting method and spec is the same for your 36mm as the 38mm shown in the video.

@@TokyoOffroad Thank you very much man! What would i do without your knowledge. Also thank you for having very quality videos and being here on youtube!

Probably best to ask online in a CB focused forum for clarification regarding where to measure from. The Honda manual CB125S states "the height of the float above the carburetor body should be 24mm" and the photo in the manual isn't very clear...

Yes, that's true, but it's difficult to do it accurately that way. It's fine as a rough guide though...

As I showed in the video when the I moved the carburetor by hand the float doesn't move significantly over a fairly broad range of angles. 60 degrees is probably close to the middle of the range. However, if you get closer to horizontal (float moves freely), or closer to vertical (valve damper spring starts to compress) the angle does matter. When trying to make accurate measurements it's a good idea to eliminate as many variables as possible and the holding the carb at a known angle is a good idea IMO. It's makes the measurements more repeatable. That's not to say that you can't get similar result by just holding it in your hand and eyeballing it.

What's the purpose of the dampener plunger in the needle?

What 's your question?

@@TokyoOffroad when the float is lowered to the specified level per the manual, is it when the needle valve just closes completely to take the measurement or when the small button is compressed as well? I’m guessing when only the needle valve closes, and the tab is just bumping the button - but I’m not clear on it. Thanks for your help!

@@BOPilot5517 The most accurate way is to check the float at when the carb is at 60degrees as shown in the video. But the quick and easy way is to rotate the carb until the valve just contacts the seat and check. If you continue to rotate the carb the button in the valve will eventually start to compress, but if you take your float height reading when the button is compressed it will give the wrong reading.

@@TokyoOffroad thank you. I meant to say keeping the carb at 60 degrees while the needle valve just seats. Excellent video by the way! Now if I just get my 1987 TRX250R to come up on the pipe I would be a happy. This is where the float adjustment came from. Trying to diagnose why it doesn’t want to come into the powerband.

Yes, the spec is 6.5mm. I would aim for 6.5-7.0mm

Michael Pinkston -yes it means the same thing.

I don’t have any experience with TMs, but a regular 2-stroke if setup and running well it should start instantly when warmed up. Does it run OK once warm?

Yes it runs great. Only thing is it takes several seconds of cranking when warm.BTW if I set my float level per your video, then at 16mm the needle is bottomed out.

Found the float was grossly out of adjustment. Probably more like 20+ vs 16. I bent the tang back straight and rechecked. It was now 16. Thanks for all the help. I just dont understand how the only symptom was hard starting after warm. Bike ran great no hesitation, bogging ect. ?????

The Mikuni TMX38 standard float height spec is 8.7mm +/-1mm. Out of the KTM factory the Mikuni typically have the fuel level too high. It’s a good idea to measure and set it as early as possible. In this video I show the Mikuni spec: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-h3J6EUR45es.html (1:24)

I'll add the video idea to my list. To determine the health of the piston ring to cylinder seal my preferred method on a 2-stroke is to do compression testing. You could use a leak down tester to confirm if the head gasket is leaking (air would appear in the coolant) with the piston at TDC. To debug crankcase leaks you'd need to plug off the exhaust port and intake. Will definitely make a video sometime in the future...

The Keihin PWK documentation I have states 60 degrees and I believe this to work well so haven't tried other angles.

Thank you

LOL. I wouldn't describe my fingers as fat, but they are often greasy and are very hairy. The pointing tool is certainly preferable to looking at my fingers so I will continue to use it :-)

Yes, that's correct. The upper most hoses which point are are vents and the ones at the side act as overflows when the bike is leaning over on it's side. The top vents you can route up underneath the tank if you fit longer hoses. I do this as there is a risk when doing deep water crossing if the vents hang down into water it can get sucked up into the carb.

@@TokyoOffroad if those vent lines can suck up water wouldn't they also suck in dust and clog things? I'm confused on what they actually vent. I can see it if the fuel level changed alot but the float should keep it close to constant. What am I missing?

@@otpipe Those pipes connect to the cast body of the carb, the side and top hoses are connected to the same channels on each side of the carb. When you lean the bike over like when you lay it down, the fuel in the carb can run out of those hoses to prevent the engine from flooding. If you really screw up and the bike gets inverted the top hoses will function the same way allowing fuel to exit the carb. In addition, fuel naturally evaporates, the pressure caused by evaporation in the bowl has to be vented somewhere or it pressurizes the carb and you get poor man's fuel injection.

@@TokyoOffroad i tried this on my 300exc pwk38 but found on a mx track that after a series of jumps the bike would rev when the throttle was shut. I assumed the fuel was shaken up the tube and fed back in when the bumps stopped. Very scary! Any thoughts on this?

@@jamesc2109 - What you experienced is certainly not normal. Do not ride the bike again until the problem has been identified and fixed. I suspect that either the slide is staying open for some reason after you release the throttle, or the carb is running lean and you're experiencing hanging idle. With the bike not running, try opening and closing the throttle. It should be smooth and when you release the throttle you should be able to hear the slide fully close (it makes a mechanical noise when it fully closes and hits the stop). It it's sticking check the throttle and throttle cable and clean or replace it there's an issue. If the problem is with the carburetor then disassemble and inspect and clean. Check that the cable is installed correctly in the slide. With the carb off the bike and throttle cable connected test by opening and closing the throttle and ensure that the slide movement is correct. If you suspect hanging idle (bike continues to rev very high after throttle is closed due to a lean condition) then there could be several different causes including carburetor not getting enough fuel (blockage), float level too low, carburetor setting is too lean.

16mm? Or 6mm? 6mm is a standard float height for Keihin PWK. 7mm is a little lower and will help reduce fuel spillage and run very slightly leaner too.

Thanks for the reply mate 👍 Yeah 16mm the pdf says for a PWK 36! What do you recommend? I actually went on a ride today, while riding fire roads for a little while, bike was gutless(just top end) ,as if it was low on fuel, could the float height actually be to low and its emptying the bowl?

@@lukerobins5537 16mm doesn't make sense to me. They must be using a different way of measuring float height. If the float height is too low it will tend to run lean, lack power and feel like it's running out of fuel. My recommendation is to set the float height at 7mm as shown in the video, which will result in a good performance setting while minimizing fuel spillage.

@@TokyoOffroad if I could attach a photo of the PDF I would, yes it is confusing, it states the flat surface of the carb to top of float,as per your video! What you describe is spot on, as if it’s running out of fuel, I’m going to set it to your recommendation, it’s got to improve! I’ll report back to you 🤘 merry Xmas 🎄

I use a 7mm thick piece of aluminum

@@TokyoOffroad Thank you!

Thanks for your interest. Yes, the Keihin 38 with STIC is a great upgrade from the Mikuni on the 300 and results in strong, responsive power which increases in a linear manner from bottom to top. Low rpm lugging performance is very good and the engine doesn't load up, even after extended periods of lugging. Top end is extended with more over rev. I set up each carb specifically for the customer's bike so the carb can be installed, idle speed set then ridden. Your ride elevation range is similar to mine (slight higher than mine) and I am able to use the same jetting year round and only tweak the airscrew setting. The STIC makes the carb far less sensitive to elevation and temp changes. I sell the kit on the Tokyo Offroad webshop here: www.tokyooffroad.com/product/stic-keihin-pwk-38mm-air-striker-short-kit/

The carburetor in the video is a Keihin PWK 38mm

I use a 6.5 slide. This size usually comes stock in the Keihin PWK 38mm Air Striker short carbs.

Tokyo Offroad yeah ok, a guy on the forums is helping me figure out why I have a rich spot 0-1/4 range but mid and top rip like none other. Necj 3rd clip 38 pilot 168 main float is 8.5mm slide is 6.5 I’ve adjusted as from 1/2-3 gets better w higher but then comes the knock/ping over 2.5 60:1 golden spectro 91 junk ca pump 🤣 Rk tek low elevation insert. Going to try 35 pilot if it doesn’t fix it I’m going to pull head and put stock back on and try all over.

Jef Groff - what are the symptoms you’re experiencing of the low down richness? Yes, using a smaller pilot, and/or air screw adjustment should help if it’s running rich. Check that the needle valve is not damaged and seals properly (or the fuel level in the bowl will become higher and the engine will run rich). If damaged replace with a new one.

Tokyo Offroad so off idle through 1/4 throttle it hesitates/ surges and bogs once past that it cleans up and runs very very strong. If I wick the throttle in neutral it will bog and then rev out. Pipe has decent oil ring on tip after 20 min test ride and plug is blackish Mid to top are amazing. Need to add more up top even as it signed off early n got flat with a 165 main. Going back to 168 or 170.

Yes, my experience with the Mikuni TMX 38 on KTMs is that from the factory the float level is set very high. This creates problems when going downhill (bike stalls) and also wastes a lot of fuel as it easily spills out from the overflow on rough ground. Definitely recommend checking and setting your float height. Mikuni recommend 8.7mm (which is what I used when I ran a Mikuni), but I bet yours is higher if it's never been adjusted...

@@TokyoOffroad it was set on 7.3mm ...i've set it on 8.2mm for the last ride and it was abit better going downhill (did not Stall as quickly) u reckon i should go for 8.7mm?

the stock settings on all BETAs is too high. Adjust as per this video and your fuel mileage will improve, you won't leak fuel when on the side stand and your bottom end response will be cleaner

And bike is 2011 ktm exc 125

If you have a jointer, you can make one from wood.

The machined surface which the float bowl seals on needs to be at 60 degrees to the horizontal (the bench in my case). If you look at the angle of machined surface it's closer to vertical than it is to horizontal. It's definitely 60 degrees in the video and not 30 degrees. And if you look carefully at the protractor tool I use to measure the angle you can see it's set to 60 degrees.