Keihin 38mm PWK Air Striker Leak Issue

[El Camexican]

I have an issue that befuddles me.

My carb is just over a year old with 60 hours on it, no issue till recently. The float was set bang on at 16mm on day one and there is nothing funny about the angle of its mount. Everything is stock (it’s on a bike not a trike).

So on my last trip I noticed that my mileage had dropped off about 10% and my initial thought was that it was time for rings. However another trip on much flatter terrain yielded fantastic mileage.

I had also noticed on the first trip that I could smell fuel at times, but never saw any leakage from the tubes.

Long story short I see now that if I shake the bike side to side for a few seconds the overflow tube leaks at least 5 to 10 drops and suspect that over rough terrain I’ve been losing fuel. I will be opening it up shortly to inspect, but before I do have any of you had an issue like this? BTW, I did open it once suspecting the needle and seat were dirty, but they were not. I’m thinking that if I can’t find anything wrong I’ll just adjust the float to 17mm, but I can’t see that changing much it terms of the leak. Your thoughts?

[3wheelrider]

ElCamex- I'm curious if your carb is a Quad Vent A/S? Thats what I have on mine & have wondered if Im losing gas thru the tubes just because I waste 4 gallons of fuel in about 5 hrs of hard riding. The Quad Vent I BELIEVE spills fuel out by design-on jumps & bumps (uneven terrain) so that it dont bog out on you. But Ive never smelled gas or noticed fuel coming out on mine unless I tip it to one side?- So IDK? How did you calculate the "about 10% anyway"?

[yaegerb]

That's a new one to me El. I have the same carb on my 85r and probably have 20-30 hours on mine. Let us know what you find out.

[El Camexican]

ElCamex- I'm curious if your carb is a Quad Vent A/S? Thats what I have on mine & have wondered if Im losing gas thru the tubes just because I waste 4 gallons of fuel in about 5 hrs of hard riding. The Quad Vent I BELIEVE spills fuel out by design-on jumps & bumps (uneven terrain) so that it dont bog out on you. But Ive never smelled gas or noticed fuel coming out on mine unless I tip it to one side?- So IDK? How did you calculate the "about 10% anyway"?

- Yes, quad vent. Standard issue KTM
- Fuel is NOT coming out of those fangled vents, but they may be facilitating it in some way. I just find it hard to believe anyone would design a carb to leak over bumps and my old carb didn't
- The 10% lose is based on mileage. I have a Trail Tech Vapor and check my consumption every time I fill up. I'm off at least 10% when I'm not on smooth terrain.

My pre-teardown thoughts are:
- Brass overflow tube is at the wrong angle, or not pressed in properly
- My float height was set wrong
- My gas cap vent isn't working correctly and is allowing the tank to pressurize
- Overflow tube is somehow getting negatively pressurized due to its location and drawing fuel up (yea, pretty far fetched, but I once saw a sink drain on a ship spew water out the 3 deck sink drain, so I guess anything is possible)

Can't imagine its anything more complicated than this, but ya never know.

[DohcBikes]

You covered my first thought, which would be the overflow tube not seated, but then i thought, 60 hours? At that point, it likely wasnt the installation of the tube. I think... 3wheelrider 'burning' or losing fuel with the same carb is interersting. Well placed Go pro evidence maybe?

[El Camexican]

Pulled the carb off today and started looking for the problem beginning with setting my float height tool. My float was bang on 16mm and I couldn’t see any other reason for the drip.

I decided to bump it to 17mm and also to try and see if I could move the overflow tube up a mm or two. Turns out that carb is designed so that the tube fits very precisely between two different points on the casting, well, make that three after the attempted mod. Basically I tried to bend the whole tube up, but when I did that the bowl wouldn’t sit on the mounting surface without resistance.

I determined where it was contacting and there looked to be a little casting flash on the points, so I took them down with a small tungsten file. Once I had clearance the round bowl that protects the main jet was making contact, so I put a groove in it to.


Still didn’t seem like much height was gained, so I put a substantial vertical bend near the top of the tube and also pinched about 25% of its opening to make it harder to catch fuel. I realize that straight up catches more fuel than horizontal, but it was height that I was mainly after.

Put the thing together, fired second kick and ran great around the block, but best of all I can shake the heck out of it and not a single drop comes out. Off to test it!

Check out my newly discovered carb repair bench. It’s what was a cheesy top on the tool boxes I used to make my bench and removed. Turns out they are resistant to fuel and carb cleaner, nothing can roll off and they have lots of little compartments for screws and such. I was going to toss them, but not anymore.