Anybody got any experience with running flatslide carbs?

[oberling]

My carbs are in rough shape and I bought a pair of Mikuni style flat slide carbs (same size as OEM) that I want to eventually swap onto the bike. I'm wondering if anybody has any idea what would be a good jet size to start out with to make tuning slightly less painful. My engine is stock.

Thanks!

 

[Team Junk]

Need to be a little more specific about the carbs. There are several companies that make flat slide carbs. Did these come from another bike or bought raw.


Manufacturer.
Size.

 

[oberling]

They are 34 mm. They are Chinese carbs visually similar to a TM34 mikuni, although not exactly the same. I bought them because they were extremely cheap and got decent reviews, including from someone who put them on an xs650. The casting says KOSO on the side if that means anything.

Current carbs are in a pretty sorry state; one of them is broken where the float pivots so the float can get stuck, and both carbs are full of corrosion. Since starting the thread I found somebody's notes on using tm34s; they were running 200 main jets but their engine was modified to displace more.

I may start in that general vicinity and see where I end up, unless I can find better data.

 

[arcticXS]

I have Mikuni RS/TM 34 flatslides on my otherwise mostly stock XS. But I bought them set up specifically for an XS, from Mikuni Topham in Germany.
So I have actually never had them apart or checked what jets I have. They worked great right out of the box.

 

[grizld1]

He doesn't have radial smoothbore pumpers, Arctic; he has the conventional carbs that come with 2-stroke jetting.

Oberling, I've run Mikuni flat slide pumpers for years, but not conventional TM34s. But they're used fairly often on the XS650, and take baseline jetting that's very similar to the usual VM34 baseline: PJ 17.5 to 25 depending on the slide used, P-5 or P-6 needle jet, 180 to 190 MJ. That usually works at sea level up to around 700 feet. If you're at high altitude you'll need leaner jetting. Good luck with those Chinese carbs.

 

[grizld1]

BTW although bore size may be 34 mm. in both OE and aftermarket carbs, they aren't really the same size when it comes to breathing. In the OE carbs the venturi is obstructed by the throttle plate and shaft. A carb with the same bore diameter but without obstruction is effectively larger.

 

[timbeck]

 

[oberling]

Thanks so much y'all!

I'll try to update with how it goes.

 

[grizld1]

If the KOSO casting is genuine, I'd find that a bit reassuring. KOSO gauges are good quality items, and if the carbs are made by the same company they're likely to be decent. I'm looking forward to reading about your experience with them.

 

[gggGary]

If the KOSO casting is genuine, I'd find that a bit reassuring. KOSO gauges are good quality items, and if the carbs are made by the same company they're likely to be decent. I'm looking forward to reading about your experience with them.

You are disappointing me Grizzly bear!
Hope you're able to get out and enjoy the heat.

 

[grizld1]

Haven't seen the KOSO carbies, Gary. If I'd inspected a pair and found the kind of crap we've seen from certain sources, I'd have said so loud and clear, but I haven't so I didn't. The Chinese can and do make some good products; I have a pair of Chinese Minolta binoculars that are amazing. But too often they make crap to a price point or a contracted price. When we get burned we have nobody to blame but our own cheap selves for buying based on price at the expense of quality. As long as we create a demand for substandard goods the Chinese will be happy to fill it.

Yeah, the heat. Spent a couple of hours with a pole saw this morning before things warmed up cutting honeysuckle and Russian olive that was threatening to grow up into my power line. The joys of country living never end.

 

[oberling]

I doubt they're genuine koso carbs. They were under $30 on amazon. I am impressed by the fit and finish for the price, but I was wrong about what they were; they seem to be similar to keihin carbs, not mikuni, which I know nothing about. I started out with a mikuni 180 main jet but that was far too rich. It was surging at idle and I ended up dumping the bike over while messing with it because of this issue. Good thing I haven't painted it yet lol.

I installed a 140 main jet (that came with the carbs) and that seems to be a much better baseline. Pilot jet is still wrong but I'm not sure if it's too rich or too lean as of right now. Once I am passed about a quarter throttle the engine stops spluttering and starts pulling. One complication is that the throttle cable I ordered online doesn't fit at all. I cut it down so that it sticks out far enough to get idle but the section of the cable that splits one input into two outputs doesn't have enough throw to go to full throttle.

Weirdly enough my starter motor no longer engages since I put the engine back together so I guess I still have a lot of work to do before this thing gets sorted lol.

 

[grizld1]

Ouch. There are plenty of posts here about Keihin PWK knockoffs. A quick search will turn up mrriggs' excellent thread, "Keihin flatslide PWK carburetors." Bottom line: in 4-stroke applications those are troublesome at best, and that's putting it mildly. The PWK series was designed for 2-stroke applications and Keihin offered only one needle jet (AKA atomizer, AKA emulsion tube) for each venturi size, so the PWK knockoffs sold for the XS650 are actually 28mm. carb bodies bored out to 32 mm. in an attempt to get a NJ in place that's lean enough for 4-stroke use, and even so they tend to run rich. I have no idea what's in those 34s; in genuine Keihin carbs venturi sizes larger than 30 mm. are odd numbered, there is no PWK34. But have a look at mrriggs' thread and the hoops he had to jump through to get a pair of 32s functional; it's an unbiased hands-on report by an expert tuner.

 

[kopcicle]

Ouch. There are plenty of posts here about Keihin PWK knockoffs. A quick search will turn up mrriggs' excellent thread, "Keihin flatslide PWK carburetors." Bottom line: in 4-stroke applications those are troublesome at best, and that's putting it mildly. The PWK series was designed for 2-stroke applications and Keihin offered only one needle jet (AKA atomizer, AKA emulsion tube) for each venturi size, so the PWK knockoffs sold for the XS650 are actually 28mm. carb bodies bored out to 32 mm. in an attempt to get a NJ in place that's lean enough for 4-stroke use, and even so they tend to run rich. I have no idea what's in those 34s; in genuine Keihin carbs venturi sizes larger than 30 mm. are odd numbered, there is no PWK34. But have a look at mrriggs' thread and the hoops he had to jump through to get a pair of 32s functional; it's an unbiased hands-on report by an expert tuner.

https://www.xs650.com/threads/keihin-flatslide-pwk-carburetor-oppinions-issues.6592/

 

[RustiePyles]

They were under $30 on amazon.

Every experience I've ever had with Chinese knockoff carbs was bad, I've never been able to get one/set to run properly for more than a few hours. My DT175 came with a bootleg VM22, it cracked around the enrichener valve just sitting on the work bench. I've also found that the Chinese carb parts might look like their genuine counterparts they will not interchange. Hope you have better luck than I have.

 

[gggGary]

One thing for sure you'll be going on a trip.....

 

[Jarrettwright]

I got the pwk flat slides from mikes xs running like a dream. I live in eastern Ohio, running stock exhaust, vape pma and electronic ignition and velocity stacks…. these are a must for excellent performance out of these carbs.

To set float height:
I drilled a hole into a tapered rubber stopper, shoved a clear tube into the hole and filled the carb with fuel via the fuel inlet, hold the open end up to the side of the carb and the bottom of the meniscus should be about 3/32” from the mating surfaces of the float bowl and carb body.

I’m running a 40 pilot and a 110 main that came with the kit, needle clip is set 3rd notch from top, and air screws are adjusted until you can see about a 5/16” of space under the slide.
During summer I can run the fuel screw at about 2 1/4 turns out from bottoming, at temps around 40 degrees that screw can go in anywhere from 1 to 2 turns out.

Had this setup for a year and a half, had to clean the carbs once and I ride a lot. After cleaning still No weird noises at all, instant power no matter what gear or where you are on the throttle, seems to always have more to give. I don’t think fuel injection would even run or sound this good. Hope you have luck, don’t give up, they’re definitely frustrating at first but so worth every bit of frustration

 

[gggGary]

I got the pwk flat slides from mikes xs running like a dream. I live in eastern Ohio, running stock exhaust, vape pma and electronic ignition and velocity stacks…. these are a must for excellent performance out of these carbs.

To set float height:
I drilled a hole into a tapered rubber stopper, shoved a clear tube into the hole and filled the carb with fuel via the fuel inlet, hold the open end up to the side of the carb and the bottom of the meniscus should be about 3/32” from the mating surfaces of the float bowl and carb body.

I’m running a 40 pilot and a 110 main that came with the kit, needle clip is set 3rd notch from top, and air screws are adjusted until you can see about a 5/16” of space under the slide.
During summer I can run the fuel screw at about 2 1/4 turns out from bottoming, at temps around 40 degrees that screw can go in anywhere from 1 to 2 turns out.

Had this setup for a year and a half, had to clean the carbs once and I ride a lot. After cleaning still No weird noises at all, instant power no matter what gear or where you are on the throttle, seems to always have more to give. I don’t think fuel injection would even run or sound this good. Hope you have luck, don’t give up, they’re definitely frustrating at first but so worth every bit of frustration

Good info, great first post. Can we get a little info about your bike? yah know we like pics.........

 

[toglhot]

I read somewhere the Chinese Mikuni knockoffs have different threads on the jets to Mikuni. True/false?