Very Good !

Another reference here -

http://diabolomx.free.fr/download/LC4_BST_40_Bible.pdf

 

Thank you so much for this article, it help me tremendously in solving my hard start/ no start issue.

 

I have this carb on my Kawasaki motorcycle. My mechanic re-jetted it for me. It performs much better over a short distance but becomes 'fuel starved' under higher loads. I'm no expert on carbs but the BST34 does appear complex. I don't know what we can do to alleviate this problem. It may not be a quick fix. In a 'worst case' scenario, is there another carburetor that can replace the BST34?

 

What model of Kaw do you have it on?

 

Kawasaki Super Sherpa KL250G4
Only 225 miles, been in storage. Got it out to find that some fuel gummed the carb. Years ago I got the next size larger jets. So when I brought it in to have the carb cleaned, I brought the larger jets along. Starts up MUCH better, is very responsive and more powerful for a short ride, but becomes fuel starved and almost stalls under load.

 

OK - I had to do some home work to find out the Kaw uses a BST34 style carb, but it is vastly different in construction than the BST34 Polaris used. Now with that in mind, do you have OEM Kaw carb on the bike or the Polaris carb? Also, have you made any modifications to the intake or exhaust systems?

 

It's the OEM Kawasaki BST34 (possibly -250). No modifications to intake or exhaust. As mentioned, both jets were made one size larger (as recommended in various older forums). Thanks in advance for any advice you can provide. I plan to take my Kawi back to my mechanic in a few days.

 

Kawasaki Model is actually KL250G9 (I used to have an older KL250G4; apology for my typo.)

 

Thanks for the advice. Full schedule but will try to get to it this week--or suggest to my mechanic when I get the bike back to him. Sounds like you're recommending doing more simple checks before pulling the carb apart again. Makes sense as the bike sat for several years. Along those lines I was thinking about checking the tank valve and the fuel line into the carb to assure that adequate fuel is getting into the carb from the tank.

[BTW, agreed that the BST34 isn't a simple carb. I'm no expert but vaguely remember working on a Solex carb on my first car as a teenager, a VW Beetle. A friend and I wound up replacing it with a Holley carb after putting a free flow exhaust on it. We played with re-jetting and float level. Decades ago... possibly those carbs were more simple. Also a reason why I asked about a replacement for the BST34 if worst comes to worst.]

 

All a carburetor is is a mixer - it mixes fuel with the air allowed past the air control valve - It doesn't make any difference what name is on it as long as it fits and is jetted correctly for the application. Yes, the Solex was a simple carb as is the Zenith that was on my VW.

If the only problem is fuel starved under load (wide open throttle) the main just needs to be larger.

 

Agreed and understood: the carb is supposed to achieve the proper stoichiometric air/fuel mix for good combustion (14.5:1). Remember, as noted earlier, I took my just-taken-out-of-storage bike to the shop to have it's gummed-up carb cleaned. Since its carb was to going be taken apart for cleaning, I supplied two next-size larger jets (from Kawasaki) based on several past recommendations. My mechanic installed the jets and replaced the cleaned carb back on the bike. My first short test drive was VERY good.

The problem is that it works well only until more power is demanded over a distance, like when heading up a long hill. The first time this happened I was accelerating when it started to sputter, almost stalling. It was sudden enough that if I didn't have a good grip I could have flown over the handle bars. It seems to recover if I drive it slowly, turn it off, or let it idle. It can later achieve full power--but the situation repeats when power is again demanded over a longer distance. This makes me think that the carb is not getting adequate fuel quickly enough to replenish what was just used when the engine worked harder.

Yes, fuel supply and the air intake will be checked out. If all these check out OK, another possibility is that something is wrong with the carb. Possibly the float is too low? Or possibly something got damaged or misaligned during re-assembly? I mentioned a replacement carb should we determine that its BST34 is damaged and not easily repairable. Seems good to know about an alternative replacement carb, just in case, especially if BST34 is not easy to get parts for and is unusually complex. Thoughts?

 

After that detailed description, I am leaning to a fuel delivery problem also. Either a restricted petcock, fuel tank not venting properly or carb float bowl not venting properly. Worst case scenario is something in the the engine is getting tight under load and loosening up as the speed decreases. Your description of decreasing sever enough to feel like you are going to go over the bars sounds like something getting tight. Perhaps a brake caliper is getting hot and the expanding fluid is not being relieved to the master cylinder. This can happen if the brake pedal/lever is not returning fully, is not correctly adjusted, if corrosion is blocking the relief hole in the master cylinder or if the MC piston is not returning fully due to expanded seals on the piston.

 

Just spoke with my mechanic. The problem was with the carb. Sounds like a small part was missing that impacted needle valve/movement. This caused flooding. He learned this by looking at an assembly diagram then obtained the small part (after working as a mechanic for decades he has lots of spare carb parts on hand, fortunately). After re-assembly and taking a ride on a highway, it runs well. Looking forward to picking it up. Thanks for your thoughts!

 

Does the BST34 on the Polaris have an air/fuel adjustment screw? Or is it all just in the clip placement on the needle?

 

to the forum

 

Hi, thanks for the reply. Where would I find this screw. I don’t seem to be able to find one other than the idle screw.

 

#38 in the breakdown in post #1 - it's in a well on the bottom of the spigot side of the carb - from the factory it was capped and to get to the screw the cap has to be extracted. Use care when removing the cap - if you drill through the cap/plug and the drill bit touches the screw it will drive it to the bottom of the hole usually damaging the needle and the tapered hole it seats into. You need to count the turns to lightly seat the screw to set it back to it's original setting when done working on the carb.

 

Thanks mate. Iv got a problem with my Hawkeye. I can get it to idle fine but there is no response from the throttle. Butterfly is opening. Have you come across this before?

 

Yes - the carb needs serviced or at least the water drained out of the float bowl.

 

Water? Iv pulled it apart and cleaned it 3 times. Might be best to get someone a bit smarter than myself to take a look. Im obviously missing something

 

I was not aware you have had the carb apart and attempted to clean it, but now that you have revealed that, yes you can discount having water in the carb, unless there is water in the fuel tank. The pilot jet is above the main jet so sometimes it will idle off fuel floating on top of the water while the main jet is immersed in the water and the surface tension of water can prevent it from being pulled through the main jet.

 

No worries at all. Does this still make you lean towards a carb issue?

 

In my mind - yes.

Generally when an engine will start and idle fine, but stalls as the throttle is opened it is because the carb fails to meter the correct amount of fuel with the increased air supply and the mixture becomes un-ignitable. It may possibly be an ignition issue and a new spark plug may help, but if not it's back to the carburetor, intake or exhaust with carb at the top of the list. In a really off the wall case it could be a cam problem, but a really bad cam won't cause the engine to stall, only limit it's speed.

 

Yeah that’s the thing, it doesn’t stall there’s no reaction at all. Just keeps idling.

 

Are you certain the throttle cable is not broken or the operator connected to the thumb lever is pulling on the cable?

 

Hi Lateburd,
Swallowed my pride and took it to the local Polaris dealer which in North Queensland, Australia is pretty handy. They’ve diagnosed the CDI as the culprit. Thanks for your help and insight. I’ll be sure to annoy you again if anything else comes up 😂

 

Having a reoccurring issue with my carb on a Polaris 500 sportsman. 1999 model. The diaphragm part 6 above keeps tearing. Started to have issues in 2018 and had it changed out and then it happened again a couple of years ago and now happened again. The symptom that I know it torn is I only get half throttle response. above half throttle no response from the engine Has anyone else experienced this issue? Think that something else is wrong and the result is the diaphragm gets stressed and tears.

 

I've read this thread through. And it's gotten way off the origina topic. But I'd like to bring it all the way back.

@lateburd in the op you reference the jet block for the bst34 as a $57 item....here can I buy it? I can't find it anywhere. I can't even find clones of the jet block. Every supplier lists it as unavailable. Do you or any of you guys know where I can buy one?

Hank you for letting a first time poster get right to it.

R

 

I've read this thread through. And it's gotten way off the origina topic. But I'd like to bring it all the way back.

@lateburd in the op you reference the jet block for the bst34 as a $57 item....here can I buy it? I can't find it anywhere. I can't even find clones of the jet block. Every supplier lists it as unavailable. Do you or any of you guys know where I can buy one?

Hank you for letting a first time poster get right to it.

R

Welcome to the forum

 

I have installed the Chinese counterfeit (not cloned) carbs on vehicles where the OE carb was NLA or was a Chinese carb to begin with (all the less than 400cc Polaris models are made in Asia now and while the 200, 300 and 400cc models may have Mikuni carbs, all the youth models (110, 90, 70 and 50cc models) have Asian produced carburetors or EFI fuel systems installed.

While most of the Asian produced carbs can be tweaked to work satisfactorily, it is done by correcting the manufacturing deficiencies of the unit that was chosen to be installed. Some Asian carbs are better than others. WYG CV carbs with electric by pass starters for 50cc 4 stroke scooters, KunFu, Sun, Standard of Japan, KeiKhinkt (Keihin counterfeit for two stroke scooters), Heng Sheng (PZ19 carb for 100cc and less vertical cylinder 4 stroke dirt bikes) seem to be easier to tune and the PZ19 has repair kits available. SunSun, Sun World and most unbranded carbs are not worth the material used to make them.

I have not found any aftermarket carburetors that claim to be a direct fit for Polaris 400, 500, 600 or 700 models to be worth the postage to get them. I would spend $300 to repair a Mikuni before I would spend $100 for an Amazon carb.

 

I'm working on a Sportsman 700 in this circumstance. Although I do have fuel delivery issues with a 83 xr200r....pd28 carb NLA. So thanks for the information.