Motor surging issues!! Help!!

[KX5Zuki]

Well, I am using the stock 40mm Mikuni carb, round slide, and dont know what it was tuned for before. Got the motor out of a dirt track car. The motor sat for about a year and a half before I fired it up from when I bought it. So, the seals are at least that old. What should I go for as for setting up the carb for 3-4k feet. Sand Mtn., NV?

And is there a write up online on how to mess with the carb? I am not a motor guy. I am a fabricator, not a motor tuner, and all my buddies got rid of the 2 stoke bikes and quads, all 4 strokes, 450s and street bike motors. Im still stuck on the big 2 stroke thumper motor band wagon. So I need to get this running as good as possible! Help please!

[Mick]

Well, I am using the stock 40mm Mikuni carb, round slide

Stock for what??  The KX5 came with a 39mm Keihin flat slide.  Now I feel almost certain you've got carb problems...and bro, with that Mikuni I don't have a clue what to tell ya 

[KX5Zuki]

Well, I am using the stock 40mm Mikuni carb, round slide

Stock for what??  The KX5 came with a 39mm Keihin flat slide.  Now I feel almost certain you've got carb problems...and bro, with that Mikuni I don't have a clue what to tell ya 

Its an '86. And yes, the '86 came from the factory with the 40mm Mikuni.

[Mick]

Really?  I'm not sure about that...But it makes no difference, if it's what you got now it's what you got 

[KX5Zuki]

What about running the bowl dry? I am a member over at atvdragracers.com and there saying, with my 700ft full throttle run, the 500 can suck that "small" 40mm carb dry in no time, which would explain the surging, as its starving for fuel, and even hanging up a lil. I am still going to do a leak down test on the motor, but in the mean time I am going to be looking for a 44mm carb to deal with the long full throttle run. And also run dual fuel lines to the carb. Anybody else run there 500 full throttle that long? This is mostly a drag bike.

[Mick]

Oh I've had them WFOT for +700ft on occasion    No issues and I can tell you with certainty the carb isn't the limiting factor here.  I agree with the logic of those members on the atv site, but I disagree with the cause.  I disagree with them because a carb?s size isn't an indication to how large the float bowl is.  The 44mm carb's bowl isn't (if any) much larger than your 40.  These carburetor manufacturers don't machine all new components for each carb.  They all share essentially the same pieces, but the throat or bore of the carbs are obviously machined larger/smaller for each desired application.  However, I do see where they are coming from(?)  If your float is too low, then you're motor is using the fuel quicker than the gravity fed carb can replenish it.  When the fuel gets too low, the air passing through the bore has a difficult time picking it up causing a lean condition.  I've seen this happen many times to ill jetted machines.  I will say though that typically under these conditions the motor will lean out severely under power, and not when you close the throttle.  So that essentially is running the bowl dry, but not because it's too small.  The carb just needs tuned correctly.

[ID KX500]

What about running the bowl dry? I am a member over at atvdragracers.com and there saying, with my 700ft full throttle run, the 500 can suck that "small" 40mm carb dry in no time, which would explain the surging, as its starving for fuel, and even hanging up a lil. I am still going to do a leak down test on the motor, but in the mean time I am going to be looking for a 44mm carb to deal with the long full throttle run. And also run dual fuel lines to the carb. Anybody else run there 500 full throttle that long? This is mostly a drag bike.

That sounds like you're on the right track I get surging when I leave the gas off,,, so maybe check fuel flow and ventilation from your gas tank,,, to be sure enough gas even gets to the carb.

[KX5Zuki]

Hmm, well, you are probably right. I need to find out how and to what specs I need to set my carb up to. If anyone has a site on the set up of the Mikuni carbs please share. I will also check the pintle valve in the tank, and the vent up top. It might be a while until I am able to tell for sure. Dont have easy access to the quad whenever. Thanks for the help, and I hope I can get this thing set up. Banshee Eater! HaHa

[cbxracer30]

I would like to start off with a little bitch, would like to see you put some rear fenders on that machine . I watched a guy with a honda 250 get out of control , run over his foot then his leg got sucked into the rear sprocket , that dug about to inches into the meat on the back of his thigh. The only good out of it all was his helicopter ride he didn't get to enjoy. Ok then if you decide to go with a 44tron make sure it has an alcohol float valve because the stock valve is only good for 300ft WOT. A mikuni 3.5 works well when tapped to fit.CBX                                     

[Polar-Bus]

cracked/dry rotted intake boot?

[DoldGuy]

KX5Zuki,
All of the above post have good info & would like to add the "Original" setup from Kawasaki had that carb supported from both the intake and air intake sides which helps to stabilize the carb (noticed the clamp on filter). If that thing vibrates anywhere near as people say they do, it could be the harmonics causing the fuel to froth & the Main Jet will be sucking alot of air and cause very similar description especially on the old round slide Mikuni. Simply try to support the carb with ty wraps or what ever is available to see if it improves, & if it does you are going to need to fabricate a new intake runner (with more rubber) to dampen the vibes. I know of a Red 500 that had the same symptoms and took a long time finding the cause which is as described & it also had the Big round slide Mikuni. Hope this info is helpful, keep us posted.

[KX5Zuki]

Well, still crossing my fingers that its not an air leak. But the carb I had hooked up was so far from stock it was rediculous. The main jet was a 330, stock was 380. The pilot was 35, stock was 40. And the needle clip was in the 3rd position, stock was the second. All of that equaled up to a really lean motor. And apon taking the bowl off, the tangs for the floats were upside down, so the bowl already only heald very little fuel. So, that explains why I ran the bowl dry, and the surging. So, I got the carb all back together, back to stock specs, and will be testing it this weekend. I will post an update as to how it does.

[kaw rider]

cbx
you think that crazy. There guy here at the ice tracks with out no rear fenders on there quads.

[mr.5thou sand]

i had motor surging issues when i changed to racing coolant and forgot to bleed my system. after i bleeded the cylinder head it went away

[KX5Zuki]

Heres an update. I rejetted the carb back to stock, I was actually able to start it by kicking it now. Still took a few good kicks, I'm only 150 lbs, and about 5'10"! HaHa. But it actually smoked like a 2 smoke now, and pulled alot harder. My gearing is way off, 5th pinned only does me about 50-55 mph, with 100% traction.

Now, onto the surging issue, almost completely gone, full throttle run, pull in the clutch, and let off the gas completely, motor does 3 small surges and thats it, waaaaa, waaa, wa and then it idles, it actually idles now! I am sure the jetting is still slightly off, runs really crappy when cold, but a lot better when warm. I did remove the air filter and watched the big round slide in the carb, and when letting off the throttle, the slide hops up and down a few times from motor vibes, probably causing the few surges. I still have not done a leak down test, mostly just havent had the time. I live in an apartment an hour away from my parents where I keep the quad. But it does pull so much better, the true test will be when I throw the paddles on it and open it up in the sand.

Oh, and just for a good note on it, bottom end, destroyed a CRF150R and a YZ450F, both bikes, couldnt test it against my KFX450 quad, not another set of knobbies, or slicks. These races were heald on private property on asphault, ie. 100% traction. both the bike pulled up on me after I hit 5th, the races were set up about 500ft. They got me about half way! HaHa, but pulled them hard until then.

And as for my pic with no helmet, this weekend I had helmet, boots and pants, so did the other two bike riders. Thanks for all the help with my issues. And I got the quad with no rear fenders, and dont really need them, I dont jump the quad or anything else, my boots stay on the pegs and thats it. I will get fenders soon, just gotta make sure work picks up first.