2KR Carb Specs

Dids · Aug 17, 2010

Well, I've read this site until my eyes hurt.

I think I've read two different values for the pilot jets? 2-7/8 and 3 turns?
And what about the float values?

Story is, I have a 2KR which refuses to start when cold. I did the valve clearances (lots of thanks to Dave's posts) and found the inlets at .03 to .07 - so well out of spec. Did that. Guess what? Still won't start. It stammers like it wants to start, but just can't quite get there. I've got new oil and new plugs in, so I'm back to seeing what's wrong with the carbs.

I'm making sure the needles are in the middle position, and I'm cleaning the carbs with carb cleaner and compressed air.

But, if that won't fix it... what else? Someone mentioned leaking air through the choke slides. Where the heck would I get parts like this for this bike?

Cheers
Craig (CanberraRiders)

dave · Aug 17, 2010

Air can leak thru the rubber boots that connect the carbs to the head.

The idle screw turn settings you found are very close to one another. either would probably do..

Check float levels

To Check the float level you only need a piece of clear tube that fits over the carb drain outlets.Then open the drain tap with a screwdriver. Get the bike perfectly upright and test all carbs one at a time. Start the engine to see if the levels move around.

Attached pipe like such... Dimensions are for 2KR carbs...

This pic is 3LN carbs with the levels marked on the carbs

Dids · Aug 18, 2010

Thanks Dave - found that image after I posted.

Mines doesn't have the lines (or Mikuni) printed on the float bowls - they're blank. (Edit: Yeah just read it again and you mention LN - sorry) Also, I managed to break a blanking bung in the float bowl. Don't blame me - the thing was done up so tight that I got it out with an ezyout and my fingers and it was no effort.

Went the Yamaharley dealer in Canberra and looks like the part is 2 weeks ex japan and $15.

What's going on with the 2KR carbs - I see a lot of posts about them through Google? Can I put 3LN carbs on? Or even FZR400 carbs?

Cheers
Craig

dave · Aug 18, 2010

The 3LN carbs are different size to the 2KR. It may be possible to make them fit, no guarantees...

cornulio81 · Aug 20, 2010

dave said:

The 3LN carbs are different size to the 2KR. It may be possible to make them fit, no guarantees...
Click to expand...

Do you know the exactly difference? Bigger diameter at the airbox side, engine intake side or both?

regards

dave · Aug 21, 2010

Bigger bore internally and I presume at both ends as well.

2KR has Mikuni BDS26 (unconfirmed)
3LN has Mikuni BDST28

Dids · Aug 22, 2010

Re: 2KR Now running!

Right - more news. Keira (my partner and the FZR owner) and I got into the garage and put an effort into things.

I've found a leaking needle valve from #4 carb flooding into the venturi. Lo and behold, it wasn't actually the valve leaking - but the o-ring around it. Replaced. Tested OK.

Took a tiny bang with 'Start Ya Bastard' to coax it into life, but has been fine since. Time for an Italian Tune Up.

Test ride found some ongoing issues:
1. Idle very high. After warmup, idle was sitting around 4500rpm. Adjusted on return.
2. Found its hestitating pretty badly at around 9500rpm. This continues - lacking power - until about 11/12000rpm. Then it's back to going pretty dam well!
3. Occasional backfire.
4. In the lower to middle range, it's pulling really quite nicely until the 9500rpm issue appears.

Thinking back, the float levels were all at the very lowest setting. I'm seriously considering removing the carbs again, and resetting, this time to the upper specs.

Anyone have some thoughts on this?

Craig

Dids · Aug 22, 2010

Apparently, the manifolds on the 3LN are different spacings, but there's some other issue with just swapping the manifolds out as well.

dave · Aug 22, 2010

Re: 2KR Now running!

2. Found its hestitating pretty badly at around 9500rpm. This continues - lacking power - until about 11/12000rpm. Then it's back to going pretty dam well!
3. Occasional backfire.
Click to expand...

Sound like either a lean-out or flooding problem. It is not uncommon for these exact symptoms to be found on the FZR. Have a look at the long needles and the jets they run in for wear.

Here is a copy of a post by chris96 - it may be helpful. Not written specifically for a high revving 250, so the midrange they call 5-7000rpm may be 9-11000rpm on the 250.

# 1. Top end (full throttle / 7.5k to redline –

Best Main Jet must be selected before starting step 2 (needle height)!

* Select Best Main Jet

* To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the hardest pull at high rpm.

* If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.

* If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.

* In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!

* Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.

# 2. Midrange (full throttle /5k-7k)

Step 1 (Best Main Jet) must be selected before starting step 2!

* Select best needle clip position

* To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.

* If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered

* If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.

* If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.

* Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.

# 3. Low end (full throttle / 2k-3k)

Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3

* Float height (AKA fuel level & how to..)

* To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.

* Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.

* If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.

* If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.

* Example: change float height from 15mm to 14mm to richen up that area.

* REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.

* Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.

* Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.

* Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!

* If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 9,000 KM's. Check them for "oblong" wear - the needle jet orifice starts out round!

#4. Idle and low rpm cruise

* Fuel Screw setting (AKA mixture screws)

* Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation.

* Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.

* If lean surging is encountered, richen mixture screws (turn out) in 1/4 turn increments.

* Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/4 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.

* NOTE: A rich problem gets worse as the engine heats up

* If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/4 turn in, to lean the idle mixture.

* NOTE: A lean problem gets better as the engine heats up.

* If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1700 rpm, the mixture screws are probably too lean: try 1/4 turn out, to richen mixture.

Last edited by Chris69 on Fri May 29, 2009 1:19 am, edited 1 time in total.

Motorcycle Jet Kits & Carburetion Basics

How to improve, understand and troubleshoot motorcycle carburetion.
By Mark Zimmerman

Fundamentally, increasing fuel flow means installing larger jets in the carburetor. Sounds simple, doesn't it? Trust me, I've been doing it now for more than 30 years, and it really is that straightforward.

Picking the right jet, now that's the hard part. Fortunately that element of the job has been made much easier by the proliferation of jet kits now available. These kits contain all of the bits and pieces you'll need plus detailed instructions, and they are available to fit nearly every current and not-so-current motorcycle I can think of. But this would be a damn short article if we left it at that.

Rather than just show you how to install a jet kit, let me walk you through the basic steps used to jet a bike from scratch. Even though the kits greatly shortcut the jetting process, individual circumstances may dictate some fine-tuning from bike to bike. If you know how to evaluate jetting, you'll know what to look for and what changes need to be made. Since the most popular carburetor in use is the variable-venturi, slide-type carburetor, that's the model we'll use. For the sake of convention we'll assume you have either installed a jet kit and want to make sure the jetting is spot-on or you suspect some jetting changes need to be made.

Presumably you already have some fundamental understanding of a carburetor's internal workings. If you don't, I suggest you bone up on a little theory first. Mr. Bernoulli was a pretty bright guy, and his insights are well worth discovering.

It Just Ain't Running Right

Deciding where the problem lies is the first step. One of the more common mistakes the novice tuner makes is trying to jet based on engine rpm. Throttle position dictates which carburetor circuit is being used at the time, so jetting is always predicated by throttle position, never rpm.

Experienced tuners and riders are pretty good at determining which carburetor circuit is at fault. If you're new to this, a visual aid may prove helpful. A throttle index will show you exactly which circuit is in play at any given time. Using masking tape, place a datum point on the throttle housing. Next use another piece of tape to divide your throttle into quarters. As an alternative you can mark the quarters on the housing and use a colored pin stuck into the grip to indicate the throttle's position.

A jet kit typically includes a drill bit(right) to remove the plugs covering the pilot screw(s), adjustable jet needles (bottom) and the removable clips (to the left of the needles) that permit their height to be changed, perhaps some shims (above the clips), a selection of mainjets, and new slide springs.

Your markings should indicate 1/4, 1/2, 3/4 and wide-open throttle, although the last is usually pretty obvious, as is the closed position.

Too often a carburetor jetting chart seems to indicate a very clear delineation among the effects of the various jets and circuits. There isn't; in fact, there is always some overlap, and bear in mind that carburetor fuel-delivery circuits are cumulative. In other words, even when we're running wide open on the main jet, the pilot jet is still supplying some fuel. For this reason we start by addressing the pilot jet first and the main jet last.

Pilot circuit; 0 to 1/4 throttle

Start by bringing the engine up to operating temperature. Open the throttle slightly. Does the engine pick up speed smoothly? If it doesn't, the fuel-mixture screw may be out of adjustment or the pilot jet may be either too small or too large. To determine the pilot screw setting, adjust the idle-speed screw so that the engine is idling slightly fast, somewhere between 10 and 20 percent above the recommended idle speed. Using either the manufacturer's or the jet kit's recommended pilot screw setting as a starting point, turn the fuel screw left or right between 1/4 and 1/2 of a turn. Select the screw position where the engine speed increase is the greatest. Readjust the idle speed to the correct level.

Next, take the bike for a ride, preferably on a relatively quiet, straight section of road. Using your throttle index, run the bike at 1/4 throttle. If the pilot jetting is correct, the engine should run smoothly, and it should be easy to maintain a constant speed without varying the throttle position. If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut. If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

Low- to high-speed transition; 1/4 to 3/8 throttle

Although the throttle cutaway plays a significant role between 1/4 and 3/8 throttle, it is seldom changed when rejetting a streetbike, particularly a cruiser. For that reason I'll gloss over it. Basically the only time the cutaway needs changing is when the transition from the low-speed fuel circuit (pilot jet) to the high-speed circuit (needle, needle jet and main jet) gets seriously out of whack.

Medium throttle opening; 1/4 to 3/4 throttle

The fuel delivery at this stage is controlled by the jet needle and the needle jet. At one time all carburetors came with adjustable jet needles. Once the smog Nazis got involved, fixed-position needles became the standard. Jet kits will usually include new needles that can be raised (to richen mixture) or lowered (to make it leaner) to suit or, less often, shims to change the height of your stock needles.

To evaluate the needle/needle-jet performance, run the motorcycle in second or third gear. Roll the throttle on from 1/4 to 1/2 throttle only. The engine should accelerate cleanly without sputtering or bogging. If so, you're all done with the midrange; if it doesn't, you'll need to do some experimenting to determine if the needle position is too rich or too lean. Assuming you've installed adjustable needles from the kit, begin your experiments by trying some alternative needle-clip positions, perhaps a step richer to begin with. If that makes the situation worse, try a step leaner. If the jetting is stock and problems crop up, it's time to order the appropriate jet kit.

Top end; 3/4 to wide-open

At this stage of the game, the majority of the fuel flow is controlled by the main jet (remember some fuel is still flowing through the pilot jet). The time-honored method of determining main-jet size is via a plug chop. If you've never done one, a plug chop is pretty simple. Start with new spark plugs. Run the bike flat out (full throttle), in as high a gear as possible, for eight to 10 seconds (don't hold the bike at the rev limiter, as it'll give you a false reading). Without cutting the throttle or slowing down, simultaneously pull in the clutch and hit the kill switch. Coast the bike, with the clutch still in, onto the shoulder and remove the plug(s). The plug insulator should be tan to grayish-white. A white plug indicates a lean mixture; a dark gray or black one means it's rich. You then select the main-jet size accordingly.

Unfortunately, plug chops present a few problems. Some bikes require an inordinate amount of work to remove the plugs--at least more than you want to do on the side of the road with a smoking-hot bike. And in most parts of the country, the law takes a very dim view of banzai runs down the main drag followed by an impromptu tuning session on the side of the road.

The alternative, and the method I personally prefer, is to use a dyno. It's certainly a lot easier and arguably more accurate, but it may not be convenient.

Is It Lean or Rich?

One of the reasons I'm so stoked about jet kits is because they remove 95 percent of the guesswork. Be that as it may, even the best kits require some fine-tuning. Essentially a jetting problem can always be reduced to one of two choices: Is it lean or is it rich?

To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture)), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.

If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich.

Troubleshooting

A lean condition is the end result of too little fuel and too much air. Slightly lean conditions create drivability problems. Worst-case scenario: Lean conditions can and do destroy engines. Holes in the pistons, burnt valves and trashed main bearings are the direct result of lean mixtures.

Rich mixtures are the end result of too much fuel and too little air. Rich mixtures waste fuel, contribute to carbon buildup and pollute the air.

Main jets (bottom) may be exchanged for larger or smaller size to make the mixture richer (a greater amount of fuel mixed with the incoming air)) or leaner (less fuel) at full throttle.

Typical Lean Conditions:

- Poor acceleration; the engine feels flat.

- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.)

- The engine runs hot, knocks, pings and overheats.

- The engine surges or hunts when cruising at part-throttle.

- Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle.

- The engine runs better in warm weather, worse in cool.

- Performance gets worse when the air filter is removed.

Typical Rich Conditions

- Engine acceleration is flat and uneven and loses that "crisp" feel.

- The engine "eight-strokes" as it loads up and skips combustion cycles.

- The engine's idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.

- The throttle needs to be open continuously to maintain acceleration.

- Black, sooty plugs, a sooty exhaust pipe and black smoke from the tailpipe that stinks of unburned fuel.

- Poor fuel economy.

- The engine works better when cold. Performance falls off as it warms up or the ambient temperature rises.

- Engine performance improves when the air cleaner is removed.

<a class="postlink" href="http://www.motorcyclecruiser.com/tech/m">http://www.motorcyclecruiser.com/tech/m</a> ... index.html
Click to expand...

Dids · Aug 27, 2010

OK... 3 and 4 are about right. 1 is too low and 2 is too high.

They were all well low when I started. 4 was dripping fuel due to the needle seat o-ring being stuffed.

This slow, painful process has been made worse by my being clever. I set up a syphon out of a jar to get the more accurate fuel levels - the bowl height drops as fuel flows into your tube. Turns out even with a few inches of head on the syphon, that was more pressure than the pump puts on the needle valves - so I set everything too lean. Then I set it too rich - and flooded all of them. I've found that using a cleverly contructed cardboard box and hooking the the carbs back up to the fuel pump and a longer hose on the pump makes things about right for checking.

The old Heisenberg Uncertainty Principle - by measuring something, we change what we are trying to measure. I did that by not using the bike's fuel pmp to get correct pressure.

Nearly there... setting it all up a little on the rich side because that's a lot less damaging than too lean!

EDIT: Just banged the carbs on, and she started first go!!! Then they started flooding a bit. But that proves the mixture was the problem as well as the valves.

EDIT2: Finally.. the needles were sticky. Cleaned up the sides of the needles with some carby cleaner and a toothbrush, and the seats with cotton wool buds soaked in carby cleaner. Then dipped the needle ends in some fresh brake fluid. Worked like a charm. Guess you can learn something from antique tractors!
The next issue has reared its ugly head - the manifolds leak. If I can't judiciously good them up, where can I get these from, and how much will they cost??

Dave - thanks for your very excellent advice thus far. These floats are very, very sensitive it appears.

Craig

Dids · Aug 30, 2010

Anyone know what the float valves are for ordering from Mikuni?

The Yamaha dealer wants $240 for them. He will have those float valves a long, long time. I told him to marry them - at that price he will have them for life.

cornulio81 · Mar 8, 2011

Hi Guys,

I need a new Diaphragm, for my FZR250 2KR.

So, it's confirmed that the Carb-Type is a Mikuni BDS26?
Maybe anyone knows wheather the Carb-type was build in other Models of Yamaha?!?!

Greetz and i hope, someone could help <img src="{SMILIES_PATH}/icon_smile.gif" alt="" title="Smile" />

wombat75 · Aug 24, 2011

<img src="{SMILIES_PATH}/icon_biggrin.gif" alt="" title="Very Happy" /> try Advanced Diaphrams in Caboolture QLD for mikuni diaphrams, they have a website try google.

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