New Member New Member 3LN1 FZR250R

Biggest issue in the carbs is the needles and emulsion tubes wearing excessively causing poor combustion and excessive fuel consumption (I think I got 70km to reserve from a full tank once). Apart from from being over 30 years old and needing a overhaul and seal kit anyway.

Pretty much a dead giveaway is to shut the fuel tap off when it has problems... if it comes alive again before it dies from lack of fuel then you pretty much know for certain that's what the issue is.

The cause is poor retention mechanism of the needle in the diaphragm, allowing it to rattle around, wearing both the needle and the emulsion tube away. The later 3LN6/7 and 3YX Zeal carburettors fixed this problem with updated designs. The earlier 2KR FZR250's also have a better retention mechanism.

 

This is the improved needle retention design on 3LN6/7



And this is the 3YX Zeal retention mechanism



These are worn needles and tubes from 3LN1/3/5 models





The slide allows the needles to droop with gravity, as these carbs are mounted at an angle, this will happen in operation:





You can get Keyster carb kits from www.motorcyclespareswarehouse.com.au

But you need to identify which carbs your bike is fitted with, as these are all grey imports, parts can get swapped around when they assemble a bike from bits out of a container from Japan

https://2fiftycc.com/index.php?thre...zr250-bodywork-frame-engine-carburetors.3469/

You will also need to identify inner and outer carbs as their internals are slightly different

https://2fiftycc.com/index.php?threads/mikuni-bdst-28-26mm-carb-settings-specs-3ln-3yx-zeal.9787/

Links for the kits:

3LN1 outer carbs: https://www.motorcyclespareswarehou...ir-and-parts-kit?_pos=14&_sid=2c37dee21&_ss=r
3LN1 inner carbs: https://www.motorcyclespareswarehou...ir-and-parts-kit?_pos=10&_sid=2c37dee21&_ss=r

3LN3/5 outer carbs: https://www.motorcyclespareswarehou...ir-and-parts-kit?_pos=12&_sid=2c37dee21&_ss=r
3LN3/5 inner carbs: https://www.motorcyclespareswarehou...ir-and-parts-kit?_pos=10&_sid=2c37dee21&_ss=r

But the o-rings and seals in those kits are pretty ordinary, your best bet for new seals is from Litetek:

https://litetek.co/Carb_Kit_Yamaha_FZR250_3LN1.html

https://litetek.co/Carb_Kit_Yamaha_FZR250_3LN3.html

@maelstrom on this forum runs Litetek, so you can be confident in the quality of the product.

 

You can deal with the cause of the overly loose needle in the slide by getting 3mm thin wall brass tube, from a hobby store is perfect.
Ream it out to > 2.55 < 2.6mm mm, then cut a small length ~6-8mm and press it into the slide from above until it just reaches the bottom.

FWIW by far the best needle retention mechanism is in the CBR250RR carbs, those carbs also have a hole for the needle which is much smaller ~2.6mm and the needles are both approximately the same diameter at the top 2.515mm

 

Bent butterfly is a worry. High idling can be caused by either air leaks, incorrect fuel mixture screw setting our by the throttles being out of balance. I'd say that bent butterfly definitely isn't helping either.

 

Those Chinese kits are OK however on the set I purchased, the emulsion tube orifice is ~2.52mm where it should be 2.59mm, that combined with the loose needle in the slide will lead to accelerated wear.

FWIW, the needles are 2.515mm, the N-8 emulsion tubes are 2.59mm, so that's normally 0.075mm clearance - the tolerances are tiny.
That's the reason that wear is such a detrimental effect on idle fueling, effectively those are near closed at idle, any wear leads to richness.

It may not be the butterfly which is bent, it may be the shaft which with care can be straightened.
If the butterfly is bent, a brass mallet tapping it to get it back flat can work, then put it onto a piece of glass and tap it to see if it wobbles

 

Howdy

 

I gather your float bowls are flooding.
Check your needle valves and seats. If they are not Viton tipped, you might want to change them.

 

If yours aren't new:
Float needles are difficult to diagnose by just looking at them, under magnification they may show small cracks, most commonly they go hard with time.
Replacements aren't expensive.
If you suspect that the fuel level [height] is incorrect [which is the important measurement], you can check that without any disassembly.

https://www.2fiftycc.com/index.php?threads/my-fizzer.872/page-32#post-15537.
Check the second picture in this post

Simplified measurements for the 3LN1 are:
The fuel level [height] measurement is above the casting mark and for the 3LN1 it should be 9.2mm
The float height measurement to achieve the above fuel level [height] is ~16mm
Someway somehow the two measurements together add up to 25.2mm across the 3LN series.

So if you need to adjust the float level to correct the fuel level by a specific amount [height] there's and odds on chance that setting the float height to 16mm with achieve the required change.

It helps to take the guess work out of it.

Also FYI: the OEM pump is supposed to output 1.5 PSI, it's really only there to allow you to continue riding when the fuel tank gets low, so if you suspect the fuel pump output pressure, take it out and try without to eliminate that, which means one less variable.
The 3LN will run fine without a pump.

 

What ruckusman said.

Fuel pump is only there because the lowest point in the tank is below the fuel bowls on the carbs, so gravity feed won't work to empty, and the FZR250 doesn't run a vacuum fuel tap. Though you could easily convert it to a vacuum tap and eliminate the fuel pump. All you'd need is a vacuum style fuel tap, and a vacuum line to one of the inlet manifold vacuum ports and maybe an adapter barb/fitting for a vacuum line.

 

New air filters are AFAIK unobtanium, well I've never seen one advertised anyway.
The foam filter replacement could be viable, is it lightly oiled?

There's all sorts of magnificently complicated resonance and other factors going on in that airbox - just look at the number of people that have attempted and failed to install pod filters on CV carbs.
Speculating: The foam filter might have an effect, but I suspect it would become evident at high revs, not at the low flow rates of idle.

Another option would be to get a replacement washable air filter element in the correct dimensions and transplant it.
I did at one stage browse the extensive range of K&N washable filters, I think they may have something of the correct or very close to correct dimensions that could drop in.