FZR400 1WG. Misfire on No3 below 4000 rpm

[RussellJupp]

Hi all,
have an irritating problem, and hoping someone can help....
Just bought an 88 1wg genesis. Been stood for a year or so. Runs, but misfire on No3 below 4000rpm!
No.3 exhaust downpipe remains cool, and idle is erratic.
So far I have changed plugs, coils, lead and cap for No.3. Also replaced No3 carb body and carb internals.
Motor sounds sweet, and will occasionally try and fire on No3 at idle. Also will fire when I spray easy start into No3 intake.
Above 4000 rpm it goes like a rocket, and No3 exhaust downpipe is normal.
I have balanced carbs with vacuum gauges, and it shows identical vacuums on all 4 cylinders.
Ive yet to check valve clearances, but would this cause my issue?
Any help appreciated!
ta
Russ

[hunter]

This sounds very much like a blocked pilot jet,
But yes it could be valve clearances.
Check the valves if ok go back and check carb,
It might be worth you getting them ultrasonic bath.

[willandrip]

If the vacuum is the same on all cylinders that should indicate that the carb/head rubbers are not leaking air;
I would also go with the pilot circuit.
Do the symptoms stay the same when the bike is upright and tilted. (paddock stand/sidestand.)?
Remove the airscrew and inspect the tip and the O ring.

[Evilchicken0]

Sounds like the pilot jet
Although you said you swaped out the carb for this cylinder. You could swap the idle jets in a good carb and see if that moves the symptom. There was a similar thing on here a week or so ago - popping on the overrun though, that turned out the be float heights.

It would be a good idea to do the valve clearances although it doesn't sound like this is the problem.

[RussellJupp]

Thanks, I'm also inclined to be thinking fuelling...even though Ive changed the carb body, and internals.
I had all airscrews out, and inspected. Dont recall seeing o rings though? Re-set the airscrews to 2.75 turns out from closed.
Which is the pilot jet please? there are two removable jets within the float bowl, and one in the carb mouth.
And the symptoms are the same on side stand, or paddock stand.
ta
Russ

[willandrip]

The jet in the carb throat (#130) is the Pilot air jet; it allows the air into the pilot circuit.(30 on the fiche)
The tiny Jet (#15) is the Pilot fuel jet; it allows fuel for the pilot circuit.(10 on the fiche)

The O ring is part of the Pilot screw set; ( 8 on the fiche.)
Composed of;
Screw
Spring
Washer
O Ring

In that order.

Google Images; 12R-14105-00-00 ( It is a common part to many Mikuni carbs.)

The anti-tamper cap (31) has to be discarded and is not required.

Try 3 turns out (IIRC some go out to 3.5)

Angle of the dangle was to test if fuel heights altered running.

[RussellJupp]

The jet in the carb throat (#130) is the Pilot air jet; it allows the air into the pilot circuit.(30 on the fiche)
The tiny Jet (#15) is the Pilot fuel jet; it allows fuel for the pilot circuit.(10 on the fiche)

The O ring is part of the Pilot screw set; ( 8 on the fiche.)
Composed of;
Screw
Spring
Washer
O Ring

In that order.

Google Images; 12R-14105-00-00 ( It is a common part to many Mikuni carbs.)

The anti-tamper cap (31) has to be discarded and is not required.

Try 3 turns out (IIRC some go out to 3.5)

Angle of the dangle was to test if fuel heights altered running.

Excellent, thanks. I'll have another look, but dont remember either an O ring or a washer on the air screws. Presumably the o rings are of some importance? Secondly where is the pilot fuel jet located please? I'm new to all this, and dont have a copy of the fiche.
Ta
Russ

[willandrip]

The washer gets lost.
The O ring gets stuck in the carb or lost.
The O ring is to prevent air leaks and the whole set up with all parts is designed to prevent vibration altering the setting.

The pilot fuel jet is in the float bowl chamber down a tunnel IIRC; If you say you have cleaned the carbs ;you should know where it is. This will be why it is not running right if it has not been cleaned. It is tiny; many replace it rather than damaging it by cleaning.

This link takes you to LitekBlair's page, who is a member on here. You need to thank him very much for providing
the fiches that can be downloaded at the bottom

http://www.litetek.co/Carb_Kit_Yamaha_1WG.html

[RussellJupp]

The washer gets lost.
The O ring gets stuck in the carb or lost.
The O ring is to prevent air leaks and the whole set up with all parts is designed to prevent vibration altering the setting.

The pilot fuel jet is in the float bowl chamber down a tunnel IIRC; If you say you have cleaned the carbs ;you should know where it is. This will be why it is not running right if it has not been cleaned. It is tiny; many replace it rather than damaging it by cleaning.

This link takes you to LitekBlair's page, who is a member on here. You need to thank him very much for providing
the fiches that can be downloaded at the bottom

http://www.litetek.co/Carb_Kit_Yamaha_1WG.html

I guess that'll be the problem then....only stripped the two obvious jets out of the body and cleaned them.
You've been really helpful, thanks!
ta
Russ

[willandrip]

Keep the thread updated; it aids others.