1993 xj600 Carbs....

so in my previous topic I couldn't get the bike to rev at all. I fixed that by taking the gas tank off and running gas from another tank because the fuel was dirty from all of the rust.

now I get it to idle and rev

The problems:

will not rev passed 7000rpm in neutral

will not rev passed 4000rpm while riding it down my street.

the carbs still backfire

there is smoke coming out of the carbs after a while when i turn the bike off

in oder to get it to rev passed 1000rpm you have to play with the throttle and then slowly accelerate


I don't know what it could be. I cleaned the carbs pretty well, the float heights seem ok. maybe a little low... and I played with the pilot screws


If anyone has any ideas what so ever, I would really appreciate them. thanks

 

I just did a set of these the other day.

One thing that is tricky on them, if you turn them so the float bowl flange is horizontal the weight of the floats will depress the spring loaded pins in the float needles. You need to tip these carburetors to 45 degrees and set the floats to 7.2 mm measured perpendicular to the flange.

Also, make sure both floats are at the same level. The set I did had about a 2mm deviation between the floats.

The main jet on these carburetors screws into a tube. The tube needs to be removed to expose the emulsion tubes. The emulsion tubes don't seem to want to come out. Make sure all 16 airholes in the sides of the emulsion tubes are open.

The pilot jets have air holes in the sides. Make sure these are open as well as the jet itself.

I don't have a 10mm colortune plug (yet). I set the mixture screws to 2 1/4 turns from bottom and got good results.

 

thanks for the advice on the float levels. Where do you measure the 7.2 mm from? (I might be doing it wrong because the haynes manual isn't too comprehensive on that topic)

do you have any ideas about the other problems?

 

Hard to do without a picture.......

Basically:

Float bowl cover removed.

Carbs held in installed position. Rotate so float is coming toward the top. When the needle closes the float stops swinging. The gasket face of the float bowl will be about 45 degrees off horizontal.

Measure perpendicular from the gasket face to the farthest spot on the float. That distance should be 7.2mm.

Your issues are almost certainaly a combination of the following:

- everything not clean
- floats set wrong (or leaking)
- idle mixture set wrong.

 

thank you so much MICarl.

what would I have to specifically clean in order for it to rev higher and/or more smoothly?

right now I have the idle mixture screw set to 1.5 turns out but the plugs seemed wet.

 

And make sure the float levels are correct because nothing else works properly if they aren't.

 

I just did the carbs... again

I took them apart sprayed every hole with carb cleaner and let the removable parts sit in it. ( the rubber pieces didn't touch the carb cleaner) and then went over everything with compressed air.

I put them back together and adjusted the floats. I did this by tilting them at a 45 degree angle and just as the tang touched the needle I measured the distance from the carb body to the highest point on the float. It measured 10mm when I started. Now they are about 7.5mm maybe 8. Hard to see with the ruler I had. I know they should be 7.2mm but since the bike was running at such a high float height that I wanted to see if it would improve the performance and I could tune it from there.

Also, I have the idle mixture screws set to 2.25 turns out.

The bike still doesn't rev paassed 6000rpm. It starts and idles ok, but once I get on the gas it stumnbles and then revs to 6000 dieing off.

Could it be a problem with something else? Like the igintion system or other electronic parts?
Could there be a problem internally with the bike's engine?
Could the bike run like that on only 2 or 3 cylinders?
Or, is it most likely the carbs and I am doing one crucial element incorrectly?

 

Possibly

Unlikely

Yes. They will seem to idle quite nicely with a cylinder or 2 knocked out, then no power as you rev. Start it up at idle and make sure all 4 pipes are heating.

This is the most likely thing. The main jets are coming into play where your trouble is.

 

maybe make sure that your jets arent reversed. that was the case on my bike.

 

Speaking of revering jets. You did clean the air jets on the back of the carburetor didn't you?

 

what are the air jets on the back of the carbs?

how is it possible to reverse the jets?

I'll check if all four of the pipes are heating and have it updated by tonight.

I also checked the spark plugs. There is a spark on all four coils.

Shouldn't the inner ceramics of the spark plug turn brown or tan. Two of mine stay white with black dots.

 

If you look at the air box side of the carburetor there are two brass jets near the bottom of the opening. They are the air jets.

Reversing them means they are switched with each other. Impossible to do with the fuel jets on Mikuni carbs, but the air jets probably can be. Unfortunately I don't know which is which.

 

That you can't get more than 6,000 rpm's has to come down to just a few things:

Main Jetting
Throttle Plate Opening
Diaphragm Piston Movement
Linkage issues

The Throttles need to be pulled to Wide Open.
If they aren't being pulled Wide Open; you won't get Full Throttle Response.
This could be cause by having the wrong Throttle Cable.
Marginally too long for your Bike

The Main Fuel Jets might be too small.
The PO might have been cheap or too scared to ride the thing as fast as it could go.

Diaphragms not opening far enough to pull-up the Needle out of the Emulsion Tube far enough to allow full fuel flow.
a) Wrong Needles?
b) Sticking Pistons within their Bores
c) Rubber Diaphragms compromised with tears or holes.

Linkage issues:
Throttle Plate Binding
Cable End attachment bent
Ill-fitting Screw substituted for Sync Screw (Physical obstruction)

Emulsion Tube(s) Air Metering Ports clogged.

Wrong Main AIR Jets (Probably not ... check anyway)

 

those are some things I definitely have to look into.

The things is however that once in a while when I ride onto the gas slowly in neutral it will rev to redline.

So I doubt that it is the throttle cable.

I checked all of the diaphrams in the sun- they all don't have any hole in them.

all of the cylinders are firing because all of the exhaust pipes get hot.

What could be causing it to rev high in neutral and keep it low while in gear and moving? Because with my logic if an engine is capable of revving the high in the first place with the setup it has, then it should be able to do it both in gear and in neutral.

 

I am really really sorry I wasted all of your time guys.
But I am also thankful for the information which all of you have given me and hopefully I can keep learning and eventually give back to the forum and the other new guys.

THis is how I fixed my bike!:

I put the air filter housing on.

It was letting in too much air.

So to anyone doing testing, please make sure the air filter housing is on the bike.

Btw, the ride is amazing. It is the hardest accelerating thing on wheels that I have ever been on.