Hi Folks,

Looking for a bit of carb help. I searched and searched other posts but couldn`t get to the bottom of my carb setup issues. My carb is the old style with the piston type secondary.

I have never messed with the carb settings as the bike has run well but has always suffered from the dreaded hiccup when the secondary kicks in. I have always just worked around this as backing off the throttle a bit from wide open typically allows the bike to rev out as it should.

My problem is the bike seems to gradually run richer over time. Typical fuel consumption was 60mpg now it`s proabably 50-55mpg. Also cold starting is suffering. The bike has always started 2nd kick when cold 1st when hot. Now it takes 5-6 kicks when cold. Hot remains at 1 kick.

I have recently completed a full teardown for new rebore,piston,rings. I am paranoid that the rich condition is going to wash the oil off the cyl. walls and result in a premature rebore the next time. That`s my motivation for getting the jetting right.

I am trying to baseline the carb settings to recommendations in the manual and Aukeboss who often seems to chime in on carb questions.

The Clymer manual talks about a Carburetor full open adjustment. Basically, setting the throttle valve(primary) to stop once the throat of the carb is fully exposed. Problem is my bike doesn`t seem to have the necessary adjusters as depicted in the manual.......and it does not stop where the manual suggests it should.......what to do.

Aukeboss has recommended to set the secondary to open at 7mm vs. 5mm as suggested in the manual. Is this correct for the old style carb. The manual also recommends to do this adjustment with the throttle cables attached. Is this necessary.

The manual says to leave the pilot air screw alone. Aukeboss recommends adjusting it. Currently it is 4 turns out from fully in. The bike is rich, should I be turning it in or out to lean.

The mains are both 130 and very likely stock, is this ok.
Aukeboss suggests needle in the 4th position. Seeing as my bike is kinda rich, would it be wise to drop the needles 1 position.

Is there some way to test that the secondary (vaccum operated) throttle valve is actually opening as it should. Does the throttle valve piston ever wear out. When operating it by hand it is nice and smooth.

The viton tip on my float valve is showing its age but......canèt get a replacement. Is this a problem.

Sorry for the lengthy discussion but I have zero carb tuning experience. Kedo says they are working on a rebuild kit for the 43F carb but not available at this time. Some of the stock Yamaha rebuild parts are becoming discontinued for this carb......getting a little worried.

Has anyone else suffered this gradual richening condition over time. What was the solution.

Sorry about the lack of proper question marks, my computer does ééééé when I hit the question icon for some reason. Wierd

Many thanks for your time and assistance,

Pawlie

Just read this

The XT600 uses a dual-carburetor setup, dubbed YDIS for Yamaha Duo Intake System. Most dual-purpose and off-road four-stroke singles now use some kind of dual-carb system, the theory being that at low rpm, where one large carburetor doesn't mix fuel and air very efficiently, a smaller carburetor will give better throttle response. At increased rpm the secondary carb kicks in to give the engine the fuel it needs for higher speeds. The XT's carbs—a 27mm slide-type primary linked to a 27mm CV-type secondary—are manufactured by Teikei. Most of the time they worked just as the theory dictates, although, there were instances when the engine would stumble, almost as if the secondary carburetor was taking a gulp before getting on with its job. The condition was especially annoying in passing situations on crowded freeways. Before reaching any firm conclusion about the performance of the carburetors, we cleaned and re-oiled the dual-stage foam air filter. This helped—it seems the new XT has inherited the TT600's trait of easily clogged air filters—but a slight hesitation was still there, between 4000 and 5000 rpm.

Hi Keny,

Interesting about the hicup/stumble persisting. But that's really not my problem. It's the rich condition I'm worried about. It is not caused by a clogged airfilter. I'm a dirtbiker. I am regularly found cleaning and oiling filters after having too much fun out on the trails. Thank goodness the trails are fun because cleaning filters is not!

Cheers,

Pawlie

I (still) have starting problems with my 43F which I suspect might be an air leak on the inlet. However, I have spent some time on my carbs recently so this is what I have done so far.

I have no problems with 'bogging down' on the second carb. Once started, it runs fine.

1. Strip and clean both carbs. Pay particular attention to the pilot jet and the little internal drillings. I ended up having to poke my pilot jet clear. I used a nylon brush bristle. Copper wire might be OK as it is softer than brass - steel wire will not.

2. Make sure all the gaskets are still in good shape. The same goes for the all the o-rings.

3. I set the cam on the secondary carb so that the butterfly is just horizontal at full opening. Mine was short of this a little due to wear on the cam.

4. Carefully check the condition of the inlet rubbers. Seal any cracks with silicone sealant. (I think I may still have a leak here). New ones are about £50!!!!

5. Set the pilot jet screw to between 4 and 6 turns out per Aukeboss' advice (richer than standard - it's an old bike now).

6. Replaced the float needle assembly. I got a genuine Yamaha one from my dealer for £30. It comes with the needle, seat, screw, o-ring and a new filter. (I suspect this might be your problem). Part number is 1JK 141 070 100 (the first character might be an I - I wrote it down in a hurry)

7. Set the float height to 6mm below the top of the float bowl. I used the clear drain pipe from the carb, shoved it on the float drain tube and made a U shape that went up the side of the float bowl. Open the float drain screw and the tube should fill up like a manometer. Measure at the middle of the side of the float bowl because the carb is not horizontal on the bike. Remember to fit the drain tube back in the right place after.

8. Start it and warm it up.

9. Screw the pilot jet screw in and out to find maximum revs (you will find a plateau of revs over a range of screw openings). Set the screw with enough turns out so that you are just still on the plateau of high revs. Turns out = richer and turns in = leaner. 5 turns out was too much on mine - it is about 3 turns out now. Nominal is 2.5 turns out.

Good luck and keep us posted.

Hi Bandit127,

Thanks for the recommended tips. I should be able to fire up the beast within the next few days to see if I have made any progress.

I'm going to pay close attention to the pilot air screw now that I know how it works. Thanks for that.

My boots are new and sealed well so no issue there. I've given the carb a good going over but couldn't remove the pilot as I didn't have the correct screwdriver and didn't want to strip it out. It's been in there for 25years!

I'll see if I can track down a float needle assembly but I seem to recall it was discontinued when I last checked. Mine is less than perfect for sure.

I'll check the float height tomorrow as you describe.

Did you happen to gander at the manual where it discusses setting the height of the primary throttle valve? I don't think my 43F has the adjusters they are talking about......bit of a mystery.

Still trying to figure out if there is a way to test the function of the vaccum controlled (secondary) throttle valve?

Thanks again for your helpful tips.

Pawlie

Mine is a 43F too. Try ringing your local dealer with the part number I posted. It worked fine on mine.

There is a screw between the 2 carbs. Open the throttle cam (where the cables connect) and you will see how this movement is transferred to the second carb, via the adjustment screw. I don't think mine had been touched from new and I gave it about 2 turns in to get the butterfly horizontal at full opening.

Good luck.

Bandit,

Thanks again for the tips.

I've made the above adjustment to the secondary carb already.

Both my OEM and Clymer manual have a section on "Primary carb full-open adjustment". Going by the description in the manual my primary is not correctly adjusted and I can't figure out where the adjuster is.

Has anyone else sucessfully adjusted the primary (Not secondary) throttle valve as detailed in the manual?

I tried the float height test yesterday and got 15mm!!!! Gonna try again as I didn't follow the proceedure exactly. I'm also going to try and source that float needle assembly.

Thanks,

Pawlie

Is there some special trick to getting a consistant reading on the fuel height measurement?

I tried it with the wheels level as Bandit 127 suggests. I also tried it with the front wheel higher than the rear. Reason for this was to get the carb vertical as recommended in the manual. Followed the rest of the directions as described in manual.

My readings varied from 15mm to 4mm......am I missing something?

Do you need to start with an empty float chamber?

I just used a small tubing. Would I get a better reading if the tube dumped into a slightly larger cylinder as shown in the manual? Doesn't seem that it should make a difference.

Perhaps there is something else I am missing?

Thanks for your help

Pawlie

I suggest you turn the bike on for a few seconds and shut it off and then check...

I've fitted new valve seats and pistons, bowl rubber o-rings, needle springs and spacers on my carb and will surely have to do the same as you're doing right now once I sort my left crankcase problem....


Vando

Hi Vando,

I did the test exactly as described in the manual. With engine running/then shut off/then measure as the level rises to maximum. ect. I also did it with other variations in an attempt to get a consistant reading. No luck at all. Quite frustrating.

For thoes that have done the fuel level test, is there something I'm missing? It doesn't make sense to me that the observed fuel level varies so much (4-15mm).

Cheers,

Pawlie

Yup, it ain't easy checking it...

Did ya forget about the fuel stop screw under the carb?? you have to open the overflow screw or it won't measure! (done that before lol and it behaved exactly like you described)

and one important thing.. you absolutely need to check the fuel height in exactly the same place on the carbs!

Keep us posted!


Vando

I never did get a consistant reading for the fuel level height. Made some adjustments and the bike is fueling better. Gonna leave well enough alone until a carb kit is available.

Pawlie