Carb' overhaul advice

[Mark]

Hi all,

I am having my girlfriends 1991 xt600E engine overhauled after 80 000 miles of use, and want to overhaul the carb's too.

I have stripped and cleaned them, and have noticed that if I push the slide up on the larger, CV carb, it stays up, instead of coming back down !
I have cleaned and lightly oiled (with WD40) the slide and the carb body where the slide fits but still it stays up. I have examined the diaphram and it has no holes, but it might be stiffer than expected.
(I have also discovered that if I block the slot above the main air passage and suck or blow on a hose coming from the top of the diaphram chamber, I can make the slide rise and fall.)

Any thoughts on the sticking slide anyone?

Also, I am planning on changing the float valve. Is there anything else I should change after so many miles?
I can't see a step in either of the needles to suggest that they are worn.
What is the method to change the needle valves, how easy is it to do?

Thanks

Mark

[gregdou]

Gday Mark,
I dont know about your float valve mods etc, but I was taught by a mechanic years ago, when working on carbs where the slide floats up and down with pressure etc, to polish the bore which the slide floats up and down in, with brasso so that it is nice and smooth and also the edge of the slide itself. keep polishing it so that it is nice and shiny and smooth and it usually works. -
Hope it helps - greg

[Bill Ryder]

How was the carb working before you took it apart? If you can find a shop that has a exhaust gas analizer, it would be worth a quick check to see how it is running especialy at idle. This will tell you if the needle and nozzel are worn because it will show up as a rich condition at idle. The needle and nozzle on most bikes are sold as a set. Change them by tapping (lightly) down on the nozzel so it comes out thru the float bowl. My experience has been that lube on the vacum slide will cause sticking a short while later on. I polish them with a scotch brite nylon pad (we use them for scrubing dishes) if you push the slide up with a finger lightly it should fall down on it's own. Really check the diaphram on the slide for any cracks or tears, they can be patched for a few thousand km with plastic and glue or buy new ones for big money. Remember repair work is 90% patience and 10% special knowledge and tools.

[aukeboss]

Mark,

1. Overhaul: the only thing to be done on XT6 carbs is, indeed, to change the needle valve: seat and needle itself. Remember to leave out the little gauze filter on top of the seat, and install an inline filter in the hose from tank to carb.
2. Sticking 2nd stage slide: are you sure the spring is still in there? If not, that's your first step.
A better test for the slide function is as follows: just hold a (switched on) vacuum cleaner nozzle to the exit side of the 2nd stage. Open throttle valve, slide should go up. After removing or switching off the vac cleaner slide should return to closed position.
If not, there is dirt somewhere or the sliding surfaces might be worn rough. This last I have never seen though.

More tips:
- Make sure all the O-rings etc are in place and in good condition. If not, replace with original Yamaha O-rings.
- I have never seen worn needles on these carbs
- Changing needles is pretty easy, although getting out and reinstalling 1st stage needle is a bit tricky.
- When removing needles and re-installing, make sure to put back the rings, little clips and springs in the correct locations.
- No lubrication required on the carb.

Auke

[winmac]

If the diaphram is twisted or jammed in the top cover it will cause the slide to stick. They can be tricky to assemble, somtimes it helps to install the cover loosely and work the slide up and down before tighting the cover.

[Mark]

Thanks for all the replies.

My secondry carb does still have the spring above the diaphram, the diaphram is located correctly and I have cleaned the slide and the body of the carb that it slides in.

If I push the slide up it stays up!
I suspect that the diaphram has gone stiff, too stiff for the spring to push it back down.

Has anyone ever had this happen to them?

Mark

[Bill Ryder]

I have had a diaphram(on a yamaha 350) degrade and get stiff to the point it would not be pulled up on acceleration. The diaphram looked fine but replacing it fixed the bad running at anything over one half throttle. How does your slide move when everything is apart? It should slide easily in and out of the carb. something else is to put everything together and leave the carb to air filter boot off. start the bike and see if the slide is moving some. No it will not run properly and may even try to spit in your eye but it is interesting to see what is happening.

[Mark]

Well, I've sorted the sticking slide issue.

I found my old Tenere carb's to compare and it's slide closed correctly
I then removed the slide to try and judge if the diaphram rubber was any less stiff, it didn't appear any different.

I did notice though that the spring that closes the slide was free to fall out on the Tenere slide, and was held captive on the 600E one. It had wound itself about 3 turns below the plate that holds the needle in the slide. I returned it to sitting on top of this plate, polished the slide and carb body as suggested and it is now behaving. Horay!

There was some ware evident in these carbs, the non stick coating on the primary slide is scuffed and worn through at the bottom edge. I will try re-using it anyway, the return spring is strong.
Also, there are some scratches / ware marks on both needles and the pilot mixture needle tip shows some erosion.

The 600E has a large breather hose from the diaphragm chamber that has a wire mesh filter in a plastic housing in it’s end. This has let sand into the carb! The Tenere does not have this breather.

Has anyone tried re-locating this breather into the air box?

Mark

[aukeboss]

As the secondary carb works on pressure difference above and below the diaphragm. If you relocate to airbox, downstream of the filter, there's a different pressure on the hose than would be with the original setup.
Better to install a piece of foam, just like air filter foam, in the housing of the mesh.

Auke

[Mark]

Hi again,

I thought that the idea behind having a large air box was to have a resovoir of still air at normal atmospheric pressure. Does it's pressure change with throttle position then?

Anyway, a piece of oiled foam is a good, simple idea.(or possibly a K & N crank case filter in place of the original wire mesh filter )

My final question is about the synchroising of the carbs.
I have a Clymer for the earlier bikes, and it states that the prmary carb slide should be raised 5mm when the secondry slide butterfly lever is just contacted.

Does this apply to the later bikes?

When these carb's came off the bike (1991 XT600E) the slide had to be raised 9mm, measured using a drill bit shaft, to get to the point where the roller on the pri carb' just contacts the arm of the secondry carb.
If I adjust it to 5mm, when the throttle is held wide open the butterfly valve goes beyond horizonal by quite a bit!

Same seems true of my old tenere carbs

As always, any thoughts / info appreciated

Mark

[aukeboss]

The air filter itself is a resistance. If there is air flow, and there is when the engine runs, the pressure in the airbox will drop (slightly).

Larger air boxes are chosen because the engine creates a pulsating flow. Larger volumes cause lower pressure drops when the same amount of air is removed form it, thus providing more air to the engine.

But, remember, in order to push the air through the filter, you need a pressure drop. As the atmosferic pressure is a given, the pressure donwstream of the filter must drop to get the air through the filter.

Auke

[Breed1229]

What worked for me was, I took the carb off of my xt 600 threw it in the garbage and installed a set off of a raptor 4 wheeler"EBAY item" very cheep.
Best thing I've done to my 600, huge difference.

[xtphreak]

Originally Posted by Breed1229

What worked for me was, I took the carb off of my xt 600 threw it in the garbage and installed a set off of a raptor 4 wheeler"EBAY item" very cheep.
Best thing I've done to my 600, huge difference.

so is this a "bolt-up" or do the carbs need any modifying?

did you use the standard XT intake or the one off the Raptor?

same question for the sirbox to carb boots?

how was the jetting, did it take much tweaking?

thanks

[Breed1229]

Not excatly a bolt on swap. There were a few mods to be made. I'm sure that I'f you have the corect carb on your bike that you could make them work just fine. I did not have that option with mine. I got the bike and had lots of trouble with the old carb set. SO installed the Raptor set. Like I said a few mods were made. Nothing a person with a little skill couldn't handle.