Carb Breathers

My XT600 is getting some major surgery before it's Portugal trip this summer.





The XT600 has a problem that when the carb breathers get blocked off, the engine stalls. This is a problem on river crossings, as the ends of the carb breathers end up under water. The fix (which is apparently a common mod on a lot of off-road bikes) is to cut the breather up near the carb, and install a T piece and a second piece of tube, which you locate under the seat below the airbox snorkel, so when the bottom of the breather is under water, the carb can still breathe via the tube under the seat.

Whilst I have easy access to the carb breathers I want to get this job done, but which two breathers are the ones I would need to T off?

Just a suggestion but as they're so close to the headstock can't you re-route and shorten them and fix them somewhere up there?

Originally Posted by monkey

Just a suggestion but as they're so close to the headstock can't you re-route and shorten them and fix them somewhere up there?

It has been suggested on the horizons unlimited forum, but the reply was that they still needed to deal with fuel overflow (?), so T'ing them off is better than re-routing them. It also says to mount them a little bit below the airbox snorkel, so that if you did end up really deep, or dropped the bike, the engine would cut before sucking in water. What it doesn't do is give you a clear picture of which two breathers.

You don't know which two would be the one's needing re-routed would you?

(BTW, that's not the headstock your looking at, its middle section of the frame where the fuel tank and the riders seat meet, but they are close enough to re-route to the airbox under the riders seat without a T piece. See the first picture and look for the carb intakes for reference and orientation)

Sorry Ralph I haven't the foggiest. I didn't even know the XT600 is a single!

Have a look through the manual if you have it. I'm sure it'll be in there. Or you could wait with someone who knows to come along!

HNTH

Originally Posted by monkey

Sorry Ralph I haven't the foggiest. I didn't even know the XT600 is a single!

Have a look through the manual if you have it. I'm sure it'll be in there. Or you could wait with someone who knows to come along!

HNTH

No worries

It's not in the manual, it's a modification not a standard config, It's basically a Yamaha design fault. The XT was one of the first 'soft roaders' (though it still has excellent off-road ability compared to the soft roaders they produce today, if you can handle the weight). Still Yamaha expected it to do more on road, and stuff like dry gravel tracks, though it has a proper snorkel on the airbox and so on, it wasn't really intended for deep water. The proper big off roader of the time was the TTR600, which is identical in many ways, but it has these breathers T'd off as standard.

I have posted on the XT section of Horizons Unlimited, but the reason I've posted it on an SV forum too, is that it's apparently common on other off-road bikes (KTM, Honda, etc), and anyone who understands the carbs, and why the engine would stall if a certain breather is blocked, should be able to deduce which breathers to modify.

[yorkie_chris]

That one on bottom of float bowl, blow in it, is it "solid" or does it wheeze into carb?


BTW that carb is "interesting", left hand is normal (manual operated) slide carb, right hand is CV vacuum operated slide (like SV) which opens progressively as revs rise. They share float bowl but each one feeds one intake valve.

You misunderstood me Ralph. I meant to look in the manual to see what hose is a breather.

[Sid Squid]

Do what Chris said, the breather you're referring to leads into the float bowl to allow atmospheric pressure to act on the upper surface of the fuel, as such they are open. If you blow through it it will pass air, unlike the hose which leads to the drain screw which won't, (unless the drain screw is undone of course - but I reckon you know that ).
This means, (although I'm not familiar with that engine so do the above test for certainty), the upper two of the 5/6mm hoses in the picture. The lower one - the one adjacent to the fat wire under the rubber boot on the starter - is the drain hose.

Originally Posted by yorkie_chris

BTW that carb is "interesting", left hand is normal (manual operated) slide carb, right hand is CV vacuum operated slide (like SV) which opens progressively as revs rise. They share float bowl but each one feeds one intake valve.

From http://www.motorcyclespecs.co.za/mod...xt600%2084.htm

"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.

Hesitation or no, the carburetors metered fuel to the engine very efficiently. On the Cycle World mileage loop the XT returned 61 mpg, giving the bike nearly a 180-mi. range from its 2.9-gal. gas tank. The Yamaha usually went on reserve between 140 and 150 mi.

Carburetion also played a part in the way the big XT started—easily for a 600cc thumper, which is not to say easily compared with most motorcycles. First off, there is no electric starter, since a kick-only design is more in keeping with the bike's off-road, less-weight theme. There is, admittedly, an automatic decompressor that opens the exhaust valves when the kick starter is depressed, but it still takes a healthy kick to get things moving. In the morning it's best to use full choke with no throttle, and the XT will rumble to life in three or four kicks. The bike can be ridden away immediately with the choke still on, since no warm-up period is necessary—a pleasant change from the cold-blooded way many four-cylinder street bikes behave.

When warm, the XT will fire with just one kick .. . usually. There were times—two or three during the 1400-mi. test period—that the XT just refused to start. Kicks, curses and push-starting had little effect. After a few minutes rest and some more kicking on the part of the rider, the XT would spring happily into its 1500-rpm idle."

Though mine is an XT600E model with an electric start.

Originally Posted by Sid Squid

This means, (although I'm not familiar with that engine so do the above test for certainty), the upper two of the 5/6mm hoses in the picture. The lower one - the one adjacent to the fat wire under the rubber boot on the starter - is the drain hose.

Thanks guys.

Where abouts did you find a good T piece? as i can only find expensive ones or sets and i need to do this mod on the dr.