tony27 Posted May 26, 2009 Report Share Posted May 26, 2009 (edited) Can anyone tell me where hose from rear of carb on exhaust side should be routed? Bike is 250c & standard vm26ss mikuni Appears to connect to the emulsion jet & I assume this is the air valve mentioned in the carb specs, at the moment the hose is approx 100mm long & have looped around front of carb to keep from being kinked shut What effect does this have? Edited May 26, 2009 by tony27 Quote Link to comment Share on other sites More sharing options...
shercoman2k8 Posted May 26, 2009 Report Share Posted May 26, 2009 Its just a breather and fuel overflow pipe, lets excess fuel out the carb to prevent flooding... just shove it along the top of the casing and down to the bottom cup of your shock absorber! Quote Link to comment Share on other sites More sharing options...
tony27 Posted May 26, 2009 Author Report Share Posted May 26, 2009 There are 3 other breathers/overflows, 1 either side of throttle slide & other from bottom of bowl The 1 I mean is on the opposite side to the air screw & air blown through it comes out at the emulsion tube, drilling looks to almost line up with the air screw so my thoughts are some form of air bypass I realise these carbs are a little different from normal mikunis so stand to be corrected For the time being I'll leave as it is & if possible check some others bikes in the weekend Quote Link to comment Share on other sites More sharing options...
stork955 Posted May 26, 2009 Report Share Posted May 26, 2009 Gday, that sounds like someone has been fiddling. VM's only ever had the 3 hoses - 2 fuel bowl vents and an overflow from the bowl at the bottom. If that extra hose goes into the main air bleed someone has definitely been playing with it. Does the bike run OK? Cheers, Stork Quote Link to comment Share on other sites More sharing options...
richty250e Posted May 27, 2009 Report Share Posted May 27, 2009 Just had a look at my carb and there are three white plastic elbows: two overflows on the side of the throttle slide housing which each have a short pipe that drops vertically and goes through a little bracket cast into the float chamber. And the third one is the one I think you are talking about which aligns with a casting in the carb inlet that leads to what I think is the pilot air or bypass air passage. The manual I have shows the hose for this being routed into the main frame tube opening above the gearbox. All the other crank case breathers are shown routed into the main frame tube as well. Not sure if your 100mm pipe would be long enough to reach that? Looks to me like an air inlet pipe that has to be at atmospheric pressure (not subjected to engine vacuum) and kept away from water/dirt. There is also a short brass pipe on the bottom of the float chamber which is an overflow, and a brass stub connection at the front on the exhaust side for the autolube pipe (blanked off). My carb has 493 62 stamped on it. Hope that helps Quote Link to comment Share on other sites More sharing options...
feetupfun Posted May 27, 2009 Report Share Posted May 27, 2009 Yes Tony the standard carby on the TY250 uses an external air supply to the emulsion tube. I think you tricked a few people saying it is a standard VM26 carby. It is a Yamaha VM26 Mikuni which is quite different to the off-the-shelf VM26 Mikuni. It is a bit of a weak point for the bike because the emulsion tube requires frequent cleaning out due to dust ingress up the hose. Yes the end of the hose needs to be at atmospheric pressure. It has nothing to do with the pilot/idle circuit. Quote Link to comment Share on other sites More sharing options...
tony27 Posted May 27, 2009 Author Report Share Posted May 27, 2009 Thanks for clearing that up, on another post about fitting a DT125 kickstart to a TY the hose is shown so that confirmed to me that my thinking was correct. Will fit a longer hose before I ride this weekend, dust won't be a problem, more likely snow in the south island of NZ. Bike was running seriously lean last night until a .55mm drill was run through pilot jet pulling out a lot of scale, no brass was removed though The airbox we had to make fills in most of the underseat space & is pretty much open on top with the filter across the top, do you think the jetting will be out wildly or in the ballpark? Quote Link to comment Share on other sites More sharing options...
richty250e Posted May 27, 2009 Report Share Posted May 27, 2009 Regarding the air hose, might be an idea to install a small filter in the pipe. I remember that some older cars had a small filter gauze on the end of the pick up pipe in the windshield washer bottle. One of those would do the job nicely. My TY came minus the original air box and with an open filter cone mounted directly on the carb. In my experience I have never found open filters to be very successful on two strokes as you lose the pulse charging effect. So don't make your air box too 'open'. A hole in the air box slightly larger than the carb choke tube (26mm Diameter) is all thats required. No point making the hole any bigger as the air filter cannot suck in any more air than the carb can flow at full throttle. The theory is that when the inlet port closes a positive pressure wave travels down the inlet, through the carb and is reflected off the end of the enclosed airbox arriving back at the inlet port as it opens on the next stroke. This helps to push the fresh charge into the crankcase. If your airbox is a similar volume to the original, and uses the same intake pipe from the carb you should not be too far out on the jetting. I have been looking for an airbox but it looks like I'll end up making one too. I've looked at some of the new go-kart airboxes but they seem too large to fit into the space available. Good luck with the jetting. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted May 28, 2009 Report Share Posted May 28, 2009 The degree of restriction that the airbox creates, whether from the cross sectional area of the airbox entry, or from the type/thickness/area of filter media will also affect the jetting. Depending on whether your TY250 Mikuni carby is from a B model (two big round holes in the side of the airbox), or from a C or D model (air entry is via a baffled slot in the airbox lid), the ideal size of the air entry to the airbox will be different. The standard jetting in the two different TY250 carbies is quite different. Quote Link to comment Share on other sites More sharing options...
tony27 Posted May 28, 2009 Author Report Share Posted May 28, 2009 Hi rich, The airbox we made pretty much fills the whole area under the seat, had a A model to look but connector tube was solid & no filter meant no option as far what to do. Would have to say maybe 1/2 as big again, will try to post a couple of photos next week after the event to show just how big With a reed valve motor there should be no pulse coming back through the carb, theory of having large airbox was to give a large body of still air to draw from meaning less pressure drop when opening throttle Just got off phone with guys I'm travelling with & they reckon it runs very well now, revs well etc though maybe still slightly overgeared so I'll probably end up treating it like my sherco except for the steering which they say should be better than anything I've ridden before Quote Link to comment Share on other sites More sharing options...
richty250e Posted May 28, 2009 Report Share Posted May 28, 2009 Hi Tony, I remember reading somewhere that you should aim for about 1 litre of airbox volume for every 15 bhp the motor produces, can't vouch for this but I guess your airbox must be around that size - especially if you are using the space vacated by the oil tank. Your right about the reed valves - they should 'trap' most of the pressure wave in the intake port, so anything that does make it through will just help to pop the reeds open again when it returns back up the system. Of course a lot of this is academic as a trials engine is not used in the top end of the rev range like a motocrosser or go-kart. Those boost bottles that add volume to the inlet port are doing a similar job - synchronising the resonance in the intake air flow to the opening and closing of the ports. This only really gives a benefit at one engine speed, so the main effect is to shift peak torque to a higher or lower engine speed. As Feetupfun said earlier, using a good quality air filter foam/oil which does not add too much restriction to air flow, combined with your still air theory to maximise snap throttle response is probably the most important thing for trials use. If you can use a standard OE filter in your airbox you stand more of a chance of the OE jetting being OK. Quote Link to comment Share on other sites More sharing options...
tony27 Posted June 2, 2009 Author Report Share Posted June 2, 2009 Got home from event about midnight & realised I didn't take any photos of the airbox so will have to remember over weekend The jetting was ok down low but not great between sections at first so raised the needle 1 clip to richen things up after checking the emulsion tube & main jet for blockages. Next day while it was snowing between sections it was close throttle to go faster so relooked at float height which we raised, still far from perfect though in sections it ran ok, tried dropping needle & found 1 climb wasn't possible due to no power at all. Upshot is a 200ish main was in the van so is now fitted which is huge improvement with the bike flying between sections, a bit more adjustment should get things right Quote Link to comment Share on other sites More sharing options...
tony27 Posted June 9, 2009 Author Report Share Posted June 9, 2009 (edited) Here are a couple of shots to show just how big the airbox ended up The filter foam fits across the top so as you can see there is a lot greater area to help with the breathing Edited June 9, 2009 by tony27 Quote Link to comment Share on other sites More sharing options...
richty250e Posted June 9, 2009 Report Share Posted June 9, 2009 (edited) Nice bit of fabrication there Tony, I think you got the maximum capacity for the space available. Edited June 9, 2009 by richTY250E Quote Link to comment Share on other sites More sharing options...
tony27 Posted June 11, 2009 Author Report Share Posted June 11, 2009 The guy who did the fabrication is a welder by trade so a good job was a given, got a little bit a tidying up to do around the back so the inner guard fits properly then the bike will be stripped down for powdercoating of the frame. Next step is turning a alloy headstem 1 night after work to lose a bit of weight, already have modern sized alloy axles which are a lot lighter & stiffer All a bit of fun so far with nothing being taken too seriously Quote Link to comment Share on other sites More sharing options...
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