gazampa Posted August 7, 2009 Share Posted August 7, 2009 how strange the change from brass floats to plastic Im changing out the brass floats from my 78 Yamaha XS 400's Mikuni BS 34 carbs to plastic floats purchased from www.mikesxs.net.( $12 each versus $50 for the brass through the local shop ). I figure they should be interchangeable, These plastics are used on later 650 models which switched to the 34's from the 38. I saw some brass on the site but for the 38 and the bowls looked differently shaped. I notice the float heights for brass are 27mm and for plastic its 22mm. Is the height different for the 400 model mikuni 34's or do they just drop in at the correct height. Anybody ever done this, it would seem a pretty common procedure as the brass ones degrade with time, it must have been done many times before. Do the plastics last longer, is that why manufacturers changed or are the plastic just less expensive and easier to come by. Also, anyone know what the 34mm referenced in the name of the carb is, is that the length of the jet ? Ive been around the board and gotten valuable info, but i had the questions above if anyones inclined. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted August 7, 2009 Moderator Share Posted August 7, 2009 if the floats drop into the bowl and are not fouled in any way, the ideal height will be 3mm from top of gasket when full of fuel. i think you need to experiment with the float heights. So to answer your other question, no, the floats would not just drop in without some sort of tang bending. I hope the floats you got are able to do that as some floats are fixed for a particular carb!! 34mm is the venturi nearest the engine. Link to comment Share on other sites More sharing options...
gazampa Posted August 8, 2009 Author Share Posted August 8, 2009 I hope the floats you got are able to do that as some floats are fixed for a particular carb!! They can move about in the bowl without restriction. However, the pins that came with the float were a tiny bit smaller and slipped out fairly easily, Im using the old ones. Also the tang that restricts how low the floats fall when right side up, not the tang that touches the needle but the one that touches the arm, needed a little bending to get it to come in contact with the arm. It was falling so low that the needle would come close to falling out when the floats fell down and would then stick in that position. Had I put them back on, they would have remained fully open. 34mm is the venturi nearest the engine. Thanks, didnt know that, i guess the carbs on the 650 could be a different model carb altogether. I found upon testing to see that the floats closed off fuel flow that one of my needle valve assemblies is leaking. Its a drop every three to five seconds. I switched needles to see if it was the needle but it wasnt. Does anyone know a good rebuild kit for a 1978 Yamaha 400xs . Ive been looking on the web for one but its hard to come by. I did find the needle assembly for some jet skis looks similiar here. link . Its used in the VM 26. Im going to order it and try it out if i cant find a kit. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted August 8, 2009 Moderator Share Posted August 8, 2009 get your carb kits from siriusonic Link to comment Share on other sites More sharing options...
gazampa Posted August 16, 2009 Author Share Posted August 16, 2009 ok, what I have found out. Having the right parts is wonderful. Having nearly right parts - not so good. These floats need a lot of coaxing into place and even then its hard to get the needle to not fall down and jam open. If the jam is not too stong when theres fuel in them, a little tap on the side of the bowl will unjam them. Fill the bowls and make sure there are no stuck floats before putting them back on the bike. Fuel Valve Assembly. - The VM26 valve assembly above doesnt fit, its a tiny bit too small so it wont screw in. You can wrap it in sealing tape and push it in to work as a stop gap. - Also, the above is a 1.8 and the specs on an xs400 call for a 2.0 which i managed to change before they were shipped, plus there is nowhere to attach a filter. It takes way too much time futzing about with stuff, GET THE KITS FIRST THING, dont mess about, its sucks up way too much time. So, back to my particular case, the gas fills up the bowl more than it did before, before it only half filled, maybe a cm above the main jet judging by the gauge level i set up and thats also where the green coloring stops. I have increased the fuel level, it falls a bit shorter than the 2mm below the lip, more like 8-10mms. Anything more and the needle jam becomes difficult to resolve. But, now i have an idle of 5k, ive read about this on the forums in places, and my idle screws have little affect. Screwing them in all the way increases the idle by about 500-1000rpm and unscrewing them all the way does nothing. Any ideas on whats going on, i thought increasing the fuel level in the bowl would enrich the mixture but the forum seems to be saying that this is a lean idle problem ? I have looked through the forum and the solutions revolve around the idle screws which ive tried. Link to comment Share on other sites More sharing options...
gazampa Posted September 2, 2009 Author Share Posted September 2, 2009 just to drop back and finish off this thread. I had a problem with the drop level using the plastic floats. The drop tang that hits off the post was too small. They could be rigged but ultimately, I bought a couple of stock brass floats from the dealer at an extra 20 bucks a pop. Small price to pay for peace of mind whilst driving around to not be thinking that a rigged tang on the float was going to somehow come asunder inside the carb. The high rpm was down to the throttle screw position. Link to comment Share on other sites More sharing options...
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