element51 Posted May 18, 2009 Share Posted May 18, 2009 Like the title says, I am experiencing a complete loss of power above 6k RPMs if i twist 1/2 throttle or beyond - it wont even rev past 7k. If I apply less than half throttle, it will happily go to redline. I have adjusted my floats to spec, and have tried it with floats adjusted to 25mm. I put in new main jets (135s), and cleaned+synced the carbs again but it still doesnt want to rev. The bike is running well other than that. any suggestions are greatly appreciated Link to comment Share on other sites More sharing options...
JimR Posted May 18, 2009 Share Posted May 18, 2009 Like the title says, I am experiencing a complete loss of power above 6k RPMs if i twist 1/2 throttle or beyond - it wont even rev past 7k. If I apply less than half throttle, it will happily go to redline. I have adjusted my floats to spec, and have tried it with floats adjusted to 25mm. I put in new main jets (135s), and cleaned+synced the carbs again but it still doesnt want to rev. The bike is running well other than that. any suggestions are greatly appreciated Hi, I would guess you have either diaphram problems or (linked) a broken/cracked vaccum pipe fromm the fuel tap. I guess you have set the timming/dwell & valve clearences to the correct specs to drop this out of the equation Regards Jim Link to comment Share on other sites More sharing options...
peteypops Posted May 18, 2009 Share Posted May 18, 2009 Like the title says, I am experiencing a complete loss of power above 6k RPMs if i twist 1/2 throttle or beyond - it wont even rev past 7k. If I apply less than half throttle, it will happily go to redline. I have adjusted my floats to spec, and have tried it with floats adjusted to 25mm. I put in new main jets (135s), and cleaned+synced the carbs again but it still doesnt want to rev. The bike is running well other than that. any suggestions are greatly appreciated Hi bud, is this happening under load or just when you're revving it in neutral? Link to comment Share on other sites More sharing options...
element51 Posted May 18, 2009 Author Share Posted May 18, 2009 Hi bud, is this happening under load or just when you're revving it in neutral? It revs freely in neutral, so it's just happening under load. I set the valve clearances already, but that didnt do anything. Dont know how to set timing, but i dont think thats the issue since it only happens under load. The vaccum pipe looks ok but i will check again. However, switching the petcock between prime, res, and on has no effect. Link to comment Share on other sites More sharing options...
peteypops Posted May 19, 2009 Share Posted May 19, 2009 It revs freely in neutral, so it's just happening under load. I set the valve clearances already, but that didnt do anything. Dont know how to set timing, but i dont think thats the issue since it only happens under load. The vaccum pipe looks ok but i will check again. However, switching the petcock between prime, res, and on has no effect. Standard air filter and airbox or aftermarket K&Ns or similar?? I've had similar problem(but may be totally different) and found that when I taped up half of the length of the K&Ns , thereby restricting tha mass of air, the problem disappeared! Maybe just lucky,who knows? Link to comment Share on other sites More sharing options...
element51 Posted May 20, 2009 Author Share Posted May 20, 2009 Standard air filter and airbox or aftermarket K&Ns or similar?? I've had similar problem(but may be totally different) and found that when I taped up half of the length of the K&Ns , thereby restricting tha mass of air, the problem disappeared! Maybe just lucky,who knows? Stock air filter and jetting Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 20, 2009 Moderator Share Posted May 20, 2009 did you check diaphrams like Jim said? Link to comment Share on other sites More sharing options...
element51 Posted May 20, 2009 Author Share Posted May 20, 2009 did you check diaphrams like Jim said? how do you check the diaphragm? i opened up the petcock, but didnt take it apart. I didnt want to tear rubber seal by prying it open. if that rubber seal is the diaphragm, then its still intact. Also, wouldn't switching to prime bypass the diaphragms? Link to comment Share on other sites More sharing options...
JimR Posted May 20, 2009 Share Posted May 20, 2009 how do you check the diaphragm? i opened up the petcock, but didnt take it apart. I didnt want to tear rubber seal by prying it open. if that rubber seal is the diaphragm, then its still intact. Also, wouldn't switching to prime bypass the diaphragms? The only way to check the diaphragms is to remove the 4 screws from the top of each carb & remove them (watch for the spring flying off) ... the diaphragm should have no holes and should have a similar consistency to a condom .. if it's dried up and/or cracked/split it will need replacement. Regards Jim Link to comment Share on other sites More sharing options...
element51 Posted May 20, 2009 Author Share Posted May 20, 2009 The only way to check the diaphragms is to remove the 4 screws from the top of each carb & remove them (watch for the spring flying off) ... the diaphragm should have no holes and should have a similar consistency to a condom .. if it's dried up and/or cracked/split it will need replacement. Regards Jim those screws are stripped. i guess i have to drill them out and get new ones. Is there any place to get the diaphagm cheaper than $60?? Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 20, 2009 Moderator Share Posted May 20, 2009 those screws are stripped. i guess i have to drill them out and get new ones. Is there any place to get the diaphagm cheaper than $60?? $60? not really, have you tried mole grips on the screw heads? sometimes you can get a purchase on them. also try sawing a groove in the head and use an impact slot driver!! Link to comment Share on other sites More sharing options...
JimR Posted May 21, 2009 Share Posted May 21, 2009 those screws are stripped. i guess i have to drill them out and get new ones. Is there any place to get the diaphagm cheaper than $60?? Hi $60 for a pair ??? in '78 they were about £40 each if so this sounds like a bargain as Drewpy says mole grips & impact drivers will remove the screws... But dont forget there is a chance the Diaphrams are OK Regards Jim Link to comment Share on other sites More sharing options...
element51 Posted May 22, 2009 Author Share Posted May 22, 2009 Hi $60 for a pair ??? in '78 they were about £40 each if so this sounds like a bargain as Drewpy says mole grips & impact drivers will remove the screws... But dont forget there is a chance the Diaphrams are OK Regards Jim I used pliers and the screws came right off.. can't believe I didn't think of it sooner. The diaphragms look good(im not sure if thats good or bad at this point haha), still rubbery and no holes or cracks. what else should i check? Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 22, 2009 Moderator Share Posted May 22, 2009 I used pliers and the screws came right off.. can't believe I didn't think of it sooner. The diaphragms look good(im not sure if thats good or bad at this point haha), still rubbery and no holes or cracks. what else should i check? do the air pistons the diaphrams attach to, move freely in the bore? you also need to pull the diaphrams a bit to see any tears. If you have the carbs off, take out the mains and see if the emulsion tube is clear. there are tiny holes in the side whch can block easily. the emulsiin tubes come out through the air piston side after undoing the main jet. it just taps out. dont turn it as its keyed!! drewps Link to comment Share on other sites More sharing options...
JimR Posted May 22, 2009 Share Posted May 22, 2009 I used pliers and the screws came right off.. can't believe I didn't think of it sooner. The diaphragms look good(im not sure if thats good or bad at this point haha), still rubbery and no holes or cracks. what else should i check? Hi, if you invert the slides (needles facing upwards) & fold the diaphram to from a well and fill with petrol if the petol leaks out the diaphrams have problems .. As Drewpy mentioned it would also be good to methodically go through the rest of the carbs ! Regards Jim Link to comment Share on other sites More sharing options...
element51 Posted May 22, 2009 Author Share Posted May 22, 2009 Hi, if you invert the slides (needles facing upwards) & fold the diaphram to from a well and fill with petrol if the petol leaks out the diaphrams have problems .. As Drewpy mentioned it would also be good to methodically go through the rest of the carbs ! Regards Jim the pistons move freely, i will to do a very thorough disassembly of the carbs. thanks for the tips! Link to comment Share on other sites More sharing options...
JimR Posted May 23, 2009 Share Posted May 23, 2009 the pistons move freely, i will to do a very thorough disassembly of the carbs. thanks for the tips! Thats good just take your time ... and explore every hole (like if you were in a brothal ) and make sure there well clean ! Regards Jim Link to comment Share on other sites More sharing options...
78XS400 Posted June 17, 2009 Share Posted June 17, 2009 Thats good just take your time ... and explore every hole (like if you were in a brothal ) and make sure there well clean ! Regards Jim What if all this is ok could it be timing? Link to comment Share on other sites More sharing options...
JimR Posted June 17, 2009 Share Posted June 17, 2009 What if all this is ok could it be timing? Hi XS400 timing is somewhat difficult to get right without the correct tools (strobe & dwell meter) if you have a dwell meter set the dwell to 22½% and then set the ignition timing to LF & RF via the strobe. If you have no tools, of that type, gap the points to 12 thou at LT & RT on the correct stroke and set the points to open at LF/RF (again on the correct stroke). tbh strobe lights are easy to get (dwell meter is not) so you could set the points to 12 thou and use a strobe to get the timing right (compromise), but I still feel the problem lies within the carbs Regards Jim Link to comment Share on other sites More sharing options...
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