78 Yamaha XS400 idles high when hot

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adwb · #1 Unread post by **adwb** » Fri Jun 23, 2006 7:11 pm

I recently came into a '78 Yamaha XS400. It has been sitting inside and outside for the last 6 years. The previous owner was nice enough to drain the fuel and put clean oil in it recently. He also gave me a new set of carbs when I bought the bike.
I cleaned everything I could get my hands on, replaced the tank, hoses, intake gaskets, air filters, carburetors, and battery then started it up. It sounds great to me but I don't really know what I'm listening to.

The problems are that it has a real hard time starting to go on a hill. I'm about 180lbs and I've got to rev it up to around 5 or 6k and let the clutch out real carefully and once it grabs it chugs along like I'm in 4th or something (which I am most certainly not, unless there are way more gears than I thought or it's going from 2nd to 4th on the down click). Once it gets up to enough speed to balance easily it feels like the horse power kicks in and i've got to shift up real quick before it red-lines.

When I come to a stop it likes to idle just below 2k... until it heats up, then it prefers 3500, unless I downshift and leave the clutch engaged nearly all the way to the stop (getting the rpms below 2k before pulling the clutch) then it stays at 2 until i start again.

I really don't know the first thing about tuning carbs. Well I guess I know the first thing: where the screws are.

xsyamahadg · Sat Jun 24, 2006 3:28 pm

When idle speed increases as the bike gets hot, that usually is an indication that the carbs are running lean and that it has an intake leak or it is sucking air somewhere other than thru the carbs. You should check the carb boots for air leaks and make sure the air filters are in place correctly. You can check the intake rubber boots for air leaks by spraying WD40 along the intake from where the carbs enter the intake all the way to the engine. If the engine either picks up or chokes down, that will indicate an intake leak. You should do this while the bike is at idle and with a fan blowing on the engine to prevent overheating.

adwb · #3 Unread post by **adwb** » Sun Jun 25, 2006 6:28 pm

xsyamahadg: I took your advice and made sure my intake was well sealed. That seems to have fixed the high idle when hot issue. Thanks.

I may have been quick to assume I knew what I was talking about in my previous post. After a few very careful downshifts I noticed that it seems to stick in 2nd and I have to push down quite hard to make sure it goes into 1st. That would explain my trouble starting up a hill.

xsyamahadg · Mon Jun 26, 2006 1:55 am

The shifting problem could be as complicated as the shifter dogs inside the transmision being worn, or as simple as a clutch adjustment. My past experience with the Yamaha clutch set up is that the clutch adjustment has to be spot on or the bike will tend to creep when in low gear, and low gear will be hard to get in and out of. Also neutral will will be a real search. Also, the Yam will need to have engine oil that is made specificly for motorcycles, the oil manufacturers add an oil additive that your bike needs for the clutch and transmission to work properly. If you have automotive oil in your bike, it could cause the same symptoms. XS

adwb · #5 Unread post by **adwb** » Mon Jun 26, 2006 11:30 am

When you say clutch adjustment what do you mean? Before I started riding the bike I pulled the clutch cable and lubed it. I put it back on and adjusted the screw on the lever to give me a little more gripping comfort. I assume this would not affect the shifting. What can I adjust to try and solve this problem?

xsyamahadg · Mon Jun 26, 2006 3:58 pm

Sorry to say I don't know the clutch adjusting procedure on an XS400. I can only tell you the proper way to adjust an XS650 clutch. First you have to relax the clutch cable at the handle by running the adjuster in so there is slack in the cable. The XS650 has another adjustment under a chrome button on the left side engine case. Under that button is a set screw with a lock nut. On the XS650 you have to loosen the locknut and run the set screw in gently until it bottoms out, then back out the set screw 1/4 turn and hold it there while you tighten the locknut. After that is done, you go back to the adjustment on the clutch handle and adjust it so there is only 1/8 " gap [or play] at the handle. I know there are other XS400 owners on this board, one of them can tell you if the procedure I described is the same as on an XS400. I shouldn't assume the XS400 and XS650 adjustments are the same, but I suspect they are. The problems you described to me are exactly how the XS650 acts when the clutch is out of adjustment. XS

halberg54 · Tue Jun 27, 2006 11:11 am

i had a 77 xs750 that had a similiar issue. I found that my manifold boots were loose and that fixed most of the problem. then I ran Seafoam through it and it was completely solved. Just a few suggestions.

halberg54 · Tue Jun 27, 2006 11:16 am

One other thing that happened on my XS750, andf if yours is a triple I understand it happenes on most of those as well, is mine had a second gear slip. I actually never did anything abour it, i just skipped the gear, but there was a fix to it with a different setup in the clutch area. You may want to check in with the dealer on yours, and see if there is an updated clutch for it.