Steelfitter

From the moment I first got the bike running it idled well but had hardly any power when accelerating and was lucky to be able to do 60mph. I had replaced the fuel valve seats along with new bowl gaskets and noticed a slight difference between the new gaskets compared to the old ones. I didn't think much of it and used the new gaskets as they were but I was still confounded as to why it seemed to be starving for fuel when under a load so the carbs came off again. In my "process of elimination" I installed the original bowl gaskets, cleaned out the main jets with a torch cleaning tool which opened up the main jets just a tad and sprayed all the passages clear again with carb cleaner and compressed air. The end result is that the bike now runs fantastic and pulls well when under a load. Was the "fix" a result of opening the main jets up a bit? Was it using the correct float bowl gasket? Was it a result of cleaning the carbs again or a result of all the above? I can't say but I'm just pleased that it's starting to run like it should and considering that the valve lash still needs to be adjusted and the carbs properly synced tells me the bike has even more to offer.

I got it all cleaned up and installed a new piston and seals in the master cyl and the front brakes actually work now. The bike had sat for 7 years before my sons friend bought it and asked me to resurrect it which took some serious carb cleaning to do and it does idle well but seems to surge when under a load. Carbs still need synced but there is at least 1 lifter that has way too much lash, it ticks pretty bad and gets worse as it's warmed up after a short ride and that needs to be dealt with first before the carbs can be dialed in correctly. Problem is that I found out that setting valve lash on the xs400 is a bit more involved than it is on my Honda cb350 and the Yamaha needs a special tool and lifter caps to get it done.

I just got a master cyl rebuild kit today from Old Bike Barn for my son's friends xs400. I've rebuilt automotive master cyl before but this will be my first motorcycle master cyl. Any tips I should be aware of or is it pretty much straight forward?

All jets were removed before soaking but were soaked with the carbs in a separate tray. About a can and a half of spray carb cleaner was used and everything was thoroughly cleared with compressed air before assembly. No pin holes or tears were found in the diaphragms either. It's almost a guarantee the carbs are out of sync but I'm hearing a noticeable ticking sound coming from the valve train so I want to check valve lash before I attempt to sync the carbs. I've had no time to mess with it yet because I've been busy working on my 18 year olds car. Had to replace the transaxle on his Saturn and once that was done and ready to take a test drive I heard an odd noise and saw the balancer/crank pulley was wobbling. Turns out my son put the balancer on wrong and shoved the keyway into the crank timing chain gear which of course ruined the gear. Oh and did I mention my 16 year has been wanting me to get his 71 Honda CB175 (that hasn't ran in who knows how long) running that my brother gave him too? Yep it's been a "fun" weekend for me.

Thank you all for the tips. Both carbs were soaked overnight in carb cleaner and special attention was made to making sure all the jets were clear however that doesn't mean I didn't miss something. I had a similar problem with my CB350 in which the right cyl wouldn't fire at idle or part throttle and it turned out the "slow" jet was plugged on the right carb. As it stands now the XS400 is a bit grumpy when starting cold but idles fine once started and has decent throttle response. It also responds reasonably well at low speeds but the moment I roll the throttle to get it up to say 55mph, that's when it throws a fit and feels like it's surging. Many things still need to be done so the problem could be a combination of many things and not just the carbs. Thanks again for the help.

Just to make things clear the screws I'm talking about are on the top front of the carbs just before the intake and a pain to get to when the gas tank is on. They are the only adjustment screws the carbs have and I would call them the idle mixture screws but the manual refers to them as synchronizer screws.

I'm working on an XS400 that my son's friend owns and I just managed to get it to fire up for the first time in 7+ years. It had a mouse nest in the air box and carbs choked up with stale/varnished gas. So far all I've done is of course clean out the air box and thoroughly soaked both carbs as well as rinse out the gas tank and so far that's what the bike needed to fire up and stay running but after taking it for a quick spin it's obvious more TLC is in order. Many things still need to be done such as checking valve lash, cleaning all electrical connections and so on but to start things off what I need to know now is how many turns should the synchronizer (low speed, idle mixture) screws be turned out in order to get the carbs in the "ball park" prior to synchronizing them? Yes I know I need to check valve lash first, I'm just trying to gather vital info.