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.

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.

you'll find all you need in the sticky at the top of this section.

I thinks its 2 1/4 turns from bottoming the adjuster outwards

drewps

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.

The syncronizer screws are on the throttle bar, and adjust the butterflies in relation to each other. The ones on top of the carb between the boot and the vacuum slide are indeed the mixture screws.

The manual spec is usually lean. I've always found 3 to 3 1/2 turns out to give the best idle and throttle response. If you did not take the carbs apart and clean all the little holes & jets it never will run right though. I dont know how well you cleaned them. I prefer a mild pressure washer once the carbs are disassembled. This works excellent.

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.

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.

Did you remove all the jets and blow carb cleaner or air through ALL the passages? These things gunk up really easily and take a bit of work to clean out, just soaking them usually doesn't do the job. It sounds like a main jet problem, either than or the vacuum slides. Check the diaphrams on them for holes. The easy check for the diaphrams is to press them up, one of the holes around the carb bell will have air come out, cover this hole and see how quick the slide goes back down. Check both, they should some down slowly and both about the same.

Also, when everything else checks, make sure the carbs are in sync, as that will also cause problems.

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.

Mine were 4 3/4 turns to get it running right

very old thread

topic closed