78 xs 400 not running right........

[scooter123]

Hey guys I took the carbs off for the third time total tear down and soaked them in varasol for a day and blew them out with 100 psi compressed air, two new air filters..... the bike will fire right up and idle perfect, but when I give it gas it bogs out and sometime dies!!!

My first question is the two little screw in jets in the air box side...did they have rubber o-rings mine dont!!!

second question the two screws on the engine side of the carbs that have the little springs on them...I read that they are to be set at 1 1/4 turns out from the seated position....is this correct???

any help would be a great!!!!

scooter

[DGXSER]

Not certain but if they are the same Mikuni BS34 style as the other XS series carbs, then the jet in the inlet bell your referring to is the pilot air jet and it does not need anything, just screws in as is. As to the mixture screw on the outlet side, that 1-1/4 turns is a starting point only. In fact, I start at 2 turns out as that is where most of mine have liked running best. Not uncommon to get to 3 turns out for it to run correct.

As to how it is running, are you setting the choke/enrichment circuit open when you start it up? Until they get warmed up which can take a good mile or so of riding, they will seem fuel starved if you do not have enough "choke" applied. As in they will idle fine but bog down and stumble when throttle is applied.

[scooter123]

It still does it even after it been running for 5 to 10 min....im kinda stumped....if I pull the choke out it will rev up...push it back in and idles nicely but when I try to give it gas it bogs out and wants to die!!!!

[scooter123]

Well still no luck tonight...I removed the carbs again for the fourth time move the needle to the third clip position and still no luck.....bike idles fine but cant give it gas!!!!
I think I will pick up some carb cleaner tomorrow and let them soak for a few days to see if that works!!!

[petejw]

a cpl of things come to mind,
have u checked the mixture screw tips
havent broken off?
also what have you set you float heights at?

[scooter123]

ahhh thanks....after looking at a couple of pics ....they are, they dont come through that little hole and there is no point on them.....this could be my problem!!!!

Thanks!!!!

[soccer4m]

partial problem

Hi Scoots
, the idle mixture tips will control the idle, but since your idle is ok,just the tips missing that would affect only adjustment of the idle, your problem on cranking up the throttle and no go is related to gas flow to the main jets. Either your carb boots are leaking, main jets are plugged or not enough gas getting to your carbs- check flow through filter if you have one, put one in asap if you dont have an inline one,,,,blow out the mains with carb cleaner, make sure the diaphrams go up and down easy,lift with finger, put finger over top hole and see if the diaphram stays up after lifted,,,if not there is a leak, and your power will be gone. See threads on diaphrams............Mike

[bikerphil]

Could be as simple as a pinched fuel line, JAT.

[brcree]

xs 400...81

Hello scooter123......I have an 81 xs400 special and had a prob with it starting and when I shifted it would rev up high. The air/fuel needle that is in between the carb body and the cyl head must have a: 1. small spring...2. very small metal washer....3. very small rubber oring. in that order. My needle was turned out over 3 turns so it was runnen lean. I looked in the book and it calls for 1 1/2 tunrs out. Be-careful not to break the tips off the air fuel needle....its a bugger to get them out!!!!!!!! Also on the side of each carb you can see the flat end of the butterfly valve shaft with a c clip behind it, there is also a rubber oring behind a washer. This can dry out and suck air in as well. My carbs had the incorrect size main jets aswell. Try all the inputs given in this forum, you will find it sooner or later,....and get a good manual for your bike. Good luck man.

[scooter123]

So I decided to step back and take a break from this bike so I disassembled the carbs again and let them soak for the last three days and I noticed one of the air screw points broken off in the hole!!!! I managed to get it out so now to order two new ones.....I hope this was part of my previous problem!!!!

I've worked on a few honda cb's in my day, but this is the first yamaha for me!!!

[brcree]

carbs 400sx

Yes Scoot....this will help 100%! Make sure when you replace the air/fuel needle you don't break the new one when you screw it in and bottom it out!!!!! Turn this screw back out 1 1/2 turns , thats where mine are now and no more up rev-ivg when I shift!!!! Starts easier as well! Make sure you place the rubber oring in first and it seats....then the metal washer , then the spring and then the needle. A good carb sync. would be in order after you have done all this.

[cywelchjr]

Yes Scoot....this will help 100%! Make sure when you replace the air/fuel needle you don't break the new one when you screw it in and bottom it out!!!!! Turn this screw back out 1 1/2 turns , thats where mine are now and no more up rev-ivg when I shift!!!! Starts easier as well! Make sure you place the rubber oring in first and it seats....then the metal washer , then the spring and then the needle. A good carb sync. would be in order after you have done all this.

Actually 1 1/2 turns is often a starting point. More turns out is richer, but if the o-ring is missing or bad you suck air through the top of the mixture screw all the time, which is why the o-ring is so important. More turns out is richer, less is leaner, but the o-ring has GOT to be good or forget having it work right. I don't know if the earlier carbs are the same, but making sure everything is there can't hurt, but the earlier carbs are quite a bit different than the later carbs.

[DGXSER]

ummm...if the screw tip snapped off, then he has the older style of carbs that do not use the o-ring and washer.

[cywelchjr]

brcree has an 81 which has the new style carbs and has the o-ring, the person he is responding to has the older carbs which I believe you are correct about does not have the o-ring, but he did have the tips broken off. I was speaking to the 1 1/2 turns that if the carb being adjusted does have the o-rings it will not adjust properly if they are bad, if it doesn't have them, then it's all different. I'm not even sure the older XS400's even use BS34's or if they use a different carb like the earlier 650's do.