77 XS650 Carb issues


XS650 Member
Reaction score
Utica, NY
Hello, everybody. long time lurker i think this is my 2nd posted question? for the most part i can usually find everything i need to know with searching but i have two issues that i haven't seen comments on exactly yet.

The bike is a 77, and these are BS38 carbs. and i have read and almost commited to memory the "carb guide" at this point.

first issue is that i have had the carbs off and rebuilt them, cleaned them, talked to them, checked them inside and out. two times thus far. first time was about 8 years ago and i never was able to put them on till just a few months ago. (build took way longer than i thought it would). second time was after putting them on i realized i had the wrong jets in them from the last rebuild.

currently they have been cleaned again, i have painfully set the floats to 25mm and now have what i feel are the correct size jets for the scenario. 27.5 pilots and 132.5 mains. the clip is on the second from the top on the needle. the needle is 4M1 and the needle jet is Z-8. the floats pass all the tests offered in previous threads, and it has new float needle and everything. i did do a bench sync with 1/8 inch drill bits and verified that there is no light coming thru the butterflys on the sides, so i would say they are centered correctly in the body.

(issue # 1) on the bench hooked up to IV bottle with filters, it was dumping out the vent tubes quite alot at first. i then left them that way for several days with a clean pan under it and that seemed to have stopped. i was shaking them and moving them to see if it was still causing it to leak from the vent tubes but after a few days i was happy with no new gas in the pan. fast forward to install and hook everything up and for sure they dumped out again. started the bike for the first time (in over 10 years) and it ran pretty good. set the air mixture screws and all seemed well. put the bike away only to find out in 2 days time, that the carbs drained all the gas from the tank onto the carpet. ( i now shut the pet cocks off when i put it away) but that leads to this circular issue, when i open the pet cocks and it dumps into the carbs i get overflow for about 5 mins then it stops. i have now been able to drive it around (first time ever) and it seems to be ok...but every time i take it out and open the pet cocks it dumps. not really sure what is going on. my fear is that if i dont shut the pet cocks off that i will just continue to find empty gas tanks and a highly undesirable scenario in my garage. side note, and something that i didnt expect is that after shutting the pet cocks off after a day or so, the lines are drained out, but there is no gas in the pie tin under it, ( i ran hose from the nipples on the bowls down past the frame to avoid getting gas all over my engine and wires (i learned quick after the first go round) if the bowls are full and the floats shut off the gas i would expect to see the gas line leading back to the tank still full? i cant for the life of me figure out where the gas is going after those line empty out.

(issue #2) now that the bike is finished and it runs and i can drive it, i took it out for the 2nd time yesterday. longer test run about 30 mins in approx 80 degree F weather. everything was going great till about 25 mins into just putting around (not really gone fast enough to take it out of 3rd gear). i came to a stop sign and the bike went FULL throttle. the cable was not stuck the butterflys were not open, and i was just sitting there. so i shut it off. started it again after a few mins of scratching my head, and it idle'd ok for a min then went right back to full throttle. i shut it off immediatly again. third time stared it and immediatly put into gear and took off. while riding everyting seemed to be ok...throttle response was ok...shifting was ok...engine ran fine, but came to a red light and pulled the clutch in and back to full throttle. shut it off immediatly again. then finally got home and put it away. i have checked the cables and butterflys, all seems to working as intended. but i have no idea what could cause it to run away like that. pulled the pods off and on the left side there was gas present inside the throttle body on the back side of the butterfly, the right side was dry. thats as far as i have gone yet. havent pulled them back off the bike yet. i am just tring to figure out how it can go full throttle with the butterflys in the closed position, and for the record the engine sounded great at full throttle, no spitting, no choking, no backfireing nothing just wide open and smooth. where is the fuel coming from and where is the air coming from to get the mixture right for it to just run like that? i am very confused to be honest.

any thoughts or ideas or anything is much appreciated. sorry for such a long post, i always feel that more info to start just helps with reducing followup questions.
Are your float needle and seat assemblies aftermarket? If so, that might explain the leaking. Many times they're no good even right out of the box. It's best to use O.E.M. replacements, expensive but they work, and it should be a one time replacement.

As far as the motor racing, have you serviced (cleaned and lubed) the advance unit and advance rod? The advance rod needs to be kept greased where it passes through the head or it can stick. If the advance sticks in the full advance position, the motor can race.
Hi 5twins, i have to say right off the bat a sincere thank you for all the information you post on the bikes. really if wasn't for you (and a few other stand out experts) i would not have been able to do the things i have been able to do. so thank you for all the knowledge transfer i have absorbed over the years.

it was a very long time ago, but i think i did get new ones from Mikes, i believe. and i apologize because i have read that before in other posts. but alot of them indicated it was a drip or two, and mine just dump out like i opened a faucet, so was really thinking it may be a different kind of issue. Actually i have another set of BS38's that i can steal them from now that i think about it.

I did install a Hughes PMA and CDI kit. so i no longer have the points and such. sorry i didn't mention that.
No advance rod/unit would send you back to the jets for the motor racing.
Genuine Mikuni parts there?
Disappearing fuel might be in the crankcase if its leaking into the cylinders.
Hi jn1639, thanks for the comment. i got the jets from Mikes, so not 100% that they are genuine Mikuni, prob not right. I am by no means an engine expert but I am going to guess that if it is going into the crank case that's not a good situation right. I already pulled the OEM float needle and seat out of my other BS38's and will get that swapped out this week as 5twins indicated would be best case
thank you,
I suppose that with the bowls over-filling and belching gas, some may have bubbled up through the needle jet into the carb's main bore, and that may be the cause of the racing motor. So yes, put some O.E.M. original float needle and seats in there. If you cure the overflowing issue, that may fix the high rev thing too.
I have been happy with my purchases of carb parts from Nichecycles and they clearly state IIRC genuine Mikuni parts
I found the website today. thank you for the info on that, if these other OEM ones dont work out thats were I will get them.
I suppose that with the bowls over-filling and belching gas, some may have bubbled up through the needle jet into the carb's main bore, and that may be the cause of the racing motor. So yes, put some O.E.M. original float needle and seats in there. If you cure the overflowing issue, that may fix the high rev thing too.
Thanks for the advise, I will swap them this week and see what happens. thanks again I appreciate your time.