Bike starts up & revs fine, then won't rev up after 30sec of running

Greasey Fingers

Greasey Fingers

XS400 Guru
Top Contributor
Messages
1,069
Reaction score
15
Points
38
Location
Australia
Calling all carb guru's...So my bike (SOHC xs360) starts and sounds really good and revs up great and it all seems like everything is fine...but after about 30sec of running it will no longer rev up, and at that point if I try to rev it, it dies???? Im guessing carbs, I'm about to rip them off and dismantle again. Any thoughts/help is most welcome, these carbs really suck I'm sick of pulling them apart :(
 
Before you do that there is a few things you can check with the carbs still on.

Might be no fuel left in the float bowl, so once it runs out it just dies. Are you running with a tank on and petcock? Try opening up the gas cap. Also, petcock might be slowing flow. Could also be a float or needle/seat problem. Easy way to see what the fuel is doing is to attach a line from the bottom port on the float bowl, and crack it open. With the line held up, you can see exactly what the fuel in the bowl is doing. The float/needle/seat should be attempting to maintain an even level of fuel, right about where the top of the bowl is. It will decrease a bit when you give it gas, but it should then creep up again.
 
Ah cool yeah I think you onto something. Im using a purpose made hanging container I use when synching/tuning carbs to feed them fuel, so I can work with the tank off...the fuel container has a tiny breather hole in it and a tight lid. Im gonna check that first, try with the lid off. So glad I asked before ripping off the carbs, thanks wolfe ill let you know how I go :) Failing that (but I think this is the problem) ill check float bowls. The lack of fuel flow would explain everything imo. I even rechecked all my valve gaps lol, thought I screwed up but they are all defiantly within spec.
 
ok so I started it this time with the cap off of fuel container and it still does it so its not a fuel flow issue (damn that would have been the easiest scenario). I noticed the left hand carb is leaking or overflowing a bit, not heaps but fuel is dripping out somewhere. Does this sound like a possible needle/seat problem?
 
when my carbs leaked it was a dirty sticking needle. Looks like another cleaning and or new needles and seats!
 
I do run a filter but not atm because I'm not using my tank atm just a hanging plastic fuel container which is clean as can be. Yeah sounds like carbs are coming off for another tear down :(
 
Make sure there are no holes in the floats. If there is fuel in them they will cause the carb to over flow.
 
Have you pulled the plugs immediately after it dies?
Might give an indication of something.
 
Ok really weird thing...I took carbs off, torn them down checked, everything all seemed cool. Put em back they did same thing. Started thinking maybe coils?? Running out of ideas, pretty sure the coils were hooked up the right way but decided to try swap them around.

So unhooked them and reattached the opposite way, hit start button and...CRACK...did not start just made a single large electrical crack sound. Oh well they were the righ way around to start with I gues lol. So hooked them back up like they were. Hit start again and the problem appears to be gone?????

Revs up just fine now, how weird is that. Maybe the coils got jolted into action or something I have no idea. Any electrical gurus out there got a theory on this one?
 
latest


now ride it like you stole it till they come back!
 
Another thought, what you said makes sense...a bad connection. Maybe after I unhooked them and put them back together I made the connection proper. So maybe all it was is I didn't push together a connection good to start with?
 
I once had an old Chevy LUV pick up when I lived in New Mexico. One day up in the mountains it started acting up. It ran fine on the flats or down hill but when I started up hill it would quit.
Turn out the wire from the coil to the distributor was a plug on connection. It felt tight when you pulled it off or pushed it on but once in place it was a bit loose. The weight of the wire held it so the connection was ok on the down hill and flat sections but pulled the wire enough on the uphill that the coil quit.
Easy fix once I discovered the problem, a bit of a squeeze with pliers and all was golden.
On any connections on the coil that plug together pull apart, clean and tighten.
Leo
 
Ok I changed out the coils and still have the same issue so it wasn't a bad connection. I pulled the plugs and they look super rich, so I'm thinking the jets I put in are too big? Its weird because at first it will rev up fine, then it wont rev up. Does this sound like its getting too much fuel and flooding?
 
When you had apart you checked the float height and needle and seat? Could be the jet needle is not seating properly with the main jet causing it to draw to much fuel? Pilot jet in the idle air circuit could be backed out to much causing it to draw to much fuel. Choke plunger stuck causing the bike to run choke on even when your pulling it out? When my bike was cold, trying to start originally it would flood out very easy and crack(backfire) big time. Just shooting ideas out there.
 
Have you run the clear lines from the carb bottoms to check what the floats are doing yet? Might get your answer there. Then I'd start looking into jets.
 
What settings and jet sizes are you running with the carbs? Are you using the correct xs360 pilot jets. They use a very specific type jet not used in any other bs34 mikuni carb.
 
Last edited:

Similar threads

C
Bike wont rev
Replies
6
Views
665
Crau
C
pulmenti
No Start - Shims / Valve Adjustment
Replies
11
Views
725
tstidham
tstidham
pulmenti
What am I Missing? (timing?)
Replies
6
Views
769
pulmenti
pulmenti
M
Please help, I need some gurus to give me direction!!
Replies
6
Views
590
JPaganel
JPaganel
T
77 xs400 starting problems
2
Replies
39
Views
2K
thecurryman
T
Back
Top