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Runs great cold, but revs high when warm
- Thread starter mil1638
- Start date
Actually, that is one thing i tried over the weekend out of interest. Screwed idle control right out while idle was flying away but made no difference whatsoever. The only way to get idle down was by forcing it. I could keep it running then with a little blip of throttle but as soon as i went too far (above 2.5-3000rpm) idle would fly away again.
I literally (or feel like) have tried every combination of settings to no avail
This is something i am unsure about and will have to look into. Engine has been completely stripped and overhauled but cant recall coming across such a thing. Saying that it has been a long time in the process so will have to get back to you on that
I am back on rota at work, but pulling carbs will probably be happening again soon so will post pics asap. I cant provide any jet sizes as the rest are illegible or not removable without force
Needle setting will be fothcoming with next carb pull
Thanks
I have come across them, through this site, and will look into shipping costs etc if it comes to that. I sure dont want to pay uk prices if i can avoid it, they are so much elevated compared to the states
I am back on rota at work, but pulling carbs will probably be happening again soon so will post pics asap. I cant provide any jet sizes as the rest are illegible or not removable without force
Needle setting will be fothcoming with next carb pull
If you can't remove them you can't fully clean them especially pilot jets.
Lol, yeah true. I have access to an ultrasonic cleaner and it has been used.Just dont tell my boss
CDNTX650
XS400 Enthusiast
Advancer is almost always over looked. They should become part of the points and condenser tune up. They also wear, springs weaken.
Actually, that is one thing i tried over the weekend out of interest. Screwed idle control right out while idle was flying away but made no difference whatsoever. The only way to get idle down was by forcing it. I could keep it running then with a little blip of throttle but as soon as i went too far (above 2.5-3000rpm) idle would fly away again.
I literally (or feel like) have tried every combination of settings to no avail
( In my mumbling that follows "bolt" means "idle control bolt", "screw" means "pilot jet screw", "butterfly" means "throttle valve" ).
On my carbs I believe if I unscrew bolt completely it should completely close butterfly and engine should stall for luck of air. I do not think my carbs have separate idle air passage as some other carbs.
I also think that screw(pilot jet) only controls amount of fuel that is sucked-in on idle, air comes through the butterfly controlled by the bolt.
In my case if bike didn't die with bolt out would mean that either my throttle cable is not letting butterfly to close all the way, or some dirt preventing it, or butterfly is bent and letting air in, or my butterflies are out of sync so that when one is closed to the limit the other one is not, or that o-ring on idle screw is bad and air is sucked in around it, or there is some other air leak . (I do not think hole in diaphragm would be relevant for this problem because diaphragm is before the valve/butterfly).
So my plan of action would be to find what causes that air leak.
Not sure if it helps, I am not even sure if you have same carbs, but the idea is the same - with everything closed bike should stall, from there screw and bolt are used to allow just enough of air and fuel for bike to idle.
Well turns out the diaphragm does have a pinhole, dont know how i missed it (5or6 times)
Tried a repair with plasti-dip and completely ruined it
Now for the wait for a new one to come from USA
I thank you all for the help and will update when i get it back together ( and hopefully running right)
Tried a repair with plasti-dip and completely ruined it
Now for the wait for a new one to come from USA
I thank you all for the help and will update when i get it back together ( and hopefully running right)
walterfrancis69
XS400 Member
I recently started a thread on same issue. My 1981xs400 would idle within a normal range only briefly before the revs increased to the point i would shut engine down. At a loss myself, i finally disassembled entire carb assembly ( floats, float needles, float valve seats, main jets- needles removed from slides, and two other needles accessed from top of carbs next to diaphram covers that i think are pilot screws) All parts including the bowls and carburettors went into pot for boil. New bowl gaskets made and all parts air compressed dry, engine performance improved greatly. Im really pleased with better throttle response. CAUTION- i also removed choke operating rod and forks and lost one spring( impossible to find and impossible for choke system to function properly without) I made new spring from a pen and all is good. The difference before and after in these parts is amazing. Idle is smooth and steady- a good baseline for further adjustment.
Well, what a deliriously happy little chappie i am
Ordered new diaphragms from www.jbmindustries.com on Monday (13th) @ 5.30pm and just home after a saturday dayshift(18th) to find a little parcel with 2 lovely bits of rubber waiting for me. What a service! From USA to UK in 5 days, ive waited longer for stuff ordered in the same country!!
Now..time to get to work
Ordered new diaphragms from www.jbmindustries.com on Monday (13th) @ 5.30pm and just home after a saturday dayshift(18th) to find a little parcel with 2 lovely bits of rubber waiting for me. What a service! From USA to UK in 5 days, ive waited longer for stuff ordered in the same country!!
Now..time to get to work
Oh well, new rubber fitted and bike would not start at all. Gave up late last night exasperated. Went out this morning gave it a go and it fired first time on left cylinder and ran briefly but no throttle response whatsoever, just a stuttery idle.
Carbs pulled again, everything double. triple and quadruple checked, refitted. Nothing!
Bowls have fuel in them but nothing seems to be getting pulled into engine, right plug is bone dry and left has a very slight dampness.
Spark is good, compression good, timing, valves etc are all good. Motor will fire with a quick spray of cold start.
Damn bike is driving me mad
Battery now dead so will need to recharge and start again. Any suggestion would be gratefully received.
N.B nothing else has been changed other than diaphragms and they are a good fit and seem to be operating as they should. Bike was running before hand albeit with the fault as per this thread
I know with those type diaphragms you need to make sure the slide faces the correct direction as the new rubbers does not have the locating tabs in them. Do the slides fit tight into the rubber without being able to spin? Did you check to see if the slide holds vacuum with your thumb over the venturi. Did you assemble the needle assembly correctly? Just a few thing I can think of
Hi xschris and thanks for reply, i can confirm that slides are positioned correctly and vacuum is consistent on both carbs.
I have, since, managed to get motor running. In fact i got it running so well i managed to tune and balance carbs and it was running sweet as a nut
Had a quick run and all was well, got back and built all other things up. Motor would not start and run again, no matter what i tried. It will will fire and run for a few seconds but dies soon after and will stop whenever throttle is applied
Almost seems that fuel is being starved??
Given up for today and will start again tomorrow, not sure where though
I have, since, managed to get motor running. In fact i got it running so well i managed to tune and balance carbs and it was running sweet as a nut
Had a quick run and all was well, got back and built all other things up. Motor would not start and run again, no matter what i tried. It will will fire and run for a few seconds but dies soon after and will stop whenever throttle is applied
Almost seems that fuel is being starved??
Given up for today and will start again tomorrow, not sure where though
Hey, i'm back again. You guys will be sick to death of me.
The non-runner is once again a runner, i'm too embarrassed to reveal cause but needless to say it was entirely my fault.
Here's where i'm at now. Got motor running, had a run to properly heat engine and returned to re-balance and fine tune. All was well.
Back into house got all the gear on, excited for my first real run. 5 miles down the road coming up to a junction and the damn bike started revving high again and seems to be backfiring through left hand carb.Thinking it was possibly lean , Out with the tools and mixture adjusted (and lots of start-stops later) and still the backfire and high revving.
These issues aside the bike runs and pulls superb. Since this was a basket case, i have no real basis for comparison as to running before and after the work i have done.
So.. back to diagnostic mode again, checking for leaks, dirt etc and maybe look to find new jets
The non-runner is once again a runner, i'm too embarrassed to reveal cause but needless to say it was entirely my fault.
Here's where i'm at now. Got motor running, had a run to properly heat engine and returned to re-balance and fine tune. All was well.
Back into house got all the gear on, excited for my first real run. 5 miles down the road coming up to a junction and the damn bike started revving high again and seems to be backfiring through left hand carb.Thinking it was possibly lean , Out with the tools and mixture adjusted (and lots of start-stops later) and still the backfire and high revving.
These issues aside the bike runs and pulls superb. Since this was a basket case, i have no real basis for comparison as to running before and after the work i have done.
So.. back to diagnostic mode again, checking for leaks, dirt etc and maybe look to find new jets
rugbywarrior89
Need more time...
Hey now. We have all done that. It's time to take the walk of shame over to the "tell on yourself" thread and give us all a good chuckle.
If you have aftermarket air filters or exhaust, you will probably need bigger pilot jets. If not, then welcome to the club. Choke cleaner is always a good start to check for air leaks. Or soak your carbs in cleaning fluid inside a ultrasonic cleaner for about 12 hours, then rinse it with carb cleaner then blow it out with air. And then do it again. And then maybe a third time just to be thorough. Only then can you say that the carbs are clean. I don't have a whole lot of room to talk though. I only cleaned mine twice and they still have the exact same problem.
Alternatively, you can just wait until your almost stopped before pressing in the clutch and the trans will force the engine back down to idle. Once it gets to idle its fine. Although pulling away from the stop can sometimes cause the engine to jump straight to 4000 rpm which can be annoying.
