Bizzy13
XS400 Addict
Mixture screws are the 1st thing you do after any mod to jets. I never touch floats but only last resort. Those mixture screw adjustments are a critically necessary adjustment.
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Run away throttle - '80SG
- Thread starter Under Dog
- Start date
Denny Scriven
Oldenny85
My bike is an 82 SJ model and I have the runaway problem.
Have removed and stripped the carbs 5 or 6 times, replaced the float needles and seats seemingly found no other problems with the other jets or needles.
Upon starting the bike idles nicely, but wait ten minutes then the revs increase to about 4k. If I blip the throttle to try and bring it down, revs go to 6k and stay there.
It has all the symptoms of lean mixture, blue exhaust pipes and popping now and again in the mufflers. It is kinda frustrating as I'd much rather be riding it.
Have removed and stripped the carbs 5 or 6 times, replaced the float needles and seats seemingly found no other problems with the other jets or needles.
Upon starting the bike idles nicely, but wait ten minutes then the revs increase to about 4k. If I blip the throttle to try and bring it down, revs go to 6k and stay there.
It has all the symptoms of lean mixture, blue exhaust pipes and popping now and again in the mufflers. It is kinda frustrating as I'd much rather be riding it.
I have a similair problem its back firing out the carbs but I have fixed some of the lean condition.I need to adjust the slide needles but Im reletivly clueless on how this is done given the fact I have to use spacers and dont have notches.Im in need of some advice also.
One thing I did not see mentioned - have you vacuum synched the carbs?
I had this problem on a different bike - idle would climb when hot. Synched carbs, and problem is completely gone.
I had this problem on a different bike - idle would climb when hot. Synched carbs, and problem is completely gone.
I have done mine yet I plan on doing this very soon.I figure as soon as the carbs are balanced properly it will aleivate the problem.
Bizzy13
XS400 Addict
That's the only way to adjust the needle is to shim it if there are no notches. But always go sync and then mixture before anything else.
Denny Scriven
Oldenny85
Thanks for the advice guys, I plan tomorrow to reassemble the carbs after I double-check there are no problems with all the jets. I then will sync the carbs with my vacuum gauge.
Who knows, maybe it will run properly this time.
Who knows, maybe it will run properly this time.
nickg
XS400 Member
mine has this problem too, but today i took it out for about an hour ride brought it home and adjusted the idle down, i noticed it was at 2000 rpm when it was hot anyway. got the idle to about 1200 and took it back out for a little and seemed to be fine, the idle didnt spike back up or anything.
Denny Scriven
Oldenny85
Same thing happened to me again when I took it out on the road, by the time I was turning into my driveway it was revving 6 grand.
Stripped it down, rebuilt it with new float needle kits, also bought new rubber intake hoses and sawed slots in the lousy phillips screws of the clamps so I could get them really tight.
Checked when running for air leaks and found outer throttle shaft seals leaking. Put in new "O" rings and made thin washers to hold them in, solved.
Our local Carb expert adjusted the idle needles for me and now it runs great, or will do when I have replaced the clutch cable and the rear brake shoes.
Denny
Stripped it down, rebuilt it with new float needle kits, also bought new rubber intake hoses and sawed slots in the lousy phillips screws of the clamps so I could get them really tight.
Checked when running for air leaks and found outer throttle shaft seals leaking. Put in new "O" rings and made thin washers to hold them in, solved.
Our local Carb expert adjusted the idle needles for me and now it runs great, or will do when I have replaced the clutch cable and the rear brake shoes.
Denny
Chainsaw
XS400 Member
So I'm having the same issue. I just re synced for the third tme today after adjusting the float height today, no fix
Here's what I posted in another forum:
So I've been having this strange issue on my bike (1980 Yamaha xs400) where I've got everything dialed up correctly (rebuilt carbs, synced carbs, adjusted float height, even throttle slides seem smooth, as well as the idle screws) but when I actually go to ride the bike, the idle doesn't return to where I had it before. I can get it to go back, but I have to adjust the throttle stop.
Sorry if I've worded it poorly, but here's what happens when I ride. Start up bike, let choke for 2 minutes, return choke, adjust idle to ca. 150 0rpm. That's all good, bike idles fine. Then I go for a ride and accelerate quickly, up to say 6k rpm and pull in clutch/go in neutral but the idle is now at ca. 4k rpm. However I can adjust the throttle stop so that it returns to 1500 rpm, but when I ride again, same thing happens. All the time, idle fine, ride, idle too high, have to adjust idle...
The last funny thing is is that its only at idle. I can pull in the clutch and it will stay at that high idle but once I start to release the clutch or engage the gear it will shoot back down to regular idle speed. So it's completely rideable, but obviously no one likes sitting stationary with the engine revving up at 4k.
Anyone ever experienced something similar or have any insights to share?
Think I'm about to take her to a mechanic..
Thanks in Advance
So I did all the leak tests as well so I think I will try and do as Under Dog said and do the timing. Will keep y'all updated.
Glad I came here. Love this forum!
Here's what I posted in another forum:
So I've been having this strange issue on my bike (1980 Yamaha xs400) where I've got everything dialed up correctly (rebuilt carbs, synced carbs, adjusted float height, even throttle slides seem smooth, as well as the idle screws) but when I actually go to ride the bike, the idle doesn't return to where I had it before. I can get it to go back, but I have to adjust the throttle stop.
Sorry if I've worded it poorly, but here's what happens when I ride. Start up bike, let choke for 2 minutes, return choke, adjust idle to ca. 150 0rpm. That's all good, bike idles fine. Then I go for a ride and accelerate quickly, up to say 6k rpm and pull in clutch/go in neutral but the idle is now at ca. 4k rpm. However I can adjust the throttle stop so that it returns to 1500 rpm, but when I ride again, same thing happens. All the time, idle fine, ride, idle too high, have to adjust idle...
The last funny thing is is that its only at idle. I can pull in the clutch and it will stay at that high idle but once I start to release the clutch or engage the gear it will shoot back down to regular idle speed. So it's completely rideable, but obviously no one likes sitting stationary with the engine revving up at 4k.
Anyone ever experienced something similar or have any insights to share?
Think I'm about to take her to a mechanic..
Thanks in Advance
So I did all the leak tests as well so I think I will try and do as Under Dog said and do the timing. Will keep y'all updated.
Glad I came here. Love this forum!
Let me sleep on it my night time cocktail is kicking in,I`ll give you an answer in the am.
Ok, my first suggestion is to stop messing around! Did the factory put a great big red knob on the idle speed screw? Is there a big sticker with an arrow that says "Turn often for maximum frustration"?
Adjusting the idle speed on a cold engine (less than 15 minutes of spirited riding) is wrong. You should be riding the bike to get it fully warmed up (the oil has to be hot). Then adjust pilot screws (idle mixture) for strongest idle. Slightly rich is what you want, lean is what you currently have. Work quickly, as the engine will get too hot just idling. Make your adjustments and then go for a ride to normalize the engine temperature. Check again to make sure you got the idle right. Then sync the carbs. If it takes more than a 1/4 turn to sync the carbs, check idle mixture again. And then re-sync. Be sure to work quickly and to ride between each adjustment to normalize the engine temps. This is a very precise balancing act, but once you have it right you won't have to mess with it for quite a while.
Chainsaw
XS400 Member
I do believe the bike was hot, but it definitely wasn't 15 minutes of riding. It would however start burn a bit of the dust/oil off if I took too long without riding... So now I do it with a fan in front...
I think I have perhaps found the culprit... The slides aren't as smooth to return as I initially thought... About a quarter of the times I push them up they will get stuck about a cm from the bottom. Not sure why adjusting the throttle stop would make them lower again but we shall see..Haven't cleaned them yet though, will report back when I do....
Quick question, the carb sync was very delicate, as in less than an 1/8 of a turn to work with... From what I've seen this is normal, correct?
I think I have perhaps found the culprit... The slides aren't as smooth to return as I initially thought... About a quarter of the times I push them up they will get stuck about a cm from the bottom. Not sure why adjusting the throttle stop would make them lower again but we shall see..Haven't cleaned them yet though, will report back when I do....
Quick question, the carb sync was very delicate, as in less than an 1/8 of a turn to work with... From what I've seen this is normal, correct?
Check the needles. If they aren't seated properly they will rub on the needle jet and cause the slides to hang-up.
Chainsaw
XS400 Member
Interesting, I hadn't thought of that. I haven't taken the slide/needles apart so not sure why they wouldn't be seated properly. Other than a visual inspection/wiggling is there any measurements I can do to verify that they're seated properly? Sorry if this seems obvious I'm not by the carbs right now (at work) so can't look at them!
The needles should be perfectly vertical and centered in the needle jet. Some carbs use a nylon washer with a locating pin to center the needle in the slides. If the pin hasn't dropped into it's hole, then the needle will be canted at an angle.
The needles could also be bent. Or the diaphragm could be swollen and hanging up. MikesXS.com should have replacement needles. If the diaphragms are shot, then JBM Industries would be your best bet for replacements.
The needles could also be bent. Or the diaphragm could be swollen and hanging up. MikesXS.com should have replacement needles. If the diaphragms are shot, then JBM Industries would be your best bet for replacements.
Chainsaw
XS400 Member
Awesome. Diaphragmns are new (previous owner must have sprayed carb cleaner into carbs without taking them apart or something) so hope it's not that! Excellent info htough. Will look at all this tonight!
Chainsaw
XS400 Member
No luck after an hour of fiddling
Cleaned up the sliders and the action seemed a lot smoother than before and both sliders were falling at the same rate. Thought I was good, but when I got her all back together and have the same problem.
This leads me to a silly question. How fast should the sliders fall? I know it's hard to answer but it's definitely not like they should fall instantaneously, right? I mean that's what the diaphragms are for? Another silly question: The needle should wiggle a bit, right? It seems to be spring-loaded so I imagine some lateral movement is normal, correct?
So if you all agree that I may have eliminated the slider issue. Could I be running the bike too lean? Simple idle adjust would solve this?
Is it the timing like the original poster of this thread found out? If so I'm not really sure how to mess with this TCI system...
Admit defeat? Is it mechanic time?
I've already put my $0.10 in. It is up to you to admit defeat or follow the advice offered...
