Runs on only one cylinder

J

jscgdunn

XS400 Enthusiast
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Hi,
I have an 83 XS 400 DOHC which I have putting together with my son. It have 15,000 Kms on it.
When we first got it would run only on the right cylinder.
We just ultrasonically cleaned the carbs and blew out all passages with compressed air. Also installed new needle and seat valves. (These carbs are actually from another 400 DOHC.) Also installed new o-rings on the carb manifold boots.
Pilot screws are set at 2.5 turns out from stop.
Here’s the info:
• Compression: 120 both sides
• Stock air box, new filter.
• Stock Exhaust
• New plugs
• Good spark on both sides
• Have changed coil on left side.
• Also have new fuel petcock on tank.

After all this it still wants to run only on right cylinder.
Throttle response is poor, but when revved up, left cylinder will fire. Also “hangs” at high rpm before coming back to idle. Will idle on right cylinder.
Vacuum gauge bounces around at idle, but does level out when revved up. This is the same for both sides.
Given that these are totally different carbs, with the same problem, I am stuck
Any advice would be most helpful….Thanks!:confused:
 
I would hazard a new battery required/charged. it needs healthy amperage and volts to operate the cdi correctly.

recheck the valve timing and check ignition timing too!
 
Thanks Everyone:
Piston valves: both were moving freely

Battery: boosting directly from car battery...lots of juice

Carbs were bench sync'd and looked pretty close

Igntion and valve timing: Have to dig into those; have not adjusted; this seems logical as same problem as before.
 
Always go in reverse. Plug, Wires, Coil, TCI. Easy way to check the first 3 is to swap the wires coming from the TCI. If the left fires now but not the right. Its the TCI. They have this nasty habbit of burning up diodes when the plug fouls.

Posted via Mobile
 
Thanks will check TCI as well, although we have another identical bike that runs well on this TCI. Could still be problem in circuit though.

As I understand it there is no timing adjustment. In terms of cam timing, if it is not correct, it seems to me that both cylinders should act the same?

Assuming TCI and wires are ok, after reading the posts on dirty carbs, seems like cleanlinness (lack of) could still be the issue, or float height.

Will check for leaks with propane, remove carbs for more cleaning, check pilot screw o-ring.

The manual says 3 +- 1 MM for fuel level. Does any body have a measure for the height of the floats when off bike? The floats are not brass.

Thaks again everyone,

Jeff
 
i know this sounds like a lot of work, but if the other bike runs good,try the carbs off that to see if it is the carbs.could be leaking boots,or could have dropped a cynlder?is compression good?could also have hole in diaphram
 
OleDoc said:
try spraying a little starting fluid in carb at idle
Click to expand...

Thanks everyone for your help. Pulled the carbs agaian and sprayed cleaner through passages. Adjusted float to match right side. Also was very careful putting piston valve in grove on top side (this may have been the issue?)

Had to use on ether to get it going, but once warmed up it ran on both sides :) and idled fine. Sprayed ether to check for vacuum leeaks while idling: no response.

Throttle response still slow and still "hangs" before dropping back to idle.

Started fine while warm

Put it all together and came back a few hours later (after it cooled off). Fired a few times but would not start. Doe not seem to be getting enough priming or choke.

Suggestions?
 
chucking spray through the carbs in XS land does not constitute cleaning. they need to be stripped out and thoroughly done. I did it twice, with the final clean being ultrasonically done.

need to have the float height within 2mm of the float bowl
 
drewpy said:
chucking spray through the carbs in XS land does not constitute cleaning. they need to be stripped out and thoroughly done. I did it twice, with the final clean being ultrasonically done.

need to have the float height within 2mm of the float bowl
Click to expand...

Had mine apart 3 times
 
hello everyone, I have an 81 special II and it simply will not start.
The only way I can get it started is with ether, I know this is hard on an engine so I try to stay away from it. And when it does get started it runs on one cylinder, the other sputters and backfires...
I have cleaned the carbs, changed the plugs, and also changed the battery.
I am getting good spark on both sides so I know that isn't the problem.
I have box filters and they are spotless on the inside...
Take has no rust, and I run premium in it.
I was convinced that maybe I was losing compression so I ran a test today and I have 130 on the running cylinder and 140 on the one not running.

I just simply don't kno where to go from here. Any help would be appreciated...thanks a lot
 
kindest said:
did you read this thread?
Click to expand...

Hi NFriday,
The folks here have been a big help in sorting out my problem.

Similar to mine, it cannot draw fuel to start, so it has to be a vacuum leak issue, or a cleanliness issue. Related to the cleanliness issue is float height.

First of all clean: You wil have to pull the carbs again....as the sages predict it will take three times to get them clean. I am on the third pull; my focus this time will be on the enrichener circuit and float height, but all passages will be checked again.

Second Vaccum: I know I did not have the piston valve diaphrams seated correctly before. Also check them using the hold up test to see if they have holes and are seated correctly....see the carb section for a recent post from me.

Float height:critical for the enerichener valve to work right.

Keep trying:bike::bike:
 
Everyone says they cleaned their carbs. I did too. But I had the same "run on one side" issue. Until I completely stripped...and I mean completely stripped the carbs, boiled them in vinegar water for 2 hours, turning them every 20 minutes, and saw what floated to the surface, I would have sworn they were clean. After they dried, I sprayed carb cleaner though each and every passage from both sides (there are a couple of posts on here how to do that) until there was a strong stream of carb cleaner at every hole there should be. Then, I put them back on.

Don't care how clean the tank looks, use and inline paper filter. $3.99 insurance.

It's been over a year now, and it runs smooth as smooth can be....for an XS400.
 
Am I the only one that has not had to clean there carbs a million times? :laugh: Pull apart, a little carb cleaner and 125lbs of compressed air, done.:shrug:
 

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