Low power in one cylinder

[paultsmith]

I am fixing up an 81 XS400 that has been sitting for about 15 years. I have cleaned the carbs and replaced the exhaust and have got the bike to idol.

I have also just took the bike out for a spin for a few miles. I have noticed that the plug on the right side looks great, however the plug on the left cylinder is almost completely covered in black soot.

Today I replaced the carb boots and when I started it back up I noticed the the right muffler was noticeably louder and pushing out quite a bit more air pressure than the left muffler. I revved the bike up to 5K and held it ther for a few minutes, white smoke was coming from the left cylinder and this cylinder backfired a few times too, however after a bit the smoke seemed to be reduced. When I examined the spark plug again, I had one good and one bad. The right cylinder and exhaust pipe was also much hotter than the left one.

I thought it might be a weak spark, but I checked the resistance on the windings and they were good on both sides, I also swaped the leads and spark plugs from each cylinder, but the problem seems to persist.

I cleaned the spark plugs and I started up the bike and let it run for jus a few seconds, removed the plugs and the plug on the left cylinder was slightly wet. It was clear and seemed to have some gas on it, but could this be oil?

I have not had a compression test done. I was wondering if my problem could be an idle mixture screw problem. I am hoping it is not a valve problem, as I will not be looking forward to taking the cylinder head off and cleaning the valves and ports or replacing the piston rings.

Is there any problem with operating the bike with one cylinder running and producing more power than the other?

Again the bike has been sitting for 15years, is there a chance that if I just add some intake cleaner to the tank and ride the bike that this problem will eliminate itself?

Thanks for your help.

[grizzlydan]

First check the valve clearances (roughly), inlet 0.10mm, outlet 0.18mm. If the problem persists get a compression test done. Another question is why was it laid up for 15 years? Could it have holed the piston?

[Cynic]

Sounds like our old favorite carb trouble again, the valve clearances would need to be cringeworthyly (new word?) out to cause that kind of issue.

When you revved it to 5k you got a bit of smoke and a few backfires.. With the wet plug i'd be looking to carb balance or possibly diaphram troubles. Float height is another one that may be affecting things. I'd be checking these before i started opening up the motor.

As far as running it as is, no its not a good idea. If you have one cylinder appreciably down in the other the engine will be braking its self which will be ineficient at best. At worst can cause crank failure, you could hole pistons due to one cylinder going massively lean or even hydraulic from flooding. No if she's porley fix it, running it like it is unless it was an emergency you were willing to sacrifice the engine for will end up costing.

[paultsmith]

So, I followed the advice and got a compression tester. I hooked it up on the right (good) cylinder and got 150psi after a few cranks.

I then hooked it up to the cylinder that was giving me trouble, I ended up getting 150 psi in this cylinder too. However I noted that it took many more cranks to get this pressure as compared to the numer it took on the right cylinder. (Perhaps the battery was just running lower on the second cylinder test??)

On the bad cylinder I got it up to 150psi and it held that pressure for 8 minutes with no leaks, until I released the pressure.

So it looks like I may be having a carb issue now?

Also, when I first did the test on the bad cylinder, it would only go up to snd hold at 120psi, but I noticed when I cranked the started, air was leaking from the spark plug hole. I hadn't screwed the tester in tight enough on this cylinder. Perhaps I had not screwed in the spark plug with enough torque?

[Cynic]

And repeat

Sounds like our old favorite carb trouble again, the valve clearances would need to be cringeworthyly (new word?) out to cause that kind of issue.

When you revved it to 5k you got a bit of smoke and a few backfires.. With the wet plug i'd be looking to carb balance or possibly diaphram troubles. Float height is another one that may be affecting things. I'd be checking these before i started opening up the motor.

As far as running it as is, no its not a good idea. If you have one cylinder appreciably down in the other the engine will be braking its self which will be ineficient at best. At worst can cause crank failure, you could hole pistons due to one cylinder going massively lean or even hydraulic from flooding. No if she's porley fix it, running it like it is unless it was an emergency you were willing to sacrifice the engine for will end up costing.

[Cy Welch]

So, I followed the advice and got a compression tester. I hooked it up on the right (good) cylinder and got 150psi after a few cranks.

I then hooked it up to the cylinder that was giving me trouble, I ended up getting 150 psi in this cylinder too. However I noted that it took many more cranks to get this pressure as compared to the numer it took on the right cylinder. (Perhaps the battery was just running lower on the second cylinder test??)

On the bad cylinder I got it up to 150psi and it held that pressure for 8 minutes with no leaks, until I released the pressure.

So it looks like I may be having a carb issue now?

Also, when I first did the test on the bad cylinder, it would only go up to snd hold at 120psi, but I noticed when I cranked the started, air was leaking from the spark plug hole. I hadn't screwed the tester in tight enough on this cylinder. Perhaps I had not screwed in the spark plug with enough torque?

Retest. Make sure that the pressure testing is tight in both sides. Also make sure that the plugs are properly torqued when you put them in (13-16 ft lbs). Also if you have seperate coils, swap the coils, plug wires and plugs between the sides to insure the trouble doesn't move. If it does, try just swapping the plugs as they can and DO go bad internally (high resistance). If compression and spark are good then it's pretty much gotta be fuel, make sure that the carbs are clean and adjusted and in sync. You WILL need some sort of metering device to do this, though there are plans for a home made one on the internet (uses plastic tubing, a piece of plywood, a ruler and automatic transmission fluid) that is supposed to work even better than dedicated guages.

I had a problem pretty much exactly like your describing and all I did was to replace the plugs as I had a bad one. I found that out by swapping the plugs and the cold side switched.

PS, I had ridden about 2500 miles before I realized that one side was basically dead. Other than leaking seals and such after sitting for 14 years, the engine runs good and stong today. It goes off the road after the 1st of the year for a teardown and gasket/seal/clutch replacement (as soon as I find the right replacement bike to ride while it's off the road, since 2 wheels are my primary transportation and I ride year round).

[G-man]

...and I ride year round).

Lucky you, wish I could say likewise.

Paul, I'm going thru exactly the same thing as you, same side as well. Gonna try swapping the plugs and see what happens.

Here's a link from a beemer website on an easy-to-build carb synch tool out of a length of tubing and a ruler: http://www.airheads.org/content/view/183/56/

You could also make one with 2 bottles, with stoppers, more hoses...more labor intensive, and I'm not for sure if it works any better.

[Cynic]

Why bother putting anything up when nobody reads the bl@@dy posts.

[paultsmith]

Thanks for your help everyone.

I did a bunch more compression tests and each cylinder was giving me the same pressure +-10psi.

I also checked the valve clearance and adjusted them. One cylinder was very loose and the other very tight. I reasembled everything and was still having the same problem!

Checked compression again and it was good again, so I decided to take apart the carbs.

I cleaned the carbs again and put them back on the bike. I will probably need to do some synchrnoization soon. One slide comes down much faster and not as smooth as the other.

I also took out the mixture screws and noted that one of the small orings had caught on the spring, so it was probably not seating correctly. I put everything back on the bike and still not much of a change.

I left the bike idle for many minutes and then held it at 4-5K rpm. The bad cylinder was sputtering and oil/mositure or gas was visible all over the inside of the left exhaust pipe. It started backfiring a bit and some white smoke was coming out. I just kept it idling at 5k and after a while it seemed to clean itself up.

It seems to be running fine now. The bike had been sitting for a number of years and I had put in some oil in the spark plug holes, before along with some carb cleaner into the carbs too. Perhaps there was just a bunch of gunk and buildup in that cylinder that was giving me the trouble.

Currently the bike is at the dealer getting safety inspected and new tires, but I drove it for about 20mins today and it sounded good, and there was a lot of heat from the bad cylinder.

[Special Man]

Thanks for your help everyone.

I did a bunch more compression tests and each cylinder was giving me the same pressure +-10psi.

I also checked the valve clearance and adjusted them. One cylinder was very loose and the other very tight. I reasembled everything and was still having the same problem!

Checked compression again and it was good again, so I decided to take apart the carbs.

I cleaned the carbs again and put them back on the bike. I will probably need to do some synchrnoization soon. One slide comes down much faster and not as smooth as the other.

I also took out the mixture screws and noted that one of the small orings had caught on the spring, so it was probably not seating correctly. I put everything back on the bike and still not much of a change.

I left the bike idle for many minutes and then held it at 4-5K rpm. The bad cylinder was sputtering and oil/mositure or gas was visible all over the inside of the left exhaust pipe. It started backfiring a bit and some white smoke was coming out. I just kept it idling at 5k and after a while it seemed to clean itself up.

It seems to be running fine now. The bike had been sitting for a number of years and I had put in some oil in the spark plug holes, before along with some carb cleaner into the carbs too. Perhaps there was just a bunch of gunk and buildup in that cylinder that was giving me the trouble.

Currently the bike is at the dealer getting safety inspected and new tires, but I drove it for about 20mins today and it sounded good, and there was a lot of heat from the bad cylinder.

Hey all. I also have a 1981 XS400 and I am having very similar problems. I've got the bike running and cleaned up, carbs torn completely apart, cleaned and put back. New plugs. . . the whole nine yards. I've noticed that when it is running, the left pipe is blowing hot and the right pipe is blowing cold. After a while the right pipe will start heating up and it spits and sputters. When it is revved up, I can hear that the right side is working but having trouble keeping it going. If I pull off the right spark plug wire, the engine continues on unaltered. If I pull off the left spark plug wire, the engine shuts down. I have swapped the spark plug wires and it won't start at all. One more thing I did notice is that the right spark plug wire shocks me slightly when I touch it. I figure that if it is a bad wire, when I swap the wires, the left wire should fire the right side, right? I have not done any compression tests and I don't really know too much about that. Any thoughts on what's going on?? and what direction I should go?? This may take some explicit instructions as I don't know much about valves and compression etc. Thanks for all your help!!

[itsnotme]

Hey all. I also have a 1981 XS400 and I am having very similar problems. I've got the bike running and cleaned up, carbs torn completely apart, cleaned and put back. New plugs. . . the whole nine yards. I've noticed that when it is running, the left pipe is blowing hot and the right pipe is blowing cold. After a while the right pipe will start heating up and it spits and sputters. When it is revved up, I can hear that the right side is working but having trouble keeping it going. If I pull off the right spark plug wire, the engine continues on unaltered. If I pull off the left spark plug wire, the engine shuts down. I have swapped the spark plug wires and it won't start at all. One more thing I did notice is that the right spark plug wire shocks me slightly when I touch it. I figure that if it is a bad wire, when I swap the wires, the left wire should fire the right side, right? I have not done any compression tests and I don't really know too much about that. Any thoughts on what's going on?? and what direction I should go?? This may take some explicit instructions as I don't know much about valves and compression etc. Thanks for all your help!!

Read Cynic's post(s) carefully, I think he's got the solution to both of your problems and he's been repeating it and none of you are reading it. You need to BALANCE AND SYNCHRONIZE your carbs. If they're not balanced then of course one side is going to be stronger than the other.

Sorry Cynic, I know you've been trying to help.

[Special Man]

Read Cynic's post(s) carefully, I think he's got the solution to both of your problems and he's been repeating it and none of you are reading it. You need to BALANCE AND SYNCHRONIZE your carbs. If they're not balanced then of course one side is going to be stronger than the other.

Sorry Cynic, I know you've been trying to help.

I have read Cynic's posts, any my diaphrams are good and I know nothing about float adjustments or valve clearances. Is there any literature or manuals that explain how to check these things in detail? Thanks.

[Special Man]

Read Cynic's post(s) carefully, I think he's got the solution to both of your problems and he's been repeating it and none of you are reading it. You need to BALANCE AND SYNCHRONIZE your carbs. If they're not balanced then of course one side is going to be stronger than the other.

Sorry Cynic, I know you've been trying to help.

I have read Cynic's posts, any my diaphrams are good and I know nothing about float adjustments or valve clearances. Is there any literature or manuals that explain how to check these things in detail? Thanks.

[drewpy]

I have read Cynic's posts, any my diaphrams are good and I know nothing about float adjustments or valve clearances. Is there any literature or manuals that explain how to check these things in detail? Thanks.

yes ther'e called;

haynes manual,

clymers manual,

yamaha manuals.

loads on ebay

[itsnotme]

yes ther'e called;

haynes manual,

clymers manual,

yamaha manuals.

loads on ebay

Not to mention all the threads on here describing how to do it with pictures!

[XRayHound]

I'm having the same problem as Special Man. It is NOT in the carbs. Here's my story:

I rebuilt the carbs, set the valves, sync'd the carbs, everything was nice. It started easy, ran good. Let it sit for awhile since I had accomplished what I set out to do. When I next try to start it, no joy. Nothing works. I get it running with brake cleaner and notice the right pipe is cold. Pull the plug wire, it's still loping along, nothing changes. Here's the goofy part: if I pull the wire off and hold it way off the plug, to where the spark is arcing to the plug, the cylinder fires and the bike runs great! I figured this was a bad wire, so I swapped in an extra wire I had. No change. After I caught onto the pulling the boot halfway off making it fire thing, I swapped the plugs side to side, and sure enough it moved to the left cylinder. Two new NGK plugs later and after a short period of running well it's back in the right cylinder. I'm stumped.

[Airhead]

Try a new plug cap on the bad side

[drewpy]

I'm having the same problem as Special Man. It is NOT in the carbs. Here's my story:

I rebuilt the carbs, set the valves, sync'd the carbs, everything was nice. It started easy, ran good. Let it sit for awhile since I had accomplished what I set out to do. When I next try to start it, no joy. Nothing works. I get it running with brake cleaner and notice the right pipe is cold. Pull the plug wire, it's still loping along, nothing changes. Here's the goofy part: if I pull the wire off and hold it way off the plug, to where the spark is arcing to the plug, the cylinder fires and the bike runs great! I figured this was a bad wire, so I swapped in an extra wire I had. No change. After I caught onto the pulling the boot halfway off making it fire thing, I swapped the plugs side to side, and sure enough it moved to the left cylinder. Two new NGK plugs later and after a short period of running well it's back in the right cylinder. I'm stumped.

thats how those "booster" ignition enhancers work. this works by letting the coil soak up more juice in the seconday coil and then it eventually jumps the gap (I'm talking milliseconds here).

either the plug is fouling and causing resistance, ht cap causing resistance or the coil is knackered and failing. could also try a new contact breaker as this makes and breaks the circuit for the coil

BTW if you can see sparks from the contacts with the cover off and the engine running, you have a knackered condensor, try earthing it properly or buy a new one.

Drewps

[paultsmith]

To check Ignition Coil Measure Ohms

Primary coil resistance 4.0 ohms, red/white & Orange Wires

Secondary coil 8.6-9.5KOhms orange and spark plug lead

[XRayHound]

pault: Thanks, I'll check that.

Drewpy: Thanks for the insight, I did replace the coil but there's no telling if it was any better than the last. I'll check the resistances on all of them. I would follow through with more of your advice, but I've got the breakerless ignition. I really hope that bit isn't screwed up.

Oldgit: Will do.

Thanks all.