1978 XS - Cold left cylinder/Hot right cylinder

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Atxrider

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Hey guys,

So I've got a 1978 XS650 that Im currently bringing back to life and it's giving me some issues.

Issue:
-Right exhaust gets really hot and pressure coming out feels somewhat low, not blowing my hand if I hold it loose
-Left exhaust is completely cold to where I can touch it after riding 10 min and it has lots of pressure coming out of it to where my hand gets blown back
-Excess gas is sometimes coming out of left cold exhaust too, but no oil or anything else
-Generally feel there is a lack of power; Takes awhile (15-20 seconds) to get to 55-60MPH -If I don't let out the clutch super slow when taking off it kinda wants to die from what it feels like; I have to keep the throttle 3/4 to full when taking off which doesn't feel right.

Facts:
-1978 XS650
-BS38 carbs; Completely cleaned w/ carb/brake cleaner and air compressor
-New jets; Pilot and Main are close to the richest side on the scale due to exhaust and air filter mods
-Carbs synced on the butterfly opening, but not with an actual gauge
-Right exhaust is yellowing on header by engine mount/Left is untouched
-Mikes XS Performance Reverse Cone Megaphone Muffler 17.25" x dual exhaust
-Mikes XS pods
-Pamco electronic ignition and black 2 out coil
-145-150PSI range compression on both cylinders; Tested 3 times
-Have swapped plugs/plug wires; Both plugs getting perfect spark
-Left spark plug is always wet with gas


Any ideas? I know the right cylinder tends to run a bit leaner, but I know it shouldn't be this different :wtf:
 
It does sound as if you are running on one cylinder. Reverse your spark cables and see if the problem follows. It could be the cables or coil. Start with new plugs before you test and post the results. Some fellow members have a problem with the magnets embedded in the rotor. They seen to lose their magnetism.
 
So I've already bought new plugs, and switched the wires. Don't think it's a spark issue.

Compression is solid too. Carbs seem to be good.

Any other ideas I'm not seeing?
 
Ya, floats are sitting at 24mm, the air fuel screw is 1 3/4 out. I haven't viewed that but will double check throughout it!

Any other ideas?
 
If your 'normal' service inspections come up empty, it's time to look for the unusual, which really isn't unusual for old machinery.

Float heights may be right, but fuel bowl levels may not. Try the forum's Google search on 'clear tube method'.

Have you done the 'slide drop' test mentioned in the carb guide?

Lastly, the seal of the needle jet emulsion tube o-ring needs to be good, as well as the bowl gasket sealing in that central zone...
 
I believe the left side should be at least somewhat hot as I know the right side tends to run leaner.
 
Your discription sounds as if your carb is leaking. A quick way to clean a dirty float valve is to drain the carb bowl so the valve opens. Turn on the petcock/prime and the flowing fuel will carry away any dirt that was between the needle and seat.

You have vacuum operated petcocks if it is a 78, so the diaphragm inside could be leaking through the vacuum hose into the manifold giving way too much fuel.

While those are two practical reasons for floading fuel, the most common is the carb rebuild is screwed up.

The only way to tell a difference in temp in a properly running engine is with instruments.

Tom
 
Hey Tomterrific,

That's a good idea. I could see the float valve getting some form of dirt or something stuck to keep it open. Out of everything else that seems like it could be the culprit.

I will test tonight and post results from what I find.

Also, I don't believe my petcock is a vacuum operated one, as it only has the one line out for fuel.
 
Well there was internal work done. All out back to spec and it has 150 psi on both cylinders after three kicks
 
Also, I took apart the carbs yet again. Everything looks good, if not great.

Just took it for a spin and I still feel like the left is wayyyyy cooler. But I think there's still excess gas left in there.

Last thing I need to check is the pamco ignition to make sure all magnets are there.
 
The reason I asked about the cam chain was because of an off the wall thought I had. I was thinking one cylinder might be running advanced, and the other retarded. If the the cam chain was off a tooth, and since the electronic ignition fires at the same time each rotation, it could be firing too early for one cylinder, to late for the other.
 
That's a really good thought! I'll check it out. Can't think of anything else that would be causing this.

Hmusket, if it's off a tooth would it still have good compression?
 
*******UPDATE:

So I tested something out by marking TDC for the left cylinder and then rotating the advanced rod 180º to find that now the left cylinder runs perfect and hot, but the right is now cold....ideas?

I have now leveled it down to knowing it's something with the spark/timing/pamco/advance unit. All else is good to go.

What am I missing here that could cause this?
 
This is really Pete's department, but it sounds like one half of the Pamco rotor isn't triggering the Pamco hall sensor.

One cyl could be coming up on compression, and receive the correct spark, but the other (unfired) cyl would still have its exhaust valve open, and the waste spark could be igniting its unburned fuel, going out the exhaust.

On the next revolution, when the other cyl comes up on compression, there may be no spark, so it doesn't produce power, and the missing wasted spark doesn't matter to the other exhausting cyl.

So, you may want to inspect the rotor and its magnets. Pete shows the 90° and 60° rotors on his website:

http://www.yamahaxs650.com/6090.JPG
 
So I have confirmed at this point that:

-I have perfect compression and tuned carbs
-I have a 60º PAMCO Dwell, not the 90º Dwell
-If I set the engine at TDC and move the advancing rod 180º it will switch to firing right on one side, but not the other (Like the first time I switched it the left now fires and is hot and the right is cold to touch)
-I don't have a re-phased engine


My thoughts of it now are:

-That I have gotten the issue down to knowing it's something to do with the timing/PAMCO
-I may need the 90º Pamco Dwell for it to fire correctly


Thoughts? Idea's? Heard of this before?
 

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