I'm just finishing a spell of night shifts so I have barely had chance to do anything, except source some stuff for my project bike. I have lots else to do as well but over next couple of days I hope to start a few jobs.

Its an old "naked twin" Yamaha. (XS400 Special-'82) There is a seperate exhaust pipe from each cylinder. When I start her up she sounds very throaty, much beefier than the GS. I have not noticed any smoke or contaminants coming from either pipe. Both pipes very soon get hot so I assume it's firing OK, it still has the old plugs and I will replace very soon.

If I cover the right hand pipe she soon starts to lug and would stall, I think this is normal, a good sign. The left pipe is another matter, block it and the engine goes on with no discernable change. What is the most likely cause, Gasket ? ? or what ?

valves or rings.

No I'd say plugs or carbs ... my virago 1100 does this ... rear cylinder quits when hot ... too rich ... start blipping the throttle and it catches up and runs like a bear ...
Cool.
Srinath.

Carbs or valves.

Carbs: floats reset level to each other and vacuum sychronized.

Valves - I believe the valves for the XS400 are the threaded adjuster type, which means no shims, but you'd better be good with the feel of a feeler guage. Now, with threaded adjusters, I prefer to err to the tight side, because I don't like them hammering away at each other (you don't get that with shims). One way to check that you have the right clearance is to attempt to put in the next size up feeler guage. If you can't, you're good.

Compression - poor man's method of checking compression is to pull both plugs (ground them), and put your thumb over the hole, and then press the start button. If you can't keep your thumb over the hole, you have good compression.

a good way to check to see if your rings are bad is to get the bike running, warm and at normal idle.  Then, with the bike on the center stand, un-screw the oil dip-stick and take it out.  If the rings are bad, the compression my suck air into the cylinders from the crank case and your idle will go way up right away.  This is a bad sign.

Quote from: Jake Da good way to check to see if your rings are bad is to get the bike running, warm and at normal idle.  Then, with the bike on the center stand, un-screw the oil dip-stick and take it out.  If the rings are bad, the compression my suck air into the cylinders from the crank case and your idle will go way up right away.  This is a bad sign.

another is to drop a bit of oil in the cylinder and let it sit for a min or two... if compression goes way up == bad rings

I changed the plugs, what do you know? It seems to have sorted that particular problem. The "old" ones were actually fairly new but black and sooty, also gaps seemed too small.

Quote from: Cal PriceI changed the plugs, what do you know? It seems to have sorted that particular problem.

I love it when that happens.  

Oh yes! It's SO nice to get a pleasant surprise. I suspect that a lot of other stuff may not be that simple. I am pretty sure the alternator is just there for decorative purposes, does not seem to do anything, Ill have to sort that before moving on to other electrical jobs. I have decided that as it seems to run reasonably well to prioritise brakes then fit turn signals (indicators to me) Steering bearings etc,. to check and try and get it through the M.O.T. test so I can take it here and there which will make some jobs a lot easier (I wish!!)

I still can't figure why the turns and possibly all the wiring was removed but we have to have a little mystery in our lives don't we.