Topic: only one piston firing - cb360 twin

[annie bacon]

so i realize this isn't exactly the place for a question about a twin, but there's so much brain in these forums, and i need a little bit of it . . .

i've been working on a '76 honda cb360 twin for a few months (my first bike ever, so i'm slow moving). when i got it (for free) the engine had not been turned over in 20 years. it had been in someone's driveway rusting away. i have rebuilt or replaced almost everything except the engine. things seemed to be going okay mechanically (if not aesthetically) until it got out on the road when it was discovered that  a) it had no fourth gear and b) after a short while (maybe a mile or so? or a few minutes) it was only firing one piston (not sure which). the piston bit has been consistent. we'll get it back in the shop, poke around, fix this or that little thing, get it back out and it'll be running with both, and then after a bit it'll go back to just one.

any ideas why this might be happening? and how to fix it?

i've got the best mechanic in oakland helping me on this,  but i thought i should take advantage of whatever genius i could find . . . i'm very stubborn and want to figure this out.

other possibly relevant information: the left (brand new) spark plug was trashed after the first run around the block, the top all covered in soot-like substance; the left and right idle-mix screws have vastly different sensitivities; and i'm sure a thousand other things i wouldn't think to mention . . .

thanks
anne.

[mlinder]

Plug is fouling. Adjust/Fix the carb

Old electrical will also crap out as it heats up. Coil, plug wire, etc..

[annie bacon]

thanks for the reply.

that's the trouble, however. we keep adjusting the carbs, and then it'll run with both pistons and then after a little while it'll revert to one.

if you absolutely think it is the carbs causing the problem, maybe i need to pull them out and completely rebuild them again? make sure everything's tight?

[mlinder]

thanks for the reply.

that's the trouble, however. we keep adjusting the carbs, and then it'll run with both pistons and then after a little while it'll revert to one.

if you absolutely think it is the carbs causing the problem, maybe i need to pull them out and completely rebuild them again? make sure everything's tight?

Can you take a picture of the offending spark plug after it fouls? That may give the gurus here a better chance of diganosing the issue for you.

[byidesign]

Bad sync,dirty slo,jet,
    high/misadjusted float level/leaky needle/seat
          come to mind, especially with great adjustment
      differences between the two carbs............
             Hope it helps
                   Bruce

[crazypj]

 If you did'nt pull carbs right off to clean them, they aint clean. check carb diaphrams and both the needles are the same lengthas well. sounds like float level is off or sticking  though. are the points set right for both cylinders? Place a 2"x1" under left foot of mainstand so you dont loose so much oil when your checking timing. (remove round cover on left side for flywheel/timing marks)
PJ

[bryanj]

because there are two sets of contacts, one will frequently short out on the cover, try running without the points cover at the top of the engine --GENERATOR COVER MUST ALWAYS BE IN PLACE.
Also carb diaphragms do go and these are rare and EXPENSIVE

[dusterdude]

has anyone mentioned swapping coils from side to side?