Topic: My CB360 is back firing in the left Cylinder

[luvhonda750]

I suspected a bad valve and ran a compression check, sure enough, it 20 lbs lower in that cylinder, the clearence's have been adjusted to spec, before I adjusted the valves, they were all slightly tight, I needed to add about .001 on each valve. They must have seated and were never adjusted, the bike only has 5k miles on it.

So I think the exhaust valve is the issue, the exhaust pipe is also slightly discolored on that side. I tried adding some marvel mystery to the gas and crank case, at idle, there is no backfire, its only above 1500 RPM

Any thoughts on what to do? My CB750 had that issue on one cylinder because some carbon broke loose, but it corrected itself.

[mattcb350f]

If the left side pipe is more discoloured than the right, (i.e blued) than that side has a higher exhaust temp. Lean conditions will cause higher exhaust temps, sounds to me like the left side carb is too lean. It's likely got some crud in the main jet since it idles fine.
 Matt.

[cb650]

Mine backfires out the left.  Maybe they should meet.


                      Terry

[mack808]

If you set valves at proper adjustment and at the proper spot on crank rotation, you may just have a valve that is either stuck in the guide or maybe bent from some other item. Try squirting a little oil in the low compression cylinder and than retaking the compression test. If compression goes up, it rings, if it stays the same, the valve is still partially open for some reason. If compression stays low try readjusting the valves, the way the tappets adjuster screw with the fine thread tend to hold when you snug the nut down they maybe loosing the clearance. Make sure the cylinder in question is at top dead (the "T" mark for that cylinder) by the indicator on the rotor under the l/s engine cover. Exhaust clearance is either .003" or .08mm. If compression goes up, timing maybe advanced on the cylinder

[luvhonda750]

Mac808, I wanted to report back, things are improving very much, And what I did to help it was very similar to your suggestion. I actually set the leaking exhaust valve to a slightly higher clearence of .005, I squirted a teaspoon  marvel mystery in the cylinder, I ran it that way a while, I added about 6 ounces to the crankcase and some to the gas as well. I just kept running it, it slowly started getting better, then I poured some directly into where you adjust the valve. I think it was a sticking valve guide that just needed some extra lube, compression is also up to close to the other cylinder. The bike sat for 10 years prior to running, the tank was full of rust before I used electrolisys to clean it out.