Color of exhaust valves

Doing some other work on my 71 Commando, I noticed the exhaust valve when viewed thru the left exhaust port was whiter than I would have expected. This compelled me to check the right side and it was darker than I expected. After doing some searching on this form (exhaust-valve-color-t14024.html) and on the Web, it seems the white coloring may be more normal (or a bit too lean?) and the black an indication of oil leaking past the valve stems or too rich. Lots of discussion on carbs/jetting being too rich/too lean, which I'm sure could use some tweaking, but I don't think it's that far off that one cylinder would be so different from the other (I have new Amal 930 Premiers with 220 mains, 106 needles and center needle notch). In general, the bike runs strong through all the gears and idles fine (although I have the Power Arc ignition from Old Britts, so starting can be a bit tricky!).

The question is, which port, if either, is normal and what is the most likely issue with the other?

Thanks,
Mike

Left port:



Right port:


And the plugs look like this (after a normal run - not a chop test, ). The lighter plug (on the left) was from the whitish (left) port):

Ugh I would assume some intake guide oil being sucked in more than carb issue but could also be some head gasket leaks too. Exht valve should be sealed against intake draw in as when open the pressure is outward from chamber, unless exht seat issue. To check for air leak leaning of slide/bore wear can smear with grease which lasts about 100 miles and may be enough to see if that matter or not. The thermal evidence on plug hooks implies about spot on spark timing and heat range of plug.

hobot said:

The thermal evidence on plug hooks implies about spot on spark timing and heat range of plug.

Click to expand...

Interesting comment, given the different colors of those 2 plugs.
A hotter plug may well burn off that oil deposit on the darker plug.
A compression test could be useful, see if both cylinders are working equally.

It does look like oil fouling on that darker plug.
I have ridden behind several Commandos puffing smoke out of one pipe or the other,
has anyone ridden behind you lately to comment. ?
OEM Commando pistons were a little prone to ring sticking, smoking on acceleration,
and valve guides could be the culprit too, puffing smoke mostly on the overrrun.

I have not seen a smoke issue upon start-up or out and about nor does the bike consume oil in any conspicuous way (it is rare event to have to top off the oil tank, and even then, it's just ounces). But, as you say, I may need someone to ride behind me to see what is going on under acceleration.

I had an 850 where the valve guide seal 'fell off' one inlet valve, and it colored the spark plug just like that.
No amount of playing with the jetting did anything, including changing carbs, but removing the head and fitting a new seal did an instant cure.
I wasn't aware of any oil burning.

750 engine doesn't have those inlet seals (?), so you are looking for wear or oil ingress somewhere.
Checked how much oil is sloshing around the inlet rockers area while running ?

Having said that, it may be worth checking that one carb isn't flooding.....

Actually its the white exht valve that alerts me most as implies too dam hot. Should be dark tan or dry sooty. If spark time well the thermal change on hooks should be just before the end bend and these pretty much show that, which is separate issue from being sooty on rim or not. Being a Commando entirely possible one side too lean d/t fuel or air lean and the other too oil rich from a leak. Btw 750 or 850 Commando all have 6 start oil pumps over oiling heads so have factory provision for intake seals but they come off or may not have installed. If takes a big oil leak to be seen at speed but a follower a few bike lengths back can smell the oil on throttle ups. A compression leak down test might reveal by sound if rings or valve seats leak but might not. Peek in chambers and tell us what ya see.

Hello,

the darker plug is not ok, so I would check carb setting, etc.. If all settings are ok, you could try new spark plugs. Then I would ride the bike as long as it runs well and no smoke is seen.

Ralf

It may be worth taking a few minutes out to pop off the rocker covers to see if the valve seals have split. When i did my engine over haul I had the left valve seal take a crap. I could see that it was rock hard and split in half. Just my .02

hi, IMHO there is no seals on exhaust valve guide.........!