Oil smoke on trailing throttle

[Brian G]

Hi All -

Went for a Sunday blast with my step-son just before I came back to work on the ship. His '84 Interceptor is undergoing a rebuild so I loaned him my '77 XS650 and followed along on the BSA. It's the first time I've been behind her - was looking forward to hearing the exhaust note but my triple out-shouted her - and I was somewhat chagrined to see blue smoke emanating from the right hand exhaust.

Interestingly enough, it was only on trailing throttle. We stopped for coffee and I watched carefully on the way home and at first I thought I'd been seeing things as there was nothing until we got back into town when it started again - and only on trailing throttle. I had a good look and there's nothing on startup, even after a few days standing, and nothing on acceleration - and the kid doesn't hang about; took a good handful of Rocket 3 throttle to keep up with him!

This should tell me something, but I'm not sure what. Sucking oil past the head gasket when firing pressures are low?

Thanks for the advice,
Brian

 

[Pat D]

I've not been inside that many bike engines, but I've had plenty experience with automotive applications, and typically smoke on deceleration points to worn out valve guide seals, or taken further, worn valve guides. Someone with more specific experience with these XS powerplants may be able to offer a more specific explanation.

 

[ANLAF]

Some info needed - has the bike been stored or in regular use? What maintenance or work have you done recently, and did the rider notice the blue smoke. I say that because if the rider didn't notice then it might have been smoking like that for a while.

Any symptoms while riding - miss-firing, and what's it like on start-up?

Sounds like it is burning oil - any loss?

Have those valve guide seals ever been replaced?

Anlaf

 

[Brian G]

The bike is new to me so I don't know its engine history. The P.O. did a fine job redoing the brakes - new master cylinder, S.S. brake lines, added an oil cooler, PMA and Pamco, steering head bearings, needle bearing swingarm conversion, clubman bars, pod filters (rejetted), Loaded Gun rearsets - she looks really nice and he told me compression was up around 160 on both.

She has run well for the short time I've had her - no missing, pulls hard and starts easily with no smoke even after sitting for 5 weeks while I'm at work. The rider didn't notice the smoke and as I say, it's the first time I've ridden behind her so it may have been that way from the time of purchase but I don't know. Goes through a little oil - 'bout the same as the BSA; more than the Hondas - but nothing outrageous. Didn't see any oil in the muffler or on the plugs but if only on trailing throttle, it would probably burn off fairly quickly.

I'll retorque the head when I get home - if only because its the easiest fix and I usually try the easiest thing first. If not, I guess I'll have to schedule some engine work for next winter

 

[TwoManyXS1Bs]

Usually smoke on decel is oil pulled up past the rings, but if compression is good, no discernable oil film in exhaust or on plugs, and little/no blowby thru crankcase vent, then rings could be good. Could pull carb and exhaust on that side, flashlight up the ports, and look for telltale signs of worn guides/seals, like coating of coke on backside of valve faces, build-up and/or streak lines on stems (rotate engine to open valves to inspect stems)...

 

[Brian G]

Righty-oh, then. I'll do a compression check myself and take a peek up the ports as well. It can only come from one of three places so it's just a matter of elimination, I guess. If retorquing doesn't help, the head will have to come off anyway so if that's the case, I'll probably go the big-bore route. The proverbial silver lining...
Thanks for the comments.

Regards,

 

[ANLAF]

Things do wear out, and seal replacement is not a big job - and you can go for the bigger set-up while you are in there.

Let us know how things progress.

Anlaf