Strange problem

I lost a plug in a similar manner -- different kind of car, but I don't see the brand as a significant issue here -- and had to have a heli-coil installed to hold the plug. The plug had come loose, and it damaged the threads.

I also learned that I was supposed to use Anti-Seize on the threads. When I installed the plugs, they felt tight, but the torque spec is not very high and a bound plug can trick the torque wrench. I'd not have believed this to be true before I witnessed the result firsthand.

Apparently the aluminum head and steel threads on the plug need to be greased up a little with antiseize, or the plug might not seat well.

Always but be careful of this stuff!

I've used it for years on motorcycles, the normal procedure is to open tube of Anti-Seize, put small amount on finger, wipe around the threads of the plug, install plug, stand up (remember it's a motorcycle?), step on tube of Anti-Seize which ruptures tube and squirts Anti-Seize for surprising distances in all directions.

You'll also find the stuff is absolutely permanent on clothes, fingers, motorcycle seats, floors, etc..

As I recall, that's the model designation of a 4-cyl Yamaha from the early '80s, maybe late (very late) '70s. I had an XS750, then an XS11 Special.

My 750 was a '78, and the XS11 was an '80.

Is that why you use XS11E as your screen name?

I once had a well-used Corvair that had the same problem on three of its six cylinders. The plugs would remain in place for a little while, but would eventually pop right out. The situation worsened quickly. The solution was to install helicoils.

It's possible that the plug was never installed properly; someone accidentally neglected to tighten it down perhaps. Otherwise, I find it a stretch to believe that the plug spontaneously unscrewed itself and popped out. I think your problems with this cylinder are not over.

Correct, the XS11e was the 1978 model, considerably faster and more economical than later versions due to EPA compliance.

EPA rules hit motorcycles very hard but Yamaha shipped the entire year's supply before December 31 so all the 1978s were in the US prior to January 1, 1978 and therefore exempt from all 1978 regulations. Later models suffered timing and carburetor mods that hurt performance and, around 1980, had cylinder head, valve and muffler changes that got back some of the power but knocked the bike WAAAAAAY out of the 50+ mpg range....

The XS11f was 1979, XS11g was 1980, etc.

Yes, I had two of 'em and liked both of them, they were the ONLY Japanese motorcycles this Harley-Davidson/BMW/Moto Guzzi/Triumph fan ever liked out of the many I owned and the ONLY Japanese motorcycles I owned that didn't require a huge number of modifications before I'd consider them ridable....

Probably a good idea to remove the plugs, re-apply antiseize compound and re-torque them in the spring before driving the car.

Seems to me the XS11 was also one of the few cycles ever made to have a vacuum advance in addition to the usual centrifigal advance.

'78 Yamaha XS650 and SR500 here.

miker

I'm not sure but I believe the vacuum advance may have gone away on the later production models.

I restored an SR500 that suffered from several years of sitting uncovered. The engine was frozen solid but after removing the cylinder and inspecting the whole mess I replaced the rings and gaskets and all was well. The fuel tank was rusted out but the local wrecking yard had one for a reasonable price, I got it, painted the critter and had a really georgeous SR500!

It turned out to be a VERY nice bike but not quite up to freeway operation, speeds over 60mph seemed to be asking a bit much from it. Had I kept it I would have changed sprockets and it probably would have been a very nice all around bike.