Sportster mufflers and a warped head?

[jbbishop2]

I have an '81 XS650 street tracker, stock engine as far as I know. I got it a couple if years ago as a project in progress so don't know how many miles it had on it but I've put on13,600+ miles since. At the beginning of my time with it I cut off the stock mufflers and replaced them with a pair of nearly new stock Sportster mufflers that somebody took off when they got aftermarket cans. At the time I wondered at the restrictive-looking honeycomb just inside the inlet but they seemed to work and sounded good to me. A few months ago I began a commute home and it started running rough, apparently on one cylinder. The entry section of the left muffler got so hot it turned blue, so figuring it was blocked somehow I pulled it off. It seemed to run better and I thought I might get home with one open pipe, but a few miles later oil started spraying out under the left cylinder outlet.

You probably already figured out that the Sportster muffler has a catalytic converter in it...if I'd had half a brain I'd have knocked it out to begin with but I don't and didn't. I assume that my old motor finally put more rich exhaust or oil or both in than the cat could stand and it melted down and clogged.

So, I figure I overheated the left cylinder enough that the head distorted and let the oil out. It's taken me until now to get the motor out and take off the head. The head gasket is stuck to the top of the cylinders and did not look damaged. There was oil all over the top of the gasket and it seems clear that the oil escaped between the head and gasket.
Any suggestions for how to verify whether the head is warped or not?If it is, what next?

Thanks,
John

 

[ReycleBill]

Checking for a warped head is simple. Remove all the old gasket, clean the head throughly and check it with a good straight edge like perhaps a carpenters square if you don't have the usual machine shop straight edges. Slowly move the edges of the square all over the surface of the head looking for light under the square. No light means head is fine.

These engines are notorious for needing the head bolts retightened every few years. Let them go a little too long and the oil goes shooting out. Myself and many others learned it the hard way. I'm betting that's what happened to yours.

I can't be certain but other than making your fuel mixture a bit rich, (thus the blue pipe) I doubt the cat would have had any harmful effects. If I had the cats I'd go down a couple of jet sizes and give them a try.

I'm really thinking you had 2 separate issues that took place at almost the exact same time: Rich fuel mixture caused by the cats and the oil leak caused by the stretching of the head bolts.

PS. I just realized, on the Sporster there is probably some means to inject fresh air into the cats to make them work and that's something you and I won't have on our XSs so scratch the idea of putting the cats back on.

 

[jbbishop2]

Thanks, Bill. I happen to have a machinist's straight edge wide enough to go across the head. When I took the head nuts and bolts off they were all very tight, but removal torque doesn't equal tightening torque...

The deposits inside the chambers on the heads were quite different; the right whitish and fairly clean, the left had carbon built up on the valves and around the plug hole. Not sure whether it had any bearing on the outcome but I had switched to Iridium plugs several hundred miles earlier, which improved how it ran, then put in a new coil and wires.

Now that it's apart I should replace the valve guides, too.

 

[ReycleBill]

I did the entire top end when I went inside mine. I'm not sure how much was really necessary but being I was already inside...

I think you'll probably find your head is okay. You might need to shave it a bit. Worst case scenario: If the head has to be replaced you will note the numbers in the valve cover match the numbers on your head. You should always try to replace those as matching pairs. But odds are good you'll not have to go that far.

 

[jbbishop2]

It's kind of hard to tell whether it's too much light under the straight edge. My straight edge is 12" long, and it's hard to say whether it's being held up a trifle by the outside edges. It seems only a little different from putting the straight edge across the tops of the cylinders.

If any light is bad, then I think I do need to have it shaved. I've wanted to rebuild it for a while, anyway, not for performance but to limit oil consumption and seal the leaks so it'll look nicer. It's done great service in the rat style I've built so far, and since I have another bike I can ride for now it can get the clean-up it deserves.

Thanks again,
John

 

[ReycleBill]

Perhaps one of the real experts on these engines will weigh in. There are some limits to shaving the head because of valve clearance and timing chain adjustment but I'm unaware of what those actual limits are. Also, if your warping is minor it might be a new head gasket will do the trick as sealing uneven metal parts is the real reason gaskets were used in the first place. Case in point: Series 60 Detroit Diesel Engines are machined to such exact specs. that there are no gaskets anywhere in the engine.

Don't be too quick to rush to the machine shop.

 

[weekendrider]

You could lay it on a piece of glass, a polished granite counter top etc and see if you can slide a feeler gauge between the head and the surface.