Flaming exhaust port? See video.

Just getting into ‘74 XS650, new project to me.

Speedo was in a bucket and reads 16k miles. General shape of bike is ‘fair’. Last registered 2013. Considering how initial inspections are going, most everything needs attention.

Previous owner bought it 3 years ago, put on electronic ignition and flat slide carbs. Engine was ran back then. Gas in tank was varnished now.

I swapped gas and it fired right up. But it coughs fire out of right exhaust port. I figured it was rich and carbs needed love. I fully rebuilt the carbs and it still coughs fire in right port. Both cylinders had similar compression to each other and similar compression also to my sons XS650. I’m not sure how to check timing yet, but I’m guessing that is not the problem bc it’s wasted spark and problem is only on one side. The fire might not be every cycle, it may be rampantly intermittent. Valves? Sticking? Bike does not sound terrible, but can hear there is a miss in it.

Any suggestions where to look to straighten this up without a full rebuild? Or perhaps that’s in order considering what I’ve found up to this point?

See YouTube video. In slo mo you can see and hear it’s not every cycle.


thanks, Scott

Well, for starters running it with no pipes is going to screw the jetting all up. At least up a set of head pipes on it and try again.

That's not all it will screw up, either. OP is gonna warp an exhaust valve if he hasn't already.

But if you put a pipe on it you won't be able to see the flame which may not be too far from normal. Maybe just the result of a fuel that burns slightly slower, like good 93 high test.

Looks totally normal to me.

Um open head? Yeah you'll see flame. Pretty normal with no pipes. Look at dragsters with zoomies, they'll have flames out the pipes.

It is not a problem starting a motor without headers. Just don’t load it or heat it up.

There is a miss and the left and right side behave entirely different. It is also intermittent only on right side.

I’ve never seen straight pipes start to fire until loaded, and even that is usually a cam with more overlap than a stock grind.

Thanks for the comments. I don’t mean to be a hard head and ignore suggestions, but I’m kinda thinking a sticky exhaust valve.

Another clue... went to check compression on hot motor, and right side actually dropped to 60#. Few minutes later, back up to 95# to match left side.

Tore in.




I’ll do guides, seals, lap surfaces, and new rings n hone.

Cool motor to work on.

TheMonkey said:

I’ll do guides, seals, lap surfaces, and new rings n hone.

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Valve guides on these motors generally hold up pretty good. Be sure and check 'em obviously, but I'd guess you'll find 'em in spec. The cam chain guide on the other hand is a must change item while you're in there... that and the chain.

Look for Hoos Racing for the chain guide.

Faces of valves were in rough shape because of all the carbon. Cam chain was worn enough to retard valve train 6*. Among other reasons as well, motor got a full top end. I have gone waaay further into this bike than I thought, but I’m falling in love with it.

That right side exhaust wasn’t even shutting because of the carbon. Check out before and after...



Transgressive moment (not okay):



Wiring and clutch yet, but I got pipes on now. No tach yet, maybe idle revs too slow?

Yes carbon causing an exhaust valve to stay open is not rare. Two prime suspects are intake valve seal and leaky head gasket. You have both those dealt with. What carbs are those?
PS oil burns kinda slow..........

gggGary said:

...What carbs are those?.....

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Carbs are the Mike’s version of PWK 32 flat slide. They were inherited with the project. I’m aware it might be frustrating, but I’ll see how it goes.

gggGary said:

...PS oil burns kinda slow..........

Click to expand...

I’m feeling a bit thick here... not sure what your telling me?

TheMonkey said:

Carbs are the Mike’s version of PWK 32 flat slide. They were inherited with the project. I’m aware it might be frustrating, but I’ll see how it goes.




I’m feeling a bit thick here... not sure what your telling me?

Click to expand...

Just a guess that oil leaking into the RH cylinder was causing the flames in the exhaust cuz oil burns slow.
Lot of overlap on XS650 cams, without a resonance wave to "stuff the mix" it's easy for raw gas to blow through and be ignited by the next exhaust valve opening. 'Specially if a carb is rich for some reason.