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Twenty years ago or so, I had Krauser heads, electronic ignition and Mikuni Tm36 flat-slide carburetors installed on my R100RS. It took a long time to get the bike back, but I was just happy to see it again on one piece. It never ran right, blowing black smoke and backfiring, but with a son on the way and a beater house to work on so I left it alone.
Now I'm trying to get it sorted out, I went through the usual maintenance, replaced the ignition module, cleared the mouse out of the airbox.. Bike won't idle at all on the right cylinder with a ton of eye-watering black smoke. I did a cold engine compression test, 100 psi on the "good cylinder", 90 psi on the "bad". Moved onto a leakdown test using a cheapy Harbor freight gauge, 57% leakage on the bad one, 38% on the better one. Hissing from both cylinders audible through the oil filler.
So, re-hone?? If so, do I need new rings, even if they have about 10 miles on them?

Krauser4valve wrote:Twenty years ago or so, I had Krauser heads, electronic ignition and Mikuni Tm36 flat-slide carburetors installed on my R100RS. It took a long time to get the bike back, but I was just happy to see it again on one piece. It never ran right, blowing black smoke and backfiring, but with a son on the way and a beater house to work on so I left it alone.
Now I'm trying to get it sorted out, I went through the usual maintenance, replaced the ignition module, cleared the mouse out of the airbox.. Bike won't idle at all on the right cylinder with a ton of eye-watering black smoke. I did a cold engine compression test, 100 psi on the "good cylinder", 90 psi on the "bad". Moved onto a leakdown test using a cheapy Harbor freight gauge, 57% leakage on the bad one, 38% on the better one. Hissing from both cylinders audible through the oil filler.
So, re-hone?? If so, do I need new rings, even if they have about 10 miles on them?

I gather you don't have nikasil barrels? With that few miles on it, the rings have never been broken in properly. If the cylinders were not rusted, you may just need to get out and ride it to seat the rings. Read up on engine break in. You aren't supposed to baby them any more. If your barrels were rusted and pitted, you may just need to hone them. The rings may still be good to go with that few miles.

Cast iron sleeves. Yeah, I've tried to ride it, but the right cylinder fouls so quickly that I wind up running on only the left one, limping it back home. Valve adjustment is set loose for a Krauser at .003.

Black smoke sounds like a carb problem to me. Not familar with Mikuni .....does it have enrichener that could be stuck/installed wrong ? Maybe the idle jet is missing or something to let excess gas be sucked up with the slide closed. Maybe no needle in the right carb.

I swapped the carbs side-to-side and the results were the same. Is it possible to get a cylinder so carboned up that that nothing I do will fix it without taking it apart?

Krauser4valve wrote:I swapped the carbs side-to-side and the results were the same. Is it possible to get a cylinder so carboned up that that nothing I do will fix it without taking it apart?

If the carbs have been sitting for 20 years and you haven't rebuilt them, I would check the o-rings. You probably need to rebuild them.
You say it never ran well. Was a Krauser cam used with your kit?
Also. You need to test compression and leak down when the engine is hot. Cold pistons and rings fit looser in the cylinder than hot ones. Compression always suffers on a cold motor.

Carburetors are clean, O-rings replaced a month ago, pilot jets are #35. Enricheners are seating. Excellent spark. Static timing is good, hard to check the advance with the lousy running, but it doesn't seem stuck. Stock camshaft, I don't think those fancy krauser cam grinds were around then.

Krauser4valve wrote:Carburetors are clean, O-rings replaced a month ago, pilot jets are #35. Enricheners are seating. Excellent spark. Static timing is good, hard to check the advance with the lousy running, but it doesn't seem stuck. Stock camshaft, I don't think those fancy krauser cam grinds were around then.

Vanzen seems to know more about Krausers than anyone. Maybe he'll chime in at some point. I would imagine valve adjustment needs to be pretty precise on these 4 valve jobs. Make sure you're following the proper procedure (whatever that is). I need to ask, although you seem to know a thing or two...Are you sure you're setting them at TDC on the compression stroke? Black smoke would seem to indicate a valve or carburation issue, although you certainly need more miles on those rings.I know they came up with a specific grind for Krausers for a reason. The stock cam may create issues of it's own. Good luck, don't give up. That beauty needs to get back on the road and so do you.

mattcfish wrote:Vanzen seems to know more about Krausers than anyone. Maybe he'll chime in at some point. I would imagine valve adjustment needs to be pretty precise on these 4 valve jobs. Make sure you're following the proper procedure (whatever that is). I need to ask, although you seem to know a thing or two...Are you sure you're setting them at TDC on the compression stroke? Black smoke would seem to indicate a valve or carburation issue, although you certainly need more miles on those rings.I know they came up with a specific grind for Krausers for a reason. The stock cam may create issues of it's own. Good luck, don't give up. That beauty needs to get back on the road and so do you.

The cams are gentler ramps to protect the cam followers. There's nothing about them that would pertain to this problem.

The Major is correct. K4V heads were designed and intended to operate with a stock cam.
OK, as time goes on, it was found that the extra valve-train mass
beats the sh!t out of tappets and front cam bearings –
Not likely to be Krauser4V's current problem,
but it would certainly be worth the effort to inspect cam lobes and followers ...

Swapping the carbs left to right is the best way to eliminate a fueling concern.
You've done this with results that will lead you further into the engine.

By your description of the leak-down,
(Hissing from both cylinders audible through the oil filler.)
IMO, this sounds to be a cylinder / ring problem:
Could be a rusted bore (how long was it sitting ?)
Out-of round or otherwise out of spec bore
Incorrect piston to bore fit
Improper rings (as in rings intended for Nik bore)
Stuck rings (how long was it sitting ?)
Rings ill fitted to the piston (end gap or groove clearance)
Improperly installed cylinder studs
or even (possibly) carbon build-up

We can all speculate ad infinitum ...
But it looks like it's time to break out the tools and micrometers,
get your hands into it,
and know for sure.