riddle me this

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May 23, 2003
Backfires and sputters on right cylinder. Standard 850 with Boyer analog, dyna dual output coil, Keihin FCR carbs.

This is a spare engine I have in while rebuilding my high compression 880 unit. The backfires being confined to the right cylinder rule out timing or anything else eletrical as plugs and coil routing alternation cause no changes.

I've rebuilt the carbs, confirmed that there are no intake leaks....stumped...

She starts easy enough, but will die if not revved to 2k.

Just bought a compression gauge and will check the jugs tomorrow. The only thing left I can think of is a compression leak.

That, or she's just jealous of the newly acquired CB400f.

Head scratchin...

Have you looked at the valve adjustment, too tight or too loose, or the guides worn to the point the valves won't even seat (don't ask me how I know that one) also check the "lift" of the pushrod compared to the left side,maybe the cam has worn round.

A comperssion gauge is ok but kinda hard to read on a slowly revolving two cylinder motor, what woiuld really tell more is a cylinder leak down test. Most good garages have one I don't think they would charge too much, it would be pretty easy on a Norton.

Good luck.

Ride On
I was hoping it would be a valve adjustment. This was a "professionally rebuilt" engine I picked up off ebay in the spring......the valve adjustments were spot on. Also noticed that the tach drive had the o ring seal done which raised my confidence in the rebuild. Time and fettling will tell.....
Sounds like carburation to me.
I had one doing a similar thing when it showed up at my shop, turned out that who ever put slides back in the carbs was not careful when lining up the needle with the needle jet and managed to drive the needle up into the slide. It would start up and idle fine, back fired and spit back through the carb when revved.
Of course, that was with Amal carbs, I don't have any experience with Kiehins on a Norton, but it's something to check.



My lack of trust of Boyer low tension wires makes me add a suggestion to check these at the magnetic pickup plate. It's possible for only one of these to break inside the insulation. There's no visible indication. You have to do some type of continuity check. Often there is weak contact until the wire is pulled on.
Bingo Derek!

I happened to have a single Mikuni setup that came with the bike (never tried it). I switched my fancy flatslides for the Mikuni and the bike started right up and dropped into perfect idle. As I've already cleaned all the jets in that Keihin, I must have either some deep internal blockage or some really nasty air leak.

So, I've lost a little top end and acceleration until I get the Keihins in order but at least I've got a runnin Norton....wooHoo!
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