Idles 1 cyl, clutch-in RMP+ 2 cyl

Hyde

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Hey guys, strange one for ya. Took the old girl for a run today, ran flawless to the middle of nowhere. Grabbed coffee and let the bike sit 15mins. Kicked her over, started but really rough. Decided to get home ASAP. As I take off I can hear and clearly feel the right cylinder is dead - cant gas it out that just kills it altogether.

Here’s where its strange. Pull in clutch and up throttle - she fires even. Let the clutch out on take off and the sputter/dead cylinder comes in. If I can get RPM up out of first she fires on both cylinders again. Right side drops out of I hit the gas too hard but if slow and smooth getting to speed - she’ll keep firing both. We limped home through all the backroads successfully, stop signs and lights were highly “interesting” until I could get up to even fire.

Haven’t changed anything, we’re at 5000kms since build and rock solid. Running Hugh’s CDI with MOFSET so paranoia kicked in. Battery in motion is taking in a constant 14.2v, battery after engine off is holding 12.5 to 12.7. Nothing strange there, voltage matches what it’s done since build.

I’m thinking potentially a cooked coil? If a plug cooked, I wouldn’t think it would fire at all regardless of my “speed”. The sputter does come on if the throttle is pushed too fast, a real gentle touch to get up to speed is needed. Just looking for input or if anyone else has pointers before I go digging in, thanks!
 
Hey, Hyde. I'm thinkin' stress on the ignition system. At low throttle settings, or mid throttle with sufficient rpms, manifold vacuum is high, and compression pressures are low. Easy to *spark* thru that. More open throttle, lower manifold vacuum, compression pressures increase, harder to get a spark to jump thru that.

A fouled or faulty plug, or coil that can't push the spark. Failing internal windings, a crack in the coil body, old or arcing plug wires. Just a thought...
 
a chunk of dirt in a jet or an air leak possibilities also............
Vacuum caps in place?
 
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Hey gents, ty for the replies, appreciated! Dug in last night - it was as simple as an almost dead plug. I’ve never seen that behavior before. Was in disbelief it failed at 5k Kms but stranger things have happened I guess.

Just to be “sure” - tested my ignition system from end to end, all is good with spot on manufacturer specs.

Thank you for the input gents!
 
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