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Discussion Starter #1
Hi ya, My bike stumble/hesitates around 4500-5000 RMP
It seems to come & go at random.

Some back round: The bike is a 96 CBR900RR w/ some mods and is well taken care of. About year + ago I throw the chain. I was in the middle of building up one of my cars at the time and didn't have time to work on it so the bike sat. Maybe two mouths ago I got to work on it. Because it sat for so long, I pulled the carbs, cleaned and went over them, flushed the tank & valve, replaced the fuel filter, spark plugs, charged the batt., oil change, all the small stuff. I dropped a little 2 stroke oil down each plug hole and turned the engine over slowly by hand a little bit....Nice and smooth.

At 1st it didn't want to start, understandable considering. After it started it seemed to have a cylinder down. I pulled the carbs again to make sure I didn't miss anything, looks good. Checked the spark at the ends of the plug wires, looks good. Put it all back again & she came to life. The hotter it got the smoother.....nothing odd there.

Well now and again it seems to miss fire & stumble around 4000-5000 RPM and it just happens randomly. The batt. is kinda iffy & doesn't hold much of a charge. (I have a new one on the way)

Any thoughts? What's the odds that maybe one of the new Honda/NGK plugs are a little funky?

Any input would be great.
 

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I think you're on the right track, I would definately start with the charging system. Since it happens under load, it would indicate either a weak battery to me or fouled plugs, but you mention you've checked the plugs.

Have you also checked the carb jetting to ensure it's tuned right... i know it's unlikely but usually starting simple is the best thing to do..

if all that doesn't work, rip the engine out and drop in a blackbird motor :eyebrows:
 

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Discussion Starter #3
Yeah, Even though I checked the spark at the plug wire ends/boots, I'm still not 100% sure if one of the new plugs are bad. It's uncommon but I've had it happen once before on another machine. I have a set of Denso Iridium on the way with the battery. I hope it does the job but, with my luck, probably not.

As for jetting, it's kinda odd. The guy that had the bike before me did some work before I got it and it had a 120 main in the two middle carbs (2 & 3?) and 118 in the outside cabs (1 & 4?) I think stock is 115, I'm not sure though. The bike has always ran a tad rich, but not enough to really play much of a roll in performance or derivability. Normally, I try to lean a engine out some for better performance. I'm not sure what he thinking was behind it.
 

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Holley said:
Yeah, Even though I checked the spark at the plug wire ends/boots, I'm still not 100% sure if one of the new plugs are bad. It's uncommon but I've had it happen once before on another machine. I have a set of Denso Iridium on the way with the battery. I hope it does the job but, with my luck, probably not.

As for jetting, it's kinda odd. The guy that had the bike before me did some work before I got it and it had a 120 main in the two middle carbs (2 & 3?) and 118 in the outside cabs (1 & 4?) I think stock is 115, I'm not sure though. The bike has always ran a tad rich, but not enough to really play much of a roll in performance or derivability. Normally, I try to lean a engine out some for better performance. I'm not sure what he thinking was behind it.
So before you threw a chain, the jets were the same right? and it ran fine then... I'm wondering, the work the previous owner has done, is the bore still 71mm? or has it been overbored?
 

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Discussion Starter #5
Yep, same jetting before I threw the chain. I made sure not to change anything because the bike was fine before. I'm not sure if it has been overboard or not, I doubt it though. The bike is strong when it doesn't randomly miss, but not board strong.

These types of problems are the worse! If it was consistent, it would be much slipper for me to track down. It's the coming and going at any given time for no apparent reason that makes it more difficult.

Thanks for all your help, it's much appreciated.
 
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