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Discussion Starter #1
Soooo....

Installed my freshly rebuilt engine in BF650 SRA after my transmission grenaded and left me with the option of running H or L, but not both.

Anyways, engine is solid. Reinstalled my previous mods included full BG exhaust, crossover intakes, Twin Air Powerflow, 02 P650 velocity stacks. This time around I thought I’d retry the 12-hole mod. Had run the gauntlet on it before and decided it was too much hassle. Got it tuned and running great, but decided again...too much hassle to worry about getting water in the airbox when cleaning it etc.

Went back with the Napa hose mod as it was before the rebuild. Now comes my question, so thanks for your patience reading all the back story.

Took it for a quick spin yesterday, mostly just putting around making sure the tuning is right throughout the throttle range. All good except when I punch it full throttle and hold it for a few seconds. When I did that yesterday, after a few seconds holding it, it knocked pretty hard and I immediately let off the throttle. My gut tells me it is tune-related, but I’m terrified to lose another transmission. I’m hoping it was a one-off occurrence, like just got a big gulp of air or something and caused a big backfire through the carb.

Shifter is butter-smooth. No grinding. Bike idles excellent. Smooth transition into mid and full throttle. I can cruise at high speed, no problem. Just a few seconds of full throttle it happened like I described. I can’t remember if it had done something like this previously before the trans let go. That was a few years back and I’ve just low gear since then until the rebuild. Trans did not pop out of gear. I left off the throttle and then gave it half throttle and it carried on just fine.

Any ideas? Lower motor mount? Though I think lower mount would cause it pop out of gear or knock bad as soon as you punch the throttle and it torques. I dunno...
 

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Discussion Starter #2
FYI I’m jetted at 42 pilots, #4 shim under each needle, mains are 152f and 155r. I went heavily through the carb when I had it apart and it’s very clean. Slides are polished, slide springs clipped 2 coils.
 

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I would think that 152,155 Keihin would be a little lean with crossovers. Is that what you were running before?
 

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Discussion Starter #5
I would think that 152,155 Keihin would be a little lean with crossovers. Is that what you were running before?
To clarify, I was running 152/155 prior to the rebuild. That was stock airbox with Napa hose mod. When I got the rebuilt engine installed and did the 12 hole mod, I started at 5/64 drilled jets and ended up at 3/32.

Now that I’m back to stock airbox with Napa hose, I’m back down to 152/155 as it was before the rebuild.

Rereading some really old threads, seems that lots of folks were in the 140s with crossovers, piped, and filter. I’ll just have to run back through some jetting to be sure it’s right.

My main concern is the transmission. If I’m holding the throttle down for a good 4-5 seconds, there was this loud smack/knock sound and I could feel the acceleration stumble. I let off the gas at that time. It never came out of gear or made any grinding sound.
 

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I'm not sure about the noise.
The jetting should be the same as it was before. 140's with crossovers is lean.
 

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Discussion Starter #7
I'm not sure about the noise.
The jetting should be the same as it was before. 140's with crossovers is lean.
Yeah it’s very difficult to describe trying to type it out. I would think if it were lean I would get a bog or stumble as soon as the throttle is pressed all the way. I checked the floats when I had it apart but I’ll recheck them again. If it’s tune related, maybe the floats are set low and running the level down too quickly.
 

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A lean stumble when hitting the throttle at lower speeds is not the main jets.
 

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Discussion Starter #9
A lean stumble when hitting the throttle at lower speeds is not the main jets.
I think you misunderstood or I didn’t write it out clearly. There is no lean bog anywhere. It is what I’m hoping is a stumble when the throttle is held fully for several seconds.
 
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