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Discussion Starter #1
I've got an '07 (865cc) Bonne with AI removed, Predator pipes, airbox removed and UNI pods. I set the carbs with a 140 mains, one shim on the needles and 3 turns on the idle mixture. I also drilled out the holes in the slides from 2.5mm to 3mm.
During the test ride today after the airbox job, I noticed a serious flat spot in the acceleration around 65mph in high gear (stock 18-tooth sprocket). It also seems to surge a little at that speed, not holding steady at RPM. If I pull on a little choke, the surge pretty much goes away, but it will cough through the carbs now and then. Even with the choke, however, the flat spot remains. Granted, it's a bit cold out today, maybe high 40s, but the bike was at warm, even if I wasn't.
I'm assuming the carbs need some fine tuning here, but I'm unsure of what needs adjustment. Any advice would be greatly appreciated.
 

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You did not mention what size pilot jets you were using. The surge that you describe sounds like it could be idle mixture related. For your application, I suggest a #42 pilot and careful adjustment of the mixture. In my case, I bought the idle mixture thumbscrews. These allowed me to make small adjustments to the idle mixture without getting off the bike. It only took about five miles to get the idle tuning just right.
 

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Just went through this.

Started with 135, then 140, then 145, before that flat spot at 65 to 70mph went away.

go up on the mains--prob not the pilots or a/f.

Oh. And dyno'ing tomorrow to fine tune. Will let you know the results as you and I have the exact setup. I may be able to go up a couple points on the mains.

Jack
 

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Discussion Starter #4
I pretty much came to the same conclusion, Sito, my mainjets just aint big enough. At WOT it's acting like it's running superlean. If you're doing well at 145, I'll look up a pair of those and maybe some 148-150 as well. While I seek bigger jets, I put in a second shim and that helped the flatspot a good bit. Based on the "If two is good, three MUST be better" concept, I'll try a third shim as well.

As Sheepdog pointed out, the idle mixture also effects mixture at a steady RPM. Reading the Jenks info (like I should have been doing all along), I'm probably too lean at idle, so I may try going to 3.5 turns on the idle screws. Any more than that and I'll have to go to a 42 idle jet.
 

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Lemme know where you end up. Hopefully I'll be able to post my dynos monday or so. I think my mains be just slightly lean and might actually add a shim, but idle-1/4 throttle is just perfect--no surging at low rpms at all.

I did the "off-throttle" test at about 90mph and I detected no lean surge. So I'm thinking my mains are nearly there.

J
 

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It sounds as though sito is giving you some good advice. I still have my airbox, so the main jet was not a number I could speak to. Don't forget to delete the shims until you get the mains close. The (Thruxton) needles on your 865 are very slim and will meter your midrange best if they're as close as possible to your main jet. You may still need to shim them, but only after you dial the mains in...
 

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I respectfully suggest that you get the mains right first; 3 shims isn't the correct answer - that's only masking the real issue.

If you need larger mains I have 145's, 152's, & 155's available. Feel free to PM me.
 

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The dyno will tell what you need but 140 mains should be close.1 or 2 shims should be all you need .You might of messed up alittle drilling the slides,I dont think that helps that much and might cause troubles.going way rich on mains can fool you in cold weather you need to jet it to run at top rom first get mix around 13.0 to1 at max rpm(some bikes make the best hp at 12.8 to 1)after you get the main right play with the shims.you can check the pilots and screw settings on the dyno at very light throttle.Alot of times you can run the small pilot with the screws turned out more and get the same mix as the big pilot with the screws turned in more.If the screws have to be turned out to much with the little pilot to the point they may unscrew and fall out then you should use the big pilot.
 

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After my first 3 dyno runs we decided I needed to lose the shim--low rpms going to WOT showed extraordinary rich condition before the bike spooled up past 3k. Taking the shim out helped the problem by about 1/3d. Need to look at getting a longer needle, I think, unless you guys have another answer.
 

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Discussion Starter #10
It'ss getting there....

Thanks to all who've responded, I really appreciate the input.

Here's how it looks so far: I put in a #42 pilot jet with 2.0 turns on the mixture. I pulled the shims and went to a 147.5 mainjet. On the first run, it felt much stronger but topped out at 100mph and still felt a little thin at a steady 65mph in 5th. I put 2 shims on each needle and went to 2.5 turns on the mixture and WOW! Very strong through the gears, confident-feeling at steady speed and no flat spots anywhere in the RPM range. Starting at about 25mph in second, WOT got it to the rev limiter very smoothly and quickly in all but 5th gear, where it settled in at 110mph and wouldn't do no more. As y'all may have guessed by now, I have no tach. What is the absolute top end (rev limiter in 5th gear) using the stock 18-tooth sprocket?

I'm very happy with the results thus far and am considering going to a 150 mainjet in pursuit of the rev limiter in 5th. I understand that what I've set up here is a bit outside the norm (140 mainjets being just about right), but employing tuning tips from Jenks and others, it seems to be asking for more than a 147.5 mainjet. The only cause I can think of is those Predator pipes and the big-ass UNI pods I installed have the bike breathing REALLY well.

The nearest dyno is a couple hours away and run by someone I'd rather not deal with. Besides, unscientific though it may be, I like my elderly seat-of-the-pants method. So, whatchall think?
 

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Discussion Starter #11
Longer needle?

Sito, it may be a bit radical, but most Mikuni needles have adjustable height and a fairly wide selection of profiles. I don't see why they wouldn't fit in the Kehin slides... They're easily obtained from online outfits like bikebandit or even from a local bike scrapyard (Mikuni's were popular in alot of jap bikes). Might be fun to try.
 

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As a rule, I tend to tune air-cooled bikes a little on the rich side. Not so much that it won't carburet cleanly, but just enough to assure cool running. Therefore, I use Thruxton needles (no shims) on my '03 T100 (790cc) and a 125 main instead of a 122.5. My plugs are consistently a nice medium brown and my throttle response is great. The only clue that I'm a little rich is my fuel mileage, which is about 40-42 mpg. This is acceptable to me. I re-tuned to improve starting and throttle response anyway...not to achieve big hp gains (though it does seem a bit more responsive). Your machine seems like it is very close to ideal, but the 150s seem like they are worth investigating. Good tuning hinges on the proper selection of mains.
 

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I read some where if you have to go more then 2 turns out on your screws, you should go the next size up on pilot jets. Is this true?
 
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