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Old 03-25-2012, 10:42 AM   #1
Buffel OP
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Dunstalls on my r90/6

I just installed dunstall replica pipes on my r90/6, but now it stumbles more than often, not in a certain rpm range, except at idle. Why does it do this? do i need bigger jets because i have an aftermarket exhaust? any help is welcome
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Old 03-25-2012, 11:14 AM   #2
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this mod on many a bike causes a drop in the lower mid-range, a hole in the torque curve.

i put slip-ons on a r100rt and the same thing happened but not too bad, in 1985.

you should state what kind of carbs are on the bike... bings?

with mikuni carbs the slide cut-away can be increased iirc.

do a google on this yet?
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Old 03-25-2012, 11:21 AM   #3
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yes googled it, couldnt find that much info:/ but the rest of my bike is stock
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Old 03-25-2012, 12:32 PM   #4
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Quote:
Originally Posted by Buffel View Post
I just installed dunstall replica pipes on my r90/6, but now it stumbles more than often, not in a certain rpm range, except at idle. Why does it do this? do i need bigger jets because i have an aftermarket exhaust? any help is welcome

from this post it was already having some running issues and the dunstalls just aggravated them.

the bing cv 32mm carbs in good shape? not leaking air at the manifolds, are the diaphrams OK, are they balanced? whole bunch of stuff...
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Old 03-25-2012, 10:27 PM   #5
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Ill do some work on it this weekend, valve clearance, ignition points, balancing cabs, hope this will solve it
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Old 03-25-2012, 10:35 PM   #6
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What needle and main jets are you running? The main is simple and cheap to swap one size bigger.
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Old 03-26-2012, 03:44 AM   #7
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All stock 150 main
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Old 03-26-2012, 06:08 AM   #8
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All stock 150 main
Try 155, see what happens. They're only a few $.
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Old 03-26-2012, 07:16 AM   #9
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If the stumble is just off idle it has something to do with the idle or less than 1/4 throttle circuit. Could be as simple as the idle air adjusting screw not being set properly, or one of the small holes in this circuit being partially plugged.

With carbs you have to figure out where in the throttle range the problem is and then deal with that part of the circuit that controls that part of the throttle.
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Old 03-26-2012, 07:52 AM   #10
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Quote:
Originally Posted by cycleman2 View Post
If the stumble is just off idle it has something to do with the idle or less than 1/4 throttle circuit. Could be as simple as the idle air adjusting screw not being set properly, or one of the small holes in this circuit being partially plugged.

With carbs you have to figure out where in the throttle range the problem is and then deal with that part of the circuit that controls that part of the throttle.
Exactly! You need to focus on the throttle position when you get the stumble. Mess with your idle mixture first, but after you know everything else is set to spec. A lean idle will cause a stumble rolling on and possible popping on decel. Hopefully you won't be getting into different needle jets and main jets or combinations thereof.
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Old 03-26-2012, 10:27 AM   #11
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Freer flowing exhaust rarely calls for bigger mains although that is what they often get. It's cheap and easy! It usually robs hp and doesn't help the actual problem. Lose/lose. Freer flowing exhaust almost always actually leans out just off idle. With our Bings, that is mostly cured with richer needle jets. If it's got 266's, 268's normally do it. Sometimes it will take 270's and so on. Chances are you need smaller mains for more power and better economy but always remember that best power and engine efficiency are real close to best meltdown. That is exactly why so many aren't there! If it was easy it wouldn't be so much fun! Good luck!
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Old 03-27-2012, 12:43 AM   #12
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It does pop a little when i close the gas, so a bigger needle jet? Which One should i get? My carbs are all stock

Buffel screwed with this post 03-27-2012 at 02:59 AM
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Old 03-27-2012, 03:54 AM   #13
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Stock 90/6 needle jet is 268.
Would you blokes go a bigger needle jet or up the needle height first?
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Old 03-27-2012, 05:18 AM   #14
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Float level matters - Snowbums site has the information on how to set the actual depth in the bowl, and getting it right is important

The later carbs have a different, stiffer spring above the slides, so order them under the latter number

Try them , if it makes things too rich you will probably be able to drop the needle a notch and still be able to richen the mixture up in transition, where you need it.

I put a Dunstall Decibell on my 900 cc 75/7 with a 2 in 1, had a similar problem and the springs and dropping the needle cured it, so I am not farting in the dark --------.
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Old 03-27-2012, 06:14 AM   #15
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How do i change the float level? I found what the correct amount of petrol that should be in the bowl but dont actually know how to change it:/ and where is the needle in the carb is located? So many questions... Haha
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