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Old 07-03-2013, 01:47 PM   #1
zephyr125 OP
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Carbs running lean at top end

I just did a very basic tear down and rebuild of my carbs. After balancing and setting the idle we are still having an issue with the midrange to top end. When just normal, light cruising the bike feels great but once you give it anything more than half throttle (and over 4k rpm) the carbs feels like they are leaning out and starts choking out. There is very little top end power.
Any ideas on what could cause the carbs to run lean once it hits the midrange power?

The bike is a R65LS with 32mm Bings.
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Old 07-03-2013, 02:37 PM   #2
SOLO LOBO
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Did it run fine before you made changes, or has it always done this?

What size are the jets in the carb?

Did you remove and reinstall the jet needles ? (get them in the wrong notch?)

How is the airfilter?

how are the diaphrams? any small tears could cause this...

how is the compression?
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Old 07-03-2013, 02:40 PM   #3
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Did you mess with the floats?
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Old 07-03-2013, 02:48 PM   #4
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Quote:
Originally Posted by bmweuro View Post
Did you mess with the floats?
I agree, I think that your float heights are too low
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Old 07-03-2013, 05:19 PM   #5
wmax351
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Quote:
Originally Posted by bmweuro View Post
Did you mess with the floats?
I was helping him with this, and suggested he post here.

We replaced the floats, and set them to stock height, starting to flow when they are parallel to the carb. Pulling the bowls while the petcock is off leaves the fuel level a bit less than a centimeter or so from the top(I didn't measure, just checked). I suspected that they were the issue, but with the float needles seating with a parallel float, I didn't investigate further.

As to the Needles: I raised (richened) them one notch when we worked on this.

Air Filter: fairly new. Replaced when the bike was bought. Very few miles since then.

Diaphragms: Replaced at the same time. Were installed backwards/not lined up by OP. I corrected them, without improvement.

As to having done this before: didn't run before he bought it. We got it going, but with leaking carbs due to soaked floats. Got worse as floats got wetter. OP replaced the floats and cleaned the carb thoroughly (pine-sol, with a final wash of carb cleaner).

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Old 07-03-2013, 05:22 PM   #6
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Quote:
Originally Posted by zephyr125 View Post
I just did a very basic tear down and rebuild of my carbs. After balancing and setting the idle we are still having an issue with the midrange to top end. When just normal, light cruising the bike feels great but once you give it anything more than half throttle (and over 4k rpm) the carbs feels like they are leaning out and starts choking out. There is very little top end power.
Any ideas on what could cause the carbs to run lean once it hits the midrange power?

The bike is a R65LS with 32mm Bings.
You are transitioning from the needles up onto the mains. Check those circuits.

Check diaphram seating, cut jet o-rings, incorrect needle position, leaking air tubes to the head, etc., etc., etc., etc., etc..
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Old 07-03-2013, 05:39 PM   #7
disston
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I think your floats are too high. Floats are supposed to turn fuel off when parallel, not "just start flowing."

Set needles to stock position. You are not allowed to change this unless you've been here longer than Plaka. Setting the height of the jet needles can be tricky. It helps to wear some kind of rubber glove so you can feel them click into position.
(just thought, you may have the other system of holding the needles. I don't know about it. Just the same set the needles to the stock position.)

Vacuum leaks around the rubber sleeve to the cylinder head? Spray with something to see if idle changes.
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Old 07-03-2013, 06:53 PM   #8
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besides checking all of above ... be sure to check flow of fuel delivered to carbs. partially blocked gas line screen can cause described problems.

my R90S acts like it runs out of fuel under hard acceleration with one petcock open. problem disappears with both petcocks open. a good cleaning of fuel petcock screens is in order.
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Old 07-04-2013, 11:15 AM   #9
zephyr125 OP
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Thanks guys, I'll start checking some of these things today.
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Old 07-05-2013, 04:11 AM   #10
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Diaphragms came immediately to mind. Did you reseat them correctly?
Look for small holes or splits.
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Old 07-06-2013, 01:36 AM   #11
wmax351
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Originally Posted by boxerboy81 View Post
Diaphragms came immediately to mind. Did you reseat them correctly?
Look for small holes or splits.
Checked the diaphragms. They are good. Reseated them correctly.

Looks like it was one of the carbs missing a main jet. He must have misplaced it during the rebuild, or it ran well from the overflowing float bowls.

He ordered a pair of new 148's. The existing one was a 145. Stock is 148.
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Old 07-06-2013, 04:01 AM   #12
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Quote:
Originally Posted by zephyr125 View Post
I just did a very basic tear down and rebuild of my carbs. After balancing and setting the idle we are still having an issue with the midrange to top end. When just normal, light cruising the bike feels great but once you give it anything more than half throttle (and over 4k rpm) the carbs feels like they are leaning out and starts choking out. There is very little top end power.
Any ideas on what could cause the carbs to run lean once it hits the midrange power?

The bike is a R65LS with 32mm Bings.
Quote:
Originally Posted by wmax351 View Post
Checked the diaphragms. They are good. Reseated them correctly.

Looks like it was one of the carbs missing a main jet. He must have misplaced it during the rebuild, or it ran well from the overflowing float bowls.

He ordered a pair of new 148's. The existing one was a 145. Stock is 148.
main jet missing explains why bike would run ok for lower rpm, then engine fall flat with what OP describe as running lean/choking out. bike is running super rich at higher RPM due to main jet missing.

picture below shows correct sequence for main jet assembly. note main jet's function is to limit fuel flow when piston in fully raised position. at which point needle offers almost no restriction to fuel flow with main jet regulating flow.

note second diagram show piston partially up with needle taper to needle jet controls fuel delivery rate, not main jet.






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Old 07-06-2013, 04:40 AM   #13
ME 109
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Quote:
Originally Posted by disston View Post
Floats are supposed to turn fuel off when parallel, not "just start flowing."
Aren't 'turn fuel off' and 'just start flowing' the same point in the universe?

I think the bowl fuel level is too high as well, disston.
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Old 07-06-2013, 09:18 AM   #14
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high floats equal too rich How do you know its lean ? Is it popping or getting too hot ? What color are your headers ? Dark blue equals lean ...Dont we want lean at top end thats where the power is ??? right ? It might be fuel starving try running both petcock open for high speed ops Its a r65 how much power do they have wfo anyways ?
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Old 07-06-2013, 09:37 AM   #15
Bill Harris
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Quote:
Originally Posted by wmax351 View Post
Checked the diaphragms. They are good. Reseated them correctly.

Looks like it was one of the carbs missing a main jet. He must have misplaced it during the rebuild, or it ran well from the overflowing float bowls.

He ordered a pair of new 148's. The existing one was a 145. Stock is 148.
Am I reading this right? Zephyr (the OP) left out a Main jet?

Report back when all the parts are replaced...
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