Author Topic: carb rebuild  (Read 271 times)

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Offline James_Leach_TN

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carb rebuild
« on: May 31, 2018, 01:36:24 am »
I am in the process of rebuilding the carbs on my '91 C10.  Everything has been disassembled and cleaned.  Got the new bowl O-rings and needle valves from Murph.  Waiting on the O-rings for all of the fuel pipes that connect the carbs together.  The bike is a Cali model that has 130 main jets.  I have new 120 jets to go back in.  I recall many moons ago when I had a 49 state bike with the , I think, 125 jets, there was a bog in the throttle response just off idle. Never had that problem with the cali bike.  The cure back then was to shim the main jet needle.  Is there a better solution now.  If not, does anyone have a source for the shims.  It seems like SISFs two minute jet mod might be a better solution.  Yes/No?  Will be a few days before the rest of the O-rings arrive, so have some time to get things sorted. 

TIA

Jim Leach
Jim Leach
'91 ZG1000  still going strong
COG 1829  AMA Life Member

Offline Bob H

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Re: carb rebuild
« Reply #1 on: May 31, 2018, 01:54:02 am »
I consider it essential to mechanically clean all the tiny drilled passages in each of the brass carb pieces.  If you try to rely on solvents to do this, you will likely end up with holes that are lined with deposits making them undersized.  The end result will be inconsistent idle, and lean, erratic performance at low throttle openings.  The Concours carbs seem to be especially sensitive to this problem, and mechanical cleaning with jet needles is the only reliable solution I've found.

Use of non-ethanol fuel, addition of an ounce or so of TC-W3 oil to the gas tank at fill ups, and using fuel stabilizer when the bike sits over winter have been the only way Ive found to prevent the deposits that narrow the small jet passages. Without these steps, I could only go about a year without having problems with poor low throttle response.
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Offline Nosmo

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Re: carb rebuild
« Reply #2 on: May 31, 2018, 03:08:47 am »
Don't waste your time with any carb mods but SISF's "two minute mod", it will cure all the carb issues and she'll run like a Rolex.
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"What could possibly go wrong...click....go wrong...click...go wrong...click...go wrong..."

Offline m in sc

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Re: carb rebuild
« Reply #3 on: May 31, 2018, 05:34:23 pm »
main jets have -zero- to do with off idle. Needles do, somewhat, but more so do pilots. turn the air screws 1/2-3/4 richer. run the bigger mains. you'll be fine.   do NOT put anything harder than the brass thru the jets. you will scar them. this does affect flow. IF you use the correct carb cleaner, it will clean the jets. ie: berrymans, gunk, etc, soak them, then spray thru with carb cleaner.

the last bike i fixed up had 2 16mm mikunis that are 46 years old. this is what i did to clean mine, and the parts are super super tiny.

« Last Edit: May 31, 2018, 05:37:54 pm by m in sc »

Offline James_Leach_TN

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Re: carb rebuild
« Reply #4 on: May 31, 2018, 09:06:27 pm »
Thanks for the input.  The carb bodies were soaked overnight in Berrymans then dried with compressed air.  Just went back and verified all the passages are clear.  I am curious about the "air cut valves" on the #1 & 4 carbs.  I have read that they are supposed to prevent/reduce backfiring on deceleration.  Why are they only on two of the carbs?  Has anyone blocked the vacuum port to the diaphragm?  This isn't supposed to affect driveability, just eliminate a possible vacuum leak if the diaphragm fails.  Has anyone had any experience with these valves failing?


TIA
Jim Leach
'91 ZG1000  still going strong
COG 1829  AMA Life Member