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Very Hot Topic (More than 25 Replies) Main jets? (Read 1143 times)
DadsR65
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Main jets?
05/06/18 at 04:22:42
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I have a recently restored 1980 R65.  I've spent the last couple of weeks (~250 miles) getting the carbs tuned and synced.  All is well except for a couple of issues:

1 - Doesn't like to be cold.  I think this may just be a "feature" of airheads.
2 - Drop-off in power at wide-open throttle.

Regarding #2:  it happens when I'm in higher gears, trying to accelerated up hill at high speed (i.e. when I need max throttle).  I twist the throttle, but power doesn't increase, and actually drops a little.  If I back off throttle to ~3/4, I feel a noticeable power increase.  I'm wondering if this is a main jet issue.  Background:  timing is set, points gap good, float level 24mm, carbs tuned and balanced, new plugs, new plug leads.

Has anyone had success with slightly bigger main jets?

Thanks!
  
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Barry
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Re: Main jets?
Reply #1 - 05/06/18 at 04:36:51
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DadsR65 wrote on 05/06/18 at 04:22:42:
Regarding #2:  it happens when I'm in higher gears, trying to accelerated up hill at high speed (i.e. when I need max throttle).  I twist the throttle, but power doesn't increase, and actually drops a little.  If I back off throttle to ~3/4, I feel a noticeable power increase.


While it would be unusual for the stock main jets to be too small, what you describe is a classic symptom that they are.  There seems to be two main jet sizes for an 80 model year R65 up to 1/80 was a 140 main jet and for the rest of the model year a 145
  

Barry Cheshire, England 79 R45
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DadsR65
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Re: Main jets?
Reply #2 - 05/06/18 at 06:13:39
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Barry wrote on 05/06/18 at 04:36:51:
DadsR65 wrote on 05/06/18 at 04:22:42:
Regarding #2:  it happens when I'm in higher gears, trying to accelerated up hill at high speed (i.e. when I need max throttle).  I twist the throttle, but power doesn't increase, and actually drops a little.  If I back off throttle to ~3/4, I feel a noticeable power increase.


While it would be unusual for the stock main jets to be too small, what you describe is a classic symptom that they are.  There seems to be two main jet sizes for an 80 model year R65 up to 1/80 was a 140 main jet and for the rest of the model year a 145


My current jets are 145.  I know they're OEM, since my Dad bought the bike new.  I certainly wouldn't expect to have a lean jet issue on a completely stock bike.  The designers are smarter than that, but the evidence sure feels like it's too lean at WOT.
  
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DadsR65
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Re: Main jets?
Reply #3 - 05/06/18 at 09:08:37
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Took a short ride this morning.  Max speed in 5th gear is ~60mph, which can't be right.  Low and mid-range power seem fine.
  
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Bob_Roller
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Re: Main jets?
Reply #4 - 05/06/18 at 10:15:48
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Have you checked the rubber diaphragms under the top covers on the carbs, sounds like they may have holes in them .
  

'81 R65
'82 R65 LS  
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!
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DadsR65
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Re: Main jets?
Reply #5 - 05/06/18 at 11:23:38
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Bob_Roller wrote on 05/06/18 at 10:15:48:
Have you checked the rubber diaphragms under the top covers on the carbs, sounds like they may have holes in them .


Good thought, but those are brand-new.  As are all the o-rings.  Other than max power, everything runs great.
  
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georgesgiralt
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Re: Main jets?
Reply #6 - 05/06/18 at 11:27:05
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Have you first checked the valve lash ? And checked that the advance mechanism is apt to go up to the "Z" mark at above 4000 RPM ?
If both statements are true, try increasing the size of your main jet 'till you have full power at WOT.
Just my 2¢
  
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wilcom
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Re: Main jets?
Reply #7 - 05/06/18 at 14:37:50
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needles in the prescribed notches? Both the same?
  

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Bob_Roller
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Re: Main jets?
Reply #8 - 05/06/18 at 14:47:10
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The diaphragms, are they Bing parts, or aftermarket pattern parts ??
  

'81 R65
'82 R65 LS  
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!
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DadsR65
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Re: Main jets?
Reply #9 - 05/06/18 at 18:19:22
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georgesgiralt wrote on 05/06/18 at 11:27:05:
Have you first checked the valve lash ? And checked that the advance mechanism is apt to go up to the "Z" mark at above 4000 RPM ?
If both statements are true, try increasing the size of your main jet 'till you have full power at WOT.
Just my 2¢


Reset valve clearances last week (this is a reasonably fresh rebuild).  0.006" intake, 0.011" exhaust (per Snowbum).  Oddly, my feeler gauge set doesn't have a 0.010".  Timing was set so S is at idle and Z above ~3500rpm.
  
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DadsR65
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Re: Main jets?
Reply #10 - 05/06/18 at 18:20:34
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wilcom wrote on 05/06/18 at 14:37:50:
needles in the prescribed notches? Both the same?


Both should be at 3rd from top, but the carb rebuild was a couple of months ago, so it's probably worth rechecking.  Good call.
  
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DadsR65
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Re: Main jets?
Reply #11 - 05/06/18 at 18:28:40
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Bob_Roller wrote on 05/06/18 at 14:47:10:
The diaphragms, are they Bing parts, or aftermarket pattern parts ??


Hmmmm...interesting question.  This one took a bit of digging, but here's what I used:

https://www.eubmw.com/collections/carburetor-kits/products/bing-carburetor-kit

I can't tell if it's OEM Bing or not.
  
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Bob_Roller
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Re: Main jets?
Reply #12 - 05/06/18 at 18:31:54
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Were the new diaphragms the same thickness as the old ones ??
There were some in the past, that were thicker and stiffer than the OEM Bing part .
  

'81 R65
'82 R65 LS  
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!
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DadsR65
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Re: Main jets?
Reply #13 - 05/06/18 at 18:32:12
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DadsR65 wrote on 05/06/18 at 18:19:22:
georgesgiralt wrote on 05/06/18 at 11:27:05:
Have you first checked the valve lash ? And checked that the advance mechanism is apt to go up to the "Z" mark at above 4000 RPM ?
If both statements are true, try increasing the size of your main jet 'till you have full power at WOT.
Just my 2¢


Reset valve clearances last week (this is a reasonably fresh rebuild).  0.006" intake, 0.011" exhaust (per Snowbum).  Oddly, my feeler gauge set doesn't have a 0.010".  Timing was set so S is at idle and Z above ~3500rpm.


Also, it runs great in lower gears all the way up to redline (or at least as close as I've gotten).  This makes me think timing, points, valves, etc. are probably OK.  Issue is isolated to nearly max throttle at high speed, which makes me think about a fuel issue.

Also, I did a fuel flow test.  Measured about 500mL in one minute.  Seems OK.
  
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DadsR65
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Re: Main jets?
Reply #14 - 05/06/18 at 18:35:17
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Bob_Roller wrote on 05/06/18 at 18:31:54:
Were the new diaphragms the same thickness as the old ones ??
There were some in the past, that were thicker and stiffer than the OEM Bing part .


The bike hadn't run in 25 years.  The old diaphragms were just black goo that I scraped off.

Diaphragms aren't expensive.  Might be worth it to just replace them and see.  It's always possible I accidentally damaged one during rebuild.
  
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