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Discussion Starter · #1 ·
When attempting to make small modifications to a carburetor with primary and secondary main jets (like a CB/CL350), which jet should be changed for minor jetting modifications?
My understanding is the the primary main (the jet WITHOUT the needle) is present "to provide a smoother transition from low speed to intermediate systems." (from Honda's carburetor manual)
Does it depend on which RPM range suffers? If my plugs showed a lean condition at WOT, I would assume changing the secondary main (the jet WITH the needle) would be appropriate, but that will affect the mid, and to a much lesser extent, the low speed range as well, right?
If the motorcycle pulls poorly in the mid range but seems fine in the top end, would I change the primary main? Wouldn't this also affect the mixture at WOT?
If you suffered from an overall lean condition, except maybe at idle where you can dial in the mixture, would it be appropriate to only change the secondary main, or should both the primary and secondary be changed?
I would imagine that if you went up a size in both primary and secondary, you might risk over doing it...
Can anyone clarify this for me?
 

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Discussion Starter · #2 ·
Re: Rejetting carburetors with w/ primary and secondary main

It just occurred to me that, of course, the primary main jet feeds the slow jet (since the slow jet has a rubber plug that blocks it from picking up fuel directly from the float bowl, it has a passage from just above the primary main leading to the slow jet).
Would changing the primary main therefor enrichen the low speed/idle speed settings by virtue of the fact that more fuel is present?
I imagine it would... once I forgot to install the passage plug and the bike wouldn't idle properly. Too rich...
 

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Sensei
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Re: Rejetting carburetors with w/ primary and secondary main

Bryan..... NO... The slow jet size only allows so much flow through regardless of how fast the larger primary main can/would/could re-supply fuel to it.... That's the purpose of any restrictive orifice, (in this case a sized jet)... It WOULD be different IF the fuel was under signifigant pressure (an orifice flows based on/due to pressure differential on the two sides of the hole), but it's a negligable change when going up in main sizing in this (secondary atmospheric differential only) case....Without the plug, the slow jet is acted on by direct atmospheric... With it, it gets an already reduced atmospheric % due to the primary differential "moderating" it first....However, the differential between 68 and 70 mains isn't signifigant enough to radically alter the fuel passing the slow jet...
Technically, you are correct....It's just insignifigantly changed....
 

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Discussion Starter · #4 ·
Re: Rejetting carburetors with w/ primary and secondary main

OK. That makes sense.
Thanks for clearing that up.
What about my first post... Which main should one consider changing when rejetting and why?
Thanks always for your input, Steve.
 

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Sensei
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Re: Rejetting carburetors with w/ primary and secondary main

depends on where the rich or lean condition exists..... Essentially, once a "jet" kicks in it mitigates the previous jets flow, and the engine is running on THAT jetting alone (not actually, but effectively/realisticly) .... So change the jet that operates in the problem range...
 

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Discussion Starter · #6 ·
Re: Rejetting carburetors with w/ primary and secondary main

66Sprint said:
depends on where the rich or lean condition exists..... Essentially, once a "jet" kicks in it mitigates the previous jets flow, and the engine is running on THAT jetting alone (not actually, but effectively/realisticly) .... So change the jet that operates in the problem range...
As I suspected... Thank you.
 

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Sensei
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Re: Rejetting carburetors with w/ primary and secondary main

"As I suspected... Thank you."...

Your logic was impeccable..... ;)
 

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Discussion Starter · #8 ·
Re: Rejetting carburetors with w/ primary and secondary main

66Sprint said:
"As I suspected... Thank you."...

Your logic was impeccable..... ;)
No, really! I assumed each jet would mostly affect a given range, but how much it would change peripheral ranges... I wasn't sure...
Plus the carburetor manual's statement of the primary's existence "to provide a smoother transition from low speed to intermediate systems," was confusing. I wasn't sure if it meant then that the primary only came into play during a brief transition between the low speed orifice and the secondary main at which point the needle position (and therefore secondary jet size since the needle height can't be changed) were all that mattered.
But seriously, thanks for your help...
 

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Re: Rejetting carburetors with w/ primary and secondary main

Where did you get the carb manual? I have two different versions of the shop/service manual, but no carb manual... They both skip the carb section saying there is a separate carb manual for reference. I didn't see any mention of it in the manuals section on the boards.
 

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10,203 Posts
Re: Rejetting carburetors with w/ primary and secondary main

For all us CM400/450 and CB400/450 Hawk and Nighthawk folks here's a link to an info on the universe's leading expert on the Keihin CV carbs (he's not too shy to tell you so himself! ;) )

http://australiancx.asn.au/forum/index.php?topic=6063.0

He even sells a book on the subject (imagine that :shock: ) Might be interesting to have if one were having problems.
 

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Discussion Starter · #11 ·
Re: Rejetting carburetors with w/ primary and secondary main

brainlock said:
Where did you get the carb manual? I have two different versions of the shop/service manual, but no carb manual... They both skip the carb section saying there is a separate carb manual for reference. I didn't see any mention of it in the manuals section on the boards.
I'm the sucker who bought a scanned and printed copy because I couldn't find it to download anywhere.
I'll make it a project to scan this manual for other forum members (unless anyone knows of another place it can be downloaded).
Jason or Perry: If I scan this manual as a .pdf, is there a way it can be uploaded to the site permanently or will it have to be hosted somewhere else? (I don't have an in depth knowledge of web stuff... are these even the right terms?)
In my opinion, if members can understand the basics of carburetor function and everyone is on the same page, it would probably help quite a bit with troubleshooting, communication (when members need to ask specific questions), etc. What better textbook is there to have than the manual Honda mechanics likely had on their shelf at the dealership?
 

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Re: Rejetting carburetors with w/ primary and secondary main

You're also one of the few that has one... so I don't know how much of a sucker you are. I'll take it upon myself to incessantly nag you to scan it... for the good of the people of course :)
 

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Discussion Starter · #15 ·
Re: Rejetting carburetors with w/ primary and secondary main

That's the one.
 
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