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Honda CB900 low rider

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krasochka
L Plate Warrior



Joined: 29 Oct 2023
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PostPosted: 03:40 - 01 Nov 2023    Post subject: Honda CB900 low rider Reply with quote

Hi all, I have a 1980 Honda cb900f running on K&Ns Air Filters, the carb is a standard Keihin VB51A , the Primary main jet is 68, and the secondary jet is 98. My problem is she is running to rich and i keep burning out plugs. I have been advised to change one or other or both jets to compensate for having no Air Box. Could anybody out there help me with this one and explain the best way of compensating for no Air Box...Much appreciated, thanks
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Last edited by krasochka on 08:56 - 22 Apr 2024; edited 1 time in total
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Nobby the Bastard
Harley Gaydar



Joined: 16 Aug 2013
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PostPosted: 08:32 - 01 Nov 2023    Post subject: Reply with quote

Put it on a dyno.
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Robby
Dirty Old Man



Joined: 16 May 2002
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PostPosted: 09:10 - 01 Nov 2023    Post subject: Reply with quote

I've seen a similar question asked every few months for the last 20 years. It's very difficult to get a bike like this to run right with no airbox. The only thing that seems to be more difficult then 80s Japanese 4 cylinder engines is 90s Japanese 2 cylinder engines.

There is no quick, easy and cheap solution. You can:

1. Get it on a dyno. Fiddle around with jets and needles, do another dyno pull. Keep testing and fiddling until you have a reasonable compromise.
2. Refit the air box. If this is impossible because it's been messed around, build and fit some kind of air box.
3. Lots of trial and error. Buy a collection of jets and get very good at removing the carbs to swap them out.

There is also secret option number 4 - fuel injection - but that isn't going to happen.
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A100man
World Chat Champion



Joined: 19 Aug 2013
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PostPosted: 12:33 - 01 Nov 2023    Post subject: Reply with quote

Robby wrote:
I've seen a similar question asked every few months for the last 20 years. It's very difficult to get a bike like this to run right with no airbox. The only thing that seems to be more difficult then 80s Japanese 4 cylinder engines is 90s Japanese 2 cylinder engines.

There is no quick, easy and cheap solution. You can:

1. Get it on a dyno. Fiddle around with jets and needles, do another dyno pull. Keep testing and fiddling until you have a reasonable compromise.
2. Refit the air box. If this is impossible because it's been messed around, build and fit some kind of air box.
3. Lots of trial and error. Buy a collection of jets and get very good at removing the carbs to swap them out.

There is also secret option number 4 - fuel injection - but that isn't going to happen.

Option 3a) join a Honda CB900 specific forum and hope that someone else has managed to overcome the same fuck-up as you have.

BTW 'She is running to rich and i keep burning out plugs' doesn't really make sense rich = sooty possibly fouled plugs.
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stinkwheel
Bovine Proctologist



Joined: 12 Jul 2004
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PostPosted: 12:47 - 01 Nov 2023    Post subject: Reply with quote

Option 5 is buy a stage 3 jet kit and fit it if there's one available for that model. You might have to get them from the US.

Dynojet list one but I'm not clear if it's a stage 1 or stage 3.
https://www.dynojet.com/jet-kit-for-1980-1982-honda-cb750-cb900-stage-1-3-1127/

6 Sigma also do them.

https://www.6sigmajetkit.com/honda-cb900f-jet-kit.html

If you can find a stage 3 dynojet, that would be the one to go for. It was the goto back in the day for all these bikes when you started fucking with the airboxes and exhausts. Just be warned, sometimes they contain very little. I had one that contained a drill bit and four springs with instructions where to drill a hole in the slide. Remember you're paying for the dyno time they put in, not the contents of the kit.
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slowasyoulike
Scooby Slapper



Joined: 17 May 2021
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PostPosted: 20:40 - 01 Nov 2023    Post subject: Reply with quote

Putting the air box back on seems by far the simplest solution to me.
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