Carburator Size ?

Posted: Thu May 21, 2020 12:42 am

Hello All
i do have an dead VB56A carburator from a CB1100 mounted on my CB900F 1981...To many time up and down, changing this, trying this... yes air pod are mounted as well and they are a pain, but got to move forward and no more air box..
Either way, it is dead, so i decide to scrap it and replace it with either an CR or an Mikuni, i believe after reading both carb are good quality.
so now if anybody knows what size of CR carb i need for a CB900F is is an CR33 or the CR35? on the web site it is mention that it works for both model!!
and about a Mikuni RS, will it be the RS34-D21-K, the RS36-D3-K or the RS38-D19-K ?
i mean i measure the engine side carb inside measurement and it is 37 mm, but again they are from a CB1100 or else..
I welcome all feedback.
thank you all..

Posted: Thu May 21, 2020 2:13 am

On an otherwise stock 900 I'd recommend a CR31. Or CR33 if you're going to be hotrodding it. With flat slides you can go a size larger.

Posted: Thu May 21, 2020 3:35 am

Ok thanks for the info, so an CR 31 will fit my 1981 CB900F if not with Mikuni the RS 34 should do it, as long as the rack are adjustable correct?

Posted: Thu May 21, 2020 4:01 am

The CRs have adjustable spacing, the RSs don't, and they aren't spaced correctly for our motors, but these guys make a kit:

https://www.mikunioz.com/shop/re-spacing-kit-for-mikuni-rs-34-carb-honda-cb750-cb900-cb1100/?v=7516fd43adaa

Posted: Thu May 21, 2020 4:23 am

RickP wrote:

Ok thanks for the info, so an CR 31 will fit my 1981 CB900F if not with Mikuni the RS 34 should do it, as long as the rack are adjustable correct?

I have CR31s on my Stock (other than 4:1 Pipe) CB900F.

They run rich (at crusing and eventually foul the plugs) even with correct jetting so, the fix is to add CBR Coils and a Coil Relay Mod. This way you can get a stronger spark. I even gapped my plugs according to CBR specs. Rich condition is gone.

They look really good too...

Dave

Posted: Thu May 21, 2020 4:57 am

Yep, still love that bike Dave Wink

And I agree, using better carbs necessitates upgrading the ignition system at least with coils and a relay.

Posted: Thu May 21, 2020 8:07 am

I have an 1123 running CR33s, and it's awesome! On a stock 900, 31s. The ignition system will be the next weak point to change.

Happy spannering !

Simon

CB1100F Joined: Jun 18, 2006 Posts: 2447 Location: UK

RickP wrote:

Hello All
i do have an dead VB56A carburator from a CB1100 mounted on my CB900F 1981...To many time up and down, changing this, trying this... yes air pod are mounted as well and they are a pain, but got to move forward and no more air box..
Either way, it is dead, so i decide to scrap it and replace it with either an CR or an Mikuni, i believe after reading both carb are good quality.
so now if anybody knows what size of CR carb i need for a CB900F is is an CR33 or the CR35? on the web site it is mention that it works for both model!!
and about a Mikuni RS, will it be the RS34-D21-K, the RS36-D3-K or the RS38-D19-K ?
i mean i measure the engine side carb inside measurement and it is 37 mm, but again they are from a CB1100 or else..
I welcome all feedback.
thank you all..

The stock carb size is measured near the needle in the venturi not where if fits to the engine

Posted: Thu May 21, 2020 10:17 am

genesound wrote:

Yep, still love that bike Dave Wink

And I agree, using better carbs necessitates upgrading the ignition system at least with coils and a relay.

Yup, Black Beauty in all it's splendor.

Dave, what boots and spacers are you using with the CRs.

Posted: Thu May 21, 2020 3:50 pm

smurdoch wrote:

genesound wrote:

Yep, still love that bike Dave Wink

And I agree, using better carbs necessitates upgrading the ignition system at least with coils and a relay.

Yup, Black Beauty in all it's splendor.

Dave, what boots and spacers are you using with the CRs.

Thank you. I still love it too. LOL

Boots are stock and spacers came with the Carbs from Dynoman (carbs were already jetted and spaced for my bike right out of the box).

Dave

Posted: Thu May 21, 2020 6:47 pm

Beautiful bike Dave, wow..
question with the CR i guess you had to change as well the push/pull throttle assy no? so basically change the left command..

Posted: Sat May 23, 2020 9:01 am

RickP wrote:

Beautiful bike Dave, wow..
question with the CR i guess you had to change as well the push/pull throttle assy no? so basically change the left command..

Rick,

I didn't have to change anything. These are Plug-n-Play. But....... need to improve the spark with the mods I mentioned.

Dave