Carb retrofit onto CB450??

[jammer415]

Just got a rack of these Mikuni VM32 carbs (from a 83 GS750)

Does anyone know what it will take to take two of them and install into a 72 CB450?

Do I need a custom intake manifold? Special throttle cables?

Thank you in advance for any insights!

 

[davidricci]

Someone wrote about 32 vs 30 for this bike on a thread favoring the 30’s. You should I think, determine first if the 32s are best before all that work. -D

 

[jammer415]

Ya good call! I’ve actually read up a ton on this upgrade and the consensus I’ve come across is that the 32’s are better for the 450 (while the 30’s are more ideal for the CB350).

Furthermore, all of the sites that sell the complete Mikuni kits for the 450, include the 32mm carbs.

This being a retrofit (not a kit), I’m concenered with making them actually fit and work.

 

[NoRiders]

I'm no expert but they look like the carbs on my CBX550. They are individual carbs with fuel feed link and common throttle and choke mechanism.

From what I can make out, it'd be reasonably simple to remove the two outer carbs, therby retaining the centre mounted throttle linkage. Not to sure how the choke is arranged on your set but, the link bars holding them together should just cut down. The fuel is fed into each carb by a link pipe, again, by using the centre two it'd be a simple matter of blocking off the common fuel transfer outlets.

 

[jammer415]

Hmm ok so if I use the center two carbs, I wouldn’t be able to use the other two on a different bike without some custom fabbing? Or another linkage?

 

[jammer415]

Also, why would I need the center throttle linkage if the 450 throttle cable spits off into two anyway? Couldn’t I just attach each end of the cable to each carb? The carbs would be sitting a little bit further apart than on this rack anyway when attached to the 450, I presume?

 

[XS750AU]

it'd be reasonably simple to remove the two outer carbs

I second this strategy - but the real issue is the pitch (distance) between carbs. It looks like the to center carbs are space wider than the outer to inner carb. Which spacing suits the engine intakes??
As for then having to make up a custom linkage for the remaining 2 carbs - you are going to have to do that anyway you proceed, as you only get one throttle linkage per bank of 4 carbies!!!!!!!
I think matching the pitch between carbies and intake is the most important issue. What have others done in the research you have done?

 

[irk miller]

The butterflies and all mechanical mechanisms are connected on each outer to inner carb. So, left side and right side are linked together. Basically, the throttle linkage in the center works the inner carbs and transfers to the outer carbs through the rods. IMO, there isn't enough to gain to go through all the work on this carb. As far as slide carbs go, I think a Mikuni BST would be better, like what they run on the DR350SE or BMW F650. The F650 is a pair, so you may get lucky on spacing, or be done at just having a custom intake made.

 

[LiamG6]

Those aren't VM32's?

 

[jammer415]

Well part of the reason I was excited about these Mikuni’s was that I was hoping I could use two on my 450 rider, and the other two on the 500T brat I’m building. Doesn’t sound like that’s realistic in terms of money/time/effort to make them work and perform well.

I found these from a 97 BMW F650... If the spacing is right, could I use the original 450 intake manifolds?

 

[irk miller]

Here’s what the spacing will be on the F650...




Sent from my iPhone using DO THE TON

 

[jammer415]

I like the Mikuni kit at DimeCity since it comes with those billet intake adapters, but I just honestly don’t have $450 to shell out for it.

Does spacing the carbs away from the engine block a little bit increase low end or top end power band?

 

[jammer415]

Here’s what the spacing will be on the F650...




Sent from my iPhone using DO THE TON

AWESOME! Thank you for posting that... Just need to measure my 450..

 

[irk miller]

There is inlet and exhaust tuning that would determine the length of the intakes, but those DCC intakes may be made that way to fit those carbs in your frame.

 

[teazer]

I must have missed something here. The 4 carbs are CV design and are not VM32s which are a slide design.

The butterfly in a CV design reduces airflow and compared to a similar size slide carb they flow around 85% of the slide type.

That's not to say that they could not be used but since they are linked as a group with a single pair of throttle cables, they are a PIA to make them fit on an older twin.

In general, long inlets favor lower peak torque. The DCC set do appear to be rather long. Not sure what they were trying to achieve with that set up. Just a thought, but have you looked at getting a pair of VM32 carbs and seeing if you can fit Mikuni bolt on manifolds to your cylinder head?

like this one:

 

[jammer415]

@Irk Ya those DCC billet manifolds definitely help fit the carb into the frame (per their website description), but also seemingly affect the power band.

@Teazer Yup that would be the ideal set up, piecing together yourself seems more cost efficient, but I’m not sure if just the Mikuni rubber intake manifolds alone (photo attached) will space the carb far away enough to fit past the lower bolts of the valve cover... even in that photo you posted, there appears to be an aluminum spacer between the engine block and the rubber intake manifold. Where to get that metal spacer?

 

[jammer415]

Ok well this other thread answers some questions...

http://www.dotheton.com/forum/index.php?topic=46846.0


Either way, still seems like finding a spacer would be much easier than grinding and rebuilding choke levers.

 

[teazer]

You can make them yourself with a drill, hacksaw and round file or even faster with a dremmel. Or buy these https://www.mikunioz.com/shop/mikuni-flange-spacer-7mm-wide42mm-boresuit-64-78mm-stud-centres/

There are others on the market that are tapered to lower or raise the carbs a touch, but grab a foot of 2" wide 8mm or 10mm thick aluminum and carve a pair out. You could also get your favorite machine shop to machine a pair for you but that's more expensive.

This is what the rest of us have been doing for decades to make things fit.

 

[jammer415]

Thanks a lot for the inspiration... Gonna have to just tackle it and make it myself.