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motorcycle carbs on 79 Spitfire

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mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Hello George,
I am only seeing 38 mm as the largest CV carbs on any Honda ( Keihin) it seems that Keihin does make 42 and 45 mm carbs for Harley upgrades. There are so many Honda salvage yards around the country that will sell you old Honda carbs for under $25.00, maybe a little more for big ones. I can't seem to find a carb used by Honda bigger than 38 so I might go with four carbs. Anyone know how big the SU carbs used on the Spits? Carb boots from a Kawasaki KZ750 will work as adapters from the 38mm carbs to the stock dual carb manifold. Now, To find a used dual carb manifold used.

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carChips Avatar
carChips Victor Harnish
Kelowna, BC, Canada   CAN
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"
HS4s are 1.5" or just about 38 mm.



'S all for now
Vic

tlcgt6 Avatar
tlcgt6 Tommy Cook
Colleyville, TX, USA   USA
1971 Triumph GT6 MkIII "Banana Hannah"
1972 Triumph GT6 "The T"
Mikuni's have an accelerator pump
I don't think that Keihin's have an accelerator pump.
I run HSR 42's on my 2 liter GT6 and HSR45's on my 2.5liter GT6.
the only adjustment that I ever have to make is change the needle by one notch when going from summer to winter and back.

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GarrettWendt Avatar
GarrettWendt Garrett Wendt
Victor, NY, USA   USA
I think you should take into consideration the 4 intake holes on the block are only 1.25" Diameter which is 31.75mm.

Does that matter?

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
yeah, but there are four of them, all wanting more, gotta give em what they want,,,,,more fuel! LOL

GarrettWendt Avatar
GarrettWendt Garrett Wendt
Victor, NY, USA   USA
Gotcha.

I had the opposite problem. My 4 mikuni 38s provided too much fuel...I was using a mechanical fuel pump...had a regulator toning it down...still ended up bringing it to a much more experience hot rod shop...they did everything they could, all four set as lean as they could go, he had to physically pinch the line to get it to run right...

I suppose the next steps would be to begin experimenting with different needles and jets...at that point...I just wanted a car that ran! hahaha.

After a LOOOOOOOOONG journey on that project, I went with Twin SUs. Sounds like you are way more experienced though. Wish you well!!

They DID look sweet. Here is a pick of the dream that never was...


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clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1594050 by RobTAR I have never heard of anyone using anything other than Mikuni bike carbs on any kind of car.

Kiehin, Mikuni, whatever you can get …
The stock Euro manifold used dual 1.5" SU carbs, so look for carbs in range of 36-40 mm.
Most folks find that fitting MC carbs that lack accelerator pumps yield flat spots and bogging from idle.
Easy enough to make flat adaptor plates, use dual cable throttle setup, then dial them in.
Ignition timing management can yield performance, but the stock distributors are basically 1950's tech.
So altering the centrifugal advance curve give a lot of bang for the buck.

I built a set of quad carb Spitfire manifolds, not so hard to do for a Spitfire, 4 tubes and two plates.
Rubber hoses from manifolds to carb, stainless hose clamps.

Some pix attached.


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TriumphFan Avatar
TriumphFan Silver Member Jim S
Cumming, GA, USA   USA
1967 Triumph GT6 MkI "Cantankerous"
FWIW there are more than a few of us in the MC World who eschew the pod filters for an airbox that feeds all four for the CV carbs. Not that you can't get pods to work rather the factory airbox works all the time. I wonder if some of the problems associated with Japanese carbs on LBCs are due to running the individual filters. For instance, on my twin café bike K&N type filters work like crap compared to individual foam filters which are still a worse choice than the stock airbox.



"If you didn't build it, it's not really yours"

GeorgeOhr Nonya Business
Yes, confused, USA   USA
You’ll be fine with two 38s. I’m running a single (2bbl) 40DCOE with only 28mm main venturis.

You gotta remember, redline on that motor was 10,500 and it was a 4 valve motor with way better VE than the stock Spit motor. You are also feeding two cylinders with one carb and only one cylinder at a time. If a 38 will support one well it should be able to feed two at different times too.

tlcgt6 Avatar
tlcgt6 Tommy Cook
Colleyville, TX, USA   USA
1971 Triumph GT6 MkIII "Banana Hannah"
1972 Triumph GT6 "The T"
the 123 distributor allows you to set the ignition curve electronically with a computer of even with a phone as you drive down the road.

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Thank you for your story, Do you still have the carbs? If so I might be interested in buying them from you.
Greg

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Garrett, What were those CV carbs off of? How did you mount them and what manifold? I just ordered my dual carb manifold yesterday to mount two 38 Keihin CV carbs. Also ordered a used 4-2-1 header, cant wait.
Greg

TriumphFan Avatar
TriumphFan Silver Member Jim S
Cumming, GA, USA   USA
1967 Triumph GT6 MkI "Cantankerous"
My 1979 XS650 came with two BS38s. Are you using two or four? Seems like two would not be enough...
Where did you find a manifold?



"If you didn't build it, it's not really yours"

mtnwings Avatar
mtnwings Greg B
Ellenville, NY, USA   USA
Hi, I found the manifold on ebay.
many Honda's used the 38 mm CV carbs, the GL series, both the 4 cyl and 6 cyl models. the VT 750, Magna V65, CX 650R XL600, CRF 450, XR 650. all can be found in salvage yards across the country.
I have been reading more than a few stories on this site and other sites about guys using CV carbs, mostly Keihin. with great success.

GarrettWendt Avatar
GarrettWendt Garrett Wendt
Victor, NY, USA   USA
In reply to a post by Greg B Thank you for your story, Do you still have the carbs? If so I might be interested in buying them from you.
Greg

In reply to a post by Greg B Garrett, What were those CV carbs off of? How did you mount them and what manifold? I just ordered my dual carb manifold yesterday to mount two 38 Keihin CV carbs. Also ordered a used 4-2-1 header, cant wait.
Greg

I sold everything on eBay to recoup for the SUs. They came from an early 80's Suzuki. I made my manifolds from scratch. It was a fun and educational project. I did the 4-2-1 header too.

Sounds like you are on a good path!

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