Bike Carby's

[altezzaclub]

It looks like a Stromberg to me, similar to the SU but with a diaphram instead of a floating piston. With all the non-important bits cleared away it has the same layout as the SU.

 

You're right, the spring will give initial richening until the diaphram lifts the needle to match the airflow. SUs are actually amazingly complex in operation for such a machanically simple device. I suppose all variable jet carbs are, the mixture doesn't just depend on the needle & jet size, it depends on the velocity of air flowing over the venturi that sucks the fuel out. Even then, I'm not sure if its just velocity or if density is involved too...

 

I'll start a new topic!

[MRMOPARMAN]

Good point- how do bikes get around the lack of a pump jet? The SUs have the oil dashpot delay the piston rising so it immediately richens for a second or two when you boot it, but I haven't seen those on bike carbs.

 

yeah not sure tbh, just read other "bike carbs onto car motor" threads over the years which mentioned some don't come with accelerator pumps.

 

Surely max air flow should be-

max revs X 1500=12000 X 600.... so max revs=12000X600/1500 or about 4800rpm on your 5K will suck the same air flow as that Honda pulling 12000rpm. Those carbs could be a bit small.

 

daves calculations were based on a 4k, not a 5k. you would think the carbs would be too small, but remember, pretty much every bike head from the 60's/70's will be more efficient than a K head. like i said before, something around 34/36mm or 32ish mm in a flatslide or barrel would do the job nicely imho. i think most bike carbs this size were on 1L+ bikes.

[TOMMYS Ke 11]

Hi there if you have a look at Thomas thread we used kawazaki 900 quads on his 5k/4k engine build and got 105rhp at 6000 revs then showed signs of leaning out so we need to look at his jetting and have a play as there is still a bit more power to gain yet :yes: but i must say the induction sound is so sweet. Anayway good luck with the build Jono i look forward to here how you go.

 

Cheers:- Stuart.

[altezzaclub]

then showed signs of leaning out

 

if they're running needles then re-shaping them is easy with some #400 wet 'n dry sandpaper. You can leave the lower stations as they are and just enrich the top end. I think that's the big advantage of needle carbs, while normal fixed-jet carbs are a lot less adjustable. That's why Webers are so popular in race cars, there are quite a few ways to change mixture over the rev range with their 5 different jet setups.

[KE25 Punk]

The way i looked at it was keep it simple. What is the cross sectional area of one of you cylinder ports? That is the size carby chokes or barrells in bike carbys case. 4a small port is 38 so 38 mm mikunis work well or 45 webers with 38 mm chokes. Getting slightly more technical you could base it off the cross sectional area of the inet valves and add a few % for ram effect.