Any thoughts on using an SU HS-2 carb on a 1/2 VW?

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BillM

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I have a pile of English carbs off of MG type cars and am thinking that they would be OK for the 1/2 VW engine.
Ken Rand (KR1 & KR2) originally used an HS-4 ( 1 1/2" throat) carb on his first KR1 and it did fine.
The HS-2 (1 1/4" throat) is good for 20 to 55 HP according to the tuners of the day (David Vizard).
I know that these carbs do not like to be "blown on" and that is why the real Mini had them behind the engine so these would need to be protected from direct wind but that should be the only issue.
Any thoughts?
BillM
 

Hot Wings

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I had a lot of trouble in cold weather with my SU's (ground bound) and ended up replacing the recommended fluid in the dampers with a small bit of ATF fluid. It's also possible that you might experience some leaning of the mixture during high "G" periods due to the mass of the slide. Other than that I had considered the same thing at one point but ended up with more modern Mikuni's for the 1/2 VW and saved the SU's for my Bug Eye.
 

BillM

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Are you on the Brit car forums? If so then I probably know you.
I have a '65 Midget and a '69 Sprite
Bill
 

Hot Wings

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Are you on the Brit car forums? If so then I probably know you.
I have a '65 Midget and a '69 Sprite
Bill
No, not on the Brit forums, planes are taking up too much of my time right now. When I get time for my MGB Gt's and Mini Mk1 I'll probably be poking around.
 

Olpat

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Hello bill, In the early 1960's I built a 1936 Harley @ 96 cu,in. in those days hotrod intake systems didn't exist. I used a 1 3/4 throat SU off of a Jag "6" 3 in one oil went into the damper so it would idle. A vacuum controlled variable venturi carburetor was just the ticket for making horsepower and it was immune to altitude! However, it had a disturbing tendency to become completely encased in ice on even a slightly humid day. Cheap thrills when it stuck wide open.
 

N8053H

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I had a lot of trouble in cold weather with my SU's (ground bound) and ended up replacing the recommended fluid in the dampers with a small bit of ATF fluid. It's also possible that you might experience some leaning of the mixture during high "G" periods due to the mass of the slide. Other than that I had considered the same thing at one point but ended up with more modern Mikuni's for the 1/2 VW and saved the SU's for my Bug Eye.

Is there a big improvement in horse power from using two Mikuni's over using one zenith carb? I am seeing about 3100 rpm at WOT using a zenith carb with a 54x24 prop. I have seen RPM's improve over sometime so maybe as she gets more hrs on her they will come up even more. When I first started flying her she was producing 2900 rpm's and today she is producing 3100 rpm's.
Tony S
 

Hot Wings

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Is there a big improvement in horse power from using two Mikuni's over using one zenith carb?
Tony S
Probably not. A 92X82 2 cylinder at 3100 rpm is only drawing 120 CFM. That's the equal of a stock 1600cc VW at 2100 RPM.
 

nheistand

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Nothing wrong with an SU carb, however the Mikuni carbs are so common and easy to work with that I like to just buy a used Mikkuni on ebay. A 32mm is good for a half VW or similar. Some are running two of them. These carbs are on many snowmobiles in the US.

Norm Heistand my .02
 

dcstrng

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I have a pile of English carbs ... The HS-2 (1 1/4" throat) is good for 20 to 55 HP according to the tuners of the day
BillM
There is a guy on the Sonerai forum who swears by HS-4s for his big VW and has convinced a couple of guys to convert their Posi and Zeniths to the SU… I’m not familiar with the SUs (Keihin CV40 is my favorite – too big for your application I’m sure), but am a fan of constant velocity carbs in general and single carbs in particular, so if this is a carb you’re familiar with I’d sure give it a whirl… You’re right, CVs need to use a steady ambient pressure – only successful “ram” I’ve seen is when the entire carb was housed in a pressurized plenum – seems like overkill for many fugal-flyer applications…
 
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