Vacuum position

luke6040 · Post by **luke6040** » Mon Jan 28, 2008 9:55 pm

I have read that the vacuum cant be taken from the base of the carb , as it is on my mini.
Is this true as i have also been told that it can go here

Thanks.

KLAS · Post by **KLAS** » Mon Jan 28, 2008 10:35 pm

that depends on what kind of carb you have. there are SU carbs with and without suitable vacuum take off

Gilesy998 · Post by **Gilesy998** » Mon Jan 28, 2008 10:48 pm

You're best to take the vac signal from somewhere well beyond the throttle butterfly. As I understand it, the SU's stock vac takeoff is actually partially blocked at part throttle, meaning the signal seen is a lie. Discussed here - http://www.minifinity.com/index.php?nam ... ic&t=73009
I've added more than one takeoff to my inlet manifold tov allow testing of various sources.

luke6040 · Post by **luke6040** » Mon Jan 28, 2008 10:52 pm

is a su hs4 with the vacuum niple actually on the carb

alexander · Post by **alexander** » Mon Jan 28, 2008 10:55 pm

hi. i will make the assumption that you are referring to the vacuum advance takeoff?
if so, then it is usually not suitable. the reason is that most carburettors have a vacuum advance
takeoff that is covered by edge of the throttle plate when your foot is off the accelerator and
sees no vacuum in that position. when opened even slightly, the hole is uncovered and finds
itself on the inside of the throttle plate, and sees full vacuum. so, as the thottle plate closes,
vacuum gets higher and higher, then drops to nil as the plate closes, and that discontinuity is
difficult if not impossible to account for with an ignition map.

i cant say as a fact how many carburettors are configured that way, but i believe it is the norm.
either way, it is not hard to test for: just pull the vac advance hose from the distributor and put
your finger over the end of it at idle. is there any vacuum? if not, then you need to find somewhere
else from which to take vacuum.

fyi, this is done to make it easier to achieve stable idle.

regards
alexander.

KLAS · Post by **KLAS** » Tue Jan 29, 2008 12:15 pm

luke6040 wrote:is a su hs4 with the vacuum niple actually on the carb

there are two versions, one thats not usable, like alexanders says. this is with the nipple a bit closer to the dashpot. and then there is the version with the nipple closer to the flange, this sees "full vacuum" all the time.
the last one was more common with the later Minis over here.
sadly, i have sold every carb i owned so i can not make any pics.

Gilesy998 · Post by **Gilesy998** » Tue Jan 29, 2008 9:30 pm

This, on a HIF44, would be no good -

Likewise, on a HS4 -

luke6040 · Post by **luke6040** » Wed Jan 30, 2008 8:07 am

this has been buggin me now because i want to fit megajolt onto my road car and dont wanna mess about strippin manifolds off until i finish my new engine.
So would this solve the prob.
http://minispares.com/Product.aspx?pid=34395

Gilesy998 · Post by **Gilesy998** » Wed Jan 30, 2008 9:03 am

That's perfect. If you have the stock mainfold spacer, just bin it and fit that one in it's place. The difference in thickness will make no differenrce on a road car. If you've not got the spacer, just stick that one in, but make sure the carb won't foul the bulkhead - my HS4 fouled when fitted to an alloy inlet fannymould, so I dropped the stock spacer.

luke6040 · Post by **luke6040** » Wed Jan 30, 2008 12:09 pm

well i have the stock spacer so i can do a straight swop! Im very happy now lol, iv gotta get it off anyway as its leakin like hell from there anyway!
I think this new spacer is small so should also give me a lil more clearance

Thanks for help
Luke

KLAS · Post by **KLAS** » Wed Jan 30, 2008 3:34 pm

Gilesy998 wrote:This, on a HIF44, would be no good -

Likewise, on a HS4 -

i've been running HIF44 and HS4 successfully. thats why i said to check the carb first. there are 2 versions