1.5 inch SU carb query

Post by **maxwoods** » Thu Nov 25, 2004 8:54 pm

Can anybody fill me in a little.
I have a 1.5 SU (possibly one of a pair from a Frogeye). There is a tiny pin hole immediately next to the butterfly valve (air filter side), with a threaded access from underneath. Is this the vacuum advance access?? There is not a small pipe protruding for a push on vacuum tube ( as on 1.25 SU). If it is the case, does anybody know where I can obtain the threaded insert to enable me to complete the conversion.

There is also a tapping on the top of the inlet manifold, although I assume this is for a brake servo access. Could this replace the previously mentioned pinhole??

Any advice gratefully received

max

Post by **rayofleamington** » Thu Nov 25, 2004 9:52 pm

As far as I remember, the Vac advance take-off will be on the manifold side of the butterfly, not the air filter side.
The tapping on the top of the manifold will be for the brake servo, and if you are not using it, it needs to be carefully plugged to avoid air leaks.

Post by **Stig** » Fri Nov 26, 2004 8:10 am

If it's 1.5 inch then it's not from a Frogeye, they only had 1.125 inch, maybe it's from an MGB? Anyway, if it is one of a pair then there's a 50/50 chance that it doesn't have a connection for the vacuum advance as only the front carb has that in twin carb set-ups (on a Midget/Sprite anyway). What side are the throttle&choke connections on? If they're the opposite of the standard Minor carb then you've got a front one, otherwise it's a rear one.
Any MGB owners out there to confirm this?

Post by **rayofleamington** » Fri Nov 26, 2004 9:18 am

As far as I remember, the Vac advance take-off will be on the manifold side of the butterfly, not the air filter side.

Actually - I'm not so sure onm that..

If you have a moggy carb to compare to then you could check that yourself.

Post by **maxwoods** » Fri Nov 26, 2004 9:36 am

Thanks for your replies .
Okay have looked at carb again. I bought it from a frogeye speciialist along with the manifold which all fits very well to the 1098 head.Perhaps it came off a midget or a sprite - not too important The carb appears conventionally set up, i.e float chamber towards front of car when fitted, with throttle and choke operating levers on the rear side; these operating levers have extra linkage holes, which is what leads me to believe it may have been one of a pair. I have blocked the manifold tapping with a removable set screw, but still have the vacuum advance dilemna. The tiny hole i spoke of is very close to the butterfly. With the idle tickover adjusted to give a small butterfly opening the pin hole is just visible on the manifold side. This suggests it could be a vacuum tapping. If not, can the manifold tapping be used for vacuum advance access instead?

Post by **Stig** » Fri Nov 26, 2004 10:22 am

maxwoods wrote:The tiny hole i spoke of is very close to the butterfly. With the idle tickover adjusted to give a small butterfly opening the pin hole is just visible on the manifold side. This suggests it could be a vacuum tapping. If not, can the manifold tapping be used for vacuum advance access instead?

I don't think the manifold tapping would work well for the vacuum advance as the pressure wouldn't so change much with throttle opening. The tiny hole by the butterfly sounds about right, but I'd want to check with a standard carb to be sure. Do you have the original carb for comparison?

Post by **Cam** » Fri Nov 26, 2004 11:07 am

Anyway, if it is one of a pair then there's a 50/50 chance that it doesn't have a connection for the vacuum advance as only the front carb has that in twin carb set-ups (on a Midget/Sprite anyway). What side are the throttle&choke connections on? If they're the opposite of the standard Minor carb then you've got a front one, otherwise it's a rear one.
Any MGB owners out there to confirm this?

Right well....... on my MGB (1978) the rear carb has a vac take-off and the front doesn't.

The front carb has the float bowl at the front and the linkage for throttle and choke at the rear, which sounds similar to what you have.

Post by **Kevin** » Fri Nov 26, 2004 12:09 pm

As it came from a specialist could you not ask them.

Post by **maxwoods** » Fri Nov 26, 2004 4:20 pm

Have removed std SU from car - vacuum access is a small pinhole corresponding to the same position on the 1.5, which answers my question. Just need to find the threaded insert and a suitable piece of tube to join to the steel pipe to the dizzy.

Thanks everybody for your input

Max