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SU carb question

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JPSmit

JPSmit

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OK if a pair of SU's makes it sing, can one just hum along?

I got an SU (HS4) to replace my ZS and last night was working on it. I have adapted the float and installed it on the manifold. however, four questions.

1. when I disconnected the fuel pipe from the float, I undid the nut and the hose just slid out. Is that all there is? there didn't seem to be a compression fitting, or anything - what keeps fuel from leaking out?

2. When I disconnected the fuel pipe, the other end just slid out of the bottom of the carb - is this correct?

3. At the top of the float there is two "fuel sized" hard lines - one is slightly longer. Which is the fuel line and what is the other for?

4. Is there a vacuum connection on an SU?

thanks all
 
There should be a very small compression ring, either copper or plastic. I forget which Frank said at his tech session.
 
JPSmit said:
1. when I disconnected the fuel pipe from the float, I undid the nut and the hose just slid out. Is that all there is? there didn't seem to be a compression fitting, or anything - what keeps fuel from leaking out?
Click to expand...

I assume you mean the float bowl connection to the jet. There is an oring and wsher that seals it. Sometimes they stay in the bowl when you pull the line out. If you don't have a rebuild kit I would recommend leaving it and trying to get it to reseal. If you go in there to pull out the oring you will likely tear it

JPSmit said:
2. When I disconnected the fuel pipe, the other end just slid out of the bottom of the carb - is this correct?
Click to expand...

Yes, this is how the choke mechanism works. The linkage at the bottom of the jet keeps it from falling out

JPSmit said:
3. At the top of the float there is two "fuel sized" hard lines - one is slightly longer. Which is the fuel line and what is the other for?
Click to expand...

Depending on the fuel bowl covers it is either fuel in and fuel out to a second carb or fuel in and overflow. If you blow into them with the bowl push up against the cover you tell which is which. If you cant flow through itand / or you blow and it comes out the other pipe then that is fuel in. If you blow and it comes out the other side of the cover then that is the overflow.

JPSmit said:
4. Is there a vacuum connection on an SU?
Click to expand...

It depends on the carb, some due and soem don't. Look for an 1/16"-1/8" barbed fitting or post a bunch of pics
 
jlaird said:
There should be a very small compression ring, either copper or plastic. I forget which Frank said at his tech session.
Click to expand...

Well, I tried to reconnect the hose and of course it didn't work and I went to get a new compression ring and of course my guy didn't have one. Looking at the ring, an O-ring should do right? any idea the size?
 
Frank said, don't loose this they are unobtaineum.
 
jlaird said:
Frank said, don't loose this they are unobtaineum.
Click to expand...

tragically, it was already too late - when all else fails, read the instructions. :wall:

I can measure the outside diameter of the tube to the needle but don't have a means to measure the inside diameter of the float. Anyone in a position to measure it?
 
JPSmit,
If I understand your question correctly and you’re talking about the rubber seal at the end of the flexible jet tube and where it attaches to the float chamber then…
Don't sweat the small stuff, easy fix; the o-ring that many rebuild kits use for this seal is a 2-008, it's listed as having 3/16” ID x 5/16” OD x 1/16” cross section. Between the 2-008 generic number and the physical size given, you will get you a good replacement. Tell whom ever your buying one from it’s going to be around fuel and that nitrile / buna based is best.
Pick up a few to have on hand.
The ID of this size o-ring is small enough to give you an installed stretch and increases the OD by a small amount so that it fits the ID of the area in the float chamber nicely. Your only trick here is that a small washer (again 3/16” ID x 5/16” OD x 1/32” thick) is normally used between the threaded sleeve nut and the o-ring so that when you tighten the sleeve nut it applies a wider flat surface to compress the o-ring to seal.
Possibly look at being able to use a number 8 SAE washer for this fitment. A #8 washer will be close to the 3/16” OD of the jet tube (clearance over a #8 screw) and the OD “might” be small enough to slip into the float chamber. Given the quality of imported washers so common “maybe not”
To double check the size of your counter bored hole in the float chamber, use a 5/16” drill. Use the shank end of the drill and see if it “slides” into this area. This will help define the size listed earlier being what your after. Did that help or did I confusingly ramble?
Sorry if I did….
Dug
 
confusing ramble of course :devilgrin:

seriously, thanks - exactly what I needed to know

:cheers:
 
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