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Rebuilding SU carbs

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eejay56

eejay56

Jedi Warrior
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I am getting a set of SU HS4s for my 78 from EBay. Used, as is; I'm assuming they will need a complete refurbish. How hard is the rebuild and are there any gotchas? Is there anything special I should look for as far as wear/damage? I've got the Haynes Weber manual which has a section on each SU as well as the Zenith carbs.
 
First thing to check would be throttle shaft wear in the carb body. If it's bad, you can have them bored out and new bushings installed. Yes, you can do that effort yourself but it's not within my capabilities.
 
IMO, odds are they'll be pretty good. Clean them up, install the kit (including needles & jets), some adjustments, and good to go.

But, check that the pistons move absolutely freely in the domes (with the damper removed). You should not be able to feel or hear any rubbing at all. The domes are kind of fragile and can easily be dented; any rubbing at all will upset the mixture.

Also look for any sideplay in the throttle shaft where it goes through the carb body. Unlike the ZS, the SUs have no seal here (at least mine don't) and rely on a snug fit to prevent air leaks. If they leak, it will affect fuel mileage (but takes some serious wear to cause other problems).

New mixture needles and jets aren't a requirement usually, but it's very hard to judge their condition, so the safest course with unknown carbs is to replace them. It's also a good way to be sure you have the right needles, since there are many different ones available.
 
:iagree:

And be careful not to swap the pistons with the domes, as they are/have become matched for optimum performance.

Mickey
 
TR3driver said:
New mixture needles and jets aren't a requirement usually, but it's very hard to judge their condition, so the safest course with unknown carbs is to replace them. It's also a good way to be sure you have the right needles, since there are many different ones available.
Click to expand...

Does anyone know where to get a "minor" rebuild kit for H6 carbs -- one without jets and needles? I haven't seen any in the catalogs...
 
On-sale right now (through today 2/29 ) at VB (victoriabritish.com) p/n 3-852 $31.95 each.
 
Moss sells it as a "gasket set", 698-010. If you want to add a float valve, buy it separately.

TRF used to carry the aftermarket gasket sets as well (which ISTR included the float valve), but I can't seem to find the number at the moment. The genuine SU sets are SUAUE2 for $37.50
 
TR3driver said:
Moss sells it as a "gasket set", 698-010. If you want to add a float valve, buy it separately.
Click to expand...

Now, THAT's what I'm talkin' about! Thanks!
I rebuilt these carbs only about 3000 miles ago -- which also happens to be about 24 years ago. I'm SURE they need a good cleaning and gasket replacement. I'm also sure that the condition of the jets and needles is "near new." My recollection is that stripping and cleaning should be done much more often than jet/needle replacement needs to be done -- and I hadn't seen the gasket kits anywhere, without the expensive parts.
 
Moseso said:
My recollection is that stripping and cleaning should be done much more often than jet/needle replacement needs to be done
Click to expand...
What I found was that the jet and bowl seals started leaking well before anything else needed attention. Switching to O-rings solved the jet problem ... and I started buying just the bowl seals every 2 years or so.
 
TR3driver said:
What I found was that the jet and bowl seals started leaking well before anything else needed attention. Switching to O-rings solved the jet problem ... and I started buying just the bowl seals every 2 years or so.
Click to expand...

Are you talking about the square seal between the flexible jet tube and the bowl?
 
dklawson said:
TR3driver said:
What I found was that the jet and bowl seals started leaking well before anything else needed attention. Switching to O-rings solved the jet problem ... and I started buying just the bowl seals every 2 years or so.
Click to expand...

Are you talking about the square seal between the flexible jet tube and the bowl?
Click to expand...
Sorry; should have been more specific since we're having a bit of thread drift here. The comment about jet and bowl seals applies to the SU H6 carbs that Moseso was asking about, not the HS series that were the original topic. The H-series is an older design where the fuel actually flows through the bowl attachment (which is sealed with a pair of rubber top hat bushings that also support the weight of the bowl), then through an outer fixed housing that surrounds the movable jet. The jet is sealed to the housing with a pair of cork rings that wear every time the choke is operated. When the lower seal leaks, the fuel drips out onto the (hot) exhaust manifold; leaks at the upper seal allow the mixture to go rich.

The HS design eliminated both of these problem areas with the flexible tube from bowl to jet.
 
TR3driver said:
Moseso said:
My recollection is that stripping and cleaning should be done much more often than jet/needle replacement needs to be done
Click to expand...
What I found was that the jet and bowl seals started leaking well before anything else needed attention. Switching to O-rings solved the jet problem ... and I started buying just the bowl seals every 2 years or so.
Click to expand...

So... You use some kind of o-ring in place of the cork jet seal? Have you got a specification I could use to try to find these?
 
Moseso said:
So... You use some kind of o-ring in place of the cork jet seal? Have you got a specification I could use to try to find these?
Click to expand...
What I used were from Moss, P/N 365-420. I believe they are just ordinary Buna-N O-rings, but I don't know that for certain.
 
Those rubber high hat seals are also a sore spot. If you overtighten them, they will very slowly drip gasoline on your garage floor.

BTW, Moss lists that rubber o-rings as a modern replacement for the cork seals.
 
OK, I got the carbs and it appears they are in pretty good shape (for 61$ for a pair). They were sooty and one jet was melted but I think they will clean up OK. The throttle shafts do seem pretty sloppy though. Any insights into the rebushing process? I have a friend who may be able to help me with it. Equipment needed? Where do I get the bushings? Next question; How do I know what rebuild kit to order? BPNW lists several for different MG models. I presume they have different jets and needles. How do I know which one is going to be right for my Spit? Rimmer Bros has one but it's really pricey. The tags on the float bowls say UD465F and UD465R, front and rear right?
 
Look into Joe Curto! He's got oversize shafts. Instead of rebushing it only requires a careful re-reaming. Just a thought.

www.joecurto.com


All genuine SU bits, he's THE experienced SU go-to guy.
 
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