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rebushing SU's
https://www.ausmini.com/forums/viewtopic.php?f=5&t=41775		 Page 1 of 2
Author:  simon k [ Tue May 06, 2008 10:59 pm ]
Post subject:  rebushing SU's
I want to re-bush (or actually bush in the first place) the butterfly shaft holes in my carbs - I use a mill etc. so the equipment isn't an issue, but the right way to go about isn't clear...

first off I figure making a plate that mounts the carb dead vertical, and turned 90*, so I can hold it under the mill, then index off the body (tips?) in 4 directions to centre over the hole, then bore the shaft hole (almost through to the carb bore , but not all the way obviously), then I think I'll use a self lubricating plastic (like lubron) and put a plug in the hole I've bored, then drill through that, and ream to 5/16

am I on the right track?

does anyone do it and can give me some pointers?

BTW I'm not going to send them off to someone to do - I like doing things myself
Author:  simon k [ Wed May 07, 2008 9:54 am ]
Post subject: 
thinking about it last night when I couldn't sleep.... I think I'd make the bush with the ID already close to 5/16, and a tool to fit it, then ream it.... drilling it in place wouldn't work

I went to the bearing place this morning and the bloke there reckons non-lubricated nylon will do the job, I've got some already that I can use so I'm gonna give that a shot
Author:  Wombat [ Wed May 07, 2008 11:17 am ]
Post subject: 
When I did mine the kit came with brass bushes but I didn't install them as the new shafts were a snug fit and had a lot less slop
Author:  simon k [ Wed May 07, 2008 11:20 am ]
Post subject: 
yeah, you got lucky then... all of mine are worn in the bodies
Author:  sgc [ Wed May 07, 2008 11:45 am ]
Post subject:  Re: rebushing SU's
simon k wrote:
does anyone do it and can give me some pointers?


David Rosenthal does these in Adelaide I think, he should be able to point you in the right direction.. send him a PM ;)
Author:  Anto [ Wed May 07, 2008 3:45 pm ]
Post subject: 
Would it just be easier to buy the bush individually and fit it, rather than custom making them?

http://classicmotoring.net.au/cmos/index.php?main_page=product_info&cPath=68_78&products_id=305
Author:  Mick [ Wed May 07, 2008 4:03 pm ]
Post subject: 
I generally use the white metal bushes that can be purchased from SU midel. They are very cheap and don't require a massive step up in size to fit. These can be easily installed using normal drill bits with care.
I step them up from the original size 1/64th at a time. I use the drill shank as the guide to ensure the far side is parallel to the nearside. and once the bushes are fitted, I then check that the spindle turns without any binding.

The most important part is creating a wooden jig to support the carburettor while this machining is going on.
Author:  simon k [ Wed May 07, 2008 5:37 pm ]
Post subject: 
$21 worth of bushes there... :roll: I've already got the nylon and made a sample bush, it looks alright, but maybe you're right with buying them

can you put up a picture of your jig for me Mick, pretty please?

do you drill all the way through the carb Mick, or do you drill to about 1mm from the bore so that the spindle hole in the bore isn't disturbed (therefore interfering with airflow :D)
Author:  Mick [ Wed May 07, 2008 5:59 pm ]
Post subject: 
I'll have to find it first...I made it out of bits and it has returned to bits...but I have to remake it for the current pair of SUs I have to bush.
I believe if I could manage it, it would be better to pull up short of the inner chamber. A problem that can happen is that air slides around the butterflies via the gaps left by poorly positioned bushes.
I make sure there is no gap by pushing them up and taking away any extra overhang into the bores carefully with a half moon file. It works very well. But if I could manage a flat bottom bore, I would do this without hesitation as it seems a professional finish if you have the right tools. Having not served an apprenticeship with carb rebuilders, I'm not aware of what the industry prefers, but my method has seen me through a number of carb rebuilds and works for me.
Author:  simon k [ Wed May 07, 2008 8:22 pm ]
Post subject: 
gotcha.... I'll carry on with my jig plan then. I'll be using a nice flat bottomed milling bit, so should be able to do it

the test carb I've been playing with has a bush in one side, and it finishes about 1mm from the bore.... my 'vizarding test mule' so to speak
Author:  Daniel Butler [ Thu May 08, 2008 11:05 am ]
Post subject: 
I don't understand why you need a jig to machine the carbs. Provided the milling machine you are using is still nice and square you can just use a vice.

I recently drilled and reamed out my HS2 bodies to fit new bushes by setting it up on the milling machine with a machining vice. All I did was put the filter side against the vice jaw (this sets t sqaure along the Y axis) then used a machining square to set up the dashpot side (square across the X axis) then drilled right through then reamed right through. It seems to have worked, although I haven't got the engine running yet.

As far as the bush leaving a gap I would have thought a little air running past the throttle would be a good thing, as if there were no air bypassing it your car would stall. If you are worried about it you could pull up short, from memory the bushes are a fair bit shorter than the hole. The only problem is you would have to undo it and turn it over which would reduce your chances of getting the alignment spot on.
Author:  Mick [ Thu May 08, 2008 2:06 pm ]
Post subject: 
No, it's not a good thing. The idle screw is supposed to set the air sliding past. With negative 21 psi behind the butterfly at idle it causes all kinds of hassles getting a low idle and balancing the carbs.

On the first pair I did, I made just that mistake and wasn't getting an idle below 1300 rpm.. :x

I made a jig because I didn't have a milling machine, and keeping it aligned was important to keep the two holes drilled parallel.
Author:  Wombat [ Thu May 08, 2008 4:12 pm ]
Post subject: 
If the air flow is so critical why is there a valve in some butterfly valves to let air through at idle?
Author:  Mick [ Thu May 08, 2008 4:40 pm ]
Post subject: 
You may need to see it for yourself, it caused all kinds of problems.
The gap it leaves when the bush is not pushed up is larger than you may think because of the now oversized holes to accept the larger replacement bushes. Like I said the lowest idle I could get was 1300..

Now if it takes a few seconds longer to push the bush up and make sure it doesn't bind the butterfly, then it will take 1/2 an hour to remove the carbs, a further 1/2 hour to unstake the carb butterfly screws and then address the problem before reassembling the whole lot again with a new set of gaskets. I know what mistake I won't be making twice...
:wink:
Author:  Morris 1100 [ Thu May 08, 2008 7:43 pm ]
Post subject: 
Wombat wrote:
If the air flow is so critical why is there a valve in some butterfly valves to let air through at idle?
That was for pollution engines. It was not for idle, it was for over run.
When you shut the throttle at speed the increased vacuum would suck the fuel out of the float bowl. The valve in the butterfly was to drop the vacuum under those conditions. It is not supposed to open at idle.

I also agree about making the spindle bushes fit close against the port even if it needs a bit of cleaning up. A small gap there is really a huge gap in the idle, it will never work properly with a gap.
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