throttle shaft bushings

[John Loftus]

I have an SU master kit for HS6 carbs (1.75") which includes standard size thottle shafts and bushings. The carbs I'm rebuilding currently have bushings. Did this type of carb original have bushings or were they only added during a rebuild? I'm wondering if I can remove the old bushings with a tap or EZ-out or if milling them out is the recommended way (not that precision milling and turning is a problem)? What do you think of the idea of replacing the metal bushings with delrin .. similar to the bushings for the HD8 carb? Seems like the brass throttle shafts wear quickly and having more compliant bush material would help in this area but there may be a downside. Opinions?

Cheers,
John

 

[Keoke]

Hi John, they are original to the HD series carbs. The original bushes were a hard paper like material while the recent rebuild kits use 0.015" thick Tefelon. I am unaware of metal bushings except brass after market ones used by rebuilders when extreme wear in the housings would accur---Keoke??

 

[John Loftus]

Hi Keoke,

No these are different than the fiber or teflon bushes on the HD carbs. I searched around some more and found out the HS series have brass bushes that are cast in place.
https://www.ttalk.info/mini-Inside.jpg
https://www.ttalk.info/mini-Outside.jpg
The SU master kit appears to contain steel bushes with teflon coating on the inside (SUmidel calls them Teflon coated steel glacier bushes)
My brass bushes do not seem to be worn much but will discuss the options with a friend who is a prototype machinist tomorrow.

Cheers,
John

 

[ThomP]

I wouldn't machine the bushes out for fear of machining the carb housing. I would make a mandrel that fit the old bushings and press them out. The old bushings were pressed in, they should be fairly easy to press out. And, now that you have the manderel, you can use it to press the new bushings in too!

 

[Dave Russell]

Quote JL,
"the HS series have brass bushes that are cast in place."

TP,
If they were "cast in place" They may not press out very easily at all.
D

 

[Keoke]

Hi John I did not know you were talking about the HS series.In these carbs the usual procedure is to line bore the bushes and install overside shafts. Trying to remove and reinstall new bushes can result in breakage of the carb housing.The replacement type bush is splined and getting the interference fit correct is a bit much.--Fwiw--Keoke

 

[John Loftus]

Hi Dave, Yes, once I found out they were 'cast in place' pressing them out is not an good option.

Hi Keoke, It's odd then that the master kit contains a normal sized throttle shaft and bushes. BTW, the bushes are not splined in my kit .. smooth as glass /ubbthreads/images/graemlins/grin.gif

Cheers,
John

 

[Keoke]

Well John, in that case you might check and see if the existing bushes can be reamed just enough to allow installation of the new ones.The ones I have in my stock and dare not use are splined on the outside.They only smooth as glass on the inside---Keoke /ubbthreads/images/graemlins/grin.gif

 

[John Loftus]

Just a follow-up for the archives. As it turned out the old bushes measured out to be the same size as the new bushes so decided to leave the old ones in and just replace the thottle shaft. The new shafts were drilled (using a mill and indexer) for the forked lever that gets pinned at one end. Then zinc plated the linkage, cleaned everything up and put it all back together using the SU master kit. All went pretty smoothly although it took a couple tries to center the jet bearing on the jet (the workshop manual covers this adjustment well). Here's a few pics for documentation.

Cheers,
John

 

[John Loftus]

And just a few more tips from the rebuild (note this is my first time rebuilding SU's so this may be old hat to some).

The master rebuild kit did not include the rubber washer that goes on the long bolt that attaches the float bowl to the throttle body so was necessary to order that separately.

On the first carb I put together, noticed the jet head was not extending very far when the choke linkage was fully open. The friction from the jets being non-centered was causing a bit of this but also the jet link was not optimally shaped. This link is easy to bend by hand and I had straightened it out some of the kinks during buffing/plating operations so by pulling the jet fully out and gently bending the link towards the throttle body was able to get the full (approx. .40") travel.

The master kit came with new needle valves assemblies for the float chamber. Mine had Grose jets which (if of a high quality) are less prone to clog so I stayed with those.

I had an extra set of carbs from my spare motor and these were very handy to refer to when putting everything back together and was able to scavenge some parts to restore to original some P.O.'s minor "modifications" on the linkage. /ubbthreads/images/graemlins/hammer.gif

Cheers,
John

 

[Keoke]

OK John, seems all is well.However, there is a tool that will simplify centering the jet bearing. I think moss carries it but others certainly do.To drill the holes in shaft correctly I simply align the old and new shafts using the throttle disk. /ubbthreads/images/graemlins/cheers.gif-Keoke

 

[John Loftus]

Yes, I had heard of the jet centering tool and see now that it looks like a simple shaft turned down to fit the bearing ID and the piston ID. Why didn't I think of that? /ubbthreads/images/graemlins/grin.gif

Do you mean thottle disks in both the shafts and spacers separating them to provide a drill guide or both shafts on the same throttle disk and then some angled vise to hold them for drilling?

Cheers,
John

 

[Keoke]

Yep, Both shafts held by a single throttle disk and a typical V vice for drilling on the drill press. Cuz sometimes I don't measure too good--Keoke- /ubbthreads/images/graemlins/lol.gif