1971 XS1B BS38 Carbs - Throttle shaft bushings

[TwoManyXS1Bs]

After tearing down my carbs, examined the throttle shaft bores, and found that the inboard bores are just 8mm holes in the carb body. But the outboard (throttle lever side) bores appear to have split bushings. A magnet sticks to the bushing, so I can assume that they're steel.

Took close-up pics to see if others agree.
What appears to be a split is at the one o'clock position in the pics.
(From my carb rebuild album): http://www.xs650.com/media/albums/2684/

If these are indeed split steel bushings, I'll have to completely rethink the planned drill-and-bush project.
The inboard bores are good, no looseness.
So, gonna mull over this for awhile.
Any ideas for renewing these 8mm throttle shaft bores?

 

[gggGary]

You'll have me looking, wonder if yours have been done once???!!! While doing the photos I put the throttle shafts, seals, back in my 71 set I had on the bench.
I have bronze 8mm split bushings on the shelf can send you a couple. I also bought a bag of bronze powder to make an epoxy bronze slurry and pour new bushings in place.

 

[TwoManyXS1Bs]

You'll have me looking, wonder if yours have been done once???!!!

I don't think so. Long ago, I loosened the butterfly screws so I could bias-position the plates away from binding on the carb throat wall. The factory staking on those screws was intact back then, same as they were when I removed them now.

Stainless steel throttle shaft running in steel bushings? And I've got a sloppy fit?
I'm wondering if there was a coating in there, that wore off.

Will take bore measurements later, looking for circular or oval bore...

 

[gggGary]

I believe all throttle shafts are chrome plated brass.

I notice a difference in looseness by direction when I see a loose throttle shaft. IE the wear is all to one side, IIRC the wear was basically in the upwards direction, pretty well matched the pull from the throttle cable through the arm. That would explain re-centering the butterflies also. Enough wear and they start hitting the top of the bore.

 

[TwoManyXS1Bs]

I believe all throttle shafts are chrome plated brass.

Sure 'nuff. After cleaning and inspecting, the unplated brass appears at the threaded end.



I'm thinking nickel-plated brass. If so, then the Caswell's electroless nickel plating kit might be a suitable way to freshen-up worn throttle shafts.

I notice a difference in looseness by direction when I see a loose throttle shaft. IE the wear is all to one side, IIRC the wear was basically in the upwards direction, pretty well matched the pull from the throttle cable through the arm. That would explain re-centering the butterflies also. Enough wear and they start hitting the top of the bore.

I measured my worst one, left side carb. Viewed toward the throttle lever, from the left side of the bike, the throttle shaft bore is oblonged, with the tightest bore dimension of 0.3160" spanning from 8:00 to 2:00. The ovalled dimension is 90° to that, spanning from 11:00 to 5:00, and measures 0.3195".

My throttle shafts measure at 0.3145", and compared to that 0.3195", would allow for 0.005" of slop.
That was enough slop to allow the butterfly plate to catch on the throttle bore. I would say that slop values even less than that warrant a remedy...

 

[TwoManyXS1Bs]

The bushing is fitted into a shouldered recess in the carb body.
The thin gap behind the bushing leaves very little room to fit a punch in there.

 

[TwoManyXS1Bs]

So, decided to try a 3/8-16 NC tap.

Carefully threading into the soft bushing, it started to spin in its recess after only 1/2 turn of the tap.
Continuing the tap turning, the bushing can be slowly withdrawn from the carb body.

The bushing appears to be steel-backed sintered bronze, with a plastic (PTFE) coating.

 

[TwoManyXS1Bs]

The bushing measures 8mm ID, 10mm OD, 8mm length.
Close-ups show the manufacturer's logo and p/n, 8H 1A (or 8K 1A?).

 

[TwoManyXS1Bs]

Equivalent PTFE/bronze split steel-backed are available.
Some industry numbers are:
0808DU
M0808DU
MB0808DU
ZUMB0808ZU
PAP 0808 P10
SF1 0808

Here's a generic description:

Here's an offering from McMaster-Carr, but it's too long, at 10mm.

 

[TwoManyXS1Bs]

There's plenty of 8-10-8mm PTFE lined bushings in eBay, but from Chinese suppliers.
I'm finding plenty of 8-10-10 bushings from US sources, but those are too long.

A little more digging, and I'm finding that those 8-10-10 bushings are a popular item for rebuilding old VW Solex carbs, plus numerous threads on how-to's. Feel like I'm reinventing the wheel again...

 

[TwoManyXS1Bs]

Received a bag of M8-10-10 teflon lined, split, bronze, steel backed bushings.
Trimmed down two bushings to 8mm length.
An old bushing is at lower-right.

The trimming was done on the lathe, using a thin cut-off wheel
A bushing is pressed onto an arbor of 0.319" diameter.

 

[TwoManyXS1Bs]

Use an old intake valve as an installation tool. Slide one of the old bushings onto the valve stem, press it against the valve face. It'll serve as the press collar.

Slick everything up with a thin coat of silicone grease.

Slide the bushing onto the valve stem, insert valve into carb body. Orient the bushing with the split towards the air filter side. Press the bushing into the carb body. It only took about 10-15 lbs of thumb pressure to fully seat the bushing.

 

[gggGary]

Good stuff 2many; now for the gold test, how snug do the throttle shafts fit?

 

[TwoManyXS1Bs]

Hey, gggGary! They're much better.

At first, I got some dragging while rotating the shafts. Chucked them up in the lathe and found a little runout. Straightened the shafts and the drag/bind disappeared.

Then clamped the carb bodies (one at a time) to the lathe bed, and used the lathe's DTI against the throttle shaft. Wiggling the shaft back-and-forth, got a little over 0.001" where the shaft emerges from the bushing. It's a little more, about 0.002", out at the shaft's threads.

FYI, the outboard shaft seals are different.
See the "Throttle Shaft Seals" thread for the ensuing anguish...

 

[angus67]

On the 8-10-10 bushings... Most of us don't have a lathe to cut off the excess, could one install the bush,then with carefull hand, use a 90* roloc to flush it off?

 

[TwoManyXS1Bs]

Can't trim it after installation. It sets 3mm recessed, shaft seal sets outside of it.
See that "throttle shaft seal" thread for an up-close pic.

http://www.xs650.com/forum/showthread.php?p=434227#post434227

But, plenty of 8-10-8 bushings are available from Chinese eBayers.
I just couldn't wait on that slow boat, wanted to do it now.

 

[peanut]

excellent job twomany
This should be added to the tech section before it gets buried

 

[TwoManyXS1Bs]

Thanx, peanut.

This 'bushing' arrangement, so far, has only been found on these carbs. It would be helpful if there could be some confirmation that this exists on other early carbs as well.

FYI, I've discovered some flaws, flukes, and trivia while trying to fit/align the butterflys in the carb bores. Rather involved, number crunching, wondering if it's worth a thread...?

 

[gggGary]

Sure, why not? I'll look!
humming from the choir.

 

[TwoManyXS1Bs]

Well, I ran outta steam while trying to put together a thread on flaws, flukes, and trivia about fitting and aligning the butterflys in the carb bores. I was able to put most of that butterfly/bore fit info into my album:

http://www.xs650.com/media/albums/1971-xs1b-bs38-carburetor-rebuild.2684/

Unfortunately, much of the text describing those was lost during the forum's software upgrade. Given enuff "round tuits", I may fix the album...