Suzuki triple but not Kettle engineering question

[yeadon_m]

Folks,

Do any of you know of a good small firm you can personally recommend to re-fit bushes to my GT380B carb top?

Details below for anyone interested and doesn't know the Mikuni VM carbs on the smaller triples, quite different from our Kettles!
Cheers,
Mike

The pic called 'Scalded cat' is of my Suzuki GT380B (1977). It is a two stroke triple which I have owned since 1978. Everything on this bike has been restored to excellent working condition, with exception of one difficult (for me) piece: the throttle shaft bushes.

One pic shows the carbs and you can see the throttle shaft on top. That shaft rotates in bushes inset into the top of the carb, and those bushes are worn. The bushes are purely mechanical supports - they play no role beyond that in fuel mixing or anything complicated.

Replacing those two bushes is what I am looking for anyone to point me to a good small engineering company to address.

Carburettor operation:
-In the pic of the assembled carbs, you can see the pulley wheel towards the right of the chromed rod / throttle shaft that runs through the two alloy 'turrets' projecting from the top of the carb.
-In use, the throttle cables wrap around that pulley, so that when the throttle grip on the handlebar is rotated, so too does the pulley. The pulley is woodruff keyed onto the throttle shaft. So as the pulley rotates, it turns the throttle shaft, therefore lifting up each of three throttle arms (also keyed to the shaft) and these raise the throttle valves inside the carb bodies to speed the motor up.

Why those worn bushes matter:
-when the bike is at idle, the position of the throttle valves inside the carb bodies is absolutely critical to good steady idle. The clearance between the bottom of the throttle valve and the venturi can be as little as 0.8mm and is tuned dynamically while on the bike by rotating adjuster screws on the throttle arms.
-in order to get a great idle across all three cylinders it may be that the left carb clearance is 0.6mm, the centre 0.8mm and the right, 0,7mm. So what? well, the bushes through which the throttle shaft runs are worn and the vertical play between the bush and the shaft is a sizeable fraction of the differences in settings from one carb to another.
-net effect - it is impossible to set the throttle valve clearances and for them to be stable for a moment. As soon as the throttle is blipped and released, the worn bushes means the throttle valves do not return precisely to where they were set. So the idle is OK, but no where near as even and stable as it can be, so I have to set the idle to a higher RPM to prevent it going onto 2 cylinders at idle. (once off-idle, the degree of wear in the bushes is so small versus the several mm of upward movement on the throttle valves that the bike runs crisply and evenly across all three cylinders.

What I would like: to have those two bushes replaced in any way that provides essentially zero vertical play for the throttle shaft yet still allows that shaft to rotate freely. The shaft itself has been hard-chromed and multiple components key onto that shaft, so that shaft must not be altered in the fix.

[kettle738]

Hi Mike, I recently used this company GJG Engineering to skim 4 thou' from the cylinder head of my 750J, and I was impressed with the thoughtful approach to holding the head so it presented uniformly to the cutter without distortion. The firm was reccommended by a friend whose judgement I trust and I wasn't disappointed. The job was done within a week and it wasn't expensive (£45) and he was familiar with GT750s.

http://www.cylex-uk.co.uk/company/g-j-g ... 31106.html

They are in Brede E. Sussex, a few miles the other side of Rye from your neck of the woods.

You are presumably hoping to have the old bushes pressed out without damaging the casting, then new bushes made up to accommodate the hard chromed rod (including holes for the grease nipples) and pressed in. Just one question....has the operating rod been newly hard chromed?

Mick...............kettle738

[yeadon_m]

Mick,

First - thanks for the tip on that company, I will give them a call if I can't find a local guy.

Yes to the rod having been hard chromed, but it was wiggly beforehand (I just didn't know the significance of that when I had it done back in 2007).

Yes to hope that the bushes can be pressed out or bored out and replaced. Does that look doable to you? I was looking at it thinking if I heat or cool it, it may be possible to press it out. After all, it takes no side load so unless they overdid it, there's no reason for it to have been irreversibly presssed in?

Cheers!
Mike

[yeadon_m]

Folks,

Update - this morning a box arrived back from Ian (Astrax). What a fantastic job he has done, re-bushed and reamed to a close tolerance my throttle arm on the GT380B carb top. I didn't make it easy for him since that throttle arm had been hard-chromed a few years back and was not a stock diameter. Its now not got even a milli-gnats todger of vertical play yet rotates as smooth as. Probably better than new, in truth.

The plus will be that I will find that I can finally balance the carbs as they were designed to be, and will have a nicer slow idle at last!

I would love to know how to use all that fine engineering kit but I am probably not up to it. So I am delighted there are people like Ian who knows how and appears to have had (some kind of) fun doing it too :-)
And a nice guy also.

THANKS,

Mike

[astrax]

Hi Mike
With a slight chance of the next line being taken the wrong way by some of the readers,
"it was a pleasure doing your bush", glad you are happy with the work.

Thanks

Ian