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Balancing SUs

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qim Qim B
Lisbon, lisbon, Portugal   PRT
Hi

I am trying to understand some basics of these carbs and read the following, by«ut can't quite understand the jargon (or the English...)

In reply to a post by http://www.mg-tri-jag.net/tech16.htm Loosen the throttle linkage so that the throttles will operate individually. Back off the throttle stop screws and the fast idle stop screw so that the throttle plates hit home (closed all the way). Now carefully lock the throttle linkage back together so that there is a little free play before the linkage will open the throttles and that as you start to open the throttles, they both open at EXACTLY the same time.

Take a thin piece of cellophane and place it between the throttle stop screw and the stop that it seats on and adjust the screw until you feel a drag on the cellophane but not enough to open the throttle at all. Do this on both or all three SUs.

Now you have both throttle plates closed all the way, both stop screws adjusted so a piece of cellophane
is snug under each throttle stop screw and if you move the throttle linkage there is a little free play before it tries to open the throttles and when you do contact the throttles they will open at the same time.

I have HS6s and I don't think that I have more than one screw per carb (no fast idle screw?). Anyway, what does the text mean by "Back off" and "the throttle plates hit home"? What are the throttle plates?

The clamp bolts on the spindles keep together a small metal part that ends in a fork. Is that the throttle plate? If so, there is quite a gap in the fork; if the throttle plates are something to do with this should they hit home at the bottom of the fork, or at the top?

Any help appareciated.

qim



Edited 3 time(s). Last edit at 2017-10-05 10:26 AM by qim.

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
Back Off = Unscrew

Throttle plate = Butterfly valve

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Those instructions only work for earlier type SU carbs (eg the H6 originally fitted to TR3). The linkage on the HS6 is different. Try these instead
https://drive.google.com/open?id=0B2H2NJt34OffYmIyM2Q5ZGQtY2YyMi00ZmVkLThiYWEtZTE3MTUyZDEyMDM0

To answer your specific questions, "back off" means to turn the screw counter-clockwise (as though you were removing it) far enough that the tip of the screw does not contact anything.
The "throttle plates" are round discs inside the carbs, not normally visible, that fit through the center of the throttle shaft. They "hit home" when the top and bottom are in contact with the carb bore, meaning the throttle is closed as far as it will go.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

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qim Qim B
Lisbon, lisbon, Portugal   PRT
Thank you both.



Now, it explains why it seemed so different from the procedures I read (and used) for my carbs.

On HS6 the clamping bolts in the spindles hold a metal part that turns and twists and narrows horizontally entering another part that looks like a limited width "mouth". I expect that the narrow part is where the throttle set screw touches. is that so?; and what are those two parts called?

Thank you



Edited 2 time(s). Last edit at 2017-10-05 11:08 AM by qim.


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2017-10-04.jpg    37.4 KB
2017-10-04.jpg

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Burlen calls the piece with the notch a "lost motion lever", and the other piece the "interconnection lever". Items 39 & 40 at http://sucarb.co.uk/carbspec/carburettor/spares/id/2393/



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

CJD john durant
Southlake, Texas, USA   USA
I don’t quite agree with that procedure in general. It appears they are trying to balance the throttles at the fully closed position....which is a theoretical position at best, since the throttle never sees that position in service. For many reasons a balance fully closed may not mean a balance at the idle throttle position. I would tend to ignore that whole direction.



John
Southlake, TX

'55 TR2

qim Qim B
Lisbon, lisbon, Portugal   PRT
So, to solve the mystery, the interconnection lever is where the throttle set screw, screws down onto. is that right?

if so, why those exact constraints on the "Lost motion lever"? Is there a start point where the interconnection lever should rest when starting to tune the carbs, or it doesn't matter where it is?

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
It's hard to say for certain, as not all HS6 are identical in that area (and obviously those are not the original carbs). But from your photo, it appears that the throttle stop screw (aka slow idle adjustment) bears on the "lost motion lever" (the one with the notch in the end).

I don't know what the advantage of the lost motion is; perhaps it's an emission thing to ensure both carbs return to just the right idle position even if heat or whatever changes their relative position slightly. But the .015" measurement shown in the first reference I gave above represents the lost motion. Exact value is not so important, I think (could be .010" or .020" without hurting anything), but it is somewhat important that it be the same on both carbs. Again, I suspect that is mostly for emissions, having the carbs slightly out of sync doesn't seem to make much difference on my car. Perhaps just a bit rougher at light throttle, but not enough to notice if I wasn't looking for a difference.

But anyway, the idea is that you synchronize the carbs at idle (using the slow idle adjustment screws) as well as setting the correct idle rpm. Then you check that .015" gap, to ensure that the throttle linkage opens the throttles at the same time.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

qim Qim B
Lisbon, lisbon, Portugal   PRT
Thank you Randall

It's all starting to make sense (for a novice that has tuned the car reasonably well, without really knowing why...).

There is one point that I did not understand: you mention the .015" gap and I searched the link that you sent me and find it on p.7 (f) (I):

"Check the actuating pins of the interconnecting clamping levers are set at .015" from the lower edge of the fork (see fig. 16)"

Now, I know what is the "interconnecting clamping levers" are and I suspect that the "fork" is the "Lost motion lever" (is it?). Unfortunately I cannot see fig.16 and so I am not sure how/where to set the gap...

EDIT

I found fig.16 and I take it now that I have to create a .015" gap as in the figure.

This goes right against what I have been doing! I loosen the clamping bolts to make both carbs independent of each other, after which I mess about with the interconnecting clamping levers, and then tighten the bolt to achieve (by sight...) a situation when both pistons start rising at the same time, After a few tries I get to work reasonably well.

Now, I assume that what I have to do is to loosen the bolts as before, but undo the slow idle adjustment screws out of the way, and set the gap of.015", tighten the bolts, and only after this should I balance both carbs with the help of a balancer and by screwing in the screws.

Reading your earlier post you say it's the other way round, and I have to accept it, but right now I got confused because I thought I had read that one of the first things to do was to get the idling screws totally away from the interconnecting clamping levers.

Many thanks



Edited 5 time(s). Last edit at 2017-10-05 03:54 PM by qim.

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
I took "the fork" to mean the fork of the "lost motion lever". Does this copy of Fig 16 work better ?
(note that you can click on the image to open a separate window, then click again for a larger view)




Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

qim Qim B
Lisbon, lisbon, Portugal   PRT
You may have missed my Edit to my previous post. Meanwhile reading the instruction on p.13 it throws more confusion (from your town?...) with "a total of 1/32 end-play between the interconnecting clamping levers and the throttle nuts. What throttle nuts?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Yes, I did miss your edit. Hopefully this will make the 1/32" clearer





Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

qim Qim B
Lisbon, lisbon, Portugal   PRT
Hi Randall. Many thanks.

I found this in an online article which seems to be the same under another name..

Adjust the pegs on the ends of the interconnecting linkage until there is approximately 1/16" inch of clearance before the peg contacts the slot on the carburetor tab.

What is the peg and what is the carburetor tab? I looked everywhere but can't find the answer

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
I believe those are alternate terms for "actuating pin" (as shown in the diagram above) and the notch we've been talking about.

In other words, your alternate instructions call for .0625" instead of .015". Seems kind of large to me; but I guess you can take your choice. As I said before, I don't think the exact gap is important, only that it's the same front & rear (so the throttle plates move in unison). But my personal preference is for as little lost motion as possible, since that translates to motion at the pedal that doesn't do anything.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

qim Qim B
Lisbon, lisbon, Portugal   PRT
Thank you Randall and apologies for the delay.

I have a doubt (among many...): one way to check the mixture is to lift the piston a tiny bit and listen to see if the rpm go up or down. When I use the outside pin and lift the piston a "tiny bit" I cannot hear any difference. However, if I lift it all the way up the engine either dies or the rpm go up, So, is the "tiny bit" supposed to be "all the way up"?



Edited 1 time(s). Last edit at 2017-10-19 10:14 AM by qim.

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