SU carb idle hangs up when engine is at operating temperature

When engine is at driving temperature the the throttles hangup at high revs when idle revs are demanded

We’d love to help, but from your description it’s hard to understand what the problem is.

Can you give more information?

Welcome to the forum Doug, sounds like the spindles are not allowing the butterflies to close completely, first off have you done any work on the carbs?

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The throttles can hang up due to worn motor mounts. The engine settles, allowing the bell cranks to interfere with the frame rail.

Another common problem is the piston hanging up. This can happen due to a damaged piston, in which case, the piston and dome may need to be replaced. Or by the jet not being centralized during the reassembly process. If it can’t be coaxed into position, the jet bushing may need to be replaced.

Does the idle return to normal when you blip the throttle?

So in this case I would suspect the engine would run rough as it’s unlikely that all three carbs would jam in this way…so Doug, does the engine run rough at all when this happens?

My 2+2 used to do that. Very, very precise adjustment of the Rube Goldberg like throttle linkage eliminated the problem. Until I sorted it out a blip of the throttle brought the revs down.

Other thoughts:

As noted above, the linkage of usually the front carb can the front carb can sometimes contact the frame rail.

Check that you have all three return springs and brackets correctly mounted under the carbs.

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Hi Robin and thanks to all for responding to my question. I’ve just had the original SUs completely overhauled. The linkagesengine as well. The car runs beautifully for a short while, half hour or so. Then when gearing down the idle speed hangs up around 1800 rpm. It does return to 800 or so if I give the throttle a quick “blip” back in the garage I can replicate the situation when the engine is at normal operating temperature. I notice the center carb butterfly does not completely close even with the new return springs under tension. If I press lightly on the return linkage the butter fly closes and idle returns to normal. I’ve used a fine penetrating oil to lubtricate all linkages.

That’s a huge clue: loosen up the s linkages, between the carbs and see if you can determine if it’s just because that carb isn’t synced properly with the others, or it’s really a misaligned butterfly.

Thank you Paul. I’ll contact the specialist who overhauled the SUs and find out if an adjustment can be done to the butterfly without removing the carb.

I’ll answer that: it cant.

If the butterfly is misaligned on the shaft, that’s a carb-off situation.

Many thanks Paul. Off to spin wenches.

Before you remove the carb see if it closes with the linkage completely disconnected from that carb. It might not but it’s an easy step since you have to disconnect the linkage as part of the carb removal process.

And that the fast idle screw on that carb is not misadjusted.

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This problem is fairly common on multiple S.U installations and is usually caused by end loading on the throttle shafts. This end loading is the result of inlet manifold end wise expansion particularly on long manifolds, at running temperature. Slacken the throttle shaft couplings on the offending carburettor and reposition them to allow more end to end movement with no hot binding.

I hope that cures your problem.

Norman G

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This happened on mine. I re-installed the engine following a clutch replacement and I had to shift it a skinch the free the throttle shaft.

Izzat close to a ‘skiff,’ or closer to a ‘skosh?’


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What a great idea. I’ll do the adjustment right away. As soon as the weather permits . I will let u know how it turns out. Doug

Since the springs at the carbs themselves are relatively weak, the angularity of the spring with the adjustable lever is critical. The lever must be at the “best” angle to allow the spring the best leverage. While you suggest it is only one carb, others may need a tweak also. Look at the geometry and use your sine/cosine skills to choose the best angle.
Ooops - if yours has 3.8 carbs, They may have a different spring to lever set up. I may have committed the sin of suggesting something from another year. Still, I’d check it.

61 3.8? Were the carbs rebuilt any time recently, like the last 8 years? They started using rubber oring side seals on the spindle shafts. They get very bindy when hot. The spindles move fine when cold and when hot they feel like dragging a rubber band on glass, squeaky. The spring force becomes insufficient to close the butterflies that final couple of degrees.

Rip them out and use the original cork ones. If they are the cork ones, and they’re new, then they’ll bind a bit too, but not to the same degree and a bit of oil will allow them to move and with time they break in. The rubber ones will never break in and will continue to bind forever.