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TR4/4A Idleing

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RonR

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I am helping a friend with the restoration of his TR4.

He paid someone to rebuild the carbs, which included new throttle shafts and butterflies.

The front carb will not idle slow with the idle adjustment screw is clear of the carb body. With the butterfly closed, the gap between the circumference of the butterfly edge and the carb body is a bit wider than that for the rear carb. The throttle shaft will move back and forth (sideways) a bit more than 1/16 inch.

Also, with the throttle springs tight, and a new linkage spring near the firewall, the front carb will not return to idle without assistance. The idle is between 1500 and 1800 rpm when releasing the acceleration pedal. We can get it down to 1000 rpm with pulling up on the acceleration pedal (or pulling on the linkage at the front carb.)

Any suggestions to the cause and correcting of these problems?

Thanks.
 
It's possible that the throttle disc is not oriented properly.
It's beveled on it's perimeter.
Another possibility is the carbs are out of sync. Sometimes in the process of tightening the throttle shaft coupling, one shaft may rotate. Then once tight, that carb has a more open throttle disc than the other.
 
With the idle screw free of it's stop, and the throttle fully closed, there should be no visible gap around the circumference except right at the throttle shaft. I would be checking that the throttle disc is installed the right way around (the edges are tapered to meet the bore squarely with the disc at a slight angle to the bore, so it is possible to install it upside down) and that it is properly centered. The mounting holes are deliberately oversize, so the disc can be perfectly centered to the bore. That includes holding the shaft so the idle stop lever is up against the carb body.

Also worth checking that the idle stop lever is not hitting the carb body. New throttle shafts come undrilled, and it's easy to get the holes not quite right.

Once that is all corrected, check the throttle operation with the linkage disconnected from the shaft. If the throttle will return properly without the linkage, but binds when you hook up the linkage, then the problem is in the linkage. One of the problems I've had is a worn bell crank pivot allowing the bell crank to drag on the head of the bolt that retains the bracket to the manifold. Almost impossible to see under there.

I've also had the pivot balls worn so badly that they would bind at certain angles and hold the throttle open.

PS the screws that retain the throttle shaft are split and designed to be spread to lock the screw in place. If you are going to take the throttle plate loose, it would be best to have new screws on hand. Otherwise, if you pinch them to remove them and then spread them again, the screw may break at a later date and fall into the engine. On top of that, having a screw out can let the throttle plate rotate, and block the throttle open. _Very_ disconcerting! (not to mention dangerous).
 
To add to the above, are these the SU's with the little "accordion" linkages? If so, check to see that they're aligned properly. I had that problem on the TR3 years ago.
 
Any time a posting mentions high idle speed on SU carbs that have had the throttle shafts and the bodies re-bushed, I suspect the 2 issues might be connected.
Mis-drilled throttle stops (H type), boring through the choke (venturi),and throttle discs not centered, can cause air leaks that make setting the idle speed impossible. Also, if the 2 bushes are mis-aligned, the throttle will require very heavy spring pressure to close.
Berry
 
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