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Test for wobbly throttle shaft?

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Jim Lee

Jedi Trainee
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Hi all,

I have a 1959 TR3A.
Based on an idle that has a mind of its own where it can go from 1200 rpm down to about 600 or vice-versa with the tap of the gas pedal, is there a quick and dirty test that I can do to check the sanctity of my throttle shaft? I have a vague recollection that this is something that can cause tiny vacuum leaks that can in turn cause strange idling problems. My main problem is that sometimes, especially when I have not fully warmed up, I will come to a stop sign and the rpm's will fall until very close to stalling or actually stalling. Then my heart starts racing and I have to take a crank or two away from my starters life. I have really explored the mechanical aspect of this and do not see that I have any play there though if anyone could point out a likely suspect in the linkage I would like to know about that.

Would it be useful to spray oil on the throttle shaft while running to see if it changes rpms? I think that I have done that and did not hear a change. I guess I am kind of stuck on the 'throttle shaft vacuum leak' theory right now.

Thanks very much,
Jim Lee
1959 TR3A aka "The Black Beauty aka "My Friend Flicka"
 
Unstable idle can have several causes, vacuum leaks through the throttle shaft bushings is just one of them. You can do an advanced search on this forum for "idle", that will bring up some threads that really have helped me.
You can check for worn throttle shafts/bushings by wiggling the shaft up and down. Noticable play means you have a vacuum leak. You can confirm vacuum leaks by spraying suspect areas with carb cleaner while the engine is running. This can/should also be done for any gasketed joints on the intake side. If the idle changes noticably, you have a vacuum leak.
A common cause for an unstable idle are jet needles that need to be centered. This solved the problem for me. Smooth operation of the dashpot pistons is also a prerequisite for stable idle.
 
My experience has been that even fairly large leaks at the throttle shafts don't upset idle all that much. I didn't really have a problem until they were worn so badly that they let the throttle disc hit the bottom of the bore, and the engine wouldn't idle down unless I 'blipped' the throttle every time.

As mentioned, smooth operation of the carb pistons is essential. If you lift them and let them drop freely, they should land with a distinct 'click'. If the jet is not centered properly, the needle will bind with the piston at the bottom.

I would also check that your float levels and point gap (if you're using points) are right. Check the mixture adjustment too.

I've also had problems with the linkage pivots being worn oblong, which after some fiddling by yours truly caused them to bind right at idle.

PS, WD-40 will also work to check for leaks; and has the advantage of leaving some lubrication behind rather than flushing it out as carb cleaner (or brake cleaner
grin.gif
) will.
 
Jim,

I had the same problem - engine quitting when you come to a stop light.

The inside of the SU chambers and the pistons looked pristine but eventually I took a clean rag soaked in soapy water and cleaned them anyway. Dried them and re-assembled. Now it is a different car - no stalling whatsoever!

Cheers
 
Thanks everyone. Excellent advice.

I have no noticeable play in the throttle shaft and spraying oil on all the seals did not change the idle at all.

I will definitely clean up all I can get to with soapy water.

The 'oblong' problem with the linkage pivots...you are talking about where the ball and joint socket link is? Is it obvious that it has been 'oblonged' (sp?).
Could you actually see it bind by working the linkage manually?

I will definitely start spraying at the intake connections also.

Is the lift and drop test of the pistons sufficient to rule out an uncentered needle? I know that I have done that before but I will check it again.

A last unrelated question. Cylinders 3 and 4 look too rich judging by the plugs. Which way do I turn the jet adjusting nut to get leaner? For some reason I am thinking counter clockwise.

Thanks all,
Jim Lee
 
Jim Lee said:
...The 'oblong' problem with the linkage pivots...you are talking about where the ball and joint socket link is?
Click to expand...

Mine were so oblong it was 'intuitively obvious to the casual observer' (as my boss used to say). So oblonged that once in awhile they would pop loose. I fixed them by adding a washer that caused them to tighten up 90 degrees from where they had been.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]...Which way do I turn the jet adjusting nut to get leaner? For some reason I am thinking counter clockwise.[/QUOTE]

As viewed (or visualized) from above: <span style="font-weight: bold">R</span>ight = <span style="font-weight: bold">R</span>icher <span style="font-weight: bold">L</span>eft=<span style="font-weight: bold">L</span>eaner
 
Jim Lee said:
The 'oblong' problem with the linkage pivots...you are talking about where the ball and joint socket link is? Is it obvious that it has been 'oblonged' (sp?).
Click to expand...
That's the spot, the 'balls' looked more like footballs! As Geo says, they would occasionally pop apart; but I didn't realize they were binding until I actually took the linkage apart and tried to turn the pivots through a full circle. I managed to find some suitable replacements at the local tractor-trailer (semi truck) dealer; but of course they are readily available today from the usual suspects.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Is the lift and drop test of the pistons sufficient to rule out an uncentered needle? [/QUOTE] If you can hear the click when they land, it should be close enough.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]A last unrelated question. Cylinders 3 and 4 look too rich judging by the plugs. Which way do I turn the jet adjusting nut to get leaner? For some reason I am thinking counter clockwise. [/QUOTE]The way I think of it; the mixture nut works like a bolt, and turning it into the carb body (meaning up) sets the mixture leaner.

Assuming of course that the springs are, in fact, pulling the jet up against the nut. If the jets are sticking down, then turning the nut does no good at all.
 
Good advice already above. May also want to check that the needles aren't worn. Good time to check the carbs over looking at the correct amount and type of damper oil and springs above the piston.

I had a problem similar to yours and it was worn throttle shafts. I fixed it by putting in new shafts. The wear was all on the shafts, none in the carb body.
 
Did you put the throttle shafts in your self or did you have a machine shop do it? Did you use the original shaft and have them do some magic by redrilling the opening that the shaft goes through so it was tight? Could you wiggle the throttle shafts or spray stuff to change the idle?

Thanks very much,
Jim Lee
 
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