Triumph Repair: TR7 idling too fast after carb rebuild, stop screws, throttle shaft

Question
QUESTION: Howard,

  I recently purchased a non running 1976 TR7 - not running because the front carb (stromberg 175 CDSEV) was flooding.  I rebuilt them both, and got the engine going.  My problem is the engine idles at 1400 rpm even when the throttle stop screws are backed all the way out.  The front carb draws the most air, so I had to turn the throttle screw on the rear one 2 turns down to match it.  My timing is at 0 degrees TDC, I've even tried 16 degrees ATDC to slow it down but it just gets rough and maintains high speed.  The chokes are manual and are pushed all the way down.  I've sprayed carb cleaner all over the intake system, found 2 leaks and plugged them.  I've checked to see that the butterflys are completely closed at idle.  I've leaned the carbs to where the engine gets rough again.  These carbs have a deceleration valve which is like a throttle bypass - could that be the problem - does it need a new diaphram ?  They also have a temperature compensator but I don't think that is drawing any air.  Any suggestions ?

   Thanks,

   Steve.

ANSWER: Hi Steve,
Even though mixture can change idle it can't make an engine idle at 1400 RPM without air. So air is entering the intake somewhere. That bypass can be at fault because it's diaphragm after years gets very hard and the spring will not close the valve. You can just open it up and put something inside to hold the valve closed all the time also some had a threaded screw that allowed you to adjust it so it could not open. The temp compensator is probably not the problem but that too can be wedged closed as a test with a small piece of sponge rubber placed under the plastic cover.
If you had removed the throttle shaft or butterfly you should check to see that the throttle plate closes all the way home.
Also look for a intake leak somewhere.
Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,

 Thanks for your reply.  I had another look at it, I took the decel valve off, turning the adjusting screw all the way counter clockwise locks the diaphram so I tried that.  My idle went down about 200 rpm.  I tried different settings and at one point I got an air leak at the screw head.  So I have bought rebuild kits and will do that soon.  I also pushed hard on the throttle shaft fork for the front carb and got the idle down to 800 rpm which solves my problem - except at that point the butterfly is jammed and needs a lot of force on the linkage to free it !  So that's not so good.  I took off the carb, and had a look into it with light behind and could see daylight through the sides of the butterfly where the throttle shaft passes into the carb body.  I could also rock the throttle shaft side to side about 2 mm so I do you think the butterfly is worn too ?  I am not able to force the rear carb to jam the butterfly so I think it may be ok and it draws less air than the front.  I think I have found 2 sources of the problem.  If the diameter of the butterfly is reduced, I can see it allowing more air at the sides but is it more prone to jamming in the carb ?  I've tried centering it with different orientations but it will still jam when closed with mild force to the position I know will drop the rpms to 800.  Perhaps the reduced diameter lets it close more vertical and more prone to jam ?

  Thanks,

  Steve.

Answer
Steve, if the throttle shaft is badly worn all you can do is to replace the shaft and butterfly (throttle plate)and bore the carbs and put throttle shaft bushings in. Then when you install the new shafts, leave the two screws loose and close the throttle and lightly hold the throttle plate closed in place with one finger and tighten the two screws while holding the throttle plate closed all the way (stop screw backed off all the way) Then open and close the throttle and watch the throttle plate to see that it does close all the way. If it is not centered or rotated a little it will not close all the way.
If one carburetor is bad, both are bad. More likely reason for a throttle plate to stick is that the throttle plate is not aligned correctly.

The standard test for an air leak is to get it to idle as slow as you can and then slowly start covering up the air supply. If at any time the engine speeds up or gets smooth then that is an indication you have an air leak somewhere. There are several tests to pinpoint a leak.
Idle as slow as possible and listen with a small hose about the size of a vacuum hose. Squirt oil around suspected leak points and listen for an RPM change. Find a shop that has a smoke generator to look for leaks.

Howard