a confused idle

[1sttr6]

At start up, until pulling away the beast idles nice at about 1000+ on the tach. Returning to a stop sign or light idle slowly heads up to about 1600-1900. Will run down to 800 fine in gear, but will not idle down there. Won't kick down to there either when warm.

thanks Peter

 

[skucera]

My "new" '76 Spitfire suffers from this too, so I want to see if someone has a solution for this too. My symptoms are slightly different, though. While warming up, after I push in choke all the way, my engine will repeatedly idle at 1500 RPM when I come to a stop, and I have to push the choke knob in deliberately drop the idle to 800 RPM. I have to do this three or four times before the motor will drop to 800 RPM at idle on its own. It is as if the choke were pushing itself partially closed while the car is warming up, then stops doing this once the engine is warm.

I never had this problem with my Midget 1500 years ago. I never had this happen on my old Spitfire 4 Mk. 1 with its little SU's either.

Peter, have you cleaned the throttle linkage, return springs, and throttle shaft with carb cleaner? Sometimes gunk can prevent the throttle butterfly from closing all the way. Another sinister cause of high idle, especially if it is uneven, is worn throttle shafts or worn throttle shaft bushings. You might try spraying a little carb cleaner on the shaft ends where they come out of the carb to seal the gaps... if the idle speed comes down, you've got a vacuum leak there. If you have a vacuum leak at the throttle shaft, you can get the carb rebushed with nylon or teflon bushings by a carb rebuilder.

Oh, yet another potential cause of high idle can be leaking vacuum plugs on the many vacuum ports used for the emissions control system. Most folks have pulled their vacuum lines off the carb for the EGR and distributor vacuum advance (and others have left all this alone) but sometimes those little rubber elbows can leak vacuum. I am painfully aware from my Saab that a vacuum leak from a seemingly insignificant hose can do all sorts of odd stuff to a carefully balanced carb, so check those vacuum connections and vacuum plugs, whether you've got some of the original emissions control gear bypassed or not.

Scott

 

[Adrio]

I had that problem and it was a leaking manifold gasket.

 

[Tinster]

Peter,

I have a similar problem on brand new, professionally
rebuilt twin carbs and brand new throttle bushings on
my 69TR6. My idle rises to a constant 1200 rpms once
I pull out of the garage. I can open the hood and pull
the linkage and get back to 800.

I will soon be replacing the stock linkage with a Ratco
stainless steel cable assembly. I'll post how the new
cable linkage effects my idle.

dale

 

[prb51]

First thing to eliminate is any air leak but they normally show up continually after choke/start at idle and they tend to hunt/fluctuate.
I'm not familiar with your linkage but that is easily the culprit. Even with tight springs a worn connector (ball/socket) will allow enough play to have a high continuous idle. If you have multiple connectors like the TR3 each one being slightly worn will do the trick.

 

[TRopic6]

Couldn't the bypass valve do that? Somewhere I read that turning the adjustment fully CCW will keep it from operating. I did as soon I got my TR6 back in operation.

I'm not really sure what it was originally supposed to do, but if it opens up at the wrong time, a high idle can result. Can any experts explain fully?

Jeff
74 TR6 CF13816U

 

[TR3driver]

Couldn't the bypass valve do that?

Could, if it's opening too soon (weak spring).
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I'm not really sure what it was originally supposed to do, but if it opens up at the wrong time, a high idle can result. Can any experts explain fully?[/QUOTE]I'm no expert, but ... it's an emissions control device, intended to address the situation where the throttle is closed but inertia is forcing the engine to turn at relatively high rpm (overrun). This runs the manifold vacuum up until the combustion chamber pressure is so low that the charge won't reliably ignite and burn (or burn too slowly), resulting in unburned hydrocarbons out the exhaust. So the bypass valve is supposed to open only when the manifold vacuum is higher than normal, to limit the vacuum and keep the engine running during overrun.

If the spring is weak or misadjusted, the bypass can open at idle, and allow fuel/air into the intake, raising idle rpm.

Does anyone know if Triumph ever switched to a gulp valve ? Same principle, but instead of letting fuel/air into the manifold, it lets fresh air (only) in. This effectively kills the fuel/air through the carb and gives improved compression braking and reduced fuel consumption, compared to the bypass valve. 'Murrican cars had them before they went to fuel injection, but I don't know if Triumph/BL ever went that way.

 

[AltaKnight]

I see you have a 73 TR6 same as mine.
I had some of these same problems and discovered that the throttles were not reliably returning to the throttle stop but were hanging slightly open, a fast kick of the throttle (with the clutch disengaged) would bring it back down to reasonable idle.
I finally figured out that there are 2 small piston/plungers, one in the side of each ZS carb that are actuated by the throttle; these plungers apparently seal the fuel bowl vent at idle so that the anti-runon device works at shutdown, when I mechanically disconnected them the throttles closed properly with a good idle. You don't need those plungers anyway.

 

[poolboy]

Hey Peter (the former "wannabe"). Check for vacuum leaks, and linkage sticking and if you have adjustment screws on the TBV, push on them with your finger. There is enough spring loaded slack to allow them to move into the TBV and push the actual valve into the seated (closed) position. If the idle drops when you do this it indicates that the valve and attached diaphram is not functioning properly.
It might respond to increasing tension on the spring. When you turn the adj.screw CCW you are increasing the tension and thereby putting more pressure on the valve itself to stay closed under a mild vacuum.
However if the rubber diaphram that surrounds the valve has lost it's flexibility, some of the fuel/ air mix will still leak past the valve and into the intake manifold.
That little extra will speed up your idle speed.