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Help needed with high idle

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TR4A_IRS

TR4A_IRS

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Hi all,

I recently gave the TR4A, with SUs, a good tune up. Now that the timing and mixture are set, I cant get the engine to idle below 2000 rpm. I'm sure I'm overlooking something pretty basic, but I'm at a loss. My SUs have idle adjustment screws, obviously, and the idle is still high with them backed out all the way and the choke linkage disconnected.

Any suggestions would be very welcome.
 
Have you confirmed that the throttle plates are closing all the way?

If so, then probably time to look for vacuum leaks. What do you have for a PCV system?
 
I took a look at the throttle plates, through the carb bodies, and they appeared to be closing fully. I have a stock PCV valve. I had a proper idle before I did the tune up, so I think it is probably something I did.

Another interesting bit of info - The plugs read rich, and the carbs say slightly lean, when I lift the pins.
 
If your SU carb bushings are worn, you'll draw air through the butterfly shafts and the idle will be too high. Spray WD40 there while running and see if that slows down the idle. If so, they are a bit worn and could be replaced at a machine shop. Easy for them; about $100.00.

Two other things can help. "Rich up" the carb mixture by adjusting the mixture adjusting nuts below the carbs, or slightly retard the timing with the distributor timing adjustment screw.

A good friend of mine always says the right adjustment of the idle screws, carb mixure, and timing will "dial in" a smooth running car. But if the butterfly shaft bushings are badly worn, you could have problems getting it perfect.
 
KVH said:
If your SU carb bushings are worn, you'll draw air through the butterfly shafts and the idle will be too high. Spray WD40 there while running and see if that slows down the idle. If so, they are a bit worn and could be replaced at a machine shop. Easy for them; about $100.00.

Two other things can help. "Rich up" the carb mixture by adjusting the mixture adjusting nuts below the carbs, or slightly retard the timing with the distributor timing adjustment screw.

A good friend of mine always says the right adjustment of the idle screws, carb mixure, and timing will "dial in" a smooth running car. But if the butterfly shaft bushings are badly worn, you could have problems getting it perfect.
Click to expand...

I'll give that a try next. The carbs were rebushed not too many miles ago, but I better rule it out.

I got so frustrated I started from scratch with the mixture adjustment, and I quit for the day before I got out of the lean zone.

Funny thing is, the plugs read rich even though the carbs say lean.
 
You have to have your jets well centered, with no sticking of the jet needles, in order for everything to work well. You may have some sticking, so be sure you're getting a good metallic click as the pistons rise and fall. And be sure you've got proper oil in the carbs.

I guess while you're at it, be sure you've balanced the carbs with either an air gauge or by using the steel rod set in the carb tool kit.

If those bushings aren't badly worn, you'll figure this out.
 
It would be quite a Carb bushing leak that would bump up the idle to 2k. I vote for something sticking choke linkage or the Carb pistons not going down all the way. Pull the air filters and lift the pistons up an inch ore more withe your finger. They should. Drop with a satisfying audible clunk at the end. If they dont recenter the jets. Also if you swapped out pistons or carb tops or pistons they are matched sets and usually stick if you pull parts from one carb and put it on the other.
 
glemon said:
It would be quite a Carb bushing leak that would bump up the idle to 2k. I vote for something sticking choke linkage or the Carb pistons not going down all the way. Pull the air filters and lift the pistons up an inch ore more withe your finger. They should. Drop with a satisfying audible clunk at the end. If they dont recenter the jets. Also if you swapped out pistons or carb tops or pistons they are matched sets and usually stick if you pull parts from one carb and put it on the other.
Click to expand...

I believe the jets are properly centered, as you and KVH suggested, as I do get a proper clunk. Also, as KVH suggested, I haves synced the carbs multiple times with my unisyn. I have not recently opened the pistons, so they are not swapped. The dashpot oil is good enough, and shouldn't affect idle much, if at all. I am really scratching my head. Ive done this many times before, and never had this problem.

Ive taken the car for a spin, and it runs OK, except for the high idle.

Thanks for all the tips so far. I'm sure I'll get it soon.
 
Idle does seem way high. Did you start with the mixture nuts opened about 12 flats? That's a good start. Could your timing be way off? I can adjust mine and throw my idle up by a few hundred RPMs, but not much more.

Also, when you tested the jet needle centering, did you have the mixture nuts all the way up? That's the only way to test.

I'm no expert on the PCV, but I suppose you'd better check that, too.
 
TR4A_IRS said:
I took a look at the throttle plates, through the carb bodies, and they appeared to be closing fully.
Click to expand...

It only takes a tiny opening to make the idle too high. At least in my experience it can't be seen by just looking down the bore unless you also have a strong light on the other side. Even having the throttle plate installed backwards can cause the idle to be high (although obviously that isn't your problem).

Sorry that's not very helpful, but my point is that I would be focusing on how the extra air is getting in there. It should not be able to idle at 2000 rpm with the throttle fully closed, no matter what is going on with the ignition and mixture.

Last time I went hunting idle problems on my TR3, it turned out that I had not drilled the replacement throttle shafts just right and the idle stop lever was not firmly against the body with the throttle closed. That was allowing the throttle linkage to move the shaft ever so slightly to the side, and the edge of the throttle plate was hanging up on the bore (which apparently had developed a slight ridge just at that point). Adding a shim and re-centering the throttle plate solved the problem.

On a 4A, the PCV valve seems likely suspect to me. There is information on how to service it (every 12,000 miles!) at
https://docs.google.com/open?id=0B2H2NJt34OffYmIyM2Q5ZGQtY2YyMi00ZmVkLThiYWEtZTE3MTUyZDEyMDM0
 
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