TR2/3/3A TR3A 5000 rpm idle

[NutmegCT]

Twin H6 carbs. I was advised that my "jet lever to cam" link was set in the wrong hole of my fast idle cam, so I moved the link arm to the next hole on the cam.

Started up and the engine raced immediately to 5000rpm. Kicking the gas pedal changed nothing. I cut the switch within 5 seconds. Linkage isn't stuck anywhere on the carbs or throttle as far as I can tell.

Figured this is not the best way to treat a cold engine, so I moved the link arm back to the original hole (the middle one) in the cam.

Engine still races at 5000rpm. Then tried the hole at the opposite end of the cam. Same thing.

I haven't run it enough to warm it up, as I fear doing some damage. The fast idle screw isn't touching the cam. If it were, I'd think the idle would be even faster. I haven't turned either of the throttle adjustment screws.

Anyone have a suggestion on what I did wrong?

Thanks.
Tom

 

[Don Elliott]

The engine will run at any speed if the air/fuel ratio is about 14 to 1. This means 14 times air by weight to gas by weight. Some call this the "mixture". If it's say 12 to 1, it's running "rich" - not enough air and too much gas. And vice versa for "too lean". It sounds like yours, running at 5000 rpm, still has the right mixture.

But the gas pedal (throttle) pedal is not really a gas pedal. It's a pedal that controls the air. Step on the pedal and all the linkage does is open the round butterflies in the two carbs. This causes an inrush of air and the vacuum produced causes the pistons to rise in the carbs. The needle in attached to the bottom of the piston, so if the pistons are in the up position and the air/fuel mixture is correct, you are probably driving along the highway at 70 or 90 MPH.

If you get this with the car stopped in your driveway, it means that your pistons are wedged up at the top and your butterflies are open. Disconnect all the linkages for the throttle and for the choke and if it still runs at 5000 RPM, stop the engine and clean out the pistons for the carbs. Did you have them out recently? If not, take out the 3 screws holding down the bell-shaped top. Do them one at a time. Don't get the parts mixed. All parts for carb #1 have to stay with carb #1, etc. Clean the inside wall with solvent and a paper towel. Nothing more abrasive than that. Then do the same for the surface on the outside of the piston where it is guided in the bell-shaped top. Those grooves have to be clean. Read all the details in the mauual. Put it back together and make sure the oil in the dashpots is correct.

Those grooves form what is called a labyrinth seal. The piston gets sucked up by the vacuum and then the air moves past between the piston and the inside circular wall past those grooves. The grooves cause turbulence as the air loses its momentum in each groove as it passes the first groove, then again as it reaches the 2nd and so on. This turbulence causes the sealing result.

Let us know what you discover.

 

[NutmegCT]

Thanks Don for the suggestions.

I'm surprised that moving the cam link to the next adjustment hole has caused all this! I certainly haven't had those pistons out. As I've never taken a carb apart I sure hope I don't screw something else up.

This is going to be a long day ...

Tom

 

[MadMarx]

Remove the linkage from the fast idle cam and start without.
Make sure that the throttles are closed and the idle screws does touch their rest.

The fast idle lever should have 1.5mm clearance to the cam when not in action.

Cheers
Chris

 

[Don Elliott]

When trying to solve any problem - in any field of life - ask yourself, "What has changed ?" Find what has changed and you've found the answer to the question. Then go back one step at a time until you get back to where it was before. On another thread here today, someone mentions stamping punch marks in the carb parts to avoid him getting them mixed up. I usually only take one carb apart at a time, but if they are both apart, I put them in separate work-trays identified as to where they came from and I put them back that way.

 

[BOXoROCKS]

dont panic,post pix and we'll figure it out. linkage,all of it, both carbs.

 

[NutmegCT]

OK - here we go.

The only thing I changed was to move the "jet lever to cam" link to the next cam hole. After the rpm surprise, I put the link back where it was (middle hole). Problem remained.

Looking through the air intake of the rear carb, there's about 1/4 inch clearance between the bottom of the pistons and the base of the housing. Pushing up with a finger, piston rises about 1/8 inch and drops smoothly. The butterfly is closed. Exact same findings on front carb.

Removed the rear carb "bell top", put screws and spring in a pan, carefully pulled out piston (easy to do), cleaned off piston surface and "needle" with paper towel (some black "dust" - not much), cleaned chamber surface (same), refit to slot and replaced. Slid down easily. Refit spring and screwed top back on.

Did same to front carb.

Questions:

Chris - you mention "The fast idle lever should have 1.5mm clearance to the cam when not in action." Do you mean the fast idle *screw tip* should have clearance? Sorry - guess I don't know what the fast idle *lever* is.

How do I decide on which cam hole the "jet lever to cam" link should go in?

Thanks all!
Tom

 

[BOXoROCKS]

they are theoretically seasonal settings,summer/winter/global warming.

 

[Don Elliott]

Tom - Did I understand that with the engine off and you look into the carb throats, the bottom of the pistons are 1/4" above the bottom base of the throat? If so, there is something wedging the up. Maybe your needles need to be re-centered.

The pistons should sit be at the bottom. When you lift the pistons, (say with a long thin screwdriver) about 1/2" or more, then you pull out the screwdriver, the pistons should drop rapidly and smoothly all the way to the bottom and when they reach the bottom, they will "click" when they lightly hit the bottom base.

 

[NutmegCT]

Don - I wasn't clear. The piston does rest fully on the base. I see that "1/4 inch space" between the bottom of the piston and the bottom of the air intake opening.

Both pistons do drop quickly to the bottom and "click" when lifted and released.

Edit: note that the pistons lift *easily* about 1/4 inch. After that, resistance is felt, but they will go much higher.

How do you determine which cam hole to use on that linkage?

Tom

 

[Don Elliott]

I never did. I always put it back the way it was. At least I think that I thought I did. But I never had any 5000 RPM idle problems and since I rarely use the choke in the summer, I haven't bothered with it or played (experimented) with it.

 

[NutmegCT]

Don - thanks for the clarification. I had thought you mentioned in a different thread that I had mine on the wrong setting.

Now that I've got the piston assembly removed, I think now I'm going to go ahead and try to grease those upper and lower cork seals in the jet assembly. That may help my "tough to pull" choke problem and I'll learn more about the carbs.

I wish I could find a "step by step on how to take the jet assembly apart and refit it properly" guideline.

Thanks.
Tom

 

[MadMarx]

Questions:

Chris - you mention "The fast idle lever should have 1.5mm clearance to the cam when not in action." Do you mean the fast idle *screw tip* should have clearance? Sorry - guess I don't know what the fast idle *lever* is.

How do I decide on which cam hole the "jet lever to cam" link should go in?

Hi Tom,

as you can see on that scetch there is a clearance between the fast idle screw and the cam....do you have clearance?

 

[NutmegCT]

Thanks Chris. Yes - I've got clearance there.

Tom

 

[MadMarx]

So you better start from new.
Hope you have a synchrotester?
If not, get one, they are cheap and you'll need them from time to time.

Screw the idle screws as far out so that the butterflies are closed.
Loose the clamping between the carbs, screw the two idle screw about half a turn in and start the engine. With help of the synchrotester find the idle speed and tighten the clamp at the throttles again. Then adjust the fast idle assembly.

Best you get a SU-carbs book or an owners manual.

Cheers
Chris

 

[NutmegCT]

Found the problem.

When I was working yesterday to pull off that "extra" spring that was hooked to the generator and to the throttle shaft, somehow a twig (!) got caught between the *rear* throttle stop adjusting screw and the throttle bar. As I was working on the fast idle cam area, I never thought to look back behind the rear carb.

Idle is back to normal. Gremlins back behind bars.

Thanks all! Now to grease those cork seals in the jet assemblies. I want to get that choke to operate without having to pull with half my body weight on the choke knob.

Tom

 

[TRclassic3]

I'm wrestling with the cam position as well. If I use the top hole then I get the best idle with the choke off, but pulling the choke half way does not effectivley increase the idle. If I use the center hole then I get reasonable increase with the choke pulled half way, but with the choke in the idle is around 1300... What to do?

 

[Don Elliott]

So you found what had changed. The twig was the change and you resolved that problem. Now on valiantly to the next one. Keep us informed.

 

[NickMorgan]

Sounds like one of those excuses the garage uses when they goof something up!! A likely story!!
Just for info, I wanted to increase the fast idle on my TR3 without changing the mixture any more. This was because the car runs fine, but tends to cut off at junctions within the first four miles from home. Then I discovered that the only apparent way to do so is to change the position of that connecting rod in the fast idle cam. Mine is in the middle hole and if I change it to the bottom hole the idle speed will be permanently changed as the screw is already at the top of its adjustment. The top hole didn't seem to work either, but i can't remember why. I wonder if different length rods were available to use in the different holes.

 

[TRclassic3]

I was wondering the same thing. My experience is the same as yours with the car stalling before completely warm.It would appear that either a slightly different length rod, or adding a hole to the cam (in my case between the existing top and middle hole) could improve he situation. Wondering if anyone has tried this.