Choke linkage nuts

[Member 10617]

Can someone show me a photo of the "choke linkage nuts." (Or point them out in Moss catalog or Practical Hints).

This is probably so simple a question that the manuals don't bother to label them. I think I see them on p. 35 of Practical Hints, but they are not identified as such. I could guess, but I'd rather not.

Reason being is that I need to loosen them to adjust the mixture.

 

[TR3driver]

There isn't anything you need to loosen before adjusting the mixture; the only nuts involved are the mixture adjustments themselves.

There are some locknuts on the rod that links the choke levers together, but they only need to be loosened to adjust the synchronization of the chokes.

Here's a blowup of p.35, with the mixture nuts circled in red and the linkage locknuts in green.

 

[Member 10617]

Randall,

Many thanks. I read in a technical article on tuning SU carburettors, by Scott Fisher. He writes in reference to adjusting the mixture:

"1. Shut the car off and loosen the choke linkage nuts."

Then he covers raising the lifting pins and turning the mixture nuts, etc., etc.

... and he concludes:


"5. Tighten the choke linkage nuts so that the choke cable will pull an equal amount on both mixture nuts when you pull the knob."

 

[dklawson]

Until Randall posts again I will add that Scott Fisher is a Mini owning acquaintance of mine. Chances are very, very good that Scott wrote the SU article concerning the later HS series of carbs. Your early TR will have the H series carbs as Randall shows in the picture above.

I cannot comment on how/whether/if it is important to loosen the choke linkage on the H series carbs. I can say that if you fail to loosen the choke linkage and back off the fast idle screws on HS series carbs it is easy to make mistakes when setting the mixture and idle speed. I speculate the H series are different and Scott's article may not apply to your carbs.

 

[TR3driver]

Seems like as good a theory as any. Or possibly it's just a difference of opinion.

Always a problem with anything you read on the Internet: you have to decide what to believe. That includes what I write.

 

[Member 10617]

You may be right. Fisher doesn't make it clear, but he mentions HS-type carbs and HIF-type carbs in one section where he gives specific instructions for dropping the jet in them.

BTW. He also writes that you have to loosen the throttle linkage nuts, as well as the choke linkage nuts when adjusting the mixture.

As my 58 TR3A has original carbs, I assume they are H-type. Here is a photo:

 

[TR3driver]

That was an overall adjustment procedure, not just mixture. Loosening the throttle couplings is so you can synchronize the throttles.

To my way of thinking, synchronizing throttles (and chokes) is one of those things you do once, when the adjustments have been disturbed for some reason (like the carbs off the manifold). But touching up the mixture is a more frequent operation. I may even turn the mixture nuts a flat in a parking lot, if it seems to be running a bit lean or rich while I'm out on the road.

Carbs mix basically by volume, so the mixture can change if the density of either the fuel or air changes. At high altitude, for example, the air is less dense so the mixture tends to go rich. Same thing happens if you happen to get a tank of pure gasoline instead of E10.

Or maybe I just like to fiddle :laugh:

 

[Member 10617]

Randall,

I, too,like to fiddle. But I still know just enough to get into trouble.

The carbs were synchronized some time ago in the shop, so I haven't fiddled around with them. But adjusting the mixture with the mixture adjusting nuts and resetting the idle seems fairly straight forward.

My only remaining question is this. All the instructions say to raise the piston lifting pins and listen to the engine. If engine speed increases and doesn't fall, then that carb is too rich; if engine speed falls fast, then it is too lean; if engine speed rises momentarily and then returns to normal idle, then the carb is just right.

My question is this (and none of the manuals add this important information): while doing this operation, do you continue to hold the lifting pin up, or do you lift it up, release it, and wait for the engine noise to let you know if it is too rich, too lean, or just right. Seems simple enough, but it's an important detail they all leave out.

 

[TR3driver]

Hold the pin up while you listen to the engine rpm.

 

[Member 10617]

Randall,

Many thanks... as usual. I don't know why the manuals don't mention this, but they don't, and it seems as though it would make a difference.

This reminds me of getting around in Paris. If you are looking for something, say a museum for example, there are great signs directing you here, and there, and round and round, and step by step. But... once you are in uncharted territory, the signs suddenly disappear! No one can give you directions. You are on your own.

Fortunately the mountains around here -- the Blue Ridge Mountains of Virginia -- are not so high (whimpy "mountains," by some estimations) that I have to stop from time to time to adjust the mixture. That was something I had to contend with out west, however, in Arizona and Colorado.


One more question -- I hope my last -- about these screws. Moss instructions on balancing carbs and setting mixture speak about setting the "slow idle screws" and the "fast idle screws." The "fast idle screws" have to be set to appropriate workshop manual specifications. Could they be speaking of the "throttle adjusting screws" ("slow") and the "choke control screws" ("fast")? And if so, is the setting for the "fast idle screws" 1/16" or 1/32"?

It would be nice if there were some consistency in nomenclature.