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MG TD TF 1500 - TD/C Choke Mechanism Mystery

As I started to prepare the carburetors last night, I noticed a couple issues with the choke pieces for my TD/C7254. Both carbs are numbered 4031, indicating to me they are correct for the car. However, the choke mechanism operation is a mystery for me.

First, there aren't linkage pieces from the jet lever to increase the idle speed when the choke is pulled. Also, the parts diagrams of Moss and Abingdon spares indicate the front carb jet lever should have a hole for a link to the fast idle mechanism at the throttle shaft. Perhaps the lever is incorrect.

Second, the arm that links the jet levers appears to be too long by at least an inch. Its so long that if I were to install it now, it would force the rear jet to remain in a dropped position.

Also, the front carb now has an idle adjustment stop with two screws...one for regular idle speed adjustment and the other for cold start fast idle (which now would screw into open space). Its similar to TC throttle stop, and perhaps incorrect for a TD/C set-up.

Please help me correct the linkage and include photos of what would be correct.

Thanks,
Charlie Adams
Fairfax Station. VA


Charlie Adams

Missing Fast Idle Parts?

Charlie Adams

Sorry the photos were loaded out of order...

Charlie Adams

Charlie wrote:
"Second, the arm that links the jet levers appears to be too long by at least an inch. Its so long that if I were to install it now, it would force the rear jet to remain in a dropped position. "

Make sure that the link fits the distance between the two carburettors. The springs are not realy strong and if the choke is pushed back with its flexible drilled cable while the jets assemblies are not free in the end the jets assemblies will stuck. I guess you need a shorter link.
W_Mueller

Hello Charlie. First is your rear carb drilled for the pivot screw that holds the fast idle lever? Second the choke lever arms do not have the hole in them for the fast idle link. I can't see from the pictures if the rear lever has the hole. See the attached picture. Sorry the carbs are so dirty just an extra set I had laying around.

Butch

R Taras

Butch,

The rear carb has a boss that is drilled to a depth of 1/4 inch, the bore is smooth not threaded. I tried to capture that in the photo below. I wonder why, your photo shows a screw going into that boss. Perhaps a fast idle was never installed on this carb?

It would certainly be possible to install the set up on the carb in your photo. I'd need to thread the hole and perhaps drill it a little deeper.

I can also see from your photos that the jet link has a third hole to provide for a rod to reach upward to lever. I've no clue why the attachment points for driving the fast idle lever do not appear on my carb.

Also, I'll shorten the link between the carbs by removing the lock nuts and shortening the end pieces. By doing so I can see that I can make the rod a correct length.

Charlie


Charlie Adams

Hi Charlie. I think you have two rear carb bodies.

Butch
R Taras

The link goes in the upper hole of the back arm, not the bottom one. The bottom one has the fitting for the cable.
Christopher Couper

Thanks Chris, the link fits now that I'm using the correct holes.

Butch, Oh my gosh! How can I go about determining if I have two rear carb bodies...I have found so many issues as I've worked thorugh this TD.

Do I have any options besides searching for a correct set of carbs? Searching for 1 1/2" MKII carbs sounds to me like a search for hen's teeth.

Might the choke fast idle speed mechanism be added to the carb I have?

Charlie
Charlie Adams

Here is a photo of the front carb.



Charlie Adams

Hi Charlie. Yes the front body can be drilled and tapped to take the pivot bolt. I don't remember what the thread size is. I can look it up if you need. You will also need to get the correct choke lever and linkages. Yes, finding a set of the mark II carbs is difficult and no one wants to pay what they are worth.

Butch
R Taras


Charlie, the throttle stop (part with 2 adjusting screws) on your front carb is from an early TC, not TD.

Keep in mind all of the linkage should be TD which is identical to TDC.
Richard Cameron

I've owned my TD for almost 40 years and have never had a fast idle/choke linkage. Have I been missing out on all the fun somehow?
Lew Palmer

Lew

The link was missing from my car for 24 years. I found it in a box from the PO and installed it. Just for fun. I think it's only necessary in winter?
W_Mueller

Butch,

I'd certainly appreciate you looking up the thread size of the pivot bolt for me. I don't have a source for the carb thread sizes.

Richard, I'll get that TD throttle stop! Thanks.

I found Abingdon Spares has parts listings for TD MKII 1 1/2 in carbs. The parts altogether (jet lever, link, fast idle lever, pivot bolt, pivot bolt washer, throttle stop front, tapered pin, and a new throttle shaft in case I can't get the existing tapered pin to release) are $130.

Any suggestions for dealing with the tapered pin on the throttle shaft?

By the way, I'm planning on installing the engine and transmission tomorrow. I finished the engine transmission rebuild last week.

Thanks for the great help!

Charlie


Charlie Adams

Charlie,

It seems like you have a hotchpotch of parts on your carbs. Some are from a TC. Even the bodies have been reworked to fit your car, air cleaner holes opened to slots. You first need to sort out why the cantilevers are at different angles. From the photos I think that the black support arms are different lengths. Measure the length of the vertical section of the rear carb support and compare it to the front one.

John

J Scragg

Hi Charlie. The thread on one's that I have is 6mm x 1.00. The hole is drilled thru into the body. There are two different ones. One has a pivot area .170 long and the other one is about .265 long. The long one uses a spring washer for tension. I think one is for the T series and the other is for MGA.
The tapered pin is knocked out from the bottom side. You need to take the shaft out of the body when doing it. much harder to do assembled to the body. When putting it back together be sure to spread the slots in the screws for the throttle disk. When assembling the throttle stop to the shaft drill the hole with a #31 drill thru and ream to size with a #000 taper pin reamer, that's the proper way. Also be sure to use a .015 shim to space the throttle stop away from the body and remove when finished. If you don't, when the carb's get hot the throttle stop will jam to the body and you can have a most unpleasant experience with the carbs not closing. Most people just drill thru with a 1/8" drill and use a 1/8 roll pin.

Butch
R Taras


Oval holes on carb intake flange is correct for an early TD/C. This permitted the early aluminum air cleaner manifold to fit, but it was soon discovered that it restricted air flow.
Richard Cameron

Charlie: On a side note your engine/trans looks real nice and you are doing a great job with the finish etc.

One piece of caution if you want to maintain the look:
The clamps for the exhaust manifold should be finished like you did for the manifold (looks like you used a high heat paint or even might have had it aluminized). With the engine paint that you have coated them it will probably burn off.
Christopher Couper

Thanks to all for helping unravel this issue!

Butch, I can get that done, and will let you know how its going! I'll look closely at the pieces I receive and will go from there. Perhaps I will have a few questions. Your insight is enormously helpful.

John, I agree someone had there hands on these carbs and changed them. The car has been off the road since 1960 and had three prior owners.

The MG factory did the carb slots. My understanding is that the TD/C cars were equiped with 1 1/2 inch carbs and the slots were added to accomodate the standard TD air cleaner manifold set-up for 1 1/4 carbs. In late production, a larger air cleaner manifold was provided.

Chris, Thanks for noticing my finishing efforts. I had the manifold done by Jet Hot. Over the winter I'll get those manifold clamps done as well.

Charlie

Charlie Adams

Mark II (TD/C) carbs will ALL have the "4031/" cast in the bodies, followed by a vibra-etched "8" on the rear carb, and "12" on the front.

The 4031/12 front carb should have the fast-idle mechanism. If you need a photo, let me know. I may have fast-idle parts on an old carb; let me know if you want me to paw through them.

I also have both early and late Mark II carb sets available, completely re-manufactured.

Tom Lange
MGT Repair
t lange

Tom,

I am happy to see your note...please see the photo's below.

Both carbs have the cast 4031/ and both are marked F12.

The first photo shows the rear carb, and next the front which only a TC-like idle speed lever on its shaft.

These carburetors are servicable although there is a little looseness in the shafts. The throttle plates are new, and the pistons have good suction and drop smoothly.

I'm sending you an email.

Charlie


Charlie Adams

Here is the front carb.

Charlie Adams

John the slotted holes are correct for early TD/C’s. This was to accommodate the early std TD air cleaner.
W A Chasser

I was never really happy with the stock fast set up on my TD. Later SUs had a cam that allowed for an idle speed increase with the first 1/4 inch of travel on the choke cable without any mixture increase. I was able to use SU parts from a Triumph TR3 to use a cam to open the throttle before enrichment takes place. Photo of the original factory set up.

John Quilter (TD8986)

And here is the modification that give the fast idle without choke operation.

John Quilter (TD8986)

Hi John

How’s that work? In your first picture with the original setup when you pull on the choke the arm is pulled down thereby raising the platform that the fast idle screw acts on. In the second picture it looks like if the arm is pulled down the cam will rotate clockwise and the fast idle screw will not contact the cam. What am I missing? Otherwise it looks great.

Stuart
S Grimm

Here is another pic of the linkage lower down. The silver bar rotates counter clockwise, pushing the rod up to turn the cam and progressively increase idle speed before the slack in the linkages created by using smaller pins, starts to move the jet down. It's been a while since I figured all this out but it is really quite effective allowing me to run on a fast idle but without mixture enrichment until the engine is partially warmed up. Later SU carbs, such as my two Morris Minors, all had this type set up although with a single carb.


John Quilter (TD8986)

I see what you did. The bottom of the fast idle linkage was relocated to the opposite side of the pivot to change the direction of rotation of the fast idle cam. Nice.
S Grimm

For my supercharger kit I needed the same modification, I used this lever instead, I found one in my box of spares.



J Scragg

Don't forget Burlen
http://sucarb.co.uk/?SID=f96bpfppd2jqoj7ucldkd69gu6&___store=su I've found them extremely helpful.
JK Mazgaj

I have installed a set of new H2 carbs from FTFU on my 53 TD (52 engine).Although not a huge problem, my fast idle screw is turned down as far as it will go (without removing the spring) and the gap is still way more than the recommended. 016". Any thoughts?
W Worden

I would start by making your question a new thread - piggy-back threads get lost.

Tom Lange
t lange

This thread was discussed between 24/09/2019 and 07/10/2019

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