Mark V Throttle Linkage Questions

[Roger_McWilliams]

My left-hand drive Mark V presently has a throttle that does not quite return to low idle without a “blip” of the throttle pedal after going into neutral.

Spare Parts Catalogue Plate M and parts listed on page 21:
3264 Spindle, Throttle, Rear Carburetter M.7
199/4 Stop on Throttle Spindles M.8
4057 Stop, Sleeve, for Loose Lever M.9
4059/2 Lever, Loose, on Rear Spindle M.10
1380 Collar, Split, retaining Loose Lever M.11
3120/1 Lever, Fixed, on Rear Spindle M.12
681 Coupling, Flexible (complete with Clamps, Bolts and Nuts) M.13

Spare Parts Catalogue Plate BZ and parts listed on page 52:
C.3474 CONNECTING SPINDLE BZ.17
1942 Lever, on Spindle, adjacent to Socket BZ.18
C.3597 Coupling, Flexible from Connecting Spindle to Rear Carburetter Spindle BZ.27
307 Anchor for Carburetter Control Spring BZ.28
310 Spring, Carburetter Control BZ.29

The Service Manual repeats the parts plate at page C.3 Plate C.1.

The Service Manual on page C.10 Plate C.6 shows photograph of parts of the linkage.

The photo below shows my present linkage aft of rear carb. Order of parts on my car from rear carb to firewall appears to be (corrections welcome):

M.8 stop on spindle/M.7spindle end/BZ.27 Coupling, Flexible/BZ.17 Connecting Spindle/M.12? Lever/BZ.29 Spring/BZ.28 Anchor/BZ.18 Lever

Question 1: I don’t see M.9, M.10, M.11, or M.12 on my car. Where are these found, or are they only found on right-hand drive cars and should be absent on my left-hand drive car?

Question 2: My C.3257 Coupling, Flexible has the coupling spring wound in the direction that tightens the winding when the accelerator is depressed. When the accelerator is released from the foot, the car right now is not quite returning to low idle, but instead needs a throttle blip to seat correctly. In the garage, a finger motion of the accelerator linkage and then gentle return arrives where the 199/4 Stop on Throttle Spindles M.8 have about 0.006” free clearance unless the throttle is blipped. Should the flexible linkage be reversed so that it instead tightens when the accelerator is released?

Question 3: Do you see any other ways the linkage should be altered? I have put the 310 Spring, Carburetter Control BZ.29 towards the rear to give easier access to the distributor rotation and straight down spring motion rather than pulling slightly forward.

[Rob_Reilly]

Well, it’s late here and I’m sleepy and it’s raining and I don’t want to go out to the barn tonight, and I don’t have a photo of my full linkage, but I know I do not have parts M9, M10 or M11.
Why is part M10 called a loose lever? It is all about the hand throttle control that was on previous models. Sort of an early version of cruise control. My '38 SS has it, although I’ve never used it. AFAIK, no Mark V ever had one. These parts do not belong in the Mark V SPC.
Anyway, my M12 is on the carb shaft immediately adjacent to M8. My BZ29 return spring is hooked on BZ28 anchor tab, which is on a different bolt than yours, more forward. So it always pulls the throttle closed, even if the linkage were to be a little bit loose.
This is the only pic I have on my computer.

[ptelivuo (Pekka Telivuo)]

Hi Roger,

I think this is a problematic design, I have had the same issue the past 15 years. I had managed to tweak it away in 2017 but then after the 2019-2020 engine rebuild it returned.

I think this is why Jaguar decided to have a return spring directly on each individual throttle spindle in the triple carb setups. There are too many small variables in the whole throttle linkage so it is very likely that both plates will not close exactly the same amount.

Cheers!

Ps. But of course the H3/H4 does not have a separate idle passage like the HD8’s have.
Still I have been thinking that perhsps adding a secondary return arm and spring to the front spindle might help?

[Peter_Lloyd]

The loose lever was as Rob said - for the hand throttle. As the hand throttle is wound out it pulls the lever up under the idle stop. Its purpose is to raise and hold the idle up to prevent the coughing, spluttering and stalling so common with cold starts on all early cars. Without these you had to constantly blip the throttle until it settled down. If you remember your dad or grandad starting his car in winter, he would have to tickle the throttle for a few minutes until it smoothed out, otherwise he couldn’t get out of the driveway without stalling.

[Rob_Reilly]

Yeah, it’s a bit congested in there on LHD where the vertical rod has to go up past the plug wires.

Here’s one with the axial spring on the throttle shaft.

I’m 50% sure that these came into fashion about 1950 or so.
You can get them from Moss et al, look under MGA parts.
I recall reading somewhere that SCCA began requiring a return spring on each carb to prevent runaways in racing. This was the way MG chose to comply.

[Roger_McWilliams]

Thank you all for your remarks. I’ve moved the Carburetter Control Spring and anchor to location similar to Rob’s photo. That provides direct, secure pulldown on the carb spindle. It can be set up to just clear my distributor vacuum unit. (BTW, I also recall SCCA requiring multiple return springs).

Amusingly, the flexible coupling has the same winding direction regardless of which end goes towards the carbs.

Rob’s 1942 Lever, on Spindle, adjacent to Socket BZ.18 appears adjacent to the side of the engine block. My lever must go behind the block since the swing takes it to overlap the block edge by about half an inch (more than 1 cm). Is my lever at the socket too long and wrong part? My lever is about 2.4 inches long (end to end).

[Rob_Reilly]

My lever has a bend in it and is installed so I can’t get a good measurement, but it is about 2" measuring along the angle. It has a good 1/2" clearance to the cylinder head.
Sounds like you may have the wrong lever, maybe a RHD part #1498?

The flexible link shaft is like a left hand thread.
On the XK120 engine it is like a right hand thread.

[ptelivuo (Pekka Telivuo)]

@Rob_Reilly

I’m 50% sure that these came into fashion about 1950 or so.
You can get them from Moss et al, look under MGA parts.

Hi,

Thanks, I might add one or two!

Too bad we don’t have Moss Motors but the MG is a good hint, I’ll ask the local Anglo Parts dealer.

Cheers!

[Roger_McWilliams]

The throttle linkage shaft going towards the carbs has a connecting spindle shaft axis to block edge distance of 1.6 inches just in front of the rear lever (which may or may not be the 1942 Lever, on Spindle, adjacent to Socket, similar to the BZ.18 for the RHD 1498 bit).

Is my engine installed too close to the connecting spindle axis? I think not, it would take much thicker carb distance pieces if the engine were further offset towards the RH side. When viewed from above, the center of the rocker cover lines up with the left hand hex head on the firewall. Just under the front of the battery shelf there are two hex heads with some sort of small tube between, all going longitudinally in firewall. The rocker cover center is in line with the LH hex. I don’t know what those two hex heads or the tube do. My guess is the back end lever on the carb spindle somehow is not original even though it works.

[Rob_Reilly]

The little tube is the air bleed from the heater. When you first fill the radiator, air is trapped in the heater. If you open the air bleed valve inside the car, first air and then coolant will come out of that tube. Then you close the valve.
The hex head screws are holding the heater mounting frame to the scuttle.
It is correct that the rocker cover is not centered wrt the body, but the crankshaft is.

[Roger_McWilliams]

My throttle return spring brings the spindle rotation right down to the stop now. Thanks for input to change the assembly positions on my car.

The rear 1942 Lever, Spindle, adjacent to Socket which has the ball attachment to the control rod still is a bit of a mystery to me. Rob’s looks properly functional next to the block while mine is behind the block because there is insufficient clearance for it next to the block. Mine works fine, but I’ve grabbed the part off my parts car for comparison.

Both my car and the parts car have the part with an embossed number 1942A on the part adjacent a rectangular boss (see photo). And both my car and my parts car the lever is about 2.4 inches long with neither able to clear if adjacent to the side of the block. The parts car has the lever straight while on my car it is bent to the rear for clearance. Perhaps the linkage parts had changes over time which did not need parts renumber?

[Rob_Reilly]

Trying to read the number on mine with a mirror, it looks like 3120 on both levers.
That’s the number in the SPC for the fixed throttle lever M12 on page 21.

I thought of something else. Do you have the insulating spacers between the carbs and the head?

[Roger_McWilliams]

The photo below shows the front carb spacer assembly on my Mark V. Starting at the head there is Washer, Jointing for Carburettors Plate H.50; C.347 Piece, Distance, between Carburettor and Cylinder Head Plate H.49; Washer (again H.50). And attaching to the starting carb is 1469/B Pipe, Starting Plate M.50A with the sharp bend. The way it all fits together and matches the parts catalog diagram makes me think the spacing of the carbs is correct. However, I don’t have the thicker washer between the spacer and the carb as showing in Rob’s photo for the back carb.

Amusingly, throttle return spring on my car is attached to a lever without bend which has embossed number 1498 which is the number for the BZ.18 spindle lever that was used in the RHD linkage. My car has been LHD from the beginning and with Los Angeles as delivery point.

In service manual Plate B.40 it looks like the lever for the throttle return spring has a bend to it, which is similar to the one in Rob’s photos.

[ptelivuo (Pekka Telivuo)]

Hi,

I am not sure if the spacers between the head and the carbs look short, maybe it’s just the angle. I added a spring and plate intended for an MG (I believe MGA) that I got from a local Anglo Parts dealer.

Cheers!