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MG MGB Technical - Throttle linkage

I have completed the rebuilding of the carburettors on my 71 roadster HS4s. They have been synchronised and the mixtures have been set.
Not sure of how to set up the throttle ttle linkage. There is a fair amount of free play each side where the tightening links fit into the slotted pieces attached to the throttle spindles. I assume if not assembled correctly the throttle could open out of sync. Thanks for any advice given. Trev
Trevor Harvey

Throttle linkage is in the lower half of the above.

A and B are the clamps that are set such that the two butterflies are opened in unison as the cable is pulled. This may need to be done after the carbs have been balanced for air flow and mixture i.e. both idle screws are set correctly. If you do it the other way round then when adjusting an idle screw the butterfly cam may be 'hanging' on the clamp finger.

The cable is set such that the butterflies sit on their idle screws leaving 12 thou of play (my WSM says 0.12" i.e. 120 thou which is an error) between the finger C and the choke spindle, the butterflies should not be 'hanging' on the cable when the pedal is released. This free play will be repeated where the fingers on the clamps fit into the slots of the butterfly cams.

The other adjustment of the cable is at the pedal end so that the pedal hits its floor stop a fraction before the butterflies are fully open, to avoid stretching the cable and pulling the spindles against the bushes.


paulh4

A and A are the clamps for the butterfly levers.
Dave O'Neill 2

Sorry yes A and A!
paulh4

Thanks for that Paul. Since posting I have discovers that clamps AandA also should have a 12 though gap to the bottom when tightening, that’s the main thing I wanted to know as there is quite a lot of movement there.
I intended to have the carbs re bushed but after taking advice and the price for this I decided to just fit new spindle shafts, I then fitted mini valve stem rubbers over the ends, had to cut them to fit but up till now am pleased with the result. Trev
Trevor Harvey

For HS4 carbs 12 thou is the clearance where the finger is against the choke spindle. That will give some free play at each clamp but it will be less than 12 thou as the finger is longer than the clamps.

The WSM says at the choke spindle for HS4 and at the clamps for HIF. Are you using Haynes? In that Section 21.11 mentions and Fig.13 shows measuring it at the choke spindle. For the HIF it is 'largely as described in Section 21' i.e. the HS4 'but with reference to Fig. 3.19'. However that figure is the exploded view of the whole carb so not much help for tuning, I think it should say Fig. 3.17 which shows the clearance at the clamps and that would be correct for HIF. 3.17 is mentioned in Section 23.19 for HS4 carbs but that is for emission control equipment vehicles i.e. North America.

Also make sure there is a little end-float of the spindle so that it's not pushing the butterfly quadrants hard against the carb body, but not so much that a finger can come out of its slot.
paulh4

With your help think I have the carbs set up well now but is still not running as I expected. I think the problem is coming from the dizzy, it doesn’t seem to be increasing the timing as the revs increase, according to to the strobe light it is decreasing the advance when the revs are increased. I know the dizzy is quite old as I believe it is original to the car and has been stripped down recently and checked that the advance mechanism was working also that the points plate was not seized so I am a bit baffled by this. Maybe it’s time for a replacement dizzy.
When using the strobe the white markers on the pulley and crankcase are not steady they bobble about think this is wear on the dizzy shaft.
Trevor Harvey

Timing jitter can come from timing chain/gear wear. Also cam and drive dog gear but less likely. Usually a minor issue that can be ignored for typical usage.

What is the reference number of your distributor? Should be 41288 for high compression.

Retarding instead of advancing with revs was covered at the beginning of the month under Ignition Timing, you said you found the trigger plate loose and after that the strobe showed it advancing with revs.

Electronics can do weird things, if you can then retro-fit points and condenser as a test at least.
paulh4

Yes it is a 41288, I suppose the jitter could be from the timing chain but they were changed about 5/6 years ago when I rebuilt the engine and probably done less than 10,000 miles since.
Tightening the plate and pickup collar certainly improved things a lot as before it was retardening past tdc. Now it’s retardening to about 5/6 btdc. So it is an improvement.
Using the strobe I set it at 13 btdc at 600 revs. When the throttle is lifted should it go lower than this at any time or is this normal to eliminate pinking. Thanks Paul.
Trevor Harvey

If it's retarding as the revs rise then something is wrong somewhere. I would suggest getting someone else to look over what is happening.

The timing at any revs should be short of pinking. When the plug fires the mixture doesn't explode but burns, and that takes a finite time. This starts as the piston approached TDC, which increases cylinder pressure above the increase that is coming from the burning mixture. If the pressure gets too high the remaining mixture will spontaneously combust - which is what happens in a diesel engine - and that is the pinking you can hear which can damage the engine.

As the revs rise TDC is reached and passed sooner, and once the piston is going down on the expansion stroke the pressure reduces, so the timing of the spark and the start of the burn can be advanced further. If it wasn't more and more of the burn would occur on the expansion stroke to be lost as waste heat.

Vacuum advance is also in play which, on engines with carb vacuum, increases the timing still further at gentle throttle openings once the throttle is off idle. But at large throttle openings there is more mixture in the cylinder so you get a bigger bang and the cylinder pressures are higher so the risk of pinking increases again. However the larger throttle opening reduces the vacuum so the advance is backed off and again reduces the chance of pinking.

In a large nutshell ho ho.
paulh4

Trevor, have you checked the timing with the vacuum to the dizzy disconnected while you increase and decrease the RPMs?

Jud
J K Chapin

We have found 90++% of timing scatter is down to wear in the gear on the camshaft.
Fit a new camshaft and it goes away completely.
Paul Walbran

Trev. is your strobe light a dial back type--If it is are you setting your 13 on the light or have you got it on 0 and reading off the scale on the engine----If you're an infrequent user of a dial up light it can be easy to get mixed up which way the timing goes, specially when the marks are underneath there.
IF it is a dial up light just leave it on 0 and read off the timing marks to see what the advance is doing.

willy
William Revit

Willy, you might have something there, yes the timing light is new to me, I’m more familiar with the old basic lights this new s/hand one is more complicated even though I have the instructions they are not easy to understand. There’s a wheel underneath the handle that as it’s tuned one way the flashing light moves forwards on the pulley and the opposite way when the wheel is turned back. So I used it turned fully back. I probably need to spend a bit more time with it to fully understand how it works. It’s made by Aircomp and feels well made but instructions are in pigeon English. Trev
Trevor Harvey

Cheers Trev. They have their uses but can be confusing at times to a new driver. I have taped mine in the fully back position. Unfortunately the one I have doesn't have a click into position wheel it just rolls around. I've been caught a few times where I thought it was right back but had accidentally bumped it round a bit--very frustrating, so it's got taped up on zero. and I just use it for basic timing. If I need to go deeper checking I'm usually hooked up to my bosch machine anyway, which is a bit more of a serious unit.
I'm sure you'll master your light, practice makes perfect as they say.
willy
William Revit

Even with a dial back light, and even if the control is in the 'wrong' position, the timing should still advance when the revs are raised unless the control is being moved at the same time. Unless the light is faulty ...
paulh4

Thanks for all the comments, think it’s sorted now. Originally the electronic ignition system had come loose that was easily fixed. It improved things but was not getting the advance I was expecting. Pulled the dizzy again and all looked OK. Reinstalled it, set the timing up again using the new to me strobe light raved it up to 2500 revs and the timing had retarded. What was happening was the dizzwas moving with the vibration it just needed tightening, I should have realised this earlier. A lesson learned. Trev
Trevor Harvey

I thought you meant it was retarding while revved.
paulh4

Paul, That’s exactly what was happening. As I increased the revs the strobe was showing a retardation, the distributor was moving by the vibration. Once reset and tightened enough it shows an advance when revved as it should. Hope I haven’t wasted your and other contributors time but I have learned a lot from this thanks again Trev.
Trevor Harvey

This thread was discussed between 19/11/2025 and 25/11/2025

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