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MG MGA - Mysterious revving

Hi guys,

I have a problem. I have a 2099cc 5 bearing breathing on twin HS6 carbs. Needles, etc. all fine and it has been tuned by a proper tuner. It fires up on the button and ticks over at about 800rpm. Oil pressure sits around 70psi pretty quickly.

However, the problem: if you touch the throttle, it goes happily to 3000+rpm and stays there, regardless of throttle position, occasionally continuing to rev higher and into the red line with no foot on the throttle.

The only way that I have found to stop the mysterious revving is to give the throttle a blip. Whilst this sounds nice on the drive, it is a tad worrying when driving.

I have checked the carbs, and on release of the throttle pedal, they return to their idle position instantly, yet the engine keeps on revving until I blip the throttle.

Today was the first day that the car has been out of the garage since its rebuild and I was hoping to take it to the Bath Motor Pageant tomorrow, but I'm not sure if I want to chance it.

Any help or recommendations of a good mechanic in Bristol (UK) would be most appreciated,

thanks in advance,

Grant :-)

PS I have also cross-posted this to the MGB side of things as its a modified 5 bearing
G Hudson

Are there return springs on the cable and carb linkages?

Chris at Octarine Services

Hi Chris,

yes, two on the linkages and one on the throttle. I even added another two on the linkage just incase things were a bit sticky, but it makes no difference. The throttle linkages swing back to the idle position instantly, as they should and the throttle pedal comes back up, but the revs stay as if the throttle is depressed.

That's what is confusing me. It's as if even though the carbs are closed, they are also wide open as the engine continues to rev.

cheers,

Grant :-)
G Hudson

Also, the choke is fully in, so no choke after an initial bit for starting.
G Hudson

Sounds like the carb pistons are staying up even when the butterfly valve is closed. Are teh filters on the right way up so that the backplate is not obscuring the pressure balace airways to above the pistons? Is the oil in the dashpots the rightbweight?
dominic clancy

Hi Dominic,

dashpot oil is correct - official SU stuff from Burlen and not too much of it. Cracking idea about the carb backplates! I'll just go and check,

thanks!

Grant :-)
G Hudson

Hi Dominic,

How did you know? Yes, one of the filter back plates was on upside down. I can't test it at the moment as it would upset the neighbours, but I'll give it a go tomorrow morning.

There's still hope that I can make it tomorrow!

Huge thanks,

Grant :-)
G Hudson

Loose clamp on a throttle shaft, so the two throttle plates do not move in unison. Fast revving with throttle off means one of the throttle plates is not fully closed. Half engine speed with full throttle down means one of the throttle plates is not opening.
Barney Gaylord

Hi,

no, that wasn't it. It still randomly revs. it is possible that one (or both) of the pistons is sticking, but I don't have time to check that at the moment, as I'm of to the pageant in the normal car instead. I'll have a look this evening and report back.

thanks for the idea so far,

Grant :-)
G Hudson

Hi Barney,

the last post was to Dominic. There is a slight slack on the right hand side carb before it engages, about 0.5mm, whereas the left hand side carb has no slack. I'll check that too tonight without the filters on.

Many thanks,

Grant :-)
G Hudson

Grant,

There is supposed to be a slight delay in the second SU opening.

See attached image.

Mick


M F Anderson

Hi,
I'm back! OK, I've taken the filters off and had it running...

Mick, thanks, I noticed the second carb's piston coming up later than the first - it's good to know that it should.

looking inside the pistons rise about 2-3mm on startup and that's the position that they stay at as the lowest point until I shut the engine off.

The first carb seems to be a little slow, but not being an expert it is difficult to judge.

However, after chatting to a few people at the car show this morning, I've tracked down a local mg specialist who came well recommended on service and price, so hopefully it'll be fixed and ready for my week of driving it round Dorset in August.

Many thanks,

Grant :-)
G Hudson

There is no "delay" mechanism on the MGA throttle shafts with H-type carburetors. The shafts are locked together so both throttle plates move at the same time, and both air pistons should rise in unison. There is a delay mechanism on the HS type carbs linkage so the rear carb opens slightly before the front carb.

Any time the engine is running faster than a slow idle, a throttle plate is partly open. Close the throttle plates, and the engine will die. Even if there was a substantial air leak elsewhere, closing the throttle plates shuts off the fuel supply, and it won't run with plain air and no fuel.

Given sufficient fuel to make correct mixture, engine speed is determined by air flow versus load. When it revs 3000+rpm with foot off the throttle, a throttle plate is sticking open.
Barney Gaylord

Are both throttle plates snugly sealed when closed? If one leaks it coul be acting like it is partially open all the time.
Steven B

The H6 carburetors on the mga twin cam do have the delay mechanism.

See attached image from the Twin Cam SPL (item 93).

Mick


M F Anderson

Re the "delay" mechanism - my Twin Cam has the H6 carbs with link pin and hole as Mick describes. As I have never been able to understand why the carb throttles should open differently, indeed it would seem counter-productive to me, mine are set with the pin resting against the edge of the hole so that they both open together. It runs fine. Probably wrong! But why not?
Bruce
Bruce Mayo

Bruce,

I have been unable to find a plausible engineering reason for the delay.
However it appears in all the SU repair and overhaul documents.
I have always used the delay setting from the early 1950's when working on four cylinder Triumph TR sports cars.
As only a guess I think it may be to simplify setting the idle using one carburetor.
Only when you come off idle does the second carburetor come into use.
This is only a wild guess.

I have attached an image of the earlier system used.
I always set the 0.006" gap.

Mick


M F Anderson

Mick,

that looks like what I've got on my carbs. Might be the difference in philosophy between the H and HS style of carb.

all the best,

Grant :-)
G Hudson

Hi guys,

after getting the car transported to a local garage (who turned out to be an MG specialst about 5 miles up the road), I left slightly red faced after about a minute's inspection and a knowing "ahh, that'd be your problem then".

Yes, it was definitely a vacuum problem. Not, it wasn't the carbs or the brake servo. It was a slightly loose blanking plug on the inlet manifold - the one that kept catching my eye for some reason and I kept ignoring.

Note to self:
Lesson 1 - check EVERYTHING
Lesson 2 - assume NOTHING
Lesson 3 - check again

In the words of the great Homer Simpson "D'oh!".

thanks for all your help,

Grant :-)
G Hudson

Now I'm confused. I can see where the air came from but where did the fuel come from? Was that leak enough to cause fuel to be drawn from the carb even with the throttle closed? I would have thought that leak would have caused so lean a condition as to not run. Oh well, live and learn. D'oh, indeed.
Dave Ahrendt

Air leaks on the inlet side can cause very strong "runaway" conditions. With a supercharger it is just like putting a microphone in front of a speaker fed by the microphone, it just screams!
dominic clancy

This thread was discussed between 05/07/2013 and 10/07/2013

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