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MG MGB Technical - Stromberg carb

OK, this one has us stumped ...

We have a US spec rubber bumper B in the workshop with single stromberg carb. It arrived with us recently after of 6-7 years' storage. Our mission: get it going and roadworthy.
Surprisingly, the engine turned over freely. First base!

Checked the ignition.
Drained out the vile smalling fuel and filled the tank with fresh fuel.
Removed fuel pump and refreshed.
Fitted new fuel filter.
Hit the starter and brrrrmmmm!!! Ran perfectly, lovely smooth quiet engine. YAY we thought, that was easy.
But the car heard us ... :-(

It would be more correct to say it idled perfectly.
When the throttle is opened, the engine speed increses normally for a few seconds - and then just fades away. After droping to alomst stopped, it recovers back up to a couple of thousand RPM and the fades away again.

We have:
Checked fuel delivery - pressure holds good, and plenty of volume.

Removed air filter and watched what happens: the fuel stops coming out of the jet - just dries up completely.

Held the piston back - engine runs normally.

Opened the cold start valve (choke) when the engine starts to die - the engine then runs normally with the extra fuel fed through the valve.

All this seems to point to adequate fuel reaching the carb and a problem in the carb somewhere.

So we have stipped and checked carb. Everything is clear - main jet, bowl vent (we have found this to be the cause in the past).
The diaphragm is intact.
The piston is correctly oriented.

Varied the float settings - no change.
Tested the volume of fuel being delivered through the needle valve with the float dropped only a mm - heaps, about a pint a minute.
Blocked off the breather in case it was playing sill beggars (don't know how, but when you are stumped you'll try anything)
And a few other things besides.

We've wondered about the piston spring. But then the car was running when it was put into storage.

The answer is probably staring us in the face. Any ideas welcome.

Paul Walbran

Long shot, but maybe a hole/perished crankcase breather hose. If I understand it correctly, with the Stromberg that hose gets vac at part throttle onwards similar to a vac. advance hose and a serious leak Could case a surge on part throttle?????? maybe or
William Revit

Thanks Willy, I removed the hose and blocked it off (no difference) - would that cover what you had in mind?
Paul Walbran

yep -- still thinking
William Revit

If you have the emissions stuff connected including the anti-runon valve, disconnect the overflow hose from the port and try then. That method of anti-runon relies on vacuum derived from the inlet manifold to suck the fuel out of the jet. If there is anything wrong with the emissions plumbing it can have all sorts of peculiar effects.

FWIW my understanding of the Stromberg crankcase breathing is that it is the same as the SU i.e. just a few in. Hg. virtually regardless of operating conditions, including idle. This is different to the earlier carb vac advance port which does only supply vacuum as the throttle starts to open, rapidly rises to a high level, then tails off as the throttle is opened further. But that's by the by.
Paul Hunt

Thanks Paul, we have disconnected the vent renturn to the tank, but not the anti-run-on valve. I'll give that a try and get back.
Paul Walbran

Sadly, no anti-run-on valve. Brake servo is blanked off too.
Paul Walbran

Earlier up there you mentioned,- If you hold the piston back it runs ok
When you say --held the piston back, is that holding the dashpot piston from rising---If so that would mean that the fuel supply to the metering needle must be ok and sort of points towards the piston spring being way too light, but how that could happen is sort of impossible unless someone has swapped it out while the car has been laid up
I don't know if all Cds are like it but on my Elan the metering needle is mounted in a little holder that the mixture adjuster screws down into through the centre/top of the dashpot ---The needle is spring mounted in the holder----IF the spring in the holder happened to be broken, it would allow the needle to lift up and down a bit (don't know how much) separate to the piston and maybe vary the mixture and cause a surge----maybe
Or if the holder securing screw happened to be loose and there was a problem with the wave washer down in the centre hole that holds the adjuster screw, the whole needle/holder/adjuster could walk up and down in the piston
Both long shots but might be worth checking that the needle can't be moved up/down seperate to the piston

William Revit

Hello Paul,

You probably already know this, but John Twist at University Motors here in the States has a lot of MG related repair videos, including the Stromberg carb. In fact, I gather that he likes them. So, you might find some info there.

Also, for the diaphragm, I think it can be difficult to determine if they have a leak unless it is major. And, I suppose they could get stiff with age and not respond properly even if not leaking.

Do you have set of SUs you could substitute to rule out everything else that might cause the problem?

C R Huff

Yes, holding the piston down, and yes it does make me think it's too light or the spring is too weak ... but how that would have changed in lay-up is beyond me!
Yes, floating needle - all seems in order but will be worth a closer look just in case.

The diaphragm isn't stiff at all, but if leaks can be subtle I have a new one in stock so I'll give it a try.

No spare SU ready to go, but from what I've measured in fuel pressure and flow the fuel delivery is as it should be. Put that together with it runs fine if the choke is activated then there's certainly enough fuel getting into the bowl.

Yes, John Twist will be worth a look too.


I'll report back.
Paul Walbran

Tried the new diaphragm, no change.
Paul Walbran

An earlier comment: "When the throttle is opened, the engine speed increases normally for a few seconds - and then just fades away." indicates that when the effect of the damper is removed the mixture is weakening too much.

As does " the fuel stops coming out of the jet - just dries up completely."

This has to be something stopping fuel getting into the float chamber, or something stopping it coming up the jet.

But "it runs fine if the choke is activated" indicates the former is probably OK. I don't know exactly how enrichment occurs on the Stromberg i.e. moving the main jet as on HS or an auxiliary valve and port as on the HIF, but it looks like the enrichment path is OK but the normal path isn't.

Then again holding the piston down simulates the effect of the damper when the throttle is initially opened, which enriches the mixture by applying increased suction to the jet until the piston rises to balance the air and mixture flow again. So if gets sufficient fuel with the piston held down it isn't a blockage in the main jet either.

That does really leave either the piston coming up too far, or the needle is down too far, or the jet is up too far. What does the needle fit into on a Stromberg?
Paul Hunt

It certainly sounds like something blocking the jet.

Is it possible that when idling, the needle is preventing the jet from being blocked, but as the piston rises and the needle moved upwards, something is being drawn into the jet, stopping fuel delivery?

Dave O'Neill 2

Paul H
In the Strombergs on our Elan, which I presume to be the same as other strombergs ,the needle is mounted in a little holder which has a very small spring mounting in it allowing the needle to float sideways a tiddle similar to some SUs and has a grub screw in at rightangles like an SU to secure it.--The difference is that idle mixture is adjusted via an allen key down in the bottom of the dashpot damper piston tube--
I have seen a few that the needle has been set up like an SU, where the needle and holder have been fitted to the piston flush with the bottom face and locked there-
This causes a lean mixture as the holder is supposed to be pushed into the piston as far as it will go until it bottoms out and then pulled further up into the piston by using the allen key down through the top of the piston, winding it up at least 2 turns for a starting point and then fine tuned with the engine running.Then tighten the lock screw-

Hope that's explained it

Interesting, one of the carbs on the Elan kept swallowing it's damper oil right back from when we first got it back in the mid 70s with less than 20.000 on the clock, It took ages to find out where it was going . The idle mixture adjuster in one carb didn't have an o ring on it letting the oil get sucked out the needle holder hole. It must have been missed out when new as it had never been touched
William Revit

Floatbowl full of water would do it but Paul has had the carby apart and cleaned it out so that sort of kills that
William Revit

Paul, that's what I conclude too, but so far haven't been able to work out how any of that could have changed since the car was last run, and how to amend it.
The damper is full of oil and feels right when lifting the piston manually, the spring is present - can't see how it would have changed - the needle is mounted in the correct place, the top of the jet is well down from the bridge, and when the carb was apart I could see straight through it. Not a sign of dirt or other contaminants in the carb - clean as a whistle.

The main jet on this one appears to be fixed. That, and Willy's notes about his Elan, leads me to thinking about just what provision there is for adjusting the mixture on this version (there are several) and how it works. Next step I think.

If the owner didn't want to just get it going to sell it, I'd have suggested twin SUs by now!
Paul Walbran

How about a single HS6, HIF6, HIF44?

Mine left SU with a HIF6.
Dave O'Neill 2

I have serveral, but they are all pairs - which I'm not keen to split as people keep asking for pairs.
What needle in the single HIF6 in case I get tempted to do so?
Paul Walbran

Paul Hunt
I found this info on the Triumph site that explains how the mixture adjuster fits/works--quite a good explanation of it all---worth a read
William Revit

William - my question related to what and how does the pointy end of the needle fit into i.e. the jet on an SU, but I realise now I didn't make that clear. Haynes shows how the needle attaches to the piston, but not the jet (assuming there is one).

This$PDF$ shows a similar (to SU) arrangement for 'early' Strombergs, but that's about it. Even that seems to have plenty of scope for trapping bits.
Paul Hunt

Yes, they did vary a bit. I've checked this unit out further and yes, it is adjust as per Willy's Elan. However adjustment didn't make any difference, which is what I expected given the nature of the problem.
I think I'm ready to go back into the loop and start at the beginning for the 3rd time ...
Paul Walbran

I've been musing over your problem during a few idle moments....
Better minds than mine have inputted and I'm not sure this is going to take you anywhere but....
Have you checked the "vent hole" to the carb upper chamber is clear?
My thinking being..
The engine idles okay when there is little airflow over the bridge of the carb and the piston is at its lowest position. At 2k (ish) it runs for a while but then appears to "lean out" which seems to be caused by the piston rising too high (for the given engine speed) as confirmed by holding the piston back. Iím assuming that as the engine speed drops the piston falls back and the cycle starts over?
Items that can cause the piston to rise are:-
1, a weak spring (you confirmed this was fine when laid up and therefore not really a suspect)
2, increased airflow which causes a vacuum in the dashpot chamber by the suction effect of the airflow over the two holes in the piston. This is usually equalised (on SU's anyway) by a vent to atmosphere in the air filter chamber. A not uncommon problem with SUís is fitting the air filter gasket the wrong way around thereby blocking the holes.
I understand the Stromberg principles of operation are the same and I would be looking for blockages in its vent arrangement before going back to step one!

Best ofÖ..
M McAndrew

Just re-read your OP and noted this
"Everything is clear - main jet, bowl vent (we have found this to be the cause in the past)"

Back to square one I guess....

M McAndrew

MGmike, thanks for thinking that through. I'm 99% sure I have checked it, but I've gone round in so many circles I feel like I'm going ga-ga on this so could easily be scrambled, so I will double check that tomorrow.
I have no doubt that it will turn out to be something 101 and I'll be extremely embarrased!

We developed a theory today that maybe there's half a rat's next in the exhaust, and that's building up enough pressure to kill the manifold vacuum. It's a desparate theory (ever tried blocking off the exhaust even at idle?) but so are we. And at least it is something that could have changed while the car was laid up. So we'll chop it off at the back of the twin pipe and give it a go. Tomorrow I hope, but we have cars bulging out of the workshop and the team is flat out. And we've spent enough workshop time on it so it's down to me to think lots and nibble away at it after hours.
Paul Walbran

"and that's building up enough pressure to kill the manifold vacuum"

But pulling the choke supplies fuel again? (Sorry).
Paul Hunt

Paul may have it.
I had an MG midget doing the same thing. an exhaust 3/4 blocked with mouse houses gslore
Packed so tight that I had to replace muffler.

Quite right Paul, there I go forgetting my own symptoms ... but then again (he said clutching at straws) the choke opens up a different pathway for the fuel to flow. I think it's a long shot, but it's easy enough to cut the pipe and weld it back up if there's no change.
Paul Walbran

This thread was discussed between 18/06/2015 and 23/06/2015

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