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MG MGA - Over rich mixture

A friend of mine has had his MGA engine set up a number of times on various rolling road dynos and also on the "Sun" type of engine set-up equipment to get the mixture sorted out, but no matter whoever has done the job, the engine still seems to run rich.
He is fortunate to be able to get 25 mpg even if he runs it very steadily.
It also has that "hunting" type of tick-over and you can smell the un-burnt fuel when he revs the motor.

It is an 1850cc 5 bearing engine running on H4 carbs, a Piper fast road cam and a tubular exhaust manifold.

He has had the jets and needles changed a few times, we have checked the fuel pressure from his SU HP pump and that seemed to be ok.

I haven't personally looked at the float levels or to see if his choke mechanism is allowing the main jets to return back up into place, but to be honest, these are the only two things that I can think of that could be the problem.

Are there any other things I could have suggested to him?

Cheers
Colyn


Colyn Firth

1850cc 5 bearing engine - has he been down to Peter Burgess's rolling road?

Just thoughts, I know as much as Manuel - are all the uprated engine components a good match to each other and overall - and are all in good condition - is the carb set up battling against the timing and ignition set up.

Want's in Peter's book about uprating the engine.

My head is now empty.
Nigel Atkins

Colyn

I had identical running rich symptoms for many years. I eventually sorted it with new jets being one of the causes but as that has been done the problem lies elsewhere. You say the pressure delivery is ok but do not say what it is. I found that to be the other contributing factor in my long quest to leaning out the mixture. I eventually got it correct at 1.5psi. In my opinion much more pressure than that makes for a rich mixture.

Steve
Steve Gyles

Colyn
I had exactly that problem at one time and as you have already suggested, traced it down to choke problems
Graham V

Tend to agree with a choke problem
If it's been to several rolling roads and still has a problem there must be something sticking intermittently as I would imagine if it was a tune problem, you would hope that somewhere along the line any roller operator would have picked it up
With an engine that spec. -not far off standard
If it's running std MGB 1800 needles
the overbore would make it run a tiddle rich but the cam would need a little more so that should almost balance that out-the tubular manifold won't make a hell of a lot of change to the mixture compared to a std MGB manifold
Running a fast road cam and looking for fuel economy are sort of fighting against each other really, at say 3000+ revs they would probably give the same economy but under that there will be a sacrafice to economy with a f/r cam--it's never going to be a putter round /good economy car with a lumpy cam
The hunting type idle(depending on how hunting you mean) is a part of having a hot cam, but you shouldn't be able to actually smell raw fuel at any stage
Check the basics first, ign. timing which needs to be at around 18 deg at idle with a cam like that but then make sure it's not going over at revs, has it had the distributor modified to suit the cam,--valve clearances ,float levels etc and a good check around them carby linkages and make sure it's all set up and working properly/consistently
Could be as simple as a stuck/sticking dashpot piston----------basics first
William Revit

I had problem with my 1800. Tried everything including Steve’s suggestion, still always rich and never got above 27 mpg. A couple of weeks ago I tried a trick a trick Barney suggests and that is lower needle slightly in its fixing.

Just back from 1000 mile week long trip including the Register’s weekend in Snowdonia. Exhaust smell gone, car running beautifully and got over 30 mpg throughout including 33 mpg on last tank.

Of course jet being too low could cause this. But I am reluctant to fiddle further as I certainly have a sweet spot.

Paul
Paul Dean

Just to clarify the high pressure fuel delivery I had. I use a Facet fuel pump that delivers more than enough pressure for the standard MGA carbs. So I bought a Fuel King pressure regulator/filter that had been set up for the MGA carbs - so I was told by the supplier.

A few years rolled by with many unsuccessful attempts to lean out my mixture. I eventually bought a pressure gauge that screwed into the Pressure regulator. It had certainly not been set up for the MGA. It was allowing 4.5psi delivery to the carbs. I wound down the pressure to 1.5psi. The difference was immediate and dramatic. However, it still did not completely solve my issue. It was only this time last year when I fitted the new jets that running rich was finally resolved.

I recorded on previous threads that I had assumed all along that my reconditioned carbs from the rebuild at the end of the 90s had all new important parts in them. I am now convinced that the jet assembly was never changed and I had been running worn jets for nigh on 20 years.

Steve
Steve Gyles

Thanks for all the suggestions, they will be much appreciated.
I will pass them on.

There will be a delay of a week or two before we can check the carbs out as the owner is on holiday, but we will investigate all of those suggestions as soon as he returns.

I will look out my 7/16th drill for checking the float levels and also my fuel pressure gauge to re-check the delivery pressure.

I have seen the effect of a sticking choke mechanism on my relations MGB where one of the main jets wouldn't slide back up into position when he pushed the choke back in.
I spotted this problem a week or two before his car was due to have its MOT and I pointed it out to him.

However, he decided to take his MGB to his local MOT station who charged him £300 to "fix" his over-rich mixture!

Great suggestion about the needles Paul, also fairly easy to do, so we will give it a try too.

Any more suggestions will be much appreciated.

I will let you know how things go.

Cheers
Colyn
Colyn Firth

Interesting one. We normally tune our carbs at tick over, as you probably did. With the correct needles on a standard set up all should be fine.
But you have had the mixture corrected at the top end on a rolling road, with modern fuels and a different engine.
We assume the engine is now right at speed. To do this the carbs were probably set richer which in turn makes the tick over richer.
As I see it you need richer needles, so that the mixture is correct at both tick over and at the top end.
PeteT
PeteT

needs the FX needles according to my tuning man
Dominic Clancy

Paul,
I have been looking at my own SU H4 carbs just to refresh my memory for when we start to sort out my friends car. My SUs are stored in a box as I now run a Weber 45 DCOE.

I have taken the needles out to have a look how easy it would be to lower them a little.

Barney suggests lowering them by 1/32" and I wondered how much you lowered your needles by and how you got them to be lowered by exactly the same amount.

Dominic, my carbs were on an 1850cc 3-brg MGB engine which was slightly tuned to give 106 bhp.
My carbs run No 6 needles and I wondered if the FX you mentioned were slightly weaker than my needles?

(hopefully I will be able to have a look at my friends carbs in a week or so)

Cheers
Colyn

Colyn Firth

FX are leaner than a 6 throughout the range
Chris at Octarine Services

I take it that the car has a decent exhaust to suit the larger 1800 engine, what air cleaners are on it.
If it has something like K&N filters or open ram tubes the std MGB needles will be too lean and if adjusted up to get correct power at revs then yes, it will be too rich at idle and needs a needle change
William Revit

Colyn

I really only did it as an experiment so I did it by eye. I had also read Barney’s suggestion and I would say I did drop them about as he suggests. After adjusting front carb by a flat they seem to be in balance. We have a long drive this week for a night in Applecross, of course including the best climb in UK. After that I will compare plugs.

Paul
Paul Dean

Colyn
If you have to drop the needles ,then it has the wrong ones
If it does have freeflow air cleaners then try a pair of AAA or even AAB might be needed if AAA aren't enough to sort it
William Revit

William

The standard MGA exhaust is fine on my 1800. Been on it for 19 years, also the standard MGA carbs, filter needles, but of cause returned to suit. All of my previous carb issues on richness were not due to engine size and we're prevalent on the original 1500.

Steve
Steve Gyles

Yeah
Really we are only guessing as we don't know all the details including what needles are in the car now
It will be interesting to see what Colyn reports back with--------------till then--???
William Revit

Thanks again everyone, some good information there and you have given me some great ideas of what to look at when I finally get my hands on the car.

I never thought about the air filters Willy.

I have ordered a 7/16" drill bit to use to check the fuel levels

I will let you know how we get on.

Cheers
Colyn
Colyn Firth

Colyn

Don't bother with the drill bit. Get the pucker SU kit. Many uses all in one small package. See my write-up: http://www.mgaroadster.co.uk/su_carburettor_tool.htm

Steve

PS. The MOGOC link needs updating by the looks of it. Task for this evening.
Steve Gyles

It looks like the kit is NCA Steve,
I have checked with MGOC, Moss and Burlen and it is out of stock.

I already have the jet spanner, it is a relic from my first MGA that I ran in the 80s that I bought from Speedwell, the then well known engine tuners.

I also have a jet spanner that I used to use on the Amal carb that I used to have on my Villiers engined racing Kart.

So I will have to make do with the 7/16" drill for now to set the float level.

Cheers
Colyn
Colyn Firth

Well, after a trip to Peter Burgess and a run or two on his rolling road, it turned out to be a sticking jet.

It just wasn't returning back up into place when the choke was pushed back in.

The air fuel ratio is now back to normal, the car is running much better and it seems like the fuel economy is back up to around 30 mpg.

Peter suggests that maybe the carbs are due for a rebuild and so probably that is next winters project.

Thanks for all your suggestions, very much appreciated.

Cheers
Colyn
Colyn Firth

Thanks for reporting back Colyn
It gets a bit frustrating when we don't hear the end result/fix of problems---------
William Revit

While the usual first suspect - a sticking jet - turned out to be the guilty party there is potentially a contributing factor ...

I have found that some parts AUC 4667, "Spring - jet lever return" are not correct.
Incorrect free length &/or spring rate mean that the spring exerts little or no pull when the jet is nearly fully back to the normal height.

[ Of course we all know that the H type carburettors don't actually have a "choke" ]

Jim
J N Gibson

No problem William,
I agree that it can be frustrating when you have gone through this process and not finding out what the problem was.

Thanks for that Jim, I didn't know about the jet lever issue, I will tell him to watch out for that when he gets the carbs done.

Cheers
Colyn
Colyn Firth

Triple carb kit available under
GAC6101X at M-E
Dominic Clancy

This thread was discussed between 22/05/2019 and 22/07/2019

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