Carb jetting

[Kilroy]

Just on the offchance - would anyone have any suggested needle or jet sizes for a 1900cc B series with MGB camshaft.?

Exhaust is standard Marina TC version - basically MGB also - although it has a single Cooper muffler.

The 1900 seems to draw more air (figures) and consequently runs rich for most throttle settings. I tried some other needles but they were just a shot in the dark - so I still am.

Carbs are standard 1.5" HS type as per TC also.

Cheers.

[MarinaCoupe]

Hi Ken

I suspect that Balmy's TC is the nearest to your spec.

I use AAA needles with my cam and head mods on my Coupe twin carb.

[ClaytonSpeed]

You could try red springs (medium) too, for the dash pots.

[MarinaCoupe]

As what you have is essentially a big-bore MGB engine, try googling some of the MG tuning websites for the UK and US.

[ClaytonSpeed]

Most sites will say take it to a rolling road. It really is the best way to set it up, but you know that so I won't try and teach you to suck eggs..

Try the AAA needles and red springs. You can't be that far off the mark. That is if you are running aftermarket air cleaners BTW.

[Kilroy]

Thanks guys.

I am running the stock air filter as it seems to filter best, and with our city still being rebuilt the dust content in the atmosphere is worse than the Kalahari.

Being an auto has a big part to play - as the torque converter does not normally allow the engine to run at optimum revs, and often the kickdown is too severe while hard acceleration in the next gear up is too low revs.
I think this makes it harder to tune for all conditions.
Running 11+/1 compression also means it is on the verge of pinking if the revs are too low.

Having said that - it has covered 300,000 odd K's and is still running great, so something must be right.

I just know that it does not like full throttle - and improves if you lift your foot off a bit. If you pull the choke out it makes no difference - so I think it is too rich rather than too lean.

Observations.?

(actually - the fact that it does not like full throttle has probably saved its life..lol)

[MarinaCoupe]

It sounds like it's running rich if the choke makes no difference. Have you ever put a gas analyser on it?

[balmy]

Mine did the same as you describe-run better with less throttle/choke made no difference. Was way too lean though.

[Kilroy]

Never have put a gas analyser on it.

Analyser couldn't keep up...

[ClaytonSpeed]

I would try ABY needles if you are running standard filters, along with the red springs.

Sounds like it is running too lean.

Standard 1.8 needles AAS V's ABY

[Kilroy]

Thanks for the input guys.

My gut feeling is that the engine is running rich.
Because it has a displacement of 1905cc it draws more air through the same carbs, and the higher gas speed draws more fuel through the same size jets.
It 'sounds' rich - sort of splashy if you know what I mean.
Using the chokes when cold is difficult because it burp burp burps even though it is too cold to run properly with much throttle.
I have a colourtune spark plug thing that I will use to see if I can get some idea of what is going on.
I did previously have leaner needles fitted which helped, but having changed the camshaft back to standard MGB I thought it might be wise to return to standard carb settings.

This thing has a lot of potential performance which I would like to unleash..

[dialup_2001]

I wish you were nearer and you could borrow my AEM Air Fuel gauge, I have it installed in my turbo and I managed to achieve a very good result.

[ClaytonSpeed]

Thanks for the input guys.

My gut feeling is that the engine is running rich.
Because it has a displacement of 1905cc it draws more air through the same carbs, and the higher gas speed draws more fuel through the same size jets.
It 'sounds' rich - sort of splashy if you know what I mean.
Using the chokes when cold is difficult because it burp burp burps even though it is too cold to run properly with much throttle.
I have a colourtune spark plug thing that I will use to see if I can get some idea of what is going on.
I did previously have leaner needles fitted which helped, but having changed the camshaft back to standard MGB I thought it might be wise to return to standard carb settings.

This thing has a lot of potential performance which I would like to unleash..

Colour tune will only tell you at idle and not much help at lift under load. It won't give you needle suggestions. Your own foot, brain and adjusting spanner is a better bet than C/T.

If you want a gauge, a Hi-Lo gauge connected into the 'full vac' port on the manifold is better than a colour tune. You want the highest reading at idle, but as before won't give you needle and spring suggestions for the setup.. Again, it's those eggs..

I use WinSu http://www.winsu.co.uk/ it is quite good at engine specs to get you going. Then RR the car to dial it in along with a decent electronic distributor and electric fan.

I wish you were nearer and you could borrow my AEM Air Fuel gauge, I have it installed in my turbo and I managed to achieve a very good result.

An air/fuel gauge is only good at telling you if the mixture is fat or lean at any given throttle position. An SU carburettor's mixture is set at idle and sets the needle position throughout the range. The gauge can't tell you what needle to use for the application.

[david painter]

The best way of setting up any carbuation , is on an engine dyno. In the real world this is not always practical. An SU carb is to a degree self tuning, if the volumetric efficency of the engine isnt changed, ie the engine has just more CC without any camshaft lift/duration & valve size altered. Yes more Jet will be uncovered at any given RPM and throttle load. This will mean that the dashpot piston will also be higher allowing more air to enter the engine this means that the fuel air ratio will still be the same. The amount of dashpot piston lift at a given manifold depresion can by changed by altering dashpot spring tension. In day gone the air fuel ratio was measured with a CO meter ususally measured by sampling ports is the exhaust ports on the exhaust manifold, or less suitably by meausing exhaust gas at the tail pipe but a delay in display is caused by the time the exhaust gases take to travel down the exhaust system then back up to the CO meter ( Exhaust Gas Analizer). There are two main disadvantages are 1, you cannot set an engine up on the road as its not practical to to carry a gas analizer in the car, with a pipe running outside to the tail pipe. 2. the delay in reading as that piont in engine RPM/Load has passed by the time it is displayed. But today we can use a Wide Band Oxygen Sensor with a boss welded into the front pipe as close as possible to the engine. When connected to a volt meter or air air fuel ratio gauge, this will give an almost instantaious reading. This can then be cross referenced with a SU needle chart or an electoronic computer program as already suggested to allow selection of a needle with a diffrent profile to give the correct air fuel raito throughout the entire RPM/ load range ( well almost thats why we went to fuel injection and engine manegment systems) . When all setup is finished the Oxygen Sensor can be removed and the hole plugged with a threded plug. Of course its easy to drive the car on the road with a Air fuel ratio meter wired up inside the car.
Dave