Single carb vs Twin Carb

[kevindean]

Hi,

This is my first post. I really appreciate any comment and expertise out there.

I currently have a B series 1800 in a LWB roadster. The carb is a single HIF6 (44mm) which is okay and just about fits. A twin carb solution will give a little more grunt but I do not think they will fit.

Has anyone used twin carbs under the bonnet without having to cut the bonnet or side panels?

Thoughts apprieciated.

Kev.
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[martinclan56]

Re: Single carb vs Twin Carb

I have just bought a roadster with the B series engine and twin carbs. The bonnet has a very small bulge to clear the dash pot of the front carb. Talking to the previous owner you can get away without this by using an alternative front engine mount although I didn't ask him what exactly.

Cheers Robin

[kevindean]

Re: Single carb vs Twin Carb

Thanks Robin,

I will have a look at the engine mounts and consider that.

Has anyone else got any great ideas?

[stevejgreen]

Re: Single carb vs Twin Carb

Since the roadster power plant is essentially B series engine that has been tweaked with many different carb setups by the MGB boys, have a look on their forums.
If the engine is still stock, standard cam, standard valves, no porting etc there is little power benefit from a carb swap alone apart from a badge on the back saying TC which improved the performance by 10% to anyone seeking it in the 1970’s.

[lil_red_roadster]

Re: Single carb vs Twin Carb

From the original build manual:


"Some TC's were fitted with thick spacers between carb & manifold. This must be sawn down to 1/4" thick or obtain thin ones from your S.U. carburettor dealer."


Not sure whether it was "some" or "all"...but the TC setup I acquired definitely had thick spacers (as per MGB) and cutting them down meant butchering the heat shield and left the throttle linkage prone to catching on the exhaust manifold.


As Steve says, in stock configuration, twins are unlikely to significantly out perform a well set up single, certainly for everyday normal usage.


For chapter & verse on tuning a B series get hold of a copy of Peter Burgess's "How to Power Tune MGB 4-Cylinder Engines"...highly recommended.


A few other things to consider; the single carb Marina engine has a softer cam than the TC/MGB and also don't overlook the dizzy. With twins the Marina TC and rubber-bumper MGB have a vacuum take-off on the inlet manifold rather than in the carb body, needing a totally different advance curve, an issue which the factory didn't adequately address (according to Burgess). Then there is the need to accomodate the 2 into 1 TC exhaust manifold.


@Robin - Marlin advice was to use Land Rover mounts (NRC 2054) to lower the single-carb engine. Using these necessitated the drilling of a 13/32 hole in between the existing holes of the front chassis bracket.


Cheers,
Jez

[martinclan56]

Re: Single carb vs Twin Carb

A few other things to consider; the single carb Marina engine has a softer cam than the TC/MGB and also don't overlook the dizzy. With twins the Marina TC and rubber-bumper MGB have a vacuum take-off on the inlet manifold rather than in the carb body, needing a totally different advance curve, an issue which the factory didn't adequately address (according to Burgess). Then there is the need to accomodate the 2 into 1 TC exhaust manifold.




Cheers,
Jez

That's interesting. The Marlin I just bought has twin carbs but fitted to what was originally a single carb engine. As the car hasn't turned a wheel in 20 years I was planning a complete overhaul of the engine. It looks like I better carefully check what I have exactly.
+1 for the Peter Burgess book.

Cheers, Robin

PS Apologies if I am hijacking the thread....

[kevindean]

Re: Single carb vs Twin Carb

Thanks to those who have commented on this tread.

Some good advice indeed.

I am just in the process of getting the car running reliably with a carb that has new seals and gaskets. I have realised that the choke must have been leaking through slightly hence the plugs were always black. Since refurb the car runs badly and the plugs are very pale. I have just taken advantage of Scott at WINSU to select a new needle for the carb which will hopefully make it run correctly. I can then consider some mods later.

Thanks again for the comment!

[Ye Ol Ripper]

Re: Single carb vs Twin Carb

As I understand it the single carb engine's camshaft gives better low end torque than the MGB's but max power peaks earlier than the MGB cam in the rev range ie. 4800RPM v 5600RPM.


The twin setup may give 10 bhp with the Single carbs engine high torque cam , if it's set up properly the engine will run smoother than the single carb.

Using the Twin Setup with the Twin Down Pipe Exhaust Manifold from the TC or MGB contributes to this and seems to apparently work as well as a LCB set up, due to its good design- provided it is ceramic coated.

On the road,it is rare to need to take the 1.8 B series above 3000 rpm as it pulls well from 1000 rpm and in such a light car the car will still be pretty quick changing up at 3000 rpm, without stressing the engine and still achieving good MPG figures.

[stevejgreen]

Re: Single carb vs Twin Carb

As I understand it the single carb engine's camshaft gives better low end torque than the MGB's but max power peaks earlier than the MGB cam in the rev range ie. 4800RPM v 5600RPM.


The twin setup may give 10 bhp with the Single carbs engine high torque cam , if it's set up properly the engine will run smoother than the single carb.

Using the Twin Setup with the Twin Down Pipe Exhaust Manifold from the TC or MGB contributes to this and seems to apparently work as well as a LCB set up due to its good design provided it is ceramic coated.

On the road,it is rare to need to take the 1.8 B series above 3000 rpm as it pulls well from 1000 rpm and in such a light car the car will still be pretty quick changing up at 3000 rpm, without stressing the engine and still achieving good MPG figures.

The problem is indeed one of matching the carburettors to a camshaft.
With engines as old as a B series it’s difficult if not impossible to tell what others have played around with previously.
There are multiple different cylinder heads and valve combinations to add to the mix. Without stripping down to component parts you will never be sure.

Beware of MGB LCB or extractor manifolds. They are much taller than the standard Marina manifold and will not turn sharply enough for a side exit exhaust.

My partially tuned 1800 TC regularly revved above 3000rpm, it was so much more fun to drive.

[marlinmick]

Re: Single carb vs Twin Carb

Hi All
I'm in the process of refurbishing my Roadster and I have fitted Land Rover engine mounts but I have also fabricated a new engine mounting for the offside identical to the original but 18mm narrower to move the engine centreline over at the front by 18mm, this has allowed me to fit full sized pancake filters to my twin SU's with the full sized heat insulators installed. The Land Rover mounts also lowered the engine slightly at the front so I have had to lower the gearbox mount to get the engine level again, in all engine and box are about 1" lower than stock Marlin. Hopefully I don't think this will cause me any great issues however I have had to completely redesign my remote gearchange as it disappeared down into the tunnel. This as been a right headache and a very time consuming problem to solve but I am there now and I'm hoping that its a lot slicker than my original attempt, which worked ok but was quire heavy in operation. Because of this I have also decided to refurbish the gearbox as well, new bearings, seals, layshaft, baulking rings etc the lockdown as a lot to answer for. In the past I seem to have accumulated two extra gearboxes I cant remember where or when I got them and or who I got them off, the interesting thing is that the casing castings are different but all have the same internals. Non of them can be identified when the were manufactured. The general refurb is going well and its exercising my engineering skills of developing, machining and welding components. I'm back enjoying it again it takes me back 30 odd years to when I built it. So thank you my Little Red Roadster for an exciting trip back to my prime.