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MG MGB Technical - Carb's, no air-filters and a tight space

Hi, I'm after some advice if I may.

I have a Marlin Roadster, which is a kit-car based on the 1.8TC engine from a Morris Marina. At the moment it is running a pair of HS4 SU's and no air-filters. In fact, it has been running like that for 8k miles (I'm guessing that means my engine is now damaged, but I'll not go there).

The Marlin has a very tight engine compartment and with the upgrade to unleaded fuel the previous owner started to experience "Vapour lock", as a cure he added a heat sheild and 3cm spacers, this unfortunately meant the carbs sit too close to the bodywork and no air filters were put on. A picture can be seen here: http://www.stefancarlton.net/pistonheads/engine.jpg

As a novice mechanic and having been told by a multitude of peope that this was fine, I accepted it but now looking towards upgrading performance slightly I've bought a few books to read, and you guessed it I find it's a complete no-no and the engine is dieing a slow a painful death at the moment so I need a solution fast.

I have looked at various ideas, asked about and it was suggested I tried here by looking the archives, but there is so much stuff in there I don't know where to start. I have looked at getting smaller spacers but this can't be done due to the throttle linkages on the HS4's. This leads to the first question - do HIF4's have a different throttle linkage? The problem with the current guise is the part where the spring drops as it extends backwards and hits the heat-sheild, are there any carbs which don't have this flaw?

I'm thinking maybe a set of HIF carbs might be the wiser choice now as with the fuel body below the SU, this gives me a space to each side where I could add a filter (with an appropiate air-box turning the 90deg corner) but what I should I be looking for, a pair of HS4's (1 1/2"?) or HS6's (1 3/4"?)? When building the Air Box what spec should it be, I don't have much space directly in front of the carb at the moment so it can't be deep unfortunately - would something like http://www.stefancarlton.net/pictures.php?menu=picture&pointer=2380 work?

I'm figuring the engine is probably going to start dieing shortly (it already uses a lot of oil) so am thinking about sourcing another 1800 engine and rebilding it to a higher standard to give a performance boost, but while retaining things like my carbs, air-filters... so want to "future-proof" any purchase.

I'm at a loss here.

thanks for your help.
Stefan Carlton 1

Stefan, unless you've been driving under very dusty conditions I wouldn't think you've done too much damage in 8K miles. Check the photos of very early MG's from the '30s and you'll see that almost none of them had air cleaners installed. Your problem is common, many Morgans also had similar clearance problems. You might find a suitable air cleaner design from one of them or you could build a box which would serve both carbs and run via a hose to a remote filter element. This would allow you to keep the carbs as they are.
Bill Young

I think that building a plenum chamber similar to the one peitured, but adapted to two carbs would be your solution....I'd run a hose from it to a remote air cleaner. I don't think that converting to HIF's is going to give you that much more space.
R. L Carleen

To your immediate issue - you may be able to get/modify some pancake type filters, or one of the custom filters that's a single filter for both carbs. HIFs would give you space at the sides, but I think the carb body is slightly deeper, which would hurt your clearance for a plenum.

In the longer term, as you do the rebuild, you might consider if it would help you to move the carbs to the other side of the engine, then go for a cross-flow head.

Steve
Steve Aichele

What we need is a blueprint for a plenum chamber, as I also have the same problem.

Jack

Hi,

thanks for the comments, its a dangerous game reading and researching into engines isn't it! Cross Flow engine would solve a fair few of problems but I fear it'd open a new set (the steering column is on the other side, and getting the dizzy/oil filter could become very awkward).

Thanks for the idea, and the re-assurance that my engine isn't going to go bang in the short-term, reading the other thread on oil usage it would sound almost normal as well :)

Are their any specific rules for creating the chamber? One presumes that you need it:
* large enough to contain enough air to be sucked in by the carbs at full revs & draw further air through
* Symetrical to ensure the same amount of air gets to each carb.
* The pipe between the filter and air-box should be central & of a large diameter (1.5" - 2" - they are 1.5" carbs). Can this be tapered on entry? Could two smaller pipes be used? Are there any pipes which can cool the air (i.e. have their insulator), could exhaust heat wrap be used to reflect heat?
* Any bends in the pipe should be as gradual as possible.

What size of airfilter should be used on the other end? Could a pipe-clamp version be used like something from the "induction" kits from K&N or would it better to terminate the pipe on base plate and have two pancake type filters?

So many questions, thanks for the replies :D
Stefan Carlton 1

Stefan, the MG TF's, 1954-55, had 1 1/2 inch carbs and used pancake air filters.
http://www.mgcars.org.uk/mgtf/Pictures/Unrestored/Thumbnails/mgtf_gallery_unrestored.htm

The front one was indented in the back to allow clearance for the tapered bonnet side. While the carbs were on the opposite side on the engine you might find that a similar setup would work for you.
Regards,
Dennis
D F Sexton

thanks for the heads-up but I'm not what you mean - the pictures in that page (http://www.mgcars.org.uk/mgtf/Pictures/Unrestored/ValveCover_c.jpg) doesn't really help.
Stefan Carlton 1

Stefan,

You can use smaller (shorter) spacers to bring the carbs closer to the heat shield. There are two options to make this work:
1. Cut slots or holes in the heat shield where the linkages interfere.
2. Dish out the heat shield by hammerforming.

I'm not a big fan of cutting holes in the heat shield. It defeats the reason for it being there. I've hammerformed the heat shields in the necessary spots by using a cut off piece of 2" x 4" rectangular steel tubing as a "form" or "buck". Place the area to be modified over the open end of the tube. Using a ball pein hammer, start dishing (stretching) the metal. The tubing will keep the edges clean and prevent distortion to other parts of the heat shield. Keep dishing out the area until enough clearance is present for the linkages. Check your progress often! You'll need to do it in three places - the linkage and at the spring mounting arm on each carb. Neatness isn't vital, but it looks better when you're done. Alternately if you can't find the tube, you could carve out something from hardwood. A piece of plywood with the hole cut out may suffice, but I've never tried it.

For the short spacers, there are two choices. Cut down the originals to about half thickness, or you can get half height spacers (they were used with the electric manifold preheaters) from Moss, etc.

Pancake filters do not flow enough air. Also a number of people have ruined engines using the "Stelling and Helling" type filters when the cover hold down stud got ingested by the engine! K&N makes a small conical filter that is usually used on left hand drive B's with brake boosters that works well. The "Longflow" foam filter may also fit ok.

Good luck,
Tom
Tom Sotomayor

Tom, it never occured to me that you could change the formation of the heat-sheild. Thats definetly food for thought!

The spacers have already been sought - the 3cm from the MGB could be replaced by 1cm versions from the MGA as that had HS4's at one point apparently. I'll have a look into changing the heat-sheild as that might just be the perfect solution.

thank you!
Stefan Carlton 1

I have not done this, but run a hose or pipe to a place where you can mount a filter
Frank

Hi all.

I remember looking at a Triumph saloon with twin SUs a while ago.

The air filter was in a large cylindrical can at the front of the car, and a hose (about 2") fed the filtered air to a pressed steel box which was bolted onto the air filters.
The box was basically rectangular, with rounded ends, and was fairly tall (top to bottom) but not very deep.
The engine was a 6 cylinder, so the carb spacing would probably be wrong for a B.

It is certainly a good idea to feed the engine with cold air in the summer, as cold air is more dense and hence gives more power.

Using some sort of scoop to ram air into the carbs seems attractive, but the mixture will tend to change with road speed as the ram effect increases / decreases.

HTH... Don
Don

Try again...

The air filter was in a large cylindrical can at the front of the car, and a hose (about 2") fed the filtered air to a pressed steel box which was bolted onto the CARBS.

Don
Don

Stefan,

Not sure whether these will fit

http://www.quillertriumph.co.uk/Quiller/Parts/parts.htm

Also check out the SU V8 system.

The idea would be the opposite of ram effect in that the whatever you use should slow air down after cool air collection and employ the use stub stacks, ignoring any pulse tuning effect.

Paul
Paul

With all your advice I once again ventured out with my dad (two heads are better than one) and came up with three things:

1) Space is severly limited - worse that I first thought due to the curvature of the bonnet.
2) We found a great place to put a air-filter (under the front)
3) How on earth is the air-box/pipe... going to fit? Without putting a bolt through the whole air-box I can't see a way to bolt the air-box/pipe to the front of the SU.

The idea of cutting the heat-sheild will not work as there is 3 obstructions, one of which sits directly over the central exhaust manifold header.

I think the best idea is going to make a pipe (ram) bend a tight 90degree's from the exit of the carb and then take it to an air-filter in the widest possible pipe (3" is possible). It'll be tricky but do-able I think. Off to have a look at modern solutions (in the scrap yard) and see if anything applies.

Regarding the above post I can't see what you were refering to, I can only presume that the extended air ram available was the idea?

Thanks for your help.
Stefan Carlton 1

Hi again.

Yes, you put bolts through the whole box, as is standard on the B filter housings.

Whether you could get them in / out with the space availible is a different matter.

Don
Don

Stefan,

The Shorties are 35mm and can be used with trumpet sock filters.

Paul
Paul

From the photos that I have seen might it be possible to manufacture a plenum with the opening to the rear. Then run an insulated tube rearwards then across the engine turning forwards on the RH side. Then with all that excess room you could mount a filter head just about anywhere.
Sandy Sanders
Hudson Florida
conrad sanders

This thread was discussed between 13/08/2004 and 16/08/2004

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