14 replies to this topic

[supermotolee]

right i have been offered a set of twin 1.5 su's to go on my soon to be 1380 but they came off an mgb i have lined them up to the head and low and behold the spacing is too wide... now hear comes the question

each of the 2 inlet manifolds is linked by a hollow tube which seems to be there mainly for servo take off and vac advance... can i modify this manifold to work as 2 separate manifolds with the use of a tig welder and a very skilled man? what i'm trying to get at is the ajoing tube there for stability? some kind of pressure balance? or both?

i work on bikes alot and as you may know most of the inline 4's have separate carbs and inlet manifolds only joined by the butterfly spindle and they work just fine will this work on a mini engine?

i know it may be easier to go out and buy new parts but we are talking mates rates and beer tokens (which is far cheaper)

thanks all

lee

[Wil_h]

If you can get it tig welded, why not cut the manifold in half then get it welded back together the right width keeping the balance pipe.

[miniboo]

if its an alloy manifold you wont be able to join it back together.

your best bet will be to get a new manifold. you wouldn't want to waste the twins su's by making them breathe badly.

[Bungle]

if it from a MGB then the float bolls will be at the erong angle

[Guess-Works.com]

if it from a MGB then the float bolls will be at the erong angle

He's right you know..... but all you need to do is remove the float bowl and get the correct part from burlen which will correct the angle, ( its like an aluminum washer with a blob on it, and fits between the bowl and the housing )

As for the manifold, you can pick them up at shows for between 10 and 20 notes...

[stormduck]

for twin 1.25 su's, minispares sell a 30degree rubber lug that sets the angle of the float chamber, they're pennies and they're for a mk3 mini....they may be the same for the 1.5.

[pikey7]

Theres an artcile on the differences between MGB and mini carbs in miniworld at the moment.

As for the manifold, there's no reason why you couldn't chop it and get it welded back together (OK it's aluminium alloy, but its possible). But would you trust it not cracking? It is petrol afterall..... (and directly over a hot exhaust)

[supermotolee]

ok as far as joining it back together the reason i didnt want to do this is its extra effort measuring it ect when it doesnt need to be there.

and you can re weld cast ally my mate has decided to go ahead and chop it at his work has made up some blanking plates to tig back onto it in his lunch tomorrow

as far as the angle how much difference is there?

i will look on minispares thanks alot for everyones advise and if it all goes wrong i will buy a propper manifold

lee

[supermotolee]

http://www.minispare....aspx?pid=35035

is that it how does that work??? is it for external floats?

guessworks what bit are you on about any pics or links?

thanks lee

[Dan]

I doubt you'll be able to get both of the angle adaptors from Mini Spares as they are different for the front and rear carbs and because even if they do sell both f+r Mini Spares sell the ones to set the bowls at the right angle on a Mini manifold not en MGB one, order them from Burlen since they are the ones who supply them to Mini Spares in the first place. They really are just pennies.

But the thing is, the reason the floats will be wrong is that the whole carbs will be at the wrong angle. This means they'll be sitting lower down in the engine bay which means they may very likely hit the bulkhead or not allow any space behind them for even the slimest air filters. That's why Mini manifolds are at a steeper angle.

And the balance pipe is needed really, yes.

It'll be far simpler to buy a genuine Mini manifold.

Edited by Dan, 28 February 2006 - 10:05 PM.

[supermotolee]

why is the balance pipe needed?

[Dan]

Because of how SU carbs work. Both carbs need to be exposed to EXACTLY the same manifold depression constantly to avoid unequal fueling which could lead to lean running in one half of the engine and to help avoid induction pulsing.

[supermotolee]

right but both left and right inlet ports are not joined in the head and cylinders 1 and 2 will draw what fuel they need from one carb and cylinders 3 and 4 will draw what they need from the other i still dont see why there needs to be a link between the 2 inlet ports i'm not saying your wrong i'm just not that experianced with su carbs i can see what u mean about induction pulsing though as the link may help to even out the pulses but i'd have to give it a go to see how much of a problem this woulld cause

the carbs are hif carbs like hif 38s i think if that makes any difference

[Dan]

Well if they're HIF carbs then ignore all the stuff above about float bowl angles and spacers, but you will have them even closer to the bulkhead than HS type carbs would have been.

People don't realise just how accurate SU carbs are at fuelling, they are extremely sensitive to very slight differences in the manifold depression meaning that if the two carbs are getting a very slightly different vacuum at certain points in the range then the pistons will be at different heights and they will be delivering different amounts of fuel. It is much better to balance the vacuum between both sides or you will never get them tuned. If you were fitting twin split Webbers then the balance pipe would not be needed, but you aren't so it is.

[supermotolee]

ok so if i were to drill and tap say for instance 2 caliper bleed nipples and joint the 2 manifolds with some plasitc tube would this have the same effect?

oh yeah and i was going to add the float bowls look to have some kind of vaccume tube joining them together its not petrol pipe more like breather pipe just to add

thanks for all the input its great to have a few more people thinking differently to how i am to cover all potential problems

thanks lee