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MG Midget and Sprite Technical - 1275 1.5 su conversion

Dear all,
am I best sourcing 1.5 su carbs from an old MGB or spitfire or 1500 midget. They all seem a little different. I think the ones from the B will use the same inlet manifold as my 1.25's? What about the special inlet that you can use on a Cooper S with 1.5's?
Any advice accepted.
S H Parson

>>I think the ones from the B will use the same inlet manifold as my 1.25's?

No - the B's head is bigger.


A
Anthony Cutler

The Cooper S manifold won't work on the Midget owing to the fact that it puts the carbs at an angle and height that will foul the bonnet.

You can't bolt HS4 carbs (1.5") to an HS2 (1.25") manifold because the mounting flanges are different.

There was an HS4 manifold that would suit the Midget (made by Titan IIRC) but these are about as rare as rocking horse poo nowadays.

The simplest way to fit HS4s is to make up a pair of adapter plates to go between the carbs and the stock manifold (and use an extra pair of gaskets).

WRT: "They all seem a little different."

There are basically two types: The earlier 'Plain Jet' and the later 'Wax Stat Jet'. I may be wrong, but I believe all HS4s had sprung biased needles which negates the need for jet centreing (unlike the early HS2s and HS1s that had fixed needles).

The wax stat jets are execrable and can cause all sorts of poor hot running issues. I always replace these jets with the plain type. You will also need to play about with the needle profiles to suit your motor.

The difference between the jets can be seen here:

http://www.rimmerbros.co.uk/Item--i-GRID005600

The wax stat jet is the one on the left.
Deborah Evans

Regarding the 'B' inlet manifold, can I have it machined to suit?

Incidentally, my engine is 90BHP (flywheel) and I think that the 1.25's may now be restrictive.
S H Parson

Another idea: is it simpler to get 1500 midget carbs and inlet manifold? Should fit.
S H Parson

SH

One option that's become very popular (with those that use it) is a single HS6 / HIF44 on MG Metro or Titan manifold.

The standard manifold's don't allow sharing of the carbs anyway, so your cyls are only seeing one 1 1/4 SU at the mo.

A single big SU on a good manifold should increase your torque and improve fuel cons.

A
Anthony Cutler

Thanks, and I've read that is the favourite, but the regulations of the historic motorsport category that I compete in, does not allow 1 3/4's HS6 or HIF carb's. So 1.5's are the only way I can increase without Webbers, which I don't want.
S H Parson

SH Parson,

The 1500 midget engine is totally different to the A series 1275, ignoring both have 4 cylinders etc, in that it has 4 inlet ports compared to the 2 siamesed ports of the 1275. Theoretically you could make the 1500 manifold fit with a lot of cutting/shutting/welding but it would be easier to get a manifold from the likes of Maniflow specifically for the 1275 and the carb.
David Billington

There doesn't appear to be any inlet manifolds on the Maniflow page, only exhaust? Anyone else make them, apart from the extinct Titans?
S H Parson

Their website doesn't look very inspiring but someone on the BBS bought one fairly recently so you may have to ask Maniflow directly for what they offer.

APTFast has a list and some images

http://www.aptfast.com/APT_Parts/Aseries_Parts/a_Intake_Manifolds.htm
David Billington

Im doing the same thing and going to HS4s with alot of my own personal modification... but that will happen after I get the engine running and broke in

several notes 1st.

1. The maniflow intake is considard to be top end, BUT Un-godly expensive for 1 pound of steel tubing and plate steel $350-$375 IIrc, with shipping over $400 if you buy thur APT, the only supplier in the USA

2, the mounting is differant from the HS2 to the HS4 the HS4 is diagnol, and the Hs2 is vertical, you might be able to modify the 1275 intake to take the HS4s

3. with HS4s you will have a slower air induction speed, to you will loose torque, unless you can figure out away to make the air flow speed up

4. you will need a bigger heat sheild

5. Hs4 intakes dont have locator rings like the HS2 intake, so this will be tricky to port match...see vizard on how to overcome.

6. there are 2 differant MGB carb set ups. I think the 1st generation ended in 1969, Its the version I went with, the next generation of HS4s had a wax stat automatic choke ??? I dont know well beyound my knowledge, but the later where more of a hassle I think they hook into the cooling system somehow to operate the automatic choke..like I said, I went with the easier earlyier model

Anyway here is what I did/am doing

I got a 67 MG INTAKE WITH CARBS off of ebay and rebuilt the carbs.

the intake is going to take some work...1st I cut only what I needed out of the middle of the intake log, then took a 3 foot long 2x6 wooden stud so it can be placed in a vise horizontaly, at the top of the board I glued an old intake gasket then mounted a piece of plexie glass over the top then lined up the 2 parts of the cut intake manifold for a good match up on the ports and where its to be mated and used dry wall screws to mount it down then took the whole thing to a welder and had it TIG welded (Not Mig), next grind the sides of the feet down so it will fit into the exhaust header flange and line up on the ports of the head...next cut the stud groves in the feet sides also... next take to a machine shop and have the feet resurfaced and milled down so its the same hieth as the exhaust header flange/and creat a true flat surface

on the jig, you want the manifold when mounted above the board so the jig can be mounted in a vise and the welder can EASILY weld all the way around and not have any obstrutions to deal with...the reason for the plaxi glass is to creat a flat non squishy surface and it also helps with light reflection when linning the manifold over the gasket ports

Hope that helps... if you need a pic of my jig let me know. it will take me a day or so to find and photograph.

Prop... yeah that $400+ maniflow intake isnt looking so expensive anymore is it? lol

Prop


Maniflow inlet manifiolds are £140, I spoke to them about 6 weeks ago, but they were waiting to make some more.
You can cut down the MGB one, as Prop suggests, thats fairly common I have one on the bench at the moment...if your at Silverstone for MGLive I could bring it for you to look at.
As Deborah says its best to get the later carbs and change the jets.

I am planning to fit a pair of HIF4's from a later MGB..........has anybody done that ??

Ian
Ian Webb '73 GAN5

just get a 45 weber and manifold.
i never looked back
Onno Könemann

Onno,

the racing rules dont allow him to use a webber
Prop

Prop
they do
he just says he does not want to
read carefully ;)

I don't understand why he does not want to so i try to convince him
Something done several times a day on discussion fora
Onno Könemann

wow, lots of ideas, though I gather it won't fit under the bonnet without a bulge. I can see this is going to take some careful thought and torque is very important to me, mmm.
S H Parson

>>>>>>>torque is very important to me, mmm<<<<<<<<<<

Theres your answer... Weber 45s!

Sorry Onno, your right. I tend to skim read when im feeling lazy.

I cant remember, But I thinks its possiable to use a webber intake and but 2 HS4s onto it. IIRC, I looked into that bUT CANT REMEber if its doable or not. granted I doulbt you could use pancake filters, just ram pipes with socks

Any insight Onno?

http://www.spridgetmania.com/web/Item/C-AHT777/InvDetail.cfm

Prop
Prop

With you on this one prop
though i vaguely remember that there was no std linkage (tinkering needed)
This is mentioned in Daniel's book but i am to lazy to walk to the bookcabinet
Onno Könemann

re: Webers, I'm bothered by the number of them I see flooding at autotests etc, and how they behave with aggressive throttle on and off. I also do need some economy as I'm driving a 1,500 mile round trip for an event if France this summer. Currently i have 90 bhp at the flywheel and 85 lbs/ft torque.
I am allowed Single Weber 40 or 45 DCOE.
S H Parson

your doing better then me Onno, at least you had the motivation to purchase danials Book... Lol

Im still mustering the drive just to buy it... LOL

Maybe Ill buy a copy ..."Tomarrow" lol.

SH...that might be an option to look into HS4s on a webber intake
Prop

Prop,

As usual you are over-complicating what is a very, very, VERY simple issue I am afraid.


"2, the mounting is different from the HS2 to the HS4 the HS4 is diagonal, and the HS2 is vertical, you might be able to modify the 1275 intake to take the HS4s."

The HS2 has HORIZONTAL bolt spacing, the HS4 diagonal. WRT modifying the stock manifold to take HS4s read on!


"3. with HS4s you will have a slower air induction speed, to you will loose torque, unless you can figure out away to make the air flow speed up."

Not necessarily. My friend Stuart has a 1310 (or 1330 can't remember) Midget running on twin HS4s on a 296 cam IIRC. He regularly drag races it. Loss of torque is NOT an issue with this car.


"4. you will need a bigger heat shield."

Why? The carb spacing will be the same and the float chambers on the HS4 are not significantly different from the HS2.


"5. HS4 intakes don't have locator rings like the HS2 intake, so this will be tricky to port match...see Vizard on how to overcome."

The SIMPLE solution, and one that I have done myself, is to machine up a pair of spacer plates of about 15mm thick alloy that have the bolt fixing holes for BOTH the HS2 AND the HS4. Ie you have a pair of plates that will bolt to the stock inlet manifold via the 'HS2' fixings and to the carbs via the 'HS4' fixings. Use countersunk bolts to hold the plates to the inlet manifold and normal bolts for the carbs. If you machine it accurately then the bolts themselves will do all the location, so port matching is automatic. If you are still worried then roll pins or dowels could also be used. Fit a gasket either side of each plate and Robert's yer Ma's brother!

This is what Stuart uses on his Midget and it works perfectly - NO carb misalignment / bonnet fouling from using Mini manifolds, NO need to shell out money on a 'trick' manifold, and NO need to bugger about cutting and shutting a manifold (which itself is liable to lead to the mother of all f*** ups!).


Having said all that I notice that the OPs car is running around 100 bhp at the flywheel. My gut feeling is that without further modification to the cylinder head and valves, as well as a 'race' camshaft, overboring, increased CR, etc, then fitting HS4s is probably a waste of time and will be of no benefit, especially given the fact that the Midget Championship Class C cars run well on HS2s.



"re: Webers, I'm bothered by the number of them I see flooding at autotests etc, and how they behave with aggressive throttle on and off."

What you are seeing is vehicles that have Webers that have NOT been set up properly. Our racer (producing nigh on 150 bhp) is on a single Weber 45. It has NEVER flooded and throttle response is seamless, even on snap opening above 2000 rpm. Note it is on a 310 degree duration cam so I wouldn't expect much torque at low rpm owing to the low gas speed.
Deborah Evans



Thanks Deb,

>>>>>>>As usual you are over-complicating what is a very, very, VERY simple issue I am afraid.<<<<<<<

You are correct there Deb! No argumeent there, unfortantly doing it the simple easy way is rather expensive for the mods I am going to be making to my induction system. So we do whats we gots ta do... 8^?

Good info, Unfortantly Im still going down this road for the 1st time, and the above info I provided is the road Ive traveled so far and how ive built this modification. The potential problem issues are the ones Ive read about as pit holes for doing a HS4 conversion from here on the forum and others ive talked to in passing about this conversion. So its great to see your experiance and knowledge put to fact and practice.

In your experiance, Id imagine there must be some sorta slow down in the speed of the air flow using HS4s...is it a neglageabe amount or does it depend on the engine set up?... I know how Im countering the effects for my own cantraption in order to retain the airspeed and torque... but now im wondering how big an issue airflow speed is after reading your synopsis. as my plan maybe a bit overkill.

I hope to find out in the next few weeks at any rate

Prop ... I need a new money bush, I think mines got worms
Prop

Isn't the whole point of SU carburettors that they have a constantly moveable venturi so shouldn't suffer so much from the airspeed problems of a fixed venturi carb (Weber/Dellorto) fitted with large chokes...

However, nothing problematical about a properly set up DCOE Weber - flooding, spitting, poor pick up are all symptomatic of poor set up - they can even be quite economical on part throttle settings (I ran a roadgoing Midget with a 1293cc race engine on a Weber for a number of years back when I was a poor student). Get the jetting right on a Weber and you will never consider going back to SU's...
James B

While I've flow tested carbs I've not actually tested the actual velocity through them. I would expect a drop in velocity on the HS4 over the HS2 (for a given motor), but this will depend upon what's downstream, ie the inlet ports and valves, and significantly the Valve Time Area (the function of valve lift, valve size and valve duration).

If you think about the sizes of the carburetors then you come up with the following figures:

HS2 = 1.25" = 31.75mm. This gives a choke area of 791mm^2

HS4 = 1.5" = 38.1mm. This gives a choke area of 1140mm^2

Thus the HS4 compares well with what you would do to a Weber 45 on a full race engine (you choke it at around 38mm for each barrel) which gives a choke area of 1143mm^2.

Now, a Weber will flow better than an SU (it's the nature of the beast) but this is only really apparent at higher power outputs (ie higher engine speeds) as achieved on race motors. The major advantage of the Weber in race applications is that it meters more accurately owing to the nature of the design (IF it's set up correctly) and it doesn't go 'out of tune', unlike an SU..

If you look at the Midget Challenge Class C cars, then these are limited to standard sized valves (albeit 'Rimflow' types are allowed) but with flowed heads and free camshaft choice (but a max 60 thou overbore and no change to rocker ratio). They are required to retain the HS2 carbs although these can be flowed and velocity stacks used (original bore size and throttle plate must be used). These cars are making around 115-120 bhp. I would warrant that the restriction here is the valve size (which limits the VTA) and that going to an HS4 would cause a loss in torque owing to a loss in inlet charge speed (because the limited VTA effectively 'chokes' the engine and prevents it from attaining the higher rpms achieved by the Class B and A cars).

Once you increase the Valve Time Area as on the Class B and A cars then any loss of charge speed through the carb venturi is more than offset by the increase in charge speed caused by the increased VTA and the concommitant ability to rev higher.


Of course the trick on the A Series (as with any motor really) is not to go down the 'old school' route of hogging out the ports to psychotic levels. The guy I get to do my heads is much more subtle than certain people. We've just put one of his heads on a Frogeye racer that has now significantly more torque (to the point where it is breaking halfshafts) than it did with the previous head supplied by an extremely well know A series tuning company.
Deborah Evans

I have used an MGB intake manifold by cutting a piece out of the balance pipe and bridging the gap with a rubber hose and jubille clips. It fitted O.K.
The Midget manifold and adaptor is a better bet though, because it has no flow interupting step where the balance pipe mets the head face.
For what it's worth, I upgraded from 1 1/4" SUs with KN filters to 1 1/2" SU's mounted on a maniflow steel inlet and custom cold air box with remote filter on a 1380 Midget. After 3 rolling road sessions I gained about 4 bhp at the top end.
SJ DEAKIN

Deborah

I agree with just about all you say, but not the claim that SU's go out of tune.

Some particular cars which I have serviced I have done so continuously for 20-30 years (ie no one else has been into them in between time) and at each tune-up, having first attended to ignition, checked compression, tappets etc, when the carbs are checked as the last component in the squence they have been spot-on other than a very infrequent and slight mixture adjustment (every 5-10 years) to compensate for wear.

Self sustaining myths get generated about twin SU's being problematic quite simply: Inexperienced tuners, having heard the myth and presented with a car which is out of tune, fiddle with the carbs first and then finding they couldn't fix it (which of course they can't when the problem is elsewhere all along) tell the next person "It's right, twin carbs ARE hard to tune!"

The phenomenon is not peculiar to twin SU's of course, Passini makes exactly the same observation about people stuffing up tuning by fiddling with Webers before checking everything else.
Paul Walbran

I forgot to add: one proviso for tuning any carb properly is that it is not stuffed, as many high milage ones can be. If they are, that's when they will be impossible to set up, what ever type they are.
Paul Walbran

In the very early 70's, my brother used to service a rather nice Scimitar Coupe (straight-6 Ford 2553 with triple SUs). He set the carbs up and told the owner NEVER to dink with them.

By the mid-70s, the rear axle (early MGB banjo) had broken, and car was left at brother's garage to 'rot' (fibreglass body!).

My mate Paul acquired it in the late 70s and fitted later MGB axle and used for a couple of years; then we 'sort of' swapped it for a TR-4 and I used it from 80-84 until my first son was born.

I don't think the carbs had been touched from when my brother set them up nearly 15 years earlier.

'twas a really cool looking car, with very smooth engine and o/d; very rarely saw another on the road.

A
Anthony Cutler

Deborah.
WRT to your response to prop..
"2, the mounting is different from the HS2 to the HS4 the HS4 is diagonal, and the HS2 is vertical....

"The HS2 has HORIZONTAL bolt spacing...

The HS2 very definitely has vertical mounting bolts to the manifold. (unless you mean the manifold to head in which case they're all horizontal).




graeme jackson

I've a spare modified Mangoletsi twin 1 1/4 -1 1/2 su manifold i played around with years ago, it does need a skim on the carb face as the tig distorted it a tad, but otherwise its fine.
Brad (Sprite IV 1380)

Paul
i've got to disagree with you here
And for the simple reason that there are much more wear parts on an su than on a weber.
Especialy on a sprung needle su where it always leans against the jet.

Head to head a wel setup weber against the wel setup su
the weber wins on power sound and reliability
the su wins in fuel economy and cold starting
Onno Könemann

Onno

My comments are my own experience with the same cars over a very long period of time, during which NOBODY else has had a fiddle with them. (Especially those belonging to my mum and sister, both of which were their only cars for a large chink of that time). I acknowledge that there is jet wear in the swing neelde type and that the lack of ball race spindle bearings like the Weber result in more spindle wear. However, despite this the carbs needed only the very occasional adjustment I described.

Obviously SU's will need overhauling more frequently in these respects than a Weber, but generally they still last the life of the rest of the engine. (Though it's surprising how many overhaul an engine and put tired carbs and distributor back on!)

My main point was that SU's do not need constant tuning, and I am sure from my long experience that this is the case. When you think about it, the wear that you and I both agree on can't happen rapidly enough to affect settings between ignition system tune-up intervals, so what other aspects of them are going to mysteriously spontaneously drift in settings so rapidly?

I agree with you that the Weber is the choice for outright power, but the margin is reasonably small so if class rules, originality, economy etc are more important to an owner then a quick reliable car can still be achieved.
Paul Walbran

sure enough, but over how many years, and how many miles, the SU will wear and need an adjustment. Maybe 30,000+ miles?

In that case, wouldn't "reliability" be perhaps a stronger word than needed? I wonder how many carburetors, regardless of design, would not also benefit from a tune after that much use (on a classic convertible, is that 10 or 20 years of use?).

Volvos used SUs for a very long time. Do they have a reputation for unreliable carburetors?

Probably the biggest factor, between Weber and SU, will be owner's preference, area of experience and personal bias.

Norm
Norm Kerr

This thread was discussed between 25/05/2010 and 27/05/2010

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