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Bigger Carbs and Manifolds

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DonPete3

DonPete3

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If anyone can adivise me on this I would appreciate it:

I've got a 66 Midget that has been upgraded with a 1275cc motor. The stock SU carbs are 1.25" throttle diameter.

What I would like to do is upgrade to a larger throttle diameter SU and switch out to a more efficient manifold.

Now, I'm on a really limited budget so I am going to "borrow" a couple of SU carbs that my dad (DonP) has for his 62 TR3.

What is the diameter on these carbs?

Can I do better with some others off of ebay?

What manifold should I use to link it all together?

Thanks,

DonPete3

/ubbthreads/images/graemlins/devilgrin.gif
 
Don a friend of mine is a Mini guy, and I used to have a Midget myself, so I'll offer an opinion, though I drive a TR4.
I think the carbs you have are quite adequate for the engine displacement, they will not be a limitation until you approach full race tune on the rest of the motor.
The carbs on the TR3 are 1.75 inch, David Vizard claims ONE of these on a suitable manifold will support outputs up to 125bhp on a 1380 A series. You might use a manifold from a Mini as it is the same block and head, but I think not wortrh changing; better to use the budget somewhere else.
Just my opinion, Simon.
 
I quite agree with Simon.
The standard 1-1/4" HS2 carbs are fully sufficient for a non-race car.
As an added point the Racing Sprite that I occasionally co-drive has a very healthy 1275 and is running the single SU carb manifold pulled from a 1275 Metro (the owner put it on to simplify tuning, but car will pull nearly 7000 RPM in top gear so it's not hurting anything). In other words, you don't need huge carbs on these things.
One of our other friends has a 1275 with two 1-1/2" MGA carbs (it's the blue one in the linked Racing Sprite picture)....he's always having carb-related problems (including terrible flat-spotting, miserable gas mileage and even an underhood fire).
Stick with the stock carbs....they're fine.
 
Just one more that agrees with both you guys. Can't see a huge gain at the cost and possible problems.

I would spend money on headers and good exhaust system first.

Bruce
 
Thanks guys.

I really appreciate the feedback. After all the work I put into this car I think I'll hold off on the intake mods.

I already have a header for it and a performance muffler (the one out of the Moss catalog).

That and some new wiring and I'll be on my way.

Later,

DonPete3
 
I put 1.75 SU's from a mgb onto my 1275 engine in my midget, I had to take custom made spacers for the bolts that are on the head to hold the manifold on, I might be able to take some pictures if it would help you, I'm not to sure on the similarites between the triumph and mgb set ups though.
 
For the record, I'm running stock 1.25 SU carb on my Mini 1380 in Sweden. Runs very strong and haven't felt the need for more.
Spend your money on the new wiring. /ubbthreads/images/graemlins/hammer.gif
 
As most people have said, stick to the standard 1ÂĽ" carbs, however the standard manifold is not brilliant in stock form; it could do with the steps and lumps and bumps removing from inside. Get hold of a copy of David Vizard's book for details.
 
A single side draft is intreging. I have been looking at my engine and tranny now out of the bucket o rust and wondered what to do.

Have any pics of this engine?

I thought about some Mikunni's but they are getting ard to find. I can make the intake tubes in my fab shop for anything.
 
Hello TLH,
I don't believe that, for a road car, you will better the performance and reliability of the original S.U. carburettors.

Alec
 
I have a highly tuned 1500 and I use dual sidedraft Weber 40 DCOM's. My take on the issue is that for a stock 1275, there's no way you'd need two (1 40mm might be better than the common option of 45mm, and 2 is impossible anyway due to the number of intake ports on an A series engine -- 2). Reliability has been good, except that since my carbs were used, the idle passages tended to varnish up when the car sat for 3+ months at a time (proper cleaning of used webers by soaking them is a good idea). However, gas mileage is supposedly worse (I never had an SU) but my mileage is excellent. The webers are fully tuneable and the manifolds are far superior to the stock 'log' manifold design. However, once again, if you happen upon a custom SU manifold, that would be a superior option (and probably cheaper and easier). Keep in mind that webers have a choke of a smaller diameter than the advertised 40 or 45mm. Mine is 34mm. Therefore, i have 4X34mm chokes. Obviously that's a lot, but on a stock motor, 2X34mm would be more realistic. Interestingly, this equates to 1.339"! So, webers are really only going to get you a better manifold design in this case. Also, jets are an added expense. You need to replace 4-8 jets (depending on original setup) per carburetor. They go for almost $10 each, so this is not too cheap. Obviously if you buy a new one, they're available already jetted, but still, this is something to consider. Webers are expensive, supposedly have reduced fuel economy, and the properly sized choke gives you nominal gain in cross-sectional area.

Matt
 
Hello Matt,
I don't know if you are aware of the single choke DCOE that were used for highly tuned Mini's. They were I suppose the ultimate in extravagance as it took two twin choke DCOE's to give two single choke DCOE's by the process of machining one choke section off each. I don't remember if it was for manifold design or that regulations required the same number of carburettors as original.
Another choice of carburettor for the Mini was used on the works four port (inlet) head using 4 single choke Amal motorcycle carburettors. The Amal is a very simple carburettor consisting of a body and a sliding piston with a needle. At full throttle the piston is fully retracted and the only air flow restriction was the needle, not many carburettors can match an Amal's capacity to flow air for a given choke size. The horrendously expensive GP Amal, which was an out and out racing carburettor even had the needle offset out of the choke completely so there was nothing in the choke at all at full throttle.

Alec
 
Alec:

The Amals are somewhat like the "S&S" carb I am running on my 1500 race-Spridget.
S&S carbs are intended for Harley-Davidson race-bikes.
I am required to use a single carb (in our local race class), so I chose the biggest "S&S" model I could find (2-1/16"). Very simple carb with adjustable idle mix, adjustable accel pump and even an optional kit so that you can convert to adjustable main jet.
Obviously my manifold is modded.
0406hrb_sscarb02.jpg
 
Actually this is exactly what I want to do to my 74. I am an old biker and have been looking at Dellorto Harley carbs. Can you help with the manifold design. Maybe some close .jpgs.

I have access to a lot of materials, lathe, all types of welding and machining centers. I also have a friend in the manderel tube bending business. He makes the tubes for Vortec and others for Mustang blowers.

I am willing to share. It's easy to build after I build the first one.
 
Hello Aeronca,
the big difference between your S and S, and perhaps I didn't make it very clear, the Amal has no butterfly throttle, that is done by the piston which is physically lifted by a cable.
At full throttle the piston is fully retracted, leaving an open choke tube.

Alec
 
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