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Offline LongTimeFordMan

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Up date on rebuilt engine and SU carburetors
« on: November 07, 2019, 12:10:34 PM »
Hi all

Update on my SU carb conversion on my 73 2.0 wagon.

As some of you all may know about a year ago I built a custom manifold to fit 2 SU HIF 4  1 1/2" carburetors from a 72 mgb onto my 2.0 in a 72 wagon.


I recently replaced the original engine which was running well but had low compression with a stock  rebiluilt 2.0 only mod was a .040 mill on the head to up the compression.

I have an adjustable cam pulley, also recurved the distributor and can adjust the carb fuel mixture.


The engine ran really strong and pulled really hard up to 6000 but i was having  some difficulty getting the proper settings with 3 variables.

So i dug out the factory holly webber 2500 32/36 and decided to use it to get basic settings.


After driving with bothe the SU and 32/36 carbs  i have some observances.

The linkages on the 32/36 open the secondary really late at about 3/4 throttle so i modded the linkage with a wire loop to open the secondary at about 1/2 throttle... big improvement.

The SUs flow.a LOT of air.. BUT with air comes fuel

With the new engine quickly revving to 6000 plus rpm fuel milage dropped from 16-18 with old engine to about 12 with new engine

As stated above the rebuilt factory engine with  pulled like a scalded cat from 2000 to past 6000

Same engine with 2 SU carbs at about 1/2 throttle (never pushed it past 3/4 throttle because revs got too high) made more power than stock 32/36 at full throttle (primary and secondary both full open).

 Throttle response is a LOT more instantanious with SU sidedrafts.

32/36 seems to flow about half as much air as a pair of 1 1/2 inch SU carbs.

Making power requires more fuel but i think i need to redesign the manifold because an MGB 1800 engine with the same carbs and compression will get 18-23 mpg.

I suspect the problem is that i was getting fuel drop in the plenum 9f the manifold i built.

Am redesigning the new manifold with no plenum and runners directly from.carbs to intake.ports.

Will follow up with report about mileage with stock 32/36 carb and progress with new manifold for the webbers.

Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles

Offline 71pintoracer

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Re: Up date on rebuilt engine and SU carburetors
« Reply #1 on: November 07, 2019, 04:17:26 PM »
Very nice! And good info. l am a long time fan of the 2.0, they are almost bullet-proof and rev like crazy! The Pintoracer originally had a modified 2.0 with a 50 shot, it ran low 13's in the 1/4. l launched at 7500 and shifted at 8800.
You have a very nice looking ride!  8)
If you don't have time to do it right, when will you have time to do it over?

Offline Wittsend

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Re: Up date on rebuilt engine and SU carburetors
« Reply #2 on: November 07, 2019, 04:20:30 PM »
I had SU (type) carbs on my '72 240Z and my Volvo 544.  I got 33 MPG out of the 240Z all be it going 50 MPH in light traffic when the limit was 55 MPH. I know it was accurate too because the Z car had a dump can sized filler neck and you surely knew when it was full. The Volvo 544 I got 36 MPG on a long trip. Not bad for a car that looked like a '46 Ford. I remember the shot glass sized float bowls would seemingly idle for a minute or two even without the fuel pump delivering fuel.

This is such a cool project. Hope you get it sorted out further.

Offline LongTimeFordMan

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Re: Up date on rebuilt engine and SU carburetors
« Reply #3 on: November 07, 2019, 05:12:44 PM »
Thanks for the comments..

Well i will be starting the manifold redesign soon.

My original design has a plenum that I thought would equalize the flow but I think that the large plenum allows for fuel drop from the carbs to the runners.

My new design will eliminate the plenum and just run runners directly from the carbs to the intake ports.

And as you all.mentioned i figure that properly tuned SUs on a 9:1 compression motor.shouod get at least 20-23 mpg minimum

Im trying to get the mileage up for the Stampede  ext summer what with as in california selling at $4 a gallon.

Meanwhile Im limping around town with the stock carb and manifold.

But i did fond a smple.way to improve the problem.with the late secondary opening with a loop of wire put onto the linkage.

I detailed that in another post here.

I assume that yiu upgraded the connecting rods on the high rev pinto. Ive heard that the stock rods are only g9od for about 6400 rpm
I will keep.updating as I proceed.
Red 1973 pinto wagon DD, SoCal desert car, Factory 4 speed, 3.40 gears, Stock engine, 14" rims and tires, 60 K original miles