Modifying SU Carbs

[Redneck Rocket]

I'm progressing fairly well with Spitfire's transformation from rolling wreck to budget racecar. I've been given a set of 1.5 inch SU's to throw on, which should work well with some of the other mods I've done.

I've heard a few people allude that you can do a few things to SU carbs to make them work better. Obviously setting them up correctly is the first step. Does anyone have any experience in tuning SU's and have any suggestions? I'm guessing that smoothing out some of the surfaces with a bit of sandpaper might improve the airflow for example.

[thebear]

There are many facets to tuning SUs that are not immediately evident. This information refers to the type used in the '50's and early 60's. Later versions used a diapragm jet. Tuning and set up are similar but I can not offer any advice other than what follows.

Air flow and vacuum in the proper locations are critical. For this reason, SU recommended that compression, timing, valve lash and all other parts of engine health be addressed before making adjustments to the carburettors.

Start with the body off the engine and insure that the butterfly plate is centered in the throttle body. Some have soldered the small hole closed however it is necessary for idle stability. Visually inspect the centering by holding the body up to the light and insuring that it can be sealed by the butterfly plate around the entire circumference or that any light around the periphery is evenly distributed. This would be an ideal time to verify that the throttle shafts are not worn and that their pass thru openings are cylindrical and not egg shaped. Bushings may be required.

The parts variables are jet diameter (0.090", 0.100" or 0.125") and needle selection. Each needle in the series has a different taper that may not be obvious to the naked eye. The purpose of the taper is to enrich the fuel mixture as manifold vacuum raises the damper in the bell. This is of course a function of the engine's `breathing' as well as RPM. A specialist tuner would be the starting point for needle selection as changes in camshaft profile and timing must be considered. Lots of trial and error involved here.

The needles are in a series for either jet diameter and with engine off they completely block off the jet. This fitting is crucial to proper operation. If the jet is not centered in the body of the carburettor the needle will `drag' when manifold vacuum raises the damper. This is evidenced by burnished areas on the side of the needle and off-round appearance of the hole in the jet. Rotating the needle in the dash pot may cure this. The shoulder of the needle should be flush with the base of the dashpot.

With the damper and bell removed and the fuel pump primed/pressurized, set the top of the jet exactly even with the surface of the casting (called the `jet bridge') and there should be a miniscus of fuel visible in the hole in the jet.

Fuel flowing at this point is an indication of a float height problem. A non-automotive manifestation of this is your toilet tank at home. If the rod is not properly bent/positioned the toilet will `run' all the time or there will not be enough water in the tank for a proper flush. Early SU floats were soldered brass and had a tendency to leak and not float. A maddening problem until the float was removed and the fuel could be heard sloshing around inside. I am sure that there are moden polymer replacements available.

The`choke' cable, when activated will draw the jet out of the casting providing a rich mixture. Make sure that when the cable is released the jet returns to the flush position.

The damper must move freely in the bell. Some would include a few drops of motor oil in the top of the dash pot. Modern lubricants such as silicones applied to the walls of the bell and the dash pot can be useful. Over oiling of this assembly will make it a dust magnet and cause irregular operation of the dashpot and premature wear of the bell.

Assemble the carburettor onto the engine and verify that the the dashpot moves freely by using a small wooden implement to lift it without effort and drop back to the closed position with a `thunk'.

Connect the throttle shafts loosely and insure that they rotate without binding then secure them lightly.

Connect the throttle return springs.

Start the engine. Operate the throttle if necessary to maintain rpm and set the idle screws on the outside of carburettors. In order to sysnchronize the carburettors an device calle a "Uni-Syn" was used. It was held over the throttle body opening and had an internal venturi that allowed a small ball to rise in a glass tube for observation. There was a rotating cone on a threaded shaft that could be used to slowly block off the throttle body and calibrate the vacuum on each carburettor. Minor adjustments are performed on the throttle shafts to aid in synchronizing. When synchronization is achieved the throttle shafts are then fixed to each other.

At this point the car may be driven carefully and throttle repose judged. If the carburettors `spit' that means there is a bind that prevents the dash pots from operating properly.

All in all, the SU carburettor is a very simple device and is patterned after the original carburettor used by Gottlieb Daimler on his first automobile.

My references for this stem fom my '57 Austin-Healey and racing days in the early 60's and having raced the Mini (in the avatar), a BMC "A" series powered Lotus 7 and a Bug(Frog)eye Sprite. Subsequent drives included the MG 1100 sedan and a '67 triumph Spitfire. You could say "been there, done that".

[thebear]

Originally Posted by Redneck Rocket

I'm progressing fairly well with Spitfire's transformation from rolling wreck to budget racecar. I've been given a set of 1.5 inch SU's to throw on, which should work well with some of the other mods I've done.

Depending upon engine displacement they may be too large. My '67 road car had 1¼" inch carbs. Later versions had a single 1½".

Originally Posted by Redneck Rocket

I've heard a few people allude that you can do a few things to SU carbs to make them work better. Obviously setting them up correctly is the first step. Does anyone have any experience in tuning SU's and have any suggestions? I'm guessing that smoothing out some of the surfaces with a bit of sandpaper might improve the airflow for example.

I wouldn't - see prior post about sensitivity to air flow. The best thing to do is make sure that the openings match those on the manifold exactly and that the gaskets do not protrude into the airflow.

[Peter Mallett]

hmm,

Shouldn't this be in Technical?

I'll move it there.

Incidently, when a ran Metros we used to "rough up" the inlet manifolds to improve the aerosol effect.

[trikesrule]

This is the technical section isn't it? I've set quite a few SU' up. I agree that the SU' and the engine must be in excellant health. After getting the engine to idle reasonably I work backwards. I start with full throttle runs for about 1/2 a km then I do plug chops. I find it easier this way. Used to have the manifold interior bead blasted to rough it up. You definately don't want any gasket material creating a hurdle for the mixture to clear. A couple of years back I found a pretty descent book by Des Hammil How To Build & Power Tune SU Carburettors www.velocebooks.co.uk.......trikes

[Peter Mallett]

It was in historics before I moved it.

[terence]

Regs will possibly rule out any serious mods but what you can do is to ensure the pistons are nice and smooth,polish them if need be,needles need care,make sure they are absolutly straight.Polish the inside of the dashpot,s and use 3in1 oil for damping,quick .cheap,and totally legal.Obviously make sure they are set up correctly.

[Redneck Rocket]

Originally Posted by thebear

Depending upon engine displacement they may be too large. My '67 road car had 1¼" inch carbs. Later versions had a single 1½".

I had thought about this myself. From what I gather, simply swapping to larger carbs will just use more fuel without making more power. However, I've made the following engine mods so far:

Intake Trumpets
Fast Road 83 Cam (280)
Full Race manifold(24 inch primaries) and straight-through exhaust
Kenlowe fan (removed the mechanical fan)
13 Row Oil Cooler

I knew that with those mods I should probably be changing the needles in the HS2's, but a friend offered me the inlet manifold off a 1500 spit, with a set of HS4's to go with it. Since I'm on a tight budget, that seemed like a possible alternative to new needles since it would allow more air & fuel flow, especially at higher revs when the cam kicks in.

[thebear]

Originally Posted by Redneck Rocket

Since I'm on a tight budget, that seemed like a possible alternative to new needles since it would allow more air & fuel flow, especially at higher revs when the cam kicks in.

But that is still attainable with only a needle change. How much are needles these days? Can't be much - maybe £1 each?

Easy to change as well.

[275 GTB-4]

Originally Posted by terence bower

Regs will possibly rule out any serious mods but what you can do is to ensure the pistons are nice and smooth,polish them if need be,needles need care,make sure they are absolutly straight.Polish the inside of the dashpot,s and use 3in1 oil for damping,quick .cheap,and totally legal.Obviously make sure they are set up correctly.

or ATF or lighter or heavier oil to get the right throtle response you want

[Redneck Rocket]

Originally Posted by thebear

But that is still attainable with only a needle change. How much are needles these days? Can't be much - maybe £1 each?

Easy to change as well.

True, but the HS4s were free!!!

[graham bahr]

i dont see a problem with twin 1.5 inch carbs, dont forget 60's A series engine tuning a swap to twin 1.5 carbs was common and they rallied minis with twin carbs as big as 1.75.

1300 spits all had 1.25 carbs, european 1500's had 1.5s, and slightly less power due to a very mid cam fitted in the name of emmisions fit a BIGGER cam in a 1500 and you will see a good power increase, and a see no reason why twin 1.5's wont give a power increase on a modded 1300 engine.

i suspect the only reason late U/S/A or Canadian spec cars had a single 1.5 carbs was again exhaust emission related.

su carbs are very forgiving, unlike a fixed jet carb going too big doesnt have much of a down side as given the correct springs they wil only open as far as needed so they wont suffer really low airspeed and poor performance compaireed to say a oversized weber or one with excessively big chokes in it

[Notso Swift]

You can "high flow" an SU quite eeasily, since that full throttle there is only the butterfly in the way, you can file down the spindle quite a bit (make sure you locktite thase screws though!) , if you are really keen you can bond the unit in place and knife edge the leading edge of the butterfly.

As far as the needles are concerned you can tweek them by turning them down (you need a very good lathe) or filing yourself, best bet is in a lathe with a bit of very fine emery, no need to buy new ones.

[esper]

so after 3 weeks , whats up with the bend needles?

[gttouring]

Funny I have a Volvo 122 now and the SU, i think may be the source of the trouble
A cold full throttle stumble, after it warms up it all good, but until then
pop pop pop crack poop gurgle vrooom!

[thebear]

Originally Posted by gttouring

Funny I have a Volvo 122 now and the SU, i think may be the source of the trouble
A cold full throttle stumble, after it warms up it all good, but until then
pop pop pop crack poop gurgle vrooom!

You may have to run thru some of the steps I have outline at the beginning of the thread.

Sounds like a set of dry dash pots. Unscrew the black knobs and put a few drops of motor oil in. Do not fill them as the oil will only squirt out the vent hole.

Next step would be remove the air cleaners with the engine of and see if you can lift the pistons with your finger. If they do not move freely, undo the screws (2 or 3, depending on model) that hold each dash pot bell in place and carefully lift the assembly out one at a time and inspect. Check for free movement of the piston. Could need a cleaning inside the dash pot bell. If it is free check for condition of needles and insure they do not have wear marks near the piston face.