The SU carb as fitted to the MG Turbos is a great piece of kit. It responds exceptionally well to the varying needs of a forced induction engine, and once prepared properly, and matched to a good ignition system, is hard to fault, giving comparable power to injection.
I won’t go into details about how the SU works, as this can be found in many books. The important differences between the turbo carb and the normal 1-¾”HIF are as follows: Dashpot spring and needle, seal under dashpot body, rubber balance pipe, throttle disc, float chamber vent pipe. There are detail differences also like the groove in the top of the dashpot, and the seals in the choke and butterfly shaft being the std. seals, but reversed! These are all details however…..
Running Problems?
Following is a list of the normal things that go wrong with the turbo SU.
A serious loss of power under boost is almost always caused by a lean out. Remember Leanout on a turbo engine means £££ expense.
There are few reasons for this, but the most common is that the carb vent solenoid (anti run in valve), which is closed when the ignition is switched, on has failed. Easy way to isolate this is to clamp the hose going to it, and take it for a run. If it cures it, block the pipe permanently with a shouldered bolt or plug (must be airtight).
Another common problem area is the rubber pipe that goes from the side to the front of the carb. This often splits, as it becomes brittle with age. Replace with hose of a similar or better grade.
The third problem happens usually with engines that are run without air filters or with a ruptured leaking air inlet tube (very common in minis where space is tight). What happens is the internal pipe in the carb that transfers the boost pressure ‘signal’ from the plenum chamber (before the restrictor) to the float chamber becomes blocked. A carb stripdown and clean cures this, but a quick solution is to connect a tube from the pipe that goes to the vent valve solenoid (or used to) to the pipe that goes from the plenum to the top of the regulator using a T-piece. This does affect the mixture slightly, so you might want to adjust it afterwards. I use this mod all the time on really high power engines, as it creates a slightly richer mixture under very high boost, making it easier to perfect needle profile.
The final thing that can go wrong is debris to collect in the bottom of the float chamber, which can restrict the jet pickup. It has to be said however, that if this happens, it is only down to the owner’s maintenance schedule. The small filter between the regulator and carb (which should be changed within 250 miles if it is a new engine/pipework install and then a max of every 10,000 miles after) should prevent any ingress.
That is everything that goes wrong with the carbs, such is the simplicity of them!
If you still have running problems, then check the dashpot oil level, plumbing of your regulator, the regulator diaphragm (can split or stick) and the ignition system. The modules on the dizzy are the cause of most other running problems, and can be difficult to isolate, because they will usually work fine under light loads, but become erratic as load and RPM increase. Replacement with a new one, making sure that the special heat conductive grease is applied between the module and Dizzy body will ensure reliability.
If your Dump valve is working badly, seeming sluggish in operation, then it is the fault of the special throttle disc fitted to the turbo carbs. This has a poppet valve built in, that is designed to work with the overboost valve in the plenum, by allowing a little air into the engine when the throttle is snapped shut, maintaining the air velocity over the jet. Remove the butterfly and replace with a solid one, or cut off the valve and solder / braze up the holes that are left. Beware of cheap dump valves that fail to seat properly. The best ones are the ‘group A’ or piston type.
Modifying the Carbs for power, economy or both?
There is an important modification that must be made to the carb if it is fitted to an engine that is different to the original specification, be it CR, Boost, Cam, rockers, whatever.
This is associated with the Rubber Balance pipe. This is actually a ‘part-throttle’ leanoff device, and is also found on some ‘economy models’ of other SU fitments. This has an accurately sized orifice that is tuned to the standard engine. As soon as any modifications are made, this becomes incorrect, and can cause excessive Leanout under part throttle or gentle acceleration.
The best all round way to deal with this is to re – calibrate this, by adding a very sensitive restrictor valve into the pipe and spending hours trying to re-calibrate it.
If you are only concerned with power and drivability, then the best action is to simply plug the two ends of the pipe.
The all-round easiest method that gives excellent mixture for power, but can give good cruising economy at a flick of a switch, is to connect a small solenoid in line with the pipe. A simple toggle switch on the dashboard can be flicked when you are sat on the motorway at a steady speed, and the engine will run slightly lean, which will do wonders for your economy, especially if you have a very high boost needle fitted. As soon as you want to accelerate, even gently, the solenoid must be closed, otherwise Leanout will occur. The small gold boost solenoid on the metros is ideal for this application.
Changing the throttle disc is a very good idea, as the poppet valve not only creates a sizeable restriction, but also can cause an erratic tickover as the spring gets weak. Dump valves should be fitted if this is removed however.
The mods that Vizard recommends for the SU – knife edging, throat profile modifying, etc, are all a waste of time on the SU Turbo as the tiny flow increase is irrelevant compared to the effort required.
If you are serious about high power with the HIF, I would recommend replacing the Bimetallic Jet holder with a fabricated one. The high heat that the carb experiences with the turbo sat 1” away from it overcompensates this ‘critical’ device and causes all manner of problems, especially in an engine bay that isn’t adequately or correctly vented above the carb. Vents want to be rear facing, so the hot air is drawn out through the forward momentum of the vehicle.
Dave Coxon, Feb 2002
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