Transcript
SU Carburettor This is a very basic guide to maintaining SU carbs. The XK120 and XK140 are fitted with a pair of H6 SU carburettors - H defines horizontal and the 6 denotes the bore size. SU's go up in size in 1/8 divisions so that an H2 has a 1 1/4 bore, an H4 1 1/2 and in the case of the 120 and 140 the H6 which is an 1 ¾ carb. The 150s are fitted with the more complicated HD6s and HD8s which I will cover in a future issue. The Basic SU Carburettor The basic SU carb is very simple but extremely cleverly designed, with a variable venturi that ensures both performance and economy. As with all good designs the SU has few moving parts. The largest and by far the most important is the piston, which is what governs the size of the venturi and therefore maintains the wonderful driveability at low engine speed. Suction chamber ( Dash pot ) Inspecting the piston and suction chamber is the first job, making sure they are scrupulously clean. Check the larger diameter of the piston, where the small groves are, for scores. Do not attempt to remove these scores, just remove any burrs formed at the side of such marks. The piston needs to seal in the suction chamber, which can also suffer from scoring and will need the same treatment. Fine wet and dry can be used to remove any burrs. The grooved part of the piston should never touch the side of the dash pot and is guided by a chrome piston rod that runs in the suction chamber. This should look like new to ensure that the piston moves smoothly up and down the dash pot. (The piston and chamber were probably matched at the factory, so it is good practice to avoid swapping pistons and suction chambers) The Damper The damper has the job of enriching the mixture on acceleration. This is done by retarding the rate of acceleration of the piston and causing a greater depression in front of the jet - in short, reducing the air and increasing the fuel. Check that the damper is not too long and that the piston can go fully into the dashpot when assembled. On most H6s there should be a small vent hole in the top of the damper. If not, drill a 1/6 hole to one side of the top. Main jet & needle The main jet should have a perfectly round 0.100 hole in the top of it - if not, it will be impossible to tune the engine. It should be changed if there is any doubt. It is harder still to recognise wear on the needle so change it if you are not sure of its condition. Centering the jet with the tapered needle centre (this should only need doing if new jets are being fitted) Remove the dash pot and piston, then adjust the jet so that it is flush with the top of the bridge (the bridge is what the piston falls on to at its lowest point). Refit the dash top and piston and let the piston fall slowly on to the bridge. The piston will probably stick just before the bridge. The needle will be fouling on the main jet. Loosen the large nut under the carb, which allows the jet to move, with the piston sitting now on the bridge re-tighten slightly, lift the piston up and allow to drop again. Repeat this until the piston drops easily then tighten the nut fully. Check again to make sure that the piston drops positively on to the bridge, then adjust the jet back to the basic setting of 2 1/2 turns anti clockwise (down) Float chamber Check for any corrosion at the bottom of the chamber. Ensure that the drilling at the base of the chamber is clear and wash with petrol. The float is soldered together and it is not uncommon for the float to fill with petrol. Do not try to salvage this as it is a lost cause (get a new one). Make sure that the float slides easily on the 1/4 centre pin. The float level is governed by the float lever, which in turn shuts the needle valve and reduces the amount of fuel. The needle valve is extremely important as it can cause unwelcome flooding and petrol smells, so if there is any doubt as to its condition change it. The float level is adjusted by bending the lever, it should be set with a 7/16 bar placed between the lever and the float chamber top. (See sketch No1) The float chamber top is held on by the centre 1/4 tube nut, which in turn holds the overflow pipe on. It is vital to use the special fibre washer under the overflow (breather pipe) as this will cause severe flooding if not used. Throttle spindle and disc The throttle spindle simply rotates in the throttle body and therefore the bushes in the body need replacing occasionally. The disc will also wear. Inspect it and change it if necessary. The bushes can be gently tapped out with a flat-ended drift and replaced in a similar way. If you are careful, the bushes will not need to be reamered. The spindle should pass through the bushes nicely. The disc can now be pushed into the slot of the spindle. The slotted screws can be fitted loosely and the spindle rotated so that the disc is closed. Look through the carburettor body to see if you can see a light past the disc. If so, move the disc around until there is virtually no light seeping through, then tighten the screws. Check once again, then bend the split ends of the screws apart. Never use second-hand screws. SETTING UP THE CARBURETTORS Cylinder compressions, spark plugs, contact breaker points, ignition timing, air filter and condition of carburettors must be checked before any attempt is made to set up the carburettors. Before adjusting the mixture it is good practice to set the jets to the recommended level. Remove the dash pots and pistons, then the dome tube nut at the bottom of the jet, screw the jet adjuster up so that the jets are level with the bridge. The basic setting is then 2 1/2 turns anti clockwise. Refit the dash pots and start the engine. The next job is to balance the airflow though the individual carbs. This can be done by placing a rubber tube in the intake of the carb and listening at the other end so that the suction can be heard. First loosen the clamp bolt of the universal jointed connector between the two carburettors so that they are working independently of one another. Rotate the throttle adjusting screws so that each carb is sucking equally and that the tickover is approximately 600 RPM. Tighten the connecting bolt and check the balance once again. To adjust the mixture, turn the adjusting nuts up to weaken or down to enrich until the fastest idling speed consistent with even running is obtained. The mixture can now be individually fine-tuned by using the lifting pins on the side of the carburettor body, listen to the engine speed and lift the pin slowly, the engine revs will either: Increase (Rich Mixture) Increase and then decrease (Correct Mixture) Immediately decrease (Weak Mixture) The carburettors are now set up. The mixture will be correct at all engine speeds, as long as the srcinal needles and the srcinal engine set-up are used. Changes such as air filters, camshafts, high compression pistons and large valves will dramatically change the air-fuel ratio. Any one of the above can damage the engine, as too little fuel will cause increased combustion temperature. This will burn pistons and valves. Too much fuel will wash the oil from the cylinder walls and increase bore wear. If your engine is modified in any way, you have no choice but to put your car on a rolling road. The mixture can then be checked at all speeds and all throttle positions. Adjustment can then be made with a huge choice of needles. Symptom Cause Remedy Erratic running Stalling at idle Lack of power High fuel consumption Sticking piston Dirty piston and dash top Jet out of centre Worn jet or needle Clean Re-centre Fit new Hesitation on pick up Low damper oil level Incorrect oil grade (too thin) Top up Replace with correct grade Float chamber flooding Dirty or worn float chamber needle valve (dirty fuel) Punctured float Incorrect fuel level Sticking float Clean or renew valve (flush system) Fit new Check and reset level SU H and HS carburettors. 1: Unscrew throttle adjusting screw until the face just clears the stop. Open by screwing in 1 1/2 turns. 2: Remove Piston and Suction Chamber, disconnect mixture control wire, and screw jet adjusting screw until jet is flush with carburettor bridge (or full up if you cant). Replace parts, and check Piston falls freely on the bridge (by using piston lift pin). Turn jet adjusting screw 2 complete turns (or 12 flats if accessibility and visibility is restricted, eg on Minis!). 3: Start engine, and adjust to desired idling speed (eg, warning light glowing). 4: Turn jet adjusting screw to obtain fastest idling speed WITH EVEN FIRING. 5: As the engine speed may have increased, readjust Throttle Adjusting Screw. 6: Check mixture strength. Use Lifting Pin to lift piston about 1/32”. o A) If the engine speed increases, and continues to increase, the mixture is to RICH. o B) If engine speed decreases, the mixture is to WEAK. o C) If engine speed momentarily increases slightly, the mixture is CORRECT. 7: The exhaust note should be regular and even. If it sounds „splashy,‟ the mixture is too weak. If there is a rhythmical misfire, together with blackish smoke, it‟s too rich. 8: Re-connect mixture control wire with about 1/16 inch free movement before it starts to pull on the cam lever. 9: Set Choke knob (on dash) to max. movement (about 5/8 inch) without moving the carburettor jet, and adjust the fast idle cam to give a fast tickover, about 1000RPM. SU Tuning This isn' t the definitive guide, but it works for me. It‟s a combination of several different pieces of literature and also personal experience. Before you tune your carbs you need to make sure that points, plug gaps, ignition leads, timing, valve clearances and the general fuel and exhaust system (i.e. leaks) are in good condition. Otherwise it‟s a head ache to tune them. For spark plugs I would recommend nothing else but NGK in your Triumph Spitfire... BPR6ES Copper Core are probably the best... but BP6E & BP6ES are fine too. NGK Technical Information Pages - lots of technical information on spark plugs NGK UK Technical Information Pages - not as detailed as above - but with good examples of plug colour and simple to understand information Check that your needles are correct and also that they‟re properly in the pistons. Note: Piston's work exclusively with their carb body - do not mix them up! Make sure you have the correct springs and also that there‟s some oil in the dashpots. If you are unsure of what oil to use, just use engine oil until you know any different - or can judge what to use. A thinner oil will richen the mixture on acceleration. 1. Go for a hearty drive to get the car to operating temperature. Go on! Drive it like you stole it! 2. Balancing the amount of air that is being taken in by each carb.
