SU Carburettor Tuning
Discussion
Can anyone recommend a garage or old school tuner in the Leics/Loughborough area? I have got my twin SU's as smooth as I can get them, but I know there is more to be done and I am out of my comfort zone now. Whilst fully driveable, the idle is poor when cold and a little lumpy if I accelerate hard.
There was a RPL in Leycroft Road, but their website is down, and telephone disconnected.
Thanks in advance.
There was a RPL in Leycroft Road, but their website is down, and telephone disconnected.
Thanks in advance.
Can't help with a local garage and it be honest it sounds like you might need a rolling road session as you could be at the stage of needing needles/springs that better match your engine. I can give you the basic is-it-as-good-as-you-can-get-it-at-home rundown.
1. Get the ignition timing set and give the distributor shaft a wiggle to see if you can feel much movement/wear. If your timing's out, or inconsistant due to wear you'll only be able to get the tuning so-so.
2. Go for a drive and get the engine warm.
3. Make sure the linkage is set correctly so both butterflies open.
4. Take off the airbox.
5. Set the idle to 1000RPM (pitting the airbox back on will dropp you a couple of hundread RPM).
6. Check the carbs are in balance. I'd recomment this type (https://tinyurl.com/y4b2hxlz) of balancer as it's just easier to use than the Gunson's one/sticking a piece of tube in your ear.
7. If you changed either of the carbs to balance then go back to step 5 until you have a 1000RPM idle and both carbs i balance.
From this point on just idling for a long period of time while you fiddle can give you a moving target so every couple of minutes rev to 3000RPM for 30s, just to blow any soot out/keep temperatures up/get the engine to re-settle to idle in case anything's sticking a little bit.
8. The carbs should have little lift pins underneath that you can yse to lift the carb piston (you can use a small screwdrive to lift the piston if not). Lift the piston just 1 or 2 mm and listen to the engine note. It should rise, then fall back to idle. If it rises and stays up that carb's too rich* so you can lean it off a little bit. If it rises and drops lower than before or just drops that carb's too lean so you can richen it a little bit.
If you get to this point/are already at this point and still not right try spraying (with the airbox back on) EasyStart at the throttle spindles where they pass through the carb bodies and also on the breather hoses. If the engine note goes up in response to any of those you've got an air leak messing things up - either wear in the carb body/spindles or lose/split breather hoses so address that (my car used to run great till I nailed it, then it would hardly run for a bit - eventually found one of the breather hoses had split right on the circlip. Normally fine but a bit of agressive revving when warm would move the hose/open the gap/flood extra air in to the engine. Stop and cool down and it would slip back in to place).
If everything's fine there then you'll need a rolling road session. Either the stages in your needles don't quite match your engine's tate of tune, giving you rich and lean spots through the rev/load range or you're dizzy's throwing sparks at the wrong times across the rev/load range. A rolling road should be able to tell you which and (if you can find one that still knows their SU needles) swap a few around to get the best match to your engine. If it's dizzy wear, or a level of tune a dizzy just can't help with then look at MegaJolt - my Spitfire wanted quite a lot of advance to pull anything below 2000RPM but setting the dizzy for that gave way too much advance abouve 4000RPM. Swap to MegaJolt and a rolling road set-up session and it pulls like never before, to the point even some of the Triumph specialist have commented on it.
1. Get the ignition timing set and give the distributor shaft a wiggle to see if you can feel much movement/wear. If your timing's out, or inconsistant due to wear you'll only be able to get the tuning so-so.
2. Go for a drive and get the engine warm.
3. Make sure the linkage is set correctly so both butterflies open.
4. Take off the airbox.
5. Set the idle to 1000RPM (pitting the airbox back on will dropp you a couple of hundread RPM).
6. Check the carbs are in balance. I'd recomment this type (https://tinyurl.com/y4b2hxlz) of balancer as it's just easier to use than the Gunson's one/sticking a piece of tube in your ear.
7. If you changed either of the carbs to balance then go back to step 5 until you have a 1000RPM idle and both carbs i balance.
From this point on just idling for a long period of time while you fiddle can give you a moving target so every couple of minutes rev to 3000RPM for 30s, just to blow any soot out/keep temperatures up/get the engine to re-settle to idle in case anything's sticking a little bit.
8. The carbs should have little lift pins underneath that you can yse to lift the carb piston (you can use a small screwdrive to lift the piston if not). Lift the piston just 1 or 2 mm and listen to the engine note. It should rise, then fall back to idle. If it rises and stays up that carb's too rich* so you can lean it off a little bit. If it rises and drops lower than before or just drops that carb's too lean so you can richen it a little bit.
- From memory so it could be the other way around - I think that's correct though).
If you get to this point/are already at this point and still not right try spraying (with the airbox back on) EasyStart at the throttle spindles where they pass through the carb bodies and also on the breather hoses. If the engine note goes up in response to any of those you've got an air leak messing things up - either wear in the carb body/spindles or lose/split breather hoses so address that (my car used to run great till I nailed it, then it would hardly run for a bit - eventually found one of the breather hoses had split right on the circlip. Normally fine but a bit of agressive revving when warm would move the hose/open the gap/flood extra air in to the engine. Stop and cool down and it would slip back in to place).
If everything's fine there then you'll need a rolling road session. Either the stages in your needles don't quite match your engine's tate of tune, giving you rich and lean spots through the rev/load range or you're dizzy's throwing sparks at the wrong times across the rev/load range. A rolling road should be able to tell you which and (if you can find one that still knows their SU needles) swap a few around to get the best match to your engine. If it's dizzy wear, or a level of tune a dizzy just can't help with then look at MegaJolt - my Spitfire wanted quite a lot of advance to pull anything below 2000RPM but setting the dizzy for that gave way too much advance abouve 4000RPM. Swap to MegaJolt and a rolling road set-up session and it pulls like never before, to the point even some of the Triumph specialist have commented on it.
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