SU carb question
Discussion
In the carb rebuild book it says the needle need to be level with the 'bridge' on the bottom of the piston but it doesn't specify which part is the 'bridge'.
So from the poor quality photos below (bloody iPhone) can you identify which is the correct setting? (HIF4 carbs for a rubber bumper MGB)
or
I currently have them set to the level shown in the bottom 2 photos
So from the poor quality photos below (bloody iPhone) can you identify which is the correct setting? (HIF4 carbs for a rubber bumper MGB)
or
I currently have them set to the level shown in the bottom 2 photos
I have proved to myself and no doubt others that I not very good at describing things but I'll give it a go.
The needle is thick at the top and thin at the bottom.
It goes into a hole in the base of the carb. Lets call that hole a jet.
When the thick bit is in the hole less fuel can come out of the hole.
When you press the throttle (I'll ignore the stuff about how and why!) and the needle move up.
As the neddle moves up it gets thinner so allowing more fuel through the hole, the jet, it's in.
So OP it does matter about the bridge (never heard that term wrt SUs before) all that matters is that the neddle is in the right place to give the right mixture.
Not 100% sure about HS4s but HS2 needles had three grooves on the neddle for the circlip to go in so you could adjust the mixture.
Hope that all makes sense!
The needle is thick at the top and thin at the bottom.
It goes into a hole in the base of the carb. Lets call that hole a jet.
When the thick bit is in the hole less fuel can come out of the hole.
When you press the throttle (I'll ignore the stuff about how and why!) and the needle move up.
As the neddle moves up it gets thinner so allowing more fuel through the hole, the jet, it's in.
So OP it does matter about the bridge (never heard that term wrt SUs before) all that matters is that the neddle is in the right place to give the right mixture.
Not 100% sure about HS4s but HS2 needles had three grooves on the neddle for the circlip to go in so you could adjust the mixture.
Hope that all makes sense!
I've never seen a circlip on the needle in an SU but then I used to work on H4 & H6 carbs not HS2s, anyway I would say the bottom picture is correct. p.s. I thought you adjusted the mixtuer by raising or lowering the jet seat (i.e. the bit the needle goes into, and by using different profile needles of which there are millions!
'Swapping for richer jets' isn't really the best way to get the most out of your SU's.
Years ago, I used Peter Burgess in Derbyshire, who for a VERY reasonable fee, tuned the needle of my single HIF44 on my Midget (as well as getting the timing right). The engine was far from stock, and he did an amazing job, well worth the money (even as a poor student). By adding wider jets, you are just making it richer everywhere in the power curve. Peter can tune your fueling correctly through the whole engine speed range, either by using different needles, or changing the profile of the existing needles with careful use of a milling machine and some emery cloth. This is definitely an old school kind of guy - perfect for old cars!!
Years ago, I used Peter Burgess in Derbyshire, who for a VERY reasonable fee, tuned the needle of my single HIF44 on my Midget (as well as getting the timing right). The engine was far from stock, and he did an amazing job, well worth the money (even as a poor student). By adding wider jets, you are just making it richer everywhere in the power curve. Peter can tune your fueling correctly through the whole engine speed range, either by using different needles, or changing the profile of the existing needles with careful use of a milling machine and some emery cloth. This is definitely an old school kind of guy - perfect for old cars!!
OK you really need an SU book, but using a steel rule or similar the shoulder of the needle should be flush with the body of the damper. The using a steel rule wind the adjuster screw so the jet is flush with the body of the carb. Make a note of the screw position this is your base point 0. Do the same to the t'other one. Then wind them down around 2 turns so the jet is below the body of the carb, make a note of direction to lower the jet. Reassemble, bodies & dampers. Remove the air filters & then run the engine until warm & if it has an electric fan gone through at least 2 fan cycles. Using a long thin screwdriver lift the piston on the carb & look for a slight increase in revs & then settling back. adjust them both as a pair until they both behave the same. If the exhaust note rises and stays high until you drop the piston, this carburettor is adjusted too rich. Adjust the mixture 1/8th turn leaner e.g. the jet higher. If the exhaust note falls and the car sounds as though it is going to stall, this carb is adjusted too lean. Adjust the mixture 1/8th richer e.g. jet lower. If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carb is set correctly. When you have achieved this setting for both carbs.
If you have put free flow filters on then all you'll need is slightly richer (thinner) needles not bigger jets.
If you have put free flow filters on then all you'll need is slightly richer (thinner) needles not bigger jets.
tr7v8 said:
OK you really need an SU book, but using a steel rule or similar the shoulder of the needle should be flush with the body of the damper. The using a steel rule wind the adjuster screw so the jet is flush with the body of the carb. Make a note of the screw position this is your base point 0. Do the same to the t'other one. Then wind them down around 2 turns so the jet is below the body of the carb, make a note of direction to lower the jet. Reassemble, bodies & dampers. Remove the air filters & then run the engine until warm & if it has an electric fan gone through at least 2 fan cycles. Using a long thin screwdriver lift the piston on the carb & look for a slight increase in revs & then settling back. adjust them both as a pair until they both behave the same. If the exhaust note rises and stays high until you drop the piston, this carburettor is adjusted too rich. Adjust the mixture 1/8th turn leaner e.g. the jet higher. If the exhaust note falls and the car sounds as though it is going to stall, this carb is adjusted too lean. Adjust the mixture 1/8th richer e.g. jet lower. If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carb is set correctly. When you have achieved this setting for both carbs.
If you have put free flow filters on then all you'll need is slightly richer (thinner) needles not bigger jets.
This is great advice but note that this is only for tuning your idle settings. Getting the misture right through all conditions needs a change in needle profile.If you have put free flow filters on then all you'll need is slightly richer (thinner) needles not bigger jets.
I've also found that balancing two SU's can be a little more tricky than above - I ended up using a piece of hosepipe from the mouth of the carb to my ear to listen to the airflow, and tweaked the idle screws to get them matched.
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