jeepguy
XS650 Addict
I have a 1961 Austin Healey 3000 and looking for someone with experience cleaning and synchronizing them. Let me know if you know anyone or you yourself can do it. thanks
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Anyone work on SU carbs????
- Thread starter jeepguy
- Start date
I had a pair on my '69 mgb but was so long ago I've forgotten everything, except keep them topped up with oil 
The official MGB manual that came with the car covered them pretty well. I tore everything down on that car multiple times using just the official manual. Which was about as thick as a phone book.
The official MGB manual that came with the car covered them pretty well. I tore everything down on that car multiple times using just the official manual. Which was about as thick as a phone book.
1974jh5
Curmudgeon.
I have had my fingers in more SU's than I care to remember. Those should be HS4's. A 1961 should have 3.
About Webers: I've been through plenty of those as well and I currently have Dellorto DHLA sidedrafts (Weber DCOE copy) on my Jensen Healey. On a 3000, about all you will find is the triple sidedraft setup and honestly that's more a race setup. It can be done and made streetable, but by the time you buy venturis, jets, etc then spend hours setting it up you will understand why the SU is actually a pretty neat instrument.
The very first thing: be 100% sure you have no vacuum leaks between the intake and head, or anywhere in the intake tract between the carb base and cylinder head. A vacuum leak will lead to all KINDS of weirdness.
First thing when rebuilding SU's: be sure the throttle shafts are not worn. If they are, it will never idle right. Spraying carb cleaner at them is not the way to check them, even perfect fit shafts will leak a tiny bit. If you can wiggle one shaft and it screws the idle up, that needs to be fixed first. That's a pretty delicate operation, it takes someone with a bit of know how to bore the carb bodies and replace the bushings, or ream the bodies and install oversize throttle shafts. Joe Curto http://www.joecurto.com/ knows his SU's inside and out.
Once that is done, the needles are next. If they have a large shiny spot on one side or the other, they are worn and will make the carbs very rich at small throttle openings. Needles are readily available and not expensive. The jets typically do not wear but should still be checked carefully, look at the opening with a mafnifying glass. If the hole is egg shaped, oval, anything but round, replace the jets too.
When installing the needles in the pistons, the 'shoulder' of the needle needs to be flush with the bottom of the piston. I always used the back side of a hacksaw blade as a straight edge. The jets need to be centered with the needles: install the needles in the pistons, then put the pistons and chambers on the carb bodies but do not install the springs. The needle should stick through the bottom of the carb body. Slide the jet and its holder over the needle, then put the nut on finger tight. Flip the carb body over, raise the piston and let it drop. It should go all the way to the bottom smoothly. If it sticks or binds, loosen the jet holder nut, reposition the jet and holder, then try it again. This will probably take two or three tries to get right.
When the jets are installed with the mixture nut and spring, the top of the jet should be even with the top of the 'bridge' in the venturi. Once you do this, mark the mixture nuts so they can easily be seen when looking over the fender. I used to use one of those little 'buzzers', make a dot or an X that could easily be seen.
The float level on an SU is CRITICAL. The fuel level in the jet will be the same as it is in the float bowl. Your carbs probably have brass floats, the hinges do not wear easily on those. If it has plastic floats, the pivot pin hole will wear making adjustment almost impossible. If you can retrofit brass floats, that's a good thing.
The piston chambers and pistons are a snug fit. If you put the piston into the piston chamber without the spring, then turn the chamber upside down, the piston should fall slowly to the bottom and you should hear a 'hiss' (air escaping around the piston). If the piston sticks, clean it and the chamber thoroughly but DO NOT use any sort of abrasive! Emery cloth etc will make junk of a piston and chamber in about five seconds.
I always prefer to assemble the float bowls to the carb bodies, then install the bodies without the pistons/chambers. That makes the next step easier.
When you install the carburetor bodies on the intake, back the idle speed screws out all the way, the throttle plate should seat against the throttle bore. Install the bodies with the throttle shaft clamps loose, snug the clamps up, then work the throttle a few times by hand. The plates should go all the way to the throttle bodies. If they don't, investigate and repair.
Once that is done, carefully turn the idle screws down till they just barely touch the stop on the lever. Then turn each one another turn down (thus opening the throttle, the way you would raise the idle).
Now go back and install the linkage, return springs, etc and finally the pistons and chambers (don't forget the springs!). Remember to put oil in the dashpots! Want to start an Internet bitch slap session? Post your favorite SU/Stromberg oil on one of the Brit car sites, then stand back to avoid the flames. Me, I always used 20w50 engine oil. Worked fine.
Once this is done, it's time to fire it up. Be 100% certain the plugs and points are clean and properly gapped! '95% of carburetor problems are in the ignition.' Also make sure the valves are properly adjusted. A too tight intake valve will have you 100% convinced SU's are the devil. BTDT.
You need one of these:
It's called a Uni Syn.
Fire the engine up, make sure there's no fuel leaks etc, then stick the Uni Syn on each carb one at a time. The red doohickey in the tube should rise to the same level on each carb. The total level is not important, just that all 3 are equal. Open the throttle a small amount (to maybe 2000 RPM) then check again.
Something to remember: whatever you do to one carburetor you have to do to all 3. So if you turn the idle screw on #1 down 1/4 turn, the others need to turn the same way and the same amount. Idle should be somewhere around 800-1000 RPM.
Mixture is easy. It will probably be a bit lean with the jets adjusted all the way up. To check this, lift each piston ~3/16" one at a time. The engine should speed up for a second, then go back down. If it tries to stumble, it's too lean. If the RPM goes up and stays up, it's too rich (not very likely). HS4's usually had a pin on the side of the carb to lift the piston, if yours don't use a pocket screwdriver or O ring pick to lift the piston (your finger is too big and will give a false reading).
To richen it up, turn the mixture nuts DOWN, i.e. clockwise as viewed from above. Turn the nuts down the same amount, same way you did the idle screws. That's why you marked the flats, so you'll know where your start point is. You will probably wind up with the mixture nuts turned down about a half turn. Count the flats to keep track.
About Webers: I've been through plenty of those as well and I currently have Dellorto DHLA sidedrafts (Weber DCOE copy) on my Jensen Healey. On a 3000, about all you will find is the triple sidedraft setup and honestly that's more a race setup. It can be done and made streetable, but by the time you buy venturis, jets, etc then spend hours setting it up you will understand why the SU is actually a pretty neat instrument.
The very first thing: be 100% sure you have no vacuum leaks between the intake and head, or anywhere in the intake tract between the carb base and cylinder head. A vacuum leak will lead to all KINDS of weirdness.
First thing when rebuilding SU's: be sure the throttle shafts are not worn. If they are, it will never idle right. Spraying carb cleaner at them is not the way to check them, even perfect fit shafts will leak a tiny bit. If you can wiggle one shaft and it screws the idle up, that needs to be fixed first. That's a pretty delicate operation, it takes someone with a bit of know how to bore the carb bodies and replace the bushings, or ream the bodies and install oversize throttle shafts. Joe Curto http://www.joecurto.com/ knows his SU's inside and out.
Once that is done, the needles are next. If they have a large shiny spot on one side or the other, they are worn and will make the carbs very rich at small throttle openings. Needles are readily available and not expensive. The jets typically do not wear but should still be checked carefully, look at the opening with a mafnifying glass. If the hole is egg shaped, oval, anything but round, replace the jets too.
When installing the needles in the pistons, the 'shoulder' of the needle needs to be flush with the bottom of the piston. I always used the back side of a hacksaw blade as a straight edge. The jets need to be centered with the needles: install the needles in the pistons, then put the pistons and chambers on the carb bodies but do not install the springs. The needle should stick through the bottom of the carb body. Slide the jet and its holder over the needle, then put the nut on finger tight. Flip the carb body over, raise the piston and let it drop. It should go all the way to the bottom smoothly. If it sticks or binds, loosen the jet holder nut, reposition the jet and holder, then try it again. This will probably take two or three tries to get right.
When the jets are installed with the mixture nut and spring, the top of the jet should be even with the top of the 'bridge' in the venturi. Once you do this, mark the mixture nuts so they can easily be seen when looking over the fender. I used to use one of those little 'buzzers', make a dot or an X that could easily be seen.
The float level on an SU is CRITICAL. The fuel level in the jet will be the same as it is in the float bowl. Your carbs probably have brass floats, the hinges do not wear easily on those. If it has plastic floats, the pivot pin hole will wear making adjustment almost impossible. If you can retrofit brass floats, that's a good thing.
The piston chambers and pistons are a snug fit. If you put the piston into the piston chamber without the spring, then turn the chamber upside down, the piston should fall slowly to the bottom and you should hear a 'hiss' (air escaping around the piston). If the piston sticks, clean it and the chamber thoroughly but DO NOT use any sort of abrasive! Emery cloth etc will make junk of a piston and chamber in about five seconds.
I always prefer to assemble the float bowls to the carb bodies, then install the bodies without the pistons/chambers. That makes the next step easier.
When you install the carburetor bodies on the intake, back the idle speed screws out all the way, the throttle plate should seat against the throttle bore. Install the bodies with the throttle shaft clamps loose, snug the clamps up, then work the throttle a few times by hand. The plates should go all the way to the throttle bodies. If they don't, investigate and repair.
Once that is done, carefully turn the idle screws down till they just barely touch the stop on the lever. Then turn each one another turn down (thus opening the throttle, the way you would raise the idle).
Now go back and install the linkage, return springs, etc and finally the pistons and chambers (don't forget the springs!). Remember to put oil in the dashpots! Want to start an Internet bitch slap session?
Post your favorite SU/Stromberg oil on one of the Brit car sites, then stand back to avoid the flames. Me, I always used 20w50 engine oil. Worked fine. Once this is done, it's time to fire it up. Be 100% certain the plugs and points are clean and properly gapped! '95% of carburetor problems are in the ignition.' Also make sure the valves are properly adjusted. A too tight intake valve will have you 100% convinced SU's are the devil. BTDT.
You need one of these:
It's called a Uni Syn.
Fire the engine up, make sure there's no fuel leaks etc, then stick the Uni Syn on each carb one at a time. The red doohickey in the tube should rise to the same level on each carb. The total level is not important, just that all 3 are equal. Open the throttle a small amount (to maybe 2000 RPM) then check again.
Something to remember: whatever you do to one carburetor you have to do to all 3. So if you turn the idle screw on #1 down 1/4 turn, the others need to turn the same way and the same amount. Idle should be somewhere around 800-1000 RPM.
Mixture is easy. It will probably be a bit lean with the jets adjusted all the way up. To check this, lift each piston ~3/16" one at a time. The engine should speed up for a second, then go back down. If it tries to stumble, it's too lean. If the RPM goes up and stays up, it's too rich (not very likely). HS4's usually had a pin on the side of the carb to lift the piston, if yours don't use a pocket screwdriver or O ring pick to lift the piston (your finger is too big and will give a false reading).
To richen it up, turn the mixture nuts DOWN, i.e. clockwise as viewed from above. Turn the nuts down the same amount, same way you did the idle screws. That's why you marked the flats, so you'll know where your start point is. You will probably wind up with the mixture nuts turned down about a half turn. Count the flats to keep track.
Last edited:
jeepguy
XS650 Addict
Thanks for the great info. It deff only has two carbs though this is the 4 seater not the two seater. It has the two bucket seats in the back. Maybe they were different. IDK.
1974jh5
Curmudgeon.
If it only has two, it's probably a later intake/carbs. Those are HD8's which have a diaphragm underneath, the same carb was used on E type Jags. For the most part, rebuilding the carbs is the same except that to adjust the mixture there is a screw on the side that adjusts the jet up/down.
