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SU Reassembly Step-by-Step [Part 1 Tear Down /Torn n Tattered /Worse for Wear n Tear]

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Lots of photos so it may take me a day or two to get them all up.

Here is a step-by-step procedure for reassembling an SU carb.

Rip it apart and fear not.

I did not take out the throttle valve plate and shaft because of the peening and I am lazy today. (I'll do a step-by-step on modifying an SU in another post in the future where the shaft is filed).

Note: I do not have my rebuild kits yet so pretend there are new gaskets



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Some Pics of the SU Carb body:

Note: I just washed with soapy water, sprayed with carb cleaner, then wire-brushed the outside with a brass wire wheel on a drill







The brass bushing around the shaft wears over time and causes a vacuum leak between the shaft and the bushing. You can put a ring of sealant around the rim of hole (but not touching the bushing or shaft) then place a flat round object (like a dime or aluminum disc or rubber disc) over the raised sealant to seal the shaft. Doing this side will cut your vacuum leak to this carb in half.


The bushing on this side of the shaft will also leak. Some fittings and linkage that will be installed on the shaft will come close to the body so there is not much you can do to seal this one. Maybe put some concentric O-ring up against the body with some silicone spark plug grease as a lubricant and a very paper-thin washer so that when the linkage presses in, the washer and O-rings will squish and the lube will prevent binding when the shaft turns.

The correct fix is to install a new bushing. Here is an idea of what you can do with the right tools:


Edited by Blue

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This is easy. Just don't loose the weeeeee itty bitty clips.

Needle nose pliers works fine to pull off; and press on the clips.


Here is the lifter rod laying just outside of its home cast in the main body.


Use a Scotchbrite or sand paper to remove the rust then grease the shaft.


Insert the rod into the body.


On the bottom side, install the spring then attach the clip retainer.


Works fine.

Edited by Blue

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Here are all the parts in relative order.


Firmly grasp the nozzle sleeve. (No I am not pink! I used colour enhancement and it enhanced me (fingers) too.)


Install the washer on the neck of the sleeve. Some grease is a good thing.

NOTE: The washer is on the wrong side in this picture. It goes under the sleeve.


Slide the Nozzle Sleeve Set Screw onto the sleeve


Install the large washer behind the Nozzle Sleeve Set Screw. Apply grease.


Install the Idle Adjust Spring behind the large washer. Apply grease.


Install (by screwing) the Idle Adjust Nut. Grease between the spring and the nut will make adjustments easy to do.


Here is where it goes at the bottom of the body. Antiseize on the threads is a good idea.


Test fit by hand tightening the Nozzle Sleeve Set Screw


Here it is in place. As you can see it is not automatically centered.

Edited by Blue

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Reference: Suction Piston


Reference: Piston bottom.

Note the centre groove and the two holes on each side of it as well as the needle hole in the centre.

Note 2: The black insert next to the needle is an inserted stand-off that prevents the piston from sealing against the bridge. This and the centre groove allow air to flow across the bridge when the piston is down


Reference: Piston front with guide groove


Reference: Top of piston.

Note the oil well and two holes on each side of it in the floor.


Insert plastic/nylon washer over oil well and push to bottom


Insert Spring. Ensure it surrounds the washer and is centered.


Reference: Suction Chamber.

Note the passage in the top front. This points away from the motor when installed.


Reference: Inside the suction chamber


Reference: Oil plunger assembly


Oil plunger installed

Edited by Blue

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You can build your own alignment tools here:


Fit alignment pin to piston. (use grease in the hole).


Install piston and chamber.


Here is where the tool aligns the Nozzle Sleeve. Note the sleeve is loose at this point and can move.


Strange photo... note that the 4 (3) rusty screws that hold the chamber are all evenly tensioned to just less than snug (see the lock washers are not compressed). This is how you want the chamber attached for the next step.


To align the chamber, spin it Counter Clock Wise (CCW) to full stop, then spin it CW. Note the limits and try to set the chamber in the middle of these.

Continue to tighten evenly like car wheel lug nuts... opposites.


You can now lift and drop the piston. It should rest flat on the bridge. Here is a photo with no flash showing the piston on the bridge. The only light is from the two hole cut in the piston's bottom (due to the dome removed) and a razor thin gap between the bridge and the piston bottom (due to the inserted stand-off mentioned above). Normally you can see through the grove in the piston bottom but the alignment tool is installed and blocks the light.


Here is how to tighten the Nozzle Sleeve Set Screw. A thin walled 19mm socket with a T handle will work even better.

Note: If you only have a stock needle to align with, install it as well as the nozzle then tighten the Nozzle Sleeve Set Screw slowly. When doing this, hold the carb vertically and move the piston up and down ensuring it hits the bridge and the needle does not bind against the nozzle.


The Nozzle and the alignment tool are same outside diameter.


Here is a shot showing the centered sleeve (left) and the centered sleeve with the nozzle in place.


If you want to double check nozzle alignment you can use this second tool that aligns the nozzle and sleeve at the same time.

Edited by Blue

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The needle can be placed at nearly any depth in the piston; however, there is only one correct depth.


Lubricate the needle hole and loosely fit the needle in the hole.

You may need to use the side screw to hold the needle if it is too loose.

The needle should be nearly fully out of the hole (Don't worry, we will push it back in).

For greatest accuracy, you can reassemble the suction chamber and secure all 4 (3) screws as mentioned above.


Turn the Idle Adjust Nut all the way up and insert the nozzle.

Drop or, in this photo, press the piston into the nozzle.

The shoulder of the needle will seat tightly against the jet at the correct depth.


Here you can see the shoulder is out of the hole at the correct depth.


Another shot.

Be careful because if the needle is loose, it can drop out or fall back in the hole. Correct tension on the needle is needed to do these steps. As mentioned above, the side screw may be required to loosely hold the needle in place.


When satisfied with the needle's depth, slap some antiseize on the side screw and install.


Lock it up.


If you are wondering if you bent a needle, here is a quick way to check:

Throw the piston and needle in the dome and spin.

You can try to straighten or you can get a new one.

You can also spin it in a drill press for inspection and straightening.

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Below are a drawing and photo showing how to set needle depth in the FSM.

The only difference in this technique is that the needle shoulder is above the bridge by the depth of the stand-off.

When the nozzle is lowered to operating location, this should not make a big difference however, with the NON-FSM method shown above in Part 7, the nozzle top is matched against the needle's shoulder, so any mechanical differences in the sleeve, washers, stand-offs,etc between carbs will be nulled. One turn of the Idle Adjust Nut on both carbs will yield the same nozzle/shoulder gap and effectively "annular fuel orifice size".





Edited by Blue

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This is the most complex assembly. I started to photocopy a step-by-step but I messed up so I got tired and reassembled and took fewer ones.


Here are the key parts loosely in their relative positions.

The long plate on the left lowers and raises the jet nozzle.

The plate on the right is connected to the choke cable.

The assembly is also connects via a small rod to the throttle assemble (as the jet is lowered i.e. choke applied, the small rod opens the throttle a little to give a faster idle.

Lots of grease helps this assembly work well.


Here are the remaining parts with the items in the above photo assembled.


A spring and washer are installed on the body.


The rest of the assembly mounts on top and looks like this:


The bolt that holds it together needs antiseize and grease.


Here is another angle with interconnection details:


And another:


Edited by Blue

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Insert the nozzle and prepare the hardware.


Install the sleeve.


Insert the screw and lock washer.


Insert the flat washer on the other side. Use lots of grease.


Move the choke lever and rotate the nozzle to position the long plate so that the screw aligns with the threaded hole in the nozzle.

Snug the screw in place.


Test the choke functioning by lifting the lever.

The nozzle should drop and no binding should occur.


Release the choke lever and the nozzle should snap up into place without binding.


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Throttle Assembly parts in their relative positions.


Throttle shaft.


The first part is installed. This is the idle lever. A set screw pushes it down to open the throttle and increase idle speed.


Next is another plate. This is just the WOT stopping plate.


A washer is installed. Grease it well on both sides.


Next is another plate with a connecting rod to the choke assembly.

You have to twist and bend a little to fit this one. The first step to fitting is to insert the rod into the choke assemble.

The way this works is very simple: As the choke is applied, it pulls the rod. In turn, the rod turns the throttle assembly which opens the throttle a little and increases engine rpms when the choke is on.


Another sleeve is installed. Note that it fits in the plate above.

Use lots of grease on the ends and inside.


This is the plate that is pushed by your foot via the throttle linkage.


A lock washer holds it all together


A final deep bolt with recess is installed.


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The limit of 10 pictures per post bumped this one:

Exploded drawing of throttle assembly


Idle set screw and spring. Note the fine thread.





Edited by Blue

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Fuel Bowl Needle Valve


Found all the parts (Everything will workout if you let it)

Note the black rubber nose on the needle.




The float.

Note: the tab in the middle is adjustable by bending.


To install the float, a free floating pin is used.


Closeup of how the float presses against the needle valve


Fuel filter and banjo joint




Locking fastener.




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Fuel bolt captive retainer in place


Fuel bowl drain installed.


Fuel bowl attached to carb body.

Note the rubber shock mounts.


That's all folks!


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I really appreciate the terrific info and great pictures on the alignment tool and procedure for using it. I have a sticking piston I hope to unstick with this information. Thanks a lot! Ace hardware (now called Thriftway here I believe) didn't have the exact parts for the alignment tool but I used (their part number) 60604-D (hitch pin- 1/8" DIA) and 65784-D (nylon spacer) as a substitute. The nylon spacer available here has a slightly larger ID than what you listed but I wrapped a couple of inches of clear packing tape around the hitch pin to make the larger ID of the nylon spacer tighter on the pin.


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Blue: I used this thread to do the work on my carbs and look what happened, great write up .





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