Basic SU Carb Maintenance

Article By: Chris Callen

Originally Published In The November 2014 Issue Of Cycle Source Magazine


So I know by now that when something goes wrong with my bike to follow a couple general rules: A. Keep it simple, or in other words start with simplest thing to check. B. Go to the last thing you messed with. Nine times outta ten you can solve a problem by going over something you missed the last time you worked on your bike. And C. I never underestimate my own stupidity. With that in mind, I started having problems with the Panhead about a month and three events ago. It was breaking up and coughing through the carb. Now, this is by no means a tech article to show you my mechanical mastery, much in the contrast to that it’s to point out that no matter how simple you start you may miss something.


So like I said above, I started simple with the usual plugs, wires, timing, points condenser. I went on to pushrod adjustment, coil and still couldn’t find the trouble. I checked the carb adjustment and eventually went back to the last thing that was fixed which was the top end. I pulled the front head since it had a bad gasket anyway but nothing was outta line.


After it ran shitty again in Sturgis I brought the Ticket home and pulled it out for some TLC. I decided to zero out the carb again and it was as I leaned down next to it to look at the adjustment knob on the carb I saw the small hole in the top bolt on my SU carb. I could hear an old friend of mine saying “Those carbs are great but remember every once in a while to put a couple of drops of 3-in-1 oil in that little hole and it will run great forever.” UGHHHHH!!!!! It couldn’t possibly be that stupid could it? Oh yes… it could. Three drops of the magic oil later and the bike fired to life and has never run so great. I figured just to be safe I would do some R&R to make sure no damage was done.


So an old SU is a Constant Velocity type carb like today’s CV on Harleys. The chamber on the top creates a vacuum that operates the slide to control the needle and deliver the fuel.


Inside that chamber is a large spring and a slide that in my case was in pretty bad shape.


After removing the slide and needle you can see how much carbon build up there was from the backfiring.


The slide was equally filthy but what’s worse is that along the stem of the slide and the chamber it goes into on the needle assembly both had signs of water damage where they may have been stuck together. We started with the outside of the needle assembly with some 600 grit wet sand paper. Trying not to remove too much material but just enough to make sure there were no sharp edges that would inhibit smooth travel.


We then moved to the inside of the slide on the chamber. With the same sand paper we lightly worked a rolled up tube worth to catch any edges as well.


Here our parts are cleaned, smooth and ready for reassembly.


I applied a light coat of graphite lubricant to the slide assembly just to see how it felt and man it was like new money.


Once the SU was back in order I finally followed the old man’s advice and applied my few drops of 3-in-1 oil. With this done the bike was once again a one-two kick pony and man the throttle response going down the road is incredible. You see, in short what a constant velocity carb does is work off the vacuum that the motor creates. So, if the vacuum drops off so does the throttle response, if the carb doesn’t deliver the right amount of fuel or does it at the wrong time, it actually works against itself. In any event the bike is back in tip top shape and hopefully my mistake can save some other poor bastard a month’s worth of pulling your hair out. Listen to the old man!

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