SU Carburetor Starting Troubles

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SU Carburetor Starting Troubles


Post by Havranasty »

So I have 93" panhead with STD cases, S&S cylinders, STD dual plug heads, Vulcan ignition, Sifton 504 cam, SU carburetor and an open belt primary. It is down at Truett and Osborn's but it won't start with the SU carburetor. Carb is rebuilt with parts from Bob at The Good Old Motorcycle Parts Co. (Thanks, Bob) Anyway, we can get fuel in the venturi using the tickler but it won't pop or cough. So the boys took the SU off and put on a borrowed S&S Super E and low and behold the thing fires off and runs. I really don't want to buy another carb since the SU has a cool factor and others seem to love them but nobody around here knows anything about SU carburetors. I downloaded all the info from the Rivera site and I am fairly certain it has been put together right but is there anything I can check that might be causing the problem? Before I took it in I made up my own leak detector to check for manifold leaks and non were found. My understanding is there air leaks cause problems with these carburetors.
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Re: SU Carburetor Starting Troubles


Post by panhead »

I don't know if this helps, but here is some SU tuning information: ... eliminator
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Re: SU Carburetor Starting Troubles


Post by Excalibur »

Sorry to hear of your starting troubles. It needs a process of elimination to solve. I had the SU on a Shovel for a number of years (sold 25 years ago). That bike was a one kick starter at any temperature. A 2nd kick was rare. I worked in shop in the 70's where 98% of cars had an SU so I suppose I had an advantage.

People do seem to love them or hate them. As we say, while some people swear by them, many swear at them. The poor opinion isn't justified as the carb makes very good power, positively famous for economy and yes is an easy starter.

Some thoughts and things to check/verify:

The dashpot piston should be 110% free for movement. Clean the dashpot assembly very, very thoroughly. If you manually raise the piston, it must fall and bottom out with a "metallic click". There should be no sound of spring binding/crunching. Lube the piston spindle with WD40/CRC. Important to set dashpot up in this way otherwise you won't be able to get the mixture screw set right.

If the engine starts with the S&S but not with the SU, "all things being equal" then the problem could be the air/fuel is out of range. Yes, there could be too much gasoline or too much air. Technically 9:1 air/fuel is about right for cold starting. If you're not getting any fire, then you appear to be some distance off this marker.

As I recall it, an exact number of primer pumps (with dashpot piston raised by little button) was required for cold start. After 25 years I can't remember how many pumps. Click the choke on which also gives some fast idle. Assuming correct number of prime pumps, engine would fire. I would continue to run engine with choke/fast idle on until a quick throttle blip would release choke automatically.

Set the mixture screw with engine hot and at a fast idle. Aim for highest engine speed then add 1/4 turn rich. Reduce idle speed back to normal.

Re ignition. It's very important to get ignition perfect before carb setting. I know nothing about your Vulcan electronic but I know not every electronic ignition is well suited to kick start engines. Any doubt and fitting a regular points timer with normal 12v/5ohm coil would verify it. In your case you've said it starts perfectly fine with S&S so it does tend to point to the SU or the manner/method/setup in which it's being operated.

Hope this helps.
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Re: SU Carburetor Starting Troubles


Post by Andygears »

The type of SU is important, posting a picture of it might help. The SU I ran in the 70's was from a Jaguar and 1 kick, fast worked well. Most out there are Rivera SU's with a different float bowl arrangement. Fuel delivery was an issue with the car models.

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Re: SU Carburetor Starting Troubles


Post by Trov »

Did you ever get your su working for you I hope so I have run them for 25 yrs on multiple bikes knuck pan and shovel maybe I can help
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Re: SU Carburetor Starting Troubles


Post by kitabel »

The Rivera data will not help starting unless you have an Eliminator (with the float bowl directly under the body.
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