Llama's quick and easy guide to SU carb diagnosis

[Llama]

This is essentially a abridged version of many books and the minds of people I have been picking over the last few weeks trying to sort out my carbs. As I finish mine, I'll post up more.

People often complain that SU carbs are hard to tune. This is not necessarily true, however tuning worn carbs is nigh on impossible. Below is a list of the few things that commonly go wrong after extended periods of time. All this is assuming that your ignition, valves etc are in reasonable condition.

This is in order of easiest things, what you should be checking first.

Symptoms: Running too rich or too lean throughout the rev range, will make it hard to tune.

Problem: Incorrect float level

Diagnosis: Hold float upside down, measure gap with a drill bit or calipers, space to factory specs. With dual carbs, the floats must be the same level! Not like the ones below!

If you want to get fancy, turn the ignition on until the fuel bowls fill up and measure them with an eyedropper or something

Note: If the float appears to have stuff inside it, replace it.

Checking the levels with a drill bit:

Some terribly balanced floats :/

Symptoms: Fuel starvation and lack of power OR overflowing float and flooding/running rich

Problem: Sticky float jets (also called needle valves, ball and needle valves)

Diagnosis: Take the float out, play with the needle valve, if it doesn't move smoothly, take it out and replace.

Symptoms: Uncertain idle (it never seems to reset to the same idle speed), lean idle and run-on.

Problem: Spindle bushing wear

Diagnosis: If in half and full throttle, you can see up and down movement in the spindle/throttle butterfly, the bushes are worn and must be replaced.

Give it a wiggle!

Problem: Worn needle

Symptoms: Surging at constant speed

Diagnosis: Surging at a certain throttle position under load indicates a wear in the needles corresponding to that throttle position. Any visible ridging means the needle must be replaced, however the needle could be worn and the wear not able to be seen.

Problem: Worn jet

Symptoms: Constantly too rich

Notes: While you can just adjust this out, there comes a point where the jet is starting to poke into the throat too much and push the piston up. Time for new jets.

Other checks you should do:

Remove the piston damper from the top of the vacuum chamber, and lift the piston. It should fall back with a clunk. If it doesn't, you need to centre the jet. Workshop manual or youtube has plenty of info on how to fix that.

<Stuff about piston levels>

If you don't have any of the symptoms associated with a problem, your car runs OK, then it's probably fine. Unless you want to spend tons of money chasing that last 0.259 hp, don't bother replacing everything.

For further reading, speed sports motorbooks has a "Tuning SU carburettors" (pdf available online) that has excellent (dis)assembly instructions.

"The SU carburettor: High-Performance manual" by Des Hammill is also a reasonable reference on how things work.

There are plenty of tuning articles out there, however they will be folly if they are improperly set up and/or worn.

Haynes also has a manual, but I haven't found a copy of that yet.