SU Carb being naughty!

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NowayPedro

NowayPedro

Active Member
Posts
114
Any ideas wot this could be?

SU carb starts racing high as engine gets warm.
It's a Rover 3.5 V8 engine with twin SU's (only 1 carb is doing it).
The tickover screw is just touching the end stop and starts off lovely at tick over. But once the engine gets warm it starts racing and makes the engine run at very high revs. A couple of turns of the tick over screw brings it back down nicely but then it wants to conk out. There is no throttle cable or choke cable attached at this point, so I'm a bit flummoxed. Anyone had this before? Any ideas?

Cheers p.
 
It sounds as though one of your carbs is drawing in too much air, either through imbalance or leakage. If imbalance have you thoroughly cleaned the SU's including the floats, pistons, jet, needle, oiled the pots and correctly set up air volume at idle, off idle and above 2000rpm? You'll need an airflow meter to do this correctly, not a rubber tube + have you slackened of the throttle linkage ensuring each SU is being set up independently?

If air leakage this will cause the symptoms you are seeing. It's common for the spindle linkage bush to wear over time, this allows unmeasured air into the SU causing over rev'ing. The only way to repair this is to have the bush pressed out and new bushes fitted. This can also be caused by a worn linkage into the bush, but is a rarer fault.

Another area here is a jet/needle that's off-centre and is causing the piston to stick/the piston not re-decking :. allowing more air to be drawn in causing higher revs/twin SU imbalance.

The only proper way of sorting SU's is to strip, clean, inspect and replace any worn components. Then rebuild, install and set-up properly. These will help you...



 
Tha
v8250 said:
It sounds as though one of your carbs is drawing in too much air, either through imbalance or leakage. If imbalance have you thoroughly cleaned the SU's including the floats, pistons, jet, needle, oiled the pots and correctly set up air volume at idle, off idle and above 2000rpm? You'll need an airflow meter to do this correctly, not a rubber tube + have you slackened of the throttle linkage ensuring each SU is being set up independently?

If air leakage this will cause the symptoms you are seeing. It's common for the spindle linkage bush to wear over time, this allows unmeasured air into the SU causing over rev'ing. The only way to repair this is to have the bush pressed out and new bushes fitted. This can also be caused by a worn linkage into the bush, but is a rarer fault.

Another area here is a jet/needle that's off-centre and is causing the piston to stick/the piston not re-decking :. allowing more air to be drawn in causing higher revs/twin SU imbalance.

The only proper way of sorting SU's is to strip, clean, inspect and replace any worn components. Then rebuild, install and set-up properly. These will help you...



Click to expand...
That is unbelievably helpful. They were ultrasonically cleaned. But I obviously have a lot more to think about here. Some bed time reading!

Thank you very much for taking the time.

P.
 
Stromberg
Screenshot_20231023-202349.png



SU

Screenshot_20231023-202505.png
 
Have a look and see if the valves (if fitted) on the butterflies are ok and not loose or something, Its a little plunger thing with a spring, anti run on or something. Anyway, one of mine got dislodged and the engine ate it, revs went up as it was getting far too much air, never did find it, prob squashed by the pistons :D
 
NowayPedro said:
Defo SU then. Thanks for the clarification.
Click to expand...

Other note is to use thick SU to manifold gaskets, ala Klingersil, and to give each face a light smear of Hylomer Blue to ensure 100% sealed gaskets. If ultrasonically cleaned it's really worth taking great care when reassembling, checking each component for wear/movement on rebuild. Same for assembly to the inlet manifold and when fitting linkages to ensure one end of the linkage doesn't lift the SU off idle.

The good news is that SU's in perfect condition are very easy to fault find.
 
landowner said:
Have a look and see if the valves (if fitted) on the butterflies are ok and not loose or something, Its a little plunger thing with a spring, anti run on or something. Anyway, one of mine got dislodged and the engine ate it, revs went up as it was getting far too much air, never did find it, prob squashed by the pistons :D
Click to expand...

These are the anti-run on valve fitted to some butterflies. @NowayPedro , if you have these fitted to your butterflies, remove these and fit std plain butterfly discs and whilst there check for the slightest movement in the spindle bushes as this is a common place uncontrolled air to enter. Replacement bushes and seals are readily available and will need a manual reaming once fitted.

Essential parts forFZX 2005 for Land Rover V8 | SU Carburetters

Su Carburetters are The Worlds Sole Manufacturer of Genuine SU Carburettors, Fuel Pumps and Spares.
sucarb.co.uk sucarb.co.uk
 
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