Series 3 1 x SU Carb does not react when adjusting the mixture

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B Raats

B Raats

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South Africa
When balancing and adjusting my twin SU HIF6 carbs, one of them (closest to the radiator), when I lift the piston, do not change the refs at all, even when I max the richness or the opposite. The throttle linkage is disconnected between the carbs and when I manually play with the throttle on this specific carb, it increases the engine refs as normal. Any idea what might be wrong? Any suggestions will be appreciated.
 
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is the adjuster under the carb actually doing anything? maybe it's just spinning on the thread or can you see it moving in or out .. I also had a very slight curved needle which prevented the (swiss) MOT from getting the CO2 right but it ran fine .. could be a combination of the two ..
 
Simon Perks said:
is the adjuster under the carb actually doing anything? maybe it's just spinning on the thread or can you see it moving in or out .. I also had a very slight curved needle which prevented the (swiss) MOT from getting the CO2 right but it ran fine .. could be a combination of the two ..
Click to expand...
Hi - Yes I checked it and can confirm the jet going down( for rich) and moving moving up (for lean). When I use the lifting pin on the side, I can even hear how the petrol is forced through the jet opening. To test it, I removed the dash-pod to view the position of the jet.
 
I like the R6 engine but it can be hard work to tune the twin carbs perfectly.
Before you adjust the mixture, you need to balance the airflow between the two carbs, otherwise one may be doing most of the work and adjusting the other one will not have much effect.
Some people can balance them by listening to the air hiss into the carb, but I found it easier with an airflow meter like a Carbalancer.
Once they are balanced for airflow, you can adjust the mix gradually, making the same change on each carb. You may find that small adjustments don't make much difference until you drive the car, also you can see from the colour of the plugs if the mix is more or less correct.
Did you overhaul the carbs or are you just doing a tune-up? If the needles are worn, it will be difficult to tune them accurately.
 
Yes you are correct. Luckily I did the balancing many times.
My challenge is one carb does not react when I change the mixture adjustment.
The jet is moving up and down, but the refs remains the same.
 
I ran a Triumph Dolomite for a number of years and keeping it in tune was a regular job with twin S U' s. A simple air balance was essential and a Gunson colour tune was a big help in getting the tuning/ fuel mix correct. If one carb is not reacting to change it may well be faulty.
 
Have your dash pots got oil in? And of the correct grade? I know a lot of people just use engine oil but IIRC they need 20w monograde oil in them. Googling SU dash pot oil viscosity brings up some healthy debate on the subject.
 
HAVE YOU CHECHECKED THAT THE PISTON ARE MOVING FREELY
 

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wireman said:
Have your dash pots got oil in? And of the correct grade? I know a lot of people just use engine oil but IIRC they need 20w monograde oil in them. Googling SU dash pot oil viscosity brings up some healthy debate on the subject.
Click to expand...
Yes I added new oil to check, but no difference unfortunately.
 
Set the butterflies using a feeler gauge (or even a nail) so they are both approximately equal.
Set the idle jets to the same number of turns.
Let it run up to temp and check the colour of the plugs and adjust accordingly.
Once the mixture is about right, then balance properly.
Blip the throttle to see how it responds and adjust if needed, but both by the same amount. Set the idle and off you go.
That's assuming no air leaks and carbs in good order
 
kermit_rr said:
Set the butterflies using a feeler gauge (or even a nail) so they are both approximately equal.
Set the idle jets to the same number of turns.
Let it run up to temp and check the colour of the plugs and adjust accordingly.
Once the mixture is about right, then balance properly.
Blip the throttle to see how it responds and adjust if needed, but both by the same amount. Set the idle and off you go.
That's assuming no air leaks and carbs in good order
Click to expand...

Thanks for the reply.
I tried to balance it a few times before, but have not yet adjusted the butterflies.
Will do it tomorrow and let you know.
 
You can make something like this quite easily to balance your carbs
DSCF2514.jpg

Just make sure you use less than a full bottle of water between them
 
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