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adjusting SUs questions 120-130 1967

When I try to adjust the idle mixture on my 'new' SU set up it seems like the jet adjustment has a very narrow range where the idle speed is at its peak. Is that normal?

When I get to the peak idle speed plus one flat, and lift the slide with the pin the engine speed drops off 200+ rpm. Getting it so the rpm does not drop so far takes me a long ways from the mixture adjustment in step 1. Any solution ?

When I turn the car off it runs-on like the idle mixture is way off. Is there a newer ( better ) procedure for adjusting the idle mixture ?

Thanks

Greg





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adjusting SUs questions 120-130 1967

Before adjusting the mixture it is essential to get the flow through each carb the same. Set the jets to 12 flats down as a baseline first, Disconnect the clamps that lock the shafts together so that each carb is separate and then use a flowmeter or a piece of hose in your ear if you are confident that you can hear small differences in hiss, and get them both equal with the individual tick over screws. When you are happy with that lock the clamps together carefully so nothing changes. Then do the mixtures. Don't expect the jet settings to be the same, they very rarely are.





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adjusting SUs questions 120-130 1967

derek uk covered the procedure for carb balance well. A piece of vacuum hose works for me. The motor should be warmed up thoroughly and the "choke" off completely.
I do take issue with your procedure for the mixture adjustment. After getting the balance and idle speed done and the linkage locked you can use a tachometer to achieve the highest idle rpm by adjusting each carb mixture control-- but leave the mixture adjuster alone at that point. When you lift the testing pin on each carb in turn you should have a momentary rise in rpm (one or two seconds) and then a drop off (the 200 rpm you site is fine). The motor should be able to continue running at that lowered rpm as long as the pin is raised. Check each carb in turn. If the rpm should drop to near stalling -- set the other carb a flat or two richer. If, instead, the rpm should rise -- set the other carb a flat or two leaner. Then go to the second carb and do the pin lift as you did with the first carb--always making the adjustment to the "other" carb. When you get a 200 rpm (or so) steady drop at idle by testing each pin you should have a proper mixture. Be sure the dashpots have oil and then road test. -- Dave





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adjusting SUs questions 120-130 1967

Hi Dave: "If the rpm should drop to near stalling -- set the other carb a flat or two richer. If, instead, the rpm should rise -- set the other carb a flat or two leaner. Then go to the second carb and do the pin lift as you did with the first carb--"

This adjusting the opposite mixture method only applies to the B20B manifold with the additional throttles that ensure that idle and part throttle mixture gets diverted through the central hot spot causing all cylinders to get identical hot mixture. It's great for emissions and might even help with cruise MPG. However, the throttle shafts wear out way to fast and the fix would be to delete the throttles and braze the holes shut. (Or go buy a 100hp DGAF conversion from some thieves)





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adjusting SUs questions 120-130 1967

"you can use a tachometer to achieve the highest idle rpm by adjusting each carb mixture control-- but leave the mixture adjuster alone at that point."
That would appear to be contradictory. You do have to play each carbs mixture settings against each other and that is a bit of an art. Not easy, as adjusting one carb affects the other so that using the pins doesn't necessarily give a totally positive indication of what is going on. Give the engine a rev between adjustments. If starting from a base 12 flats down note down the changes in the number of flats each time you make changes. You might be just changing by half a flat or so up and down. If you don't it's easy to lose track and you will have to start over. Max difference is usually about 1 flat, usually towards the weak side but maybe more. Check for vacuum leaks if there are big differences. Leaks around the carb spindles/carb body will mean you will never get a good consistent tune. In this case, the carbs are apparently reconditioned so this shouldn't be a problem but inlet manifold to head gasket leaks are possible.





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adjusting SUs questions 120-130 1967

I wrote--"you can use a tachometer to achieve the highest idle rpm by adjusting each carb mixture control-- but leave the mixture adjuster alone at that point." derek uk answered--"That would appear to be contradictory."
Perhaps I was not clear with that wording. I was reacting to the original poster's comment in his post that he finds the highest rpm and then goes a one flat farther. Setting the initial adjustment to 12 flats is a good starting point but may require a bit of fiddling richer or leaner to get the setting closer to ideal with the motor running -- and then the pin lift to check and make adjustments as necessary. I give the throttle a momentary full blast to be sure the motor accelerates instantly and smoothly and returns to idle with each adjustment. I think derek uk and I are pretty much on the same page. -- Dave





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