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Initial mixture setting for H6 carbs

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Michael Oritt

Michael Oritt

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Following a rebuild I am about to reinstall the H6 carbs on my 100. The mixture nuts are presently run all the way up to leanest position and because the nuts are somewhat difficult to access I would like to get the initial mixture setting correct before mounting up the carbs.

Four full turns down (TOWARDS LEAN) sticks in my mind as being the recommended starting point but I am not at all sure and would appreciate input here.

Thanks in advance.
 
SU CARBURETTERS TUNING TIPS & TECHNIQUES, p.14, item #3 (H Type carb): "turn down the jet adjusting nut by two complete turns". Also recommends 1-1/2 turns on the idle screw.

Flush with the bridge is lean; down is richer.

The above is an initial setting, adjustments will need to be made on the car with the engine warm.

Tom Bryant long post tune-up procedure:
https://thosbryant.wordpress.com/2015/10/04/su-carburetor-tuning/
 
Having a tiny wrench - either a special wrench made for adjusting the SU jet adjusting nuts or another very small wrench modified to reduce its size even further - helps a lot with the adjustment. With a little practice, it's possible to adjust the nuts without disassembling anything, even for someone with normal or bigger sized hands and arms. This is one time when it would be good to have slender arms and small hands.

Two full turns (or 12 flats) is a good starting point. You may not notice a lot of difference moving both ways from that setting - check the spark plugs for signs of rich or lean as well.
 
I finally got everything buttoned up this morning and started up the engine, and with the jets dropped two full turns as suggested both carbs reacted to the pin-lift with a slight increase in rpms and then a falling off. I have not yet found my Uni-Syn and so have not had a chance to balance the carbs, nor do a hot shut-down and plug read, but I am definitely in the ball park--perhaps one flat too lean.

It has been a while since I have had the carbs off and I guess I forgot the best order of reassembly, but next time I will hook up the choke linkage before remounting the carbs/manifolds: With the carbs hung on the engine getting the fiddly little clamping screws tight while keeping the wires in place was a challenge for these arthritic meathooks.

In any case, thanks all for the advice and it felt good to go for a test-spin after the car's being out of service for a month.
 
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