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72 SU's causing grief


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at the risk of being a pest......I have succesfully installed a pair of 72 carbs, linkage, grose jets etc...even went out and bought a Unisyn (found that the car industy types no longer seem to know what a carburator (What it's not fuel injection? in the pursuit of the Unisyn; I was saved though when someone suggested I check out the motorcycle and snowmobile shops as they still work on vehs with multiple carbs...bingo $48.00 CDN later I had an Edlebrock Unisyn and it works great)...

now onto the latest problem; the car runs OK...still doesn't idle too well .

It has differant needles front and rear carbs ??, I have spares from the orig carbs, should I swith to the old needles? ALSO..I set the needles by turning the jets all the way lean, loosely placing the needles in the dashpot and inserting same into the jet to allow the jet to set the needle...this has the needles about 1mm lower (leaner) than before (when needles were set using a straight edge along the bottom of the dashpot)......but after then backing the adjusting wheel out 2 1/2 turns (rich) I find I can't get the damn thing to die as I then progressively turn the wheel in again (lean)....

I know I am long winded, forgive me; anyway if I can't get the thing to stumble on the lean side how can I know when my mixture is right....am I way to rich ($$$$$ I wish!)....

PS; I have read nearly every SU tune-up post on the net.... all refer to screwing the jet in till it stumbles.....I can't seem to do that!


Tim Brown


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  • 2 weeks later...

If your SU's are to rich, it will be evident on the spark plugs(they will be black). It is very easy to check obliviously and is some I do whenever I go to adjust my carbs first.

You may have 'leaky' throttle bodies on your carbs. Were the carbs rebuilt by a competant expert befor you installed or were they only given a "kit" rebuild?

The problem is that after 27 years of wear on the linkage that connects and syncrenizes the carbs the bushing wear out causing air to leak into the carb body though the linkage. The only thing you can do to remedy the situation is to rplace these bushings.

Scott at Z Therapy has rebuilt thottle bodies at an affordable price after you send back your cores. There is aproblem with his rebuild if you plan to race your Z however. His rebuild is excellant, but not within SCCA rules...

Also, do not run different neddles or you will never actually synch your carbs mixture.

Good Luck...

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Ah the joys of SU's.I have an early 240 {HS30000219}and recall having problems tuning them initially after the car had sat for a long time from previous owner.I did have access to a tuneup machine though.The problem we had was if you set the CO reading as recommended,the car would idle fairly well but as soon as you put any real load on it i.e. drive around a corner and up a hill, it would bog down and labour and almost stall.A friend that was helping me,a mechanic with years of experience tuning british su's on triumph's etc was at a loss. We then found an expert who'd had the same problem recently and he said to set the mixture up at about 1750RPM.this was the trick and the car ran perfectly.A simple check to see if you have a vacuum leak around the throttle shafts is to spray carby cleaner at the throttle bushes and see if the car bogs or the revs drop noticeably, do this at idle.If it doesn't bog down, you don't have a problem.Do not use different needles in the carburettors,set them both at exactly the same spot in the pistons.Checking the sparkplugs is also a real easy way to check the tune of the engine especially if you don't have access to a tuneup machine.I hope this helps. regards Craig.

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