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one carb not working?


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I got this wierd issue with my su's during warm up. With the choke on and the car started my front carb has excessive vacuum while the rear has none at all. Car runs like crap till it warms up then everything is fine. Both carbs are sync'd and pull the same amount of air when at operating temp. Anyone had this issue before? Thanks for the help.

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I'm no expert with the SU's but it sounds like your choke mechanism sticks or one of your cables is not adjusted correctly. See if the cable will fully open AND close each choke. After you close the choke, try pushing up on the choke by hand to see if it closes further by hand. I doubt it is the closing since it is under choke that you have the problem.

You might look at these sites;



Sure hope you find what you need!

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Hi Chris,

It sounds to me like the throttle for the rear carb is closed. Are you measuring vacuum by placing a hand over the mouth of each carb?

The following is from the '72 engine service manual:

Starting Mechanism

By pulling the choke knob, the starting lever is moved, and the nozzle is drawn down by a link mechanism. As a result, the clearance between the nozzle and the jet needle is increased, and an increased amount of fuel required for starting is fed into the system. Moreover, the throttle valve is automatically set to proper opening for starting (approximately 6 degrees) by the connecting linkage.

The instruction sheet that came with a rebuild kit has this:

Cold Starting Device Adjustment

1. Throttle valve held completely closed

2. Choke lever held in the full choke position

3. Measure distance between throttle valve and bore of carburetor .025" (.63 mm)

4. To adjust bend connecting rod

The connecting rod is the one between the throttle valve lever and the choke lever.

It sounds like you've already adjusted the operation of the choke lever itself, and that it is pulling the nozzles down. But, have you also checked to see that when the choke is applied that the throttle valve is also cracked open? Here's how you might check: without the engine running, remove the air cleaner. Pull the choke lever all the way back. Raise the suction piston of each carb and verify the throttle is cracked open. You'll probably need a flashlight to see this. If not, you'll have to remove the carbs, but then you would need to remove the carbs anyway to properly make the measurement & adjustment mentioned above.

Hope this helps!

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Hey Ken great advice... I went out and sure enough by back carbs butterfly wasn't opening enough. I bent the bar and she came back to life. She seems really stinky though, so I was wondering if opening the butterfly's more would lean out the mixture enough and let it choke properly.

Thanks for the help


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the richness and lean is controlled by the position of the needle as it rises and falls into the nozzle. SU carbs are vacume dependant . if the buterfly is closed then the vacume being drawn into the engine is minimal. As the butterfly opens this increases the flow of air over the nozzle and that draws fuel into the engine . As this is happening the needle is further drawn up out of the nozzle and because of it's taper causes more fuel into the mix . With little vacume or air flow the piston that controlls the needle cannot rise . This is appairently what was happening . Check the adjustments on the linkage

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