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Jeff Berk

high idle speed issue

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I’m having a high idle speed issue. I’m running between 1,600 and 2,100 rpm.

Prior to this, my 74z with a modified 280z engine running a pair of SU’s was having starting problems and would sometimes stall and idle somewhat slow. I decided to try and adjust the carb’s using https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/

Because one of the fist things to do was to check the spark plug condition and gap,  I installed a new set of plugs. I’ve never adjusted the timing because I cannot find the timing mark (I have a gun and I know how to use it). I set the idle screws all the way out and then backed them in 1 ½ turns.  Then it started right up with minimal cranking, but the rpm is too high.

To get a balanced air flow from the pair of SU’s, I had to turn in the rear and back out the front SU’s idle screws. Doing so maintained the high rpm.

There was a small vacuum leak from one of the ports on the vacuum balance tube that I stopped with some Teflon tape on the M10-1.0 bolt the PO installed. I couldn’t find any other leaks between the carburetors and engine/intake manifold. The fast idle adjust screw is out all the way. The vacuum activated dashpot between the carb’s not pulling the throttle open and the choke cables are disconnected.

Could I be missing a vacuum leak or could the timing be too advanced? If so, how do I go about re-making TDC on the harmonic balance?

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