Wakes up instantly with 2800 rpm

My ‘65 FHC really liked the new 123 Tune+ dizzy and I love it. Was looking forward to opportunity to tune the advance. It ran good as usual (10 yrs) using advance data out of R. Bentley blue book for a couple of weeks and one tour, plugs looked great, just a tad high for slow idle when warm, and a little slow when cold. I was about too fix that. Then suddenly, over night it happened, instant start and right to 2800 rpm, bad for bearings. Nothings strange with any of the 50 linkage parts, nor the three H8 SUs, two of us “experts” checked. Then I removed floor mats, pinched off vacuum line to brakes, cranked it to build oil pressure and fired it up. Same result! The 123 was telling my phone the manifold is drawing a steady 11 inches mercury. Help, out of ideas. www.enjoyclassiccars.com

You’re not going to that rpm by operation of the dizzy alone. You’ve either got a substantial vacuum leak which would account for a good thousand rpm of that, excessive opening of the throttle, or a combination of the above with timing.

As you know, you can have bad timing and ‘correct’ it with carb tuning, or bad carb tuning ‘corrected’ with timing. If everything wasn’t 100% before installing the 123, an unexpected shift in the timing /advance could produce a disproportionate change in rpm.

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I think the throttles are sticking open
69 OTS with triple SUs

Thanks for thoughtful advise. I’ve put 40k miles on two S1 4.2 Etypes. The carbs were never an issue until I had to leave my “driver” outdoors for the summer. From now on I’ll cover the louvers. Although I eliminated vacuum and even checked motor mounts, I traced my problem to the choke, in particular sticking shoe/rod under the fast idle screw and subsequent misalignment of the shoe with the cam. Nothing I could find in our usual literature illustrates and discusses this aspect of the HD8 SU carb. So here is a contribution that might help the next stumped owner:

Cheers, Peter

My field hospital:

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