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Strange high idle, common issues ruled out 140-160 1968

posted by  drj434343 on  Sat Aug 1 12:58 CST 2009
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last visit: Thu Oct 24 09:46 CST 2013
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I've spent a good half day messing with my 68 145, dual SU carbs. After a 30 minute freeway drive, I got off, and noticed that the car was idling about 1600 RPM, and it would not go down no matter how long I gave it. I pulled off and began messing with everything. Blocking off either the PCV or the brake booster vacuum hoses does not alter the high idle. I messed with the mixture, idle, and plugging the vacuum hoses one at a time. Nothing seemed to fix it. At one point, with the mixture back to where I balanced it before (11 flats down), I totally unscrewed the idle screws, without changing the high idle at all. FYI, the SU's are totally rebuilt, and are totally functional.

I've tried to cover all bases. I've made sure the choke set screws are correct, and that the jets are all the way up when the choke cables are retracted. I've triple checked that the throttle linkage is not sticking and keeping the throttle plates slightly open. They definitely aren't. I've been up and down the mixture scale, and while I can kill it on the rich and lean ends, I've got it dialed in at a reasonable place, 11 flats down. High idle still persists.

So I began messing with the timing. At 1600 RPM idle it was about 21 deg, which seemed just about right. However, when I tried to adjust it, I couldn't get it to stick. What I mean is, when I would reduce the advance a few degrees, it would drift by itself all the way down to 0 deg and almost kill the engine. Then, when I tried to compensate and bring it back up a little bit, it would drift all the way back to 21 deg or so. It was like it was impossible to keep the car at any timing advance between 0 and 21 deg. I'm not sure if this is normal at 1600 RPM or not.

I'm pretty convinced I've eliminated everything else. I've pulled apart the distributor and everything looked ok. The points were clean, the spring on the points arm seemed to operate the arm just fine. All the contacts on the rotor and distributor head were good as well.

I have a hairline crack in my intake, but I'm pretty sure it's been there a while, and it seems pretty benign. I don't feel like it could be introducing enough air to make the idle this high. Also considering I know its been there in the past, without the high idle problem.

Anyone have any ideas?





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