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Old 01-27-2004, 10:16 PM
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My 350 engine idles very rough, as if some cylinders were hitting harder than others or some are intermittently missing?
Just above idle the engine smooths out and performs well. If I connect the vacuum advance to direct manifold vacuum it idles very smooth but idle timing increases to 36*.

Engine setup is;
350 bored .060 flat top pistons, Sportsman II 64cc heads 2.02/1.60 valves, Edelbrock RPM air Gap intake, Crane 216 duration cam, Comp Cams roller tip rockers, Holley 600 cfm vac secondary, 1" 4 hole carb spacer, MSD pro Billet distributor, MSD 6AL box, Accel 10mm spiral core wires, 350 TCI trans, Dana 60 411 rear gear.

Initial timing, 12*, total timing 36* full in at 2900 rpm.(no vacuum connected).

Performance is not a problem, the engine is in a 1938 Ford truck weighing less than 3400 lbs. The truck turns mid to low 13 seconds in the quarter and 0 to 60 in 4.4, but the rough idle is puzzling???
Not sure if vacuum at 5500 rpm is high enough to engage the vacuum diaphram and cause timing to advance further than 36*???and possibly cause pre detonation??? or if at 3000 rpm cruising the added advance would cause a problem???
Vacuum diaphram begins increasing advance at 9" and full increase at 13"
I drive the truck daily without the vacuum advance connected but the rough idle is very annoying????
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Old 01-28-2004, 05:17 PM
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It is hard to say, there are a number of variables that will cause a problem. I would have to say it is not a component problem but an adjustment in fuel or timing. Generally the vacuum is used only if you want to try and get better fuel economy. We normally recommend that it is hooked to a ported vacuum source and make sure that source falls off and below 5 inches as throttle increases to wide open. Try bumping the initial timing up and see if the motor smooth out. Depending on the motor combination cam compression etc., it might want more initial then 10 degrees. You might find that 14-18 provide better idle quality and as long as the motor starts easy the higher the initial the better. If you find it is better as you increase initial you might change the bushing to a larger bushing and then re adjust the carburetor to the new timing setting. These are a couple things to try.


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