But has 18 inches of va***n so I there can not be a leak,motor is a mid 70s Ford 460 with a performer intake and 750 Edelbrock and acted the same way in the donor truck. Its been sprayed all over with carb cleaner after installing in my F100 and I changed intake gaskets but still the slightly rough idle,its a Jasper reman motor with less then 30,000 miles so who knows what has been done to it but I am running out of ideas on what next to check.
I discovered a "hidden" vacuum leak in my SBC - the vacuum advance diaphragm was perforated and allowed air to enter through the vacuum port. I once had the same problem with a vacuum-operated windshield wiper motor in my Plymouth. Good luck!
Had the same problem. It turned out to be the PCV hose around the fitting on the carb. Put a clamp on it, problem solved.
It just a little rough idle that can not be adjusted out,tonight I checked the timing and set it and the idle speed to early 70s specs and adjusted the carb with the va***n gauge and got it to 18. For a few months I had another good carb on it and it was the same,pcv and booster lines have clamps. I think the motor has straight up cam timing and possibly a rv cam since it ran circles around another 460 I had in another truck that was factory stock and not touched so can a rv grind cam cause a slightly rough idle,the motor came out of a F350 so I doubt there is a performance cam but who knows.
put a qt of AFT down the carburetor at high idle and see if that smooths it out. do it after dark because it will SMOKE but it will remove carbon off valves. i do this to every used motor i install before i tune it up.
Sounds like he's got it down. But, I've sure noticed a lot of people don't, they'll post videos on YT, with the engine screaming, maybe 2500 RPM or better, but they got that ole needle in the green zone on the gauge so it must be good, right? lol An "RV" or rumpety-rump cam will not pull 18" steady vacuum at factory idle. 18"-20" is the expected figure for a mechanically healthy stock V8 at sea level.
Let it idle, air cleaner off, all open vacuum lines plugged. Place a hand over the top of the carb... like you're trying to smother your ex. If the idle picks up, you have a vacuum leak. If it tries to **** out, you have no vacuum leak.
With a handheld propane torch unlit but gas on, go around the intake gaskets, brake booster, vacuum lines, carb gasket, and such. Over the years I had a brake booster fail and the carb tb bushings leak. I also used that trick to eliminate all those things only to find a worn out timing chain. The idle will pick up when it gets a wiff of propane.
An engine with poor compression won't pull good vacuum either. A vac gauge isn't a subs***ute for a compression test, but it's close enough for most purposes, and a hell of a lot easier. It doesn't torture-test the battery and starter either.
This is a great way of checking for a leak. I think this is better than spraying carb cleaner aroun the intake/carb area. Thanks for the good tip.
Took it for a drive after tinkering with it the other day and it seems to idle better,I will have to admit sometimes I am too lazy to get the tach and just set the idle speed by ear but maybe for the first time I have a motor that needs the idle speed set to factory specs. The closest specs I have for a 460 are for a 72 Lincoln and not a mid 70s truck but its better.
Played with it some more today and seems to like 600 RPMs in drive so that is where its going to stay.
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