Cool, thanks for the input. Ya I was mainly wondering do 460's idle silky or do they always have some vibration. Mine right now acts like every so often it misses or something. Like, you will have a smooth idle for 2-3 seconds and then it will "jerk" a bit.

I've tried to clean the idle circuit - I took the idle screws out and sprayed carb cleaner through them and then some canned compressed air. Still has a random jerk when idling though.

I drove her around a bit more tonight and after warming up it idles decently again, but still has that random "jerk" while idling at a stoplight. It still does the jerk too idling in park but of course you feel it a bit more when in drive with brakes on.

I have my vac advance currently hooked up to manifold. I found some youtube videos of a guy that works on cars a lot and he recommended if your engine is a stock cam with 8 degrees or less initial timing, that you should use manifold vac, but if it's higher initial timing like 10 to 15 then go ahead and use ported.

The PCV is hooked up. I don't have the EGR hooked up, I just put a new one so it will block flow. Technically I should put a non-EGR baseplate on I think that would make things cleaner. I've disconnected the TVS from anything hooked to it and plugged all other vacuum holes. Only things on vacuum right now are brakes, transmission, PCV, and advance. I removed the cruise control since it wasn't hooked up to the throttle anyway. I also removed a bunch of mess of hoses that are supposed to run the EGR (comes off the TVS and a solenoid). The heater controls are manual cable

I sprayed carb cleaner around the base of the carb and I didn't get any reaction, I also sprayed it around the front and back of the intake manifold and also no reaction - so it *seems* like I don't have any more vacuum leaks UNLESS the EGR port on the intake is leaking underneath the carb plate. Which wouldn't surprise me I guess.

The wires, plugs, dist cap and rotor are all new and dry. Fuel pressure seems OK although I haven't tested to see if it's too HIGH since I have an Edelbrock and they hate anything over like 5 lbs.

 

Well it's going on 50 years - "who knows" is exactly correct, when it comes to just what could have been done or not done to that thing.

You'll get it. Now one thing to keep in mind no matter how well it seems to run at highway speeds or whatever, if it isn't idling right, then it isn't running as well as it could anywhere else either. If it's idling smooth, then it should run reasonably well all around.

Compression test is a good one because it determines the basic, overall mechanical health of the engine, of piston rings and valves and the basic suitability for a tuneup and further adjustment. "Divide and conquer" is a way of diagnostic troubleshooting. Good techniques will sort of split a system in half for example. Then we know the trouble lays upstream or downstream, from that point.

Check everything, don't assume anything is OK or just has to be set right because it "can't be the problem." Don't guess. Check, measure, verify.

Through a process of elimination, methodically done, eventually that which remains is the source of trouble. Again, Don't overlook simple or "rookie" things - like say, crossed or switched spark plug wires. Happens more often than one might think, and it will run reasonably well depending on what cylinders. Some people might not be able to tell. Jus' sayin', check everything.

Ignition parts are usually suspect by now, and bad grounds, loose or corroded cables, arcing plug cables, worn out plugs, weak battery... "Clean, tight, shiny bright" when it comes to anything electrical will work well for you. When ignition and spark is good and hot and engine timing is known to be correct then focus on final carburetor tuning and you'll have a really good running mo'chine.

 

Not that it will solve your problem, but you mentioned having vac advance connected to manifold (vs ported). Not looking for a debate, MY mind is made up as I've been around long enough to know what works best for ME. On a stock(ish) engine, ported vac advance will give a performance increase in light part throttle application that doesn't exist when using manifold. As throttle increases this disappears as vacuum goes away so if your driving style is mostly heavy footed, you may not notice a difference. Opinions certainly differ here, and YES I have read the technical article that's been floating around internet forums for a decade or two (by a supposed GM engineer) that claims otherwise. I know what does and has ALWAYS worked best for me. I'm not running down the 1/4 mile either.

 

Finally ran a compression test, all cylinders 119-120psi, looks good

 

Going to rebuild the carb today, opened it up and it looks pretty dirty. This has to be the problem I can't think of anything else. Double checked timing looks right to me

 

Going to rebuild the carb today, opened it up and it looks pretty dirty. Actually thought i saw the left idle bleeder plugged up with something. Would sound possible since adjisting the left idle screw never did anything. This just has to be the problem I can't think of anything else. Double checked timing looks right to me

 

Anything with a carburetor, if it's running poorly, sat for several years, and also if it's new to you - is a good candidate for a rebuild. At the least take a look see inside. Modern gasoline is corrosive it seems. Once you start checking things out at idle and test drives, keep a fire extinguisher handy.

Familiarize yourself with the proper baseline settings provided in the manual. Carbs are really simple but they need to be setup right. Hardly anyone does this so odds are, someone has thoroughly buggered the carb up along the way with the "golden screwdriver."

Compression is a little low but if it hasn't been run for a while the rings may be gummed up. A few heat cycles and highway cruise runs may seal things up. Marvel's oil in the gas and oil is a good plan too. Vintage oil and gas causes stuff to stick that shouldn't.

 

After I did the compression test I read that you are supposed to warm the engine up a bit before doing the test...I didn't do that, it was bone cold from overnight so that might be why the numbers are lower. At least cylinders are all consistent

after i got the carb horn off, a cursory examination looked like one float was definitely too high, it was less then 7/16 away from the plate quite a bit. So possibly is was micro flooding. Well see how it goes once back on the engine

i have to admit i have a feeling this isnt going to work. Something is out to defeat me on this one. Having the truck idle smooth seems like a dream

 

Just keep pluggin' away on it, you'll get it.

 

I've got a similar engine/trans configuration in a '76. One difference is mine has the infamous Motorcraft 4350 carburetor which I recently rebuilt. Mine idles smoothly, but there are many things that can cause what you describe. Your compression test sounds good. That indicates basically a "solid" engine. Try to verify performance of each component independently whenever possible. This will help to avoid costly replacement of "good" parts. Also keep in mind that these days "new" doesn't necessarily mean "good". Persistence will likely find the problem.

 

Thanks for the positive support guys! I am not a super experienced wrench turner so it's nice.

I got the carb rebuilt and while it works maybe 5% better it still seems about the same as before . At least I know the carb is in good shape!

I called my brother, who has a ton of experience working on engines and asked his opinion, he said he thought it sounds like a timing issue like a timiing chain with too much slack. Which wouldn't be surprising since this truck is 39 years old.

I found a youtube video on how to sort of test the slack by watching the dizzy (EricTheCarGuy videos) and he mentioned you don't want more than 7 degrees of slack - tested mine out watching the marks on the balancer and it looks like I have 10 degrees of slack (when you switch crankshaft direction, takes 10 degrees before you see the distributor rotor start to move the other direction), so that sounds like it could be the real issue that makes the idle behavior a bit erratic.

I don't think I'm ready to do a heavy repair like a timing chain just yet. Although, I *do* have to replace the water pump (it leaks when cold) so I could do the chain at the same time while in there. But I may just deal with and leave it for a while now, I don't have to drive every day , I work from home, so it's not a huge issue My brother said he wishes he lived nearby he could do it for me in a few hours lol (he lives 1800 miles away).

This sounds like it might also be why my vacuum gauge would hover around 15 , which say "late timing" on the dial, so maybe that's accurate assessment. I don't know if moving the dizzy can compensate for that, I don't think so, since I think it means late *valve* timing. Not sure. What's strange is when I first got this truck (only about a month ago) I tested engine vacuum and it was a solid 21! Maybe that was after it was all warmed up.


Thanks for all the support guys!

 

Engine manifold vacuum is a good indication of cylinder compression as well as ignition timing, exhaust restriction, carburetor tuning, and valve issues and more, if you know what to look for. It's also a lot easier to use, and doesn't smoke the battery/starter, no plug removal required etc.

Did you mess with the distributor at all since then? If it was a steady 21" that's a good sign.

21" Hg might even be a little on the high side, meaning ignition timing is slightly too much advanced. Normally a stock, healthy V8 will pull steady 18" to 20" at factory idle, at Sea level. There is a correction factor applied as elevation increases. At Denver, Colo. maybe 15" would be good. Start talking rumpety rumpety camshafts, forget it. 10" is doing good. Make sure ignition timing is correct, 18" to 20" would correspond or be close to optimum. Check for rattle or ping on acceleration and back off slightly if necessary.

Check for vacuum leaks - maybe a hose or plug let go to cause loss of manifold vacuum. If there are any vacuum leaks it will never run right, to cause tuning issues because you'll be trying to compensate around it by running other carb circuits excessively rich.