"19" rock solid steady" is just about perfect on the vacuum gauge. There's probably no better basic diagnostic tool, especially for the money, than a mechanic's vacuum gauge. A lot of basic defects or trouble symptoms can have several different causes and often mimic something else, that's why you'll sometimes get 10 different answers to "what could cause this?" A vacuum gauge (and ignition scope) cuts through all the guess and by golly and throwing parts at a problem.

In this case your engine's manifold vacuum tells us there aren't any major internal engine derangements (burned valves, flat cam lobe, bad head gasket, sticky valves, worn valve guides, compression rings OK, the initial timing is satisfactory, no exhaust restrictions, etc). If you dig around the intertubes there are a quite a few vacuum gauge charts to download or look at with examples of what to watch for. Engine tuning is kind of like a pyramid, the basic foundation has to be right, good compression, and then the ignition has to have a hot, high current spark at exactly the right time all up and down the RPM range. Engine timing directly affects the manifold vacuum. At the very top is the carburetion or fuel supply, Just about everything else changes or affects the way the air/fuel flows through the carb, or how much fuel the engine needs. As the specs get tightened up in one area, if you make another pass back through another area there will usually be more improvement to be had. Weak ignition might lead one to conclude the fuel mixture is too lean, incorrect ignition timing and vacuum leaks will bugger the carb adjustments.

Two things that people want to mess with when trouble starts - carburetor and base ignition timing often have nothing to do with the actual fault they are teoubleshooting. Once a carb is setup and adjusted correctly nothing further needs to be done, except maybe seasonal or adjustments for altitude. Base ignition timing doesn't change on its own. The big thing to look for is usually previous owners try to tune around or compensate for defects somewhere else usually by buggering the carburetor settings. If setting some engine adjustment back to spec makes the engine run worse, find out why.

The engine run-on might just be too high an idle, and/or the carburetor needs some idle mixture adjustment.

 

I have been reading/watching a lot of stuff on tuning lately and there are definitely layers upon layers of things. Luckily this engine isn't too terribly complicated and I think its a good one to get me back into the swing of things.

I think the idle is a little high. I had to run to the house after driving it to work (I'm less than a mile away) and it did it again at the house. It was idling high at the time.

 

One unscrupulous vacuum leak to watch for on these old carburetors is around the throttle shaft. Engine vibration and wear on the throttle shafts can create a small leak where the shaft(s) pass through the carb body. Since it is below the level of the venturi it can be harder to detect and people often try to use the carburetor adjustments to compensate but it often doesn't work in this case. There are bushing kits for many makes/models of carburetors that can be installed to take up the slack in the body and then sometimes also a new throttle shaft can help as well if it is worn. A person does have to drill the carb body some to install the bushings which I'd use a drill press at least and something to hold the carb very steady while they are drilled to do the finest job. The bodys are typically aluminum so it drills easy and fast. Some of the bushing kits come with the correct drill bit.

A good test for a vacuum leak is to spray a little carb cleaner around places and if the engine RPM increases when sprayed around the throttle shaft at the base of the carb it can help isolate any issues there.

 

Stuffing some chassis grease around it will work as a temporary test too. Somebody in another forum said they had good luck installing o-ring packings on the throttle shaft as a semi-permanent repair. I bought a remanned Autolite 2100 carburetor from Vapex and it turned out to be a good one. I had to exchange the first one because it was the wrong size, but the replacement has bushings installed, and appears to have all of its original innards complete (booster cluster etc) and it is a real gem. For some reason, pre-64 Autolite carburetors have better castings, just like Winchesters. Hm.

 

Sure, I should pull into the old Texaco station where they still do a bit of service and ask them if they can grease my carburetor. Might get a good laugh!! But, I can see that working for a quick test.

 

I had a F600 with a 361 in it that would diesel when it was real hot. Never could get it to stop doing that. If it started dieseling we just pulled the choke out to keep it from trying to run backwards.

 

I watched the Mike Carburetors videos of the rebuilds and he mentioned the throttle shaft issue on them. Thankfully there is no play in mine. There is a remanufactured sticker from 1985 on it and it was fairly clean inside.

 

Might using a higher octane fuel or non-ethanol blend fuel cure the dieseling....?

BarnieTrk

 

What is different about the pre-64 castings? Just by coincidence my 4100 happens to be a '66 like my truck, but I don't think it could run better if it had electronic fuel injection.

As far as the dieseling goes, carbon build up around the exhaust valves can also cause it. One of the combustion cleaners that you add to the gas might be worth a try as they're relatively cheap and easy to do. A higher octane gas might work as a short term fix, but I would try to find the cause and fix it.

 

I don't think it has anything to do with function too much, just that the finish or maybe porosity seems a bit better. Maybe the alloy mix changed, I don't know really. See this with all kinds of stuff, anything that has a long run in production. Most manufactured items that are labor intensive they were always trying to reduce the number of steps or processes involved along the way. Yeah they are an excellent carburetor design.

 

If you suspect a carbon build up problem, here is a simpler remedy: warm the engine to operating temp. Remove the air cleaner. Fill a pop bottle with tap water. Bring the engine RPMs up to 2500-3500. With your finger over the end of the bottle, allow the water to dribble into the carb, but not allowing the engine to die. Yes, it will cough, sputter, and spit....but dribble in the entire bottle of water and KEEP IT RUNNING! Replace the air cleaner and take it for a spin around the block. Notice it running better now? You can thank me later............

BarnieTrk

 

Barnie does mean slowly. We use to use water injectors to boost compression when racing, too much and the motor the motor did bad things. Caution with this method, but it does work to remove carbon.

 

The weekend was crazy busy so I did not get a chance to mess with it, but I will have some time tonight. Thanks for the ideas guys.

 

LOL. I can say that because I did something really dumb on the 302 in my Cougar. After getting the heads rebuilt and the engine put back together with the cam and the like I took it out and it ran like crap on the road and freeway. Put it back into the garage and let it sit after which I saw a water leak. Seems I forgot to put on the water pump gasket if you can believe that and lost no water while driving. Then while doing that, and had notice my eyes burned while the car was running in the garage, I checked the plugs. On a 302 with 7 and 8 in the firing order one separates them in the wire loom. So 8 was on 8, 7 was on 6, 6 on 7, and 5 on 5. Duh!?

I'm on sabbatical now undergoing counseling and therapy.

 

Last year I had a mysterious starting problem, until the rotor button fell out of my shirt pocket. LOL