My 1970 350/300 hp car now smokes on start up and settles down after warm up. I had the oil changed last year, due again, but it never smoked like this before. Engine has 66,000 miles on it and never been touched as far as I know. Possible valve stem seals?

 

That is typically the culprit when an engine starts doing the "startup puff".

 

I now have a similar problem with my DART Iron Eagle heads... have had the 'puff' on start-up for several years now.

On my original '624' heads prior to the engine rebuild with the DARTs, I was getting the puff on start-up and installed valve seals. Didn't help that much. This is worth a try since it is a job you can do in your own garage if you have a valve spring compressing tool, an air compressor and the fitting to force air into the cylinder via sparkplug hole (to keep the valve from dropping while you pull the spring off to get to the valve seal). I had a retired mechanic do the work on my engine. It also helps to have one of those 'wand magnets' for the 2-piece keepers that fit in the crowns of the valve spring retainers.

 

Also, could it be a fuel problem, as I had a blocked fuel filter from sediment pickup from the fuel tank (replaced rubber lines, pump, tank and rebuilt carb)

 

Could also be the valve guides themselves.

 

Doubtful, unless for some reason there's a bunch of oil in your gas.

A rich condition will be evidenced by black smoke.

Lean condition, you'll run hot, run like crap, idle like crap, etc. (but no smoke)

 

I had an '89 Dodge Caravan that got to smoking badly after start up. As you and others have suggested it was the valve seals. Any oil that leaked into the cylinders overnight had to be burned off in the first few minutes of operation. After I had them replaced there was no more smoke. Since the smoke stops fairly quickly, I think you are on the right track with the seals.

 

I know we drive Corvettes, but on this one I have to say: Not so fast!!!

What color is the smoke, and how long does it last?

White looking smoke that dissipates very quickly: this is often not smoke, just water that got into the engine overnight (common in humid areas) that gets evaporated from the heat of the engine and then condenses when it hits the cool air coming out of the exhaust pipes. I remember seeing a picture of one of our Canadian friends' cars that he started up in winter, and the back of the car was engulfed in a white "smoke". This was, of course, not smoke at all. This phenomenon will last anywhere from a minute or so in warm conditions to several minutes in cold conditions. In extreme cold, some condensate may be able to be seen coming out of the tail pipes even after the engine is at operating temperature because water is one of the products of combustion of gasoline.

Blue smoke: if it lasts for just a minute or two, see the above posts about valve seals as this seems to be the most common culprit. This is not a disaster. You will need to add more oil than you did before, but other than that, if the smoking on start-up just started, the seals are probably fine for a while. Blue smoke that is always coming out of the tailpipe is a problem that should be fixed sooner or later. If you are lucky it will be valve guide seals that have gotten completely shot, but it can also be worn-out rings. If it is the latter, it is time for an overhaul of the rotating assembly.

Black smoke: As others have said, almost always means a mixture that is too rich. If it only happens with a cold engine, my first thought would be too much choke, but I don't think I have ever seen that on a car that you could start.

 

Just FYI

I know people love love LOVE to say oil smoke is blue, but most people will look at oil smoke (myself included) and say it's white. If I squint and imagine real hard then maybe I can see a subtle hint of blue. maybe...

Just saying, don't expect blue like this ->

 

I know this is not a vette sorry, but this smoke was caused by a blown piston -> oil.

http://www.youtube.com/watch?v=RdiNDyi6zwk

We first thought it was a blown headgasket

 

You know it's funny, but that's one of the best examples I've ever seen of "blue" smoke. I've seen some oil smoke that's as white as the driven snow.

And is that an AE86 back there? Dude has good taste!

 

I know It was actually very easy to see the blue color when we watched the video afterwards but it wasnt that easy when we shot the video. Second time we started the engine it filled the whole room with thick blue smoke in 30 seconds.

Yep

 

Adjusted the choke linkage and the problem cleared, no pun intended, just was running too rich