My 73 240z belches white smoke, only at high RPM. I've read enough threads here and else where to know that white exhaust can mean a blown head gasket but I don't see any water in the oil, no bubbles in the radiator when I applied pressure to each cylinder, and compression is between 140-150. I assume these tests are just backyard mechanic quickies and it still could be a blown head gasket. Are there some more likely culprits?

Thanks for any help

Bad rings could do it also. Define "high rpm? How high are we talkin here?

I'm thinking rings, but could be head gasket. Have you smelled your exhaust lately? You can definitely get a whiff of the ethylene glycol at the exhaust pipe if you've got a gasket failure.

And as Fun asked, what does "high rpm" mean in tach numbers?

Frank

Unfortunately the tach isn't working. A rough estimate would be ~5k.

I should also mention this car is in the process of being made road worthy. I just did the 280zx dizzy mod and am still playing with the timing, getting carbs tuned, etc. This isn't a situation where the car was runnig great and then the problem suddenly appeared.

Also, concerning compression. I'm in CO at about 6400ft and I found a chart that converts compression readings base on altitude, 150 is right around 180- so I guess I wasn't considering rings.

Its not auto is it?

The previous suggestions seem more likely, but have you changed the oil lately? If so, what did you put in? Using a lower viscosity oil than the previous stuff may be the culprit. From personal experience I have seen this happen. 30 years ago at about 75K I switched from Castrol GTX 10-30 to GTX 20-50.... about 10K later, when winter set in, I went back to 10-30.... what a mess with smoke at higher rpm... It looked like white smoke in the rear view mirror (but it must have been a very light blue). Went back to 20-50 and no problem. I'm really not sure if it was blowing by the rings or the valve seals, but the problem did not return as long as I kept 20-50 in it.... ran well for another 30K until I sold it, and for a good while for the next owner, then I lost track of the car (wish I had it back).... Just a thought....FWIW

Its a 4 speed.

I believe I put straight 30wt in it. Would 20-50 be considered a higher viscosity than 30?,

under normal driving conditions?

20w-50 is 50wt oil at 100c so yeah, 30wt is a lower viscosity oil than 20w-50

multi grade oil is only really good for places that see cold temps (like, freezing cold) I live in arizona, single grade is fine.

(I run VR-1 20w-50 in my Z, but I'm in Arizona, higher viscosity helps)

How many miles are on it? as far as I know, a higher viscosity oil in an older engine isn't really a bad thing.

It has 206K on it, 100k on a rebuild.