I believe I have some type of blowby in my engine on the #1/2 side, as evidenced by occasional smoke on de-acceleration, oil vapor coming out the head vents and crankcase vent into the airbox (and making an oily mess), a slight amount of smoke coming out the tailpipe on acceleration (per my friend who drove behind me on the way back from Limerock Park), and oil around the head vent on the #1/2 side.

The engine is a 2.0 from a '74 with an unknown history that I swapped into my '75. I figured I would rebuild it this winter and do a 2056 with some Webers, similar to Jake's 2056-135 engine.

With a fully warmed up motor, I got the following compression test results:

#1 125
#2 110
#3 120
#4 135

I cranked the motor over for about 10 seconds for each test. #2 is more than 10% below the average, but just barely. These don't seem too bad, or is #2 a cause for concern? Without doing a leakdown test, I'm leaning towards bad valve guides.

Assuming it's the valve guides, does it make sense to get the heads rebuilt and put them back on, leaving everything else alone? Or am I better off rebuilding the whole motor now and having a reliable (and more powerful) motor that should last me many years?

I don't want to get into a situation where I do the heads only and then a couple years down the road, I need to do a full rebuild and I'm stuck with rebuilt but stock heads with stock valve sizes, which would limit my rebuilding choices.

Decisions, decisions.

Do a valve adjustment and take a small screwdriver and gently pry on the valve spring retainer. Good guides should have almost no deflection, bad you should see the valve stem move a bit.

The bottom end is stout on a type 4, but the cam and lifters can wear out. You can examine the cam lobes and lifters by removing the pushrod tubes. The lifter will then slide out. Keep the lifters in order.

...Did you reciently ...1. change OIL. 2. are NOW using a synthetic oil 3. what "weight "oil are you using. 4. did you add fuel, at a discount -high alcohol-content gas station, and get "regular?...5. checked for water content in you gas lately?...changed fuel filter lately?....6. adjusted the valve lash cold ..BEFORE ..the compression check?....the answers may help you determine where you are really at. 7. previous posted advice is good.....to consider

Did you do the compression test with the engine cold or hot? valve adjust recomended before test.

Valve guides and rings.
Dude...do the whole thing this winter.
These motors are only good in original form for so long..100k or so and then you just need to do it.

Rich

Fill the dipstick to the top mark and put 500 miles on it then look at the dipsctick again. That should tell you how much oil you're burning.

Test was done with a warmed up motor (it was 65 degrees here today so the top was off!!). The only oil I've used is Brad Penn 20-50. Valves were checked about 700-800 miles ago and they were fine. The blowby situation has been present for the last 1500 miles. Fuel filter not changed recently.

I usually drive the car about 4000 miles a year. Regardless of my results, with a motor like mine with unknown mileage, does it make sense to rebuild the whole thing rather than just the heads or just the P/Cs?

Rich - I have you're cell # so I may be calling you for advice if I get stuck with the rebuild this winter.

Rebuild the whole thing.
Its not rocket science.
Looking at the quality of work you do elsewhere I KNOW you will do a great job.
Get Jakes movie, Get a few books. Grow a set and rebuild it.
The difference in costs is minimal.

Rich

I'd also check the air filter, if it's dirty you will burn oil. I am guessing you did the compression test with the throttle opened?

silly.
run it till it spits chunks! I had a customer run his at AX all year long with 40, 60, 85, 145.
I have a current customer doing about the same. build something else while that one dies a slow death...but those numbers dont compel me to tear it down

PS. fix the leaky vent hose. sounds more like crank case ventilation hose routing to me.
always fix the cheap and easy first

Even if there were no vent hose leaks, I'm still getting a lot of oil vapor coming into the airbox from the heads and the crankcase. It's collecting in the top of the airbox and getting on the air filter. I can't imagine the stock air filters are supposed to be soaked in oil, which mine will be soon enough.

Also, I don't want to be too obnoxious driving down the road with a trail of smoke behind me. While I can't see it most of the time, my friend who drove behind me for 75 miles could see and smell it the whole way.

so it smokes continuously? you are right, that sounds like too much oil. no oil should actually get on teh paper element. I still question the oil breather hose routing, has it been checked to be correct? does it run to the flash pot first?

buy a core engine, and tear it down and rebuild while you can still drive this one. the compression numbers are not that bad, yet

its somehting simple...pvc check-valve in oil filler body?

...What I was trying to get you to realize is this.....your old engine has blowby past the rings possably because........1. Detonation (that you are unaware of) due to low octane fuel, and water contamination....Alcohol as a fuel additive...the sleeze bags have figured out they can add WATER in the tank and get away with it......2. Valves need adjusting ANYWAY,,,again. 3.And ...And ....You need to use a not so good oil......common ordinary off the shelf NON-synthetic quality oil,(such as VALVOLINE 20/50......which will allow your piston rings to RESEAT....your old engines, PISTON RINGS have become unseated, by the slick lubricants, in your present oil...and it may take some time for them to reseat...They may never reaseat....Crankcase blowby is due to unseated rings. The excessive crankcase pressure will put a strain on all sealing points in the engine...therefore make certain the crankcase is VENTELATED.4. check ignition timing...if advanced too much ..will aide in detonation with low octain fuel. 5. change filter also, throughly drain the oil out...6. Take care of this engine, as the compression numbers are not a disaster., it.ll go a a little longer, check -OIL pressure if you have a guage....7. make sure it,s not OVERFILLED. the additional oil "windage" will be thrown into the cylinders increasing the amount of oil to be scraped off by the OLD PISTON RINGS, oil control rings. magnifing the problem.s....to consider.

...the smaller synthetic molecule can leak past gaps that dino oil could not.
leave the flux capacitor alone till you check the simple stuff

It's tired. Build a new one with a core. Run the high mileage motor oil. I get a lot less blow by with it. castrol GTX HM or valvoline part synthetic high mileage. Don't worry about the low ZDDP, your not going to get that much more out of it anyway.