please help!!!!

k...here's the deal....burning a quart every 60 miles now....here's what i've done:
compression check----cylinders 1 and 5 at 125#, 7 and 8 @140#, 2&3 @ 145#, and 4&6 @ 150#
vacuum check----19" steady, when i rev it, it drops to zero, then goes to about 23 and back to 19 when i let off....
new pcv valve (twice just to be sure)
has baffled valve covers
new valve seals
it doesn't smoke unless i open the secondaries on acceleration
i am now lost....

 

Your compression is low in a couple of cylinders and that's a large compression variance between cylinders. 10% max is a good rule of thumb.

Check the compression again, and this time pour two tablespoons of engine oil in each cylinder before you test it. If your numbers go up, especially in the cylinders with lower compression on your prior test, some of your engine's rings have probably served well beyond their service life. At 60 miles/quart with new valve seals, and the variance, rechecking is probably a mute point....

I'd consider making it a 460 in the rebuild.

 

Could the engine be pinging under a load? It could be carboned up and creating hot spots leading to pre-ignition. If the engine has a lot of wear on it, the pistons might be jarred around enough to let oil past the rings.
I had a 70' 429 that would carbon up pretty bad with a lot of in town driving and I used a couple of different methods to clean it out. First choice was to use Marvel Mystery Oil. With the engine warm (but not hot), I would dribble it down the carb until it started to sputter and smoke, then after about a minute, shut it down and let it sit overnight. The next day, drive it pretty hard to shake the crud out. Be prepared for volumes of billowing smoke when you do this and I wouldn't suggest doing this if you live in town.
The second way was to use plain water, with the engine cold, start it up and just dump about 10 ounces of water down the carb or until it died. Again, I would let it sit over night and then drive it hard the next day. This would only create a little black smoke, nothing like the oil.
The engine was in a '76 F250 Supercab and I did this a few times a year from about '82 until I (foolishly) traded it off in '90.

alanscott


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