'70 350 CR, Gov Loc ID

[zac]

Just bought a '70 burb, put regular gas in it like I used to with my '72 and it rattles. I noticed engine has an HEI with adjustable vacuum advance. Before I screw with that, does anyone know what the stock compression ratio would have been for the optional 350 4bbl, code 4LS9AD in 1970?

Glove box also says it has 4G80AF REAR POSITRACTION A. I think that's the infamous gov loc. I asked the PO about it and he said it was just a one wheel wonder as far as he knows. Is there an easy way to check if it still has the gov loc and if it is functional without popping the rear cover? The rearend doesn't make noise but I'm curious. Thanks

[BOZO]

Does the engine have the original (or equivalant chamber cc) heads on it and pistons in it?
Sometimes people inadverdently put small cc heads (58,62,or 64) on 350 cubic inch engines with flattop pistons. This will lead to 11:1 c.r. or more depending on deck height and head gasket thickness. You can get by with these small cc heads on smaller motors (283,307,327) because they have less swept volume, but not so on flattop piston 350's & larger. 11:1 is just too
much for todays quality of gas. If your engine has dished pistons, your rattling problem is probably in the timing, as it is hard to get excessive compression with this style of piston with factory heads. Through the years your engine may have recieved enough modifications that the only way to know the real c.r. is to do a compression test.
I recently run into this because I wanted to run a set of 64cc double camel hump heads on my newly rebuilt 350 with KB flattop pistons. After doing the math, my heads would have had 11.7:1 c.r. I had to spend more money to rebuild a set of 882 heads (76cc chambers) just to get the ratio reasonable.
You can tell if it has flattop or dished pistons by looking in the spark plug hole. The only way to tell what cc chambers you have is to get the casting # off the head and look it up on a head casting # database somewhere else on this forum. Or, if you have the casting # just pm me and I'll look it up in my interchange book.

[zac]

I literally just bought this truck and noticed the issue while driving home. The engine looks like it has never been touched, but that's alot to assume on a 40 year old truck. I'm on the road at work right now and will check the casting numbers as soon as I can. My plan is to rebuild the 350 I pulled from my '72 burb and use it in this truck. I know it had 8.5:1 compression, with either 487 or 467 heads. I looked at a Motors manual for cars that I have, and it showed for 1970 350 4bbl cars with CR over 10. But I know truck engines were usually lower. I'm hoping it is just a timing issue since the engine is obviously tired but runs well. It should give me plenty of time to rebuild the new engine.

[Paul M]

friend bought a truck..whole truck rattled and couldnt get out of its own way on acceleration. going down the road it wasnt too bad. turned out the vacuum advance was locked up,so upon accel it wasnt advancing the timing. changed it to a new dist and dam thing was a monster..lol

[cdowns]

they were low compression motors from the factory//factory positracs were mostly eatons//govlocs are a far later inovation i'd say from late 80's// only real way to tell 4 sure is remove cover clean out the old and add new grease while you're at it synthetic is best

for the ping and rattle check advance and timing is a good place to start also check the feed voltage to dizzy as i see lots done with resistor wire still in place