Compression issue

[Dirka]

Hey all, what could be some causes of not having compression or lacking enough for the engine not to start/run properly? Timing off, timing chain, flatten lobe on camshaft, vacuum issue??
Thanks!

[79gmc15]

+ Warn rings, warned cylinder walls, holes in pistons, warn valve seats, bad head gaskets, cracked blocks, cracked heads, loose sparkplugs. Not so much vacuum. What's your issue?

[Dirka]

My issue is long and a continuation of last summer.
cliff notes are it's a 77 K10 305, mild cam, from what previous owner had said.
Issue with it in the beginning was rough idle and would stall out when put into drive, would have to pedal it to keep the motor running. He thought there was a seat issue with one of the valves. So I took the heads off took them to a local machine shop, engine builder, he said they were fine, put a slightly larger spring in to allow for more lift just in case cam was larger then we thought. Ever since then motor was put back together it hasn't run properly, timing was off it's mark, had to keep the motor at least half throttle for it to run would never just idle (it would stall out at idle). Then it wouldn't start. Took it to a local mechanic, he said there was no spark at first, then he put a module in and got it to run, thought maybe something wrong with carb, the next day the truck wouldn't start again. He said there was no compression, and suggested the cam is bad(flat lobe) or something is wrong with the timing. And if that's the case he won't get into that extensive of work to get it running. So that's there where I'm at.
If it's anything more then a camshaft replacement, I'm just going to pull the motor and rebuild the 400 I have waiting for when the truck was to be restored. 

[Fairlane514]

Seems like a lot of people have looked at it.......did they check to see if the valves were set too tight?  That can cause low compression. Timing chain a tooth off........

Also if the cam lobes went flat, there is enough metal floating around to ruin bearings, so you would need to go through the motor.

[Engineer]

Never seen a whole cam go flat all at once. Usually only one lobe.

If you starved your engine of oil bad enough to wipe every lobe from the cam then the crankshaft would be long gone. So much so that it would probably not even turn over.

First things first, does the engine knock in any way? When reinstalling the heads were the lifters preloaded properly? Is the timing marks aligned properly on the cam, and crankshaft gears?

If yes to all of that then it is time to run a compression test, and or leak down test.

If the compression test shows a good read, and balance between cylinders then it is time to look in other directions. Like if the timing advance is froze in your distributor, or does the carburetor need attention.


Honestly, and I have NO WAY of knowing this by reading a post on the Internet, but what you are describing, and your mechanic says it has no compression, to me sounds like the cam timing is a couple of teeth late. If the engine ran fine before in order for this to happen your cam chain has to be really bad worn, AND you over revved that engine really severe.

But the most likely scenario if it does check out to have compression, is the carb is dirty and needs rebuilt.

[79gmc15]

Or really worn chain and went opposite of rotation (durring diagnosing) and skipped a tooth.

[Dirka]

Great starting points! Thank you! I will be getting the truck towed back home this Monday, and will get to it to later this week. I will post any updates I have when I get them.
There was no knocking, both dots were aligned properly on the timing chain, the valves would be the only question mark because I did those myself, and being the novice that I am its possible to have them too tight.
I will start with just hand turning the crank and watching the lifters to make sure they are all moving up and down to see if a a lobe went flat. then I'll do the compression check and go from there. Thanks again!