No Compression after rebuild

[Darcy]

I just finished rebuilding my OMC 5.0 Chevy. Heads were machined, new exhaust valves, and a new cam, as well as new exhaust manifolds. FELPRO Marine gaskets everywhere that needed them. I used OMC gasket sealer on all paper gaskets, but none on the head gasket as that is what the OMC book said.

I followed all of the instructions per my OMC manual including torque order and force.

I primed the oil by cranking the engine with no spark plugs in. Once I attached the spark plug wires, I tried to start it for the first time, and got nothing but a backfire through the carb. Checked the compression on #1 and got nothing. No compression on any cylinders.


Any idea where I should start looking?

 

[Phantom_II]

Re: No Compression after rebuild

Valves/lifters adjusted correctly?

Been there, done that. I rebuilt the engine on my '64 Fairlaine and snugged down the rockers but never adjusted them. When I went to start it, I got no compression at all.
A few seconds of sheer panic and a quick self-inflicted slap upside the head and I realized what I had done... Or rather, didn't do.

 

[Darcy]

Re: No Compression after rebuild

Valves/lifters adjusted correctly?

I set the lash per the OMC book. I'm pretty sure I got it right...

 

[Flukinicehole]

Re: No Compression after rebuild

Check them again or bring some where and have them done. I'm willing to bet this is your problem.

 

[Darcy]

Re: No Compression after rebuild

I just read on the net that I may have bottomed out the lifter because I didn't pre-oil the lifters before I set lash.

If this is the case, how do I get the lifters oiled up so I can adjust them properly? If this is the case, is the valve lash too tight, or too loose?

Also, can I run the engine without valve covers and not get oil all over the place?

And is that backfire just a symptom of an open intake valve when the spark fires?

Thanks!

 

[Alumarine]

Re: No Compression after rebuild

I had always owned European or Japanese vehicles that used solid lifters and was quite familiar with adjusting them.

First time I adjusted hydraulic valves I had zero compression on all 6 cylinders.

 

[Maclin]

Re: No Compression after rebuild

The lifters are preoiled now that you have primed it and tried to start it.

Are you sure the cam timing gears were installed correctly.

 

[Darcy]

Re: No Compression after rebuild

The lifters are preoiled now that you have primed it and tried to start it.

Are you sure the cam timing gears were installed correctly.

Thanks for the info!

The book seemed pretty straight forward on the cam install. Lined the dot on the cam up with the line on the crank. The valves on #1 don't move at TDC, like the book says they wouldn't.

The one thing I am iffy on is the Distributor install. Does it only fully lock in to the correct position? it looked pretty lined up to the #1 plug at TDC with no valve movement, but I feel like I could have messed it up.

 

[Phantom_II]

Re: No Compression after rebuild

If you run the engine without the valve covers on, you WILL get oil all over the place.
A solution I have used in the past is to get a set of valve covers from a wrecking yard and cut holes in them to give access to the valves.
You still get a little oil splash, but not nearly as much.

And if the lash was set with dry lifters, they're too tight.

Open intake valves when the spark plug fires is a cause of backfire, but there can be several causes of the intake valve being open.

Check the timing gears for proper alignment, and recheck your valve lash.

Although it won't cause compression problems, also make sure your distributor is not off by a tooth or two (or three).

 

[Maclin]

Re: No Compression after rebuild

I know on some engines when the cam gear dots are lined up the #1 is at the top of the exhaust stroke (exhaust valve almost closed and intake valve just starting to open) while the #6 is at the top of the compression/firing stroke. You may have the distributor 180 degress out. That would not explain the zero compression though. Maybe you have both problems, rockers too tight and distributor timing off.

 

[WizeOne]

Re: No Compression after rebuild

... (If the valves aren't moving you have a serious issue.)

Bubba, he meant while the cylinder was at top dead center.

 

[joseph.signore]

Re: No Compression after rebuild

There is a valve adjust sequence to follow? Bomar posted a thread about this last week. Good Luck.

 

[800sxr]

Re: No Compression after rebuild

If the engine backfired out the carb it sounds like the timing it 180 off. Also check and see if your compression gauge works on another engine or you can place you thumb over the spark plug hole, it should build up enough pressure to move you thumb off the hole.