Proper compression for a '78 800

[1970Thunderbird]

Hey all (again),

I had a sneaking suspicion that part of my lack of power under load might have been something other than the prop or fuel, so I ran a compression check. Here are the results.

cylinder 1: 95psi
2: 95psi
3: 85psi
4: 65psi

I take it have either a bad set of rings or a bad head gasket. Perhaps both.

Anything I should know before I tear into it? I'm planning on redoing the rings for all the cylinders since I'll have the motor that torn apart anyway. Maybe get some new pistons.

What size rings do I need for stock pistons? If I get new pistons, which ones do you recommend?

And what compression should I be showing, anyway? I've heard 120psi.

Thanks for everything.

 

[Faztbullet]

Re: Proper compression for a '78 800

Time to rebuild as compression is way low. Compression should be 110-125psi on that year model if it has never been into. Your likely to find #4 damaged due to water intrusion from bad crankcase seals. Dont order anything till you get it apart!!!

 

[1970Thunderbird]

Re: Proper compression for a '78 800

To put it bluntly: is it worth rebuilding? I tend to think so, as I just redid almost all the electrical.

And I take it I should hold off on ordering so I order exactly what I need and all of it? And perhaps because I'd hate to be stuck with all the parts only to find a crack in the block.

Hope I can start tearing into it tomorrow. Any advice?

 

[194269]

Re: Proper compression for a '78 800

Just wanted to let you know, your engine does not have a removable head. That plate is just a water jacket cover.

 

[JB]

Re: Proper compression for a '78 800

Not that bad, T-bird. Could just be carbon stuck rings in #3 & 4.

Do a decarbon procedure and repeat the compression test. Chances are good that your psi will meet the REAL spec. . .10% span, never mind psi.

You will get a lot of recommendations here to do whatever is that poster's specialty. Sort of like asking a neurosurgeon what to do about your headaches.

Maybe you do need a rebuild, but you need much better evidence than you have now.

 

[1970Thunderbird]

Re: Proper compression for a '78 800

JB,

I've been looking up decarboning procedures and am a bit lost. Some people say to pour a little water into the carbs as the motor's running. Some say to add decarbon chemicals. What's your procedure? Most of the advice I've seen has been for 4-stroke car engines.

Would allowing some penetrating oil to sit in the cylinders work? I could put it in today and try a compression check tomorrow or Saturday.

Thanks.

 

[Chris1956]

Re: Proper compression for a '78 800

Most want you to mix a can of Seafoam with 1 gal 50::1 fuel and run her until she fills the sky with smoke. Then let her sit overnight. You might also spray some seafoam Deep Creep into cylinders just in case you missed a spot.

 

[1970Thunderbird]

Re: Proper compression for a '78 800

Chris,

I saw this today: http://www.thehulltruth.com/boating-forum/18222-how-decarb-engine-dunks-method.html#b

It seems to say pretty much the same thing you just said, except run it for 15 minutes with the mixture let it sit 15 minutes, and then run it again.

Looks like I'm off to Autozone to get some Seafoam. About time to buy the gallon of it, too!

 

[Jzx2sr]

Re: Proper compression for a '78 800

Most want you to mix a can of Seafoam with 1 gal 50::1 fuel and run her until she fills the sky with smoke. Then let her sit overnight. You might also spray some seafoam Deep Creep into cylinders just in case you missed a spot.

Quick one

Do you run the engine through the entire 1 gal?

Thanks!

 

[1970Thunderbird]

Re: Proper compression for a '78 800

Jzx2sr,

My understanding is that you idle it for 15 minutes, shutdown for 15 minutes, and then do it again.

 

[Jzx2sr]

Re: Proper compression for a '78 800

Great thanks!

I have similar readings and I'm hopping this will bump up the compression

 

[1970Thunderbird]

Re: Proper compression for a '78 800

I will say that it smokes like hell. I've just put my motor through two runnings. I've filled each cylinder with a large amount of Deep Creep and will crank it over tomorrow.

Good luck! I'm hoping this works.

 

[1970Thunderbird]

Re: Proper compression for a '78 800

Well, crap.

After three runnings, including an over-night cylinder soak, I have no change in compression. Looks like she'll be getting new rings and most likely new pistons.

Thanks for the help. Any last advice?

 

[194269]

Re: Proper compression for a '78 800

Be sure to install new upper and lower crankshaft seals and put kits in the carbs and fuel pumps. There much easier to do when the power head is on your bench.

 

[1970Thunderbird]

Re: Proper compression for a '78 800

194269,

Thanks for the advice. The only thing I'm not dead set on redoing are the reed assemblies. If they need attention, they will definitely get it, though.

I figured I'll end up replacing all the seals, perhaps some bearings, and basically give it a thorough exam and cleaning/repair. She's gonna purr once she's rebuilt. I'm hoping to have her done by September.

 

[194269]

Re: Proper compression for a '78 800

Check to see if there is too much play between the reed block and the c/shaft. If they are too loose you will lose crank case pressure.

 

[1970Thunderbird]

Re: Proper compression for a '78 800

Is there a proper tolerance between the reed block and c/shaft? Measurable with a feeler gauge? Or am I just going to have to guestimate?