Is there any procedure for the ignition system while doing a compression test.

I have a '75 with a '78 engine with '78 CD system.

Thanks,

Art

 

Hi Art:

As long as the ignition system has a path to ground, you'll be fine.

I pull one plug at a time and perform the test and this retains a firing path for the CD box.

 

And *ground* the plug you removed, right Steve?
High tension ignition systems do not like to be operated with the secondary open-circuited.

 

how about just unplugging the CD box?

Art

 

I have done that as well. Easy to do.

 

You can unplug the CD box, but as long 5 of the six plugs have a firing path, you'll do no internal damage to the CD box while cranking.

 

Thanks,

I took out all of the plugs just to make the test easier and I didn't feel like putting them back in!
Art

 

Well, I did the compression test and got bad news. Very low compression with no compression in 2 cyls. I then started the engine for a few minutes and did it again on one of the zero cyls with the same result.

This was a used engine that was supposed to be low mileage that sat for 10 yrs or so before I got it. I am going to scope the bores and see what may be there. I was hoping for much better results!

Art

 

Art,
Before you give up hope (completely), I would run a double-dose of Techron through the engine, with a good hard drive thereafter. There is an outside chance the bad cylinders have a valve held slightly open, by a chunk of hard carbon. I would also make sure the valves on those cylinders are *not* tight; ie: check the lash.

 

I have run that scenario through my mind and am hopeful it is something like that. We did adjust the valves before running the engine, but maybe something is in the way, or the stems are sticking. I want to do a leakdown to see where the loss is.

Thanks for the encouragement.

Art

 

If the engine sat for ten years chances are the rings had developed rust and is not tight against the cylinder wall. Squirt some oil into the spark plug hole of bad cylinder and retest. If you have some compression chances are the rings are just rusted, you can just run the engine and hope that eventually the rust will be loosen or tear the engine down and rebuild with new rings and if the cylinders are worn beyond spec. replace pistons and cylinders.

 

I would put the/a removed plug into one of the wires, ground the plug, and put the other end of that plug wire into the center of the distributor or the coil itself.

But.

Even the very early versions of CD ignitions had a hefty diode recovery circuit for energy not completely used up in the plug firing process. So I doubt if you could really do any harm with an open CD HV side circuit since the recovery circuit had to be designed, worse case, for just this circumstance.

But still, no reason to chance it.

 

if I just got an engine like yours I'd probably run 30w non-detergent oil and a few bottles of techron for a few hundred quick 3.3k-3.8k rpm miles.. and then see what I had to deal with

 

Not sure Techron in the fuel supply will reach rusty piston rings. Use too much and you risk scoring the cylinder walls. I'd squirt some Marvel Mystery Oil (old standby) in the cylinder, crank the engine a few revs, then let it sit overnight. I'd also refill (the oil reservoir, not the cylinder) with 30 wt. detergent oil. Why non-detergent as some others have suggested?

Engine cranks easier with all plugs removed. You can also ground the coil wire (distributor end).

Sherwood
http://members.rennlist.org/911pcars

 

Hi Sherwood.

Techron would help blow off any carbon on the heads & pistons.
I've used Marvel a few times on seized boat yard detroit gas engines with success.
And Bruce Anderson has suggested using 30w non-detergent to seat new rings, or rings that haven't broken in properly. I read the hard info in PANO. So my 30w non-d is a take off on that.
I used 30w non-d to break in my rings with big success. And my present new cam install will get broken in with 30w non-d.
.........Ron