Very low compression in #2 and #4 cylinders

[amishhacker]

I have a 1970 1.9 GT that is my daily driver (or at least it was). Essentially a stock engine with Weber and electronic ignition (not Pertronix). I was driving to work, stopped at a light, car running fine, good oil pressure, and temperature normal. When I pulled from the light, it just totally quit, and wouldn't start again - not even a hint that the plugs were firing. After pulling a plug wire or two on the side of the road, it looked as though there was spark from the distributor, and there was fuel getting to the carb - but it still would not fire. I had it towed home, and have been checking things for the last few days. I pulled the electronic ignition, and put the points and condenser back in and cleaned the plugs (they didn't look too bad anyway) and now the car will run, but the #2 and #4 cylinders always miss. Tonight I pulled the plugs and did a compression check. #1 130 psi, #2 30 psi, #3 140 psi, #4 40 psi. Could this be caused by the cam jumping time (the timing chain only has a few thousand miles on it). Unless someone has other ideas, I guess I am going to have to pull the head and see if I have lost a couple of pistons or valves, but why would something like this happen to two cylinders at the same time? Thanks in advance for your help.

[hrcollinsjr]

Hey Neighbor,

Check your valve adjustments first. Especially on those cylinders with low compression.

Harold

[lowkey]

While the valve cover is off, have someone turn the engine over, and see how far the rockers are moving, IE: are the valves are opening for #2. You might have a rounded off cam lobe or two.

[vikesman101]

A quick check method (although not a 100% accurate), shoot a little oil in the bad cylinder and recheck the compression. The oil will provide a temporary seal around the rings, so if the number goes up you most likely have a ring issue. If it remains the same it would probably be valves. I have had good success using this method to get a staring point.

Tom

[amishhacker]

Thanks everyone for the suggestions.

I thought I would post an update. Cam/rocker/valve system seemed to be working OK, so I pulled the head today. Looking at the top of the pistons, the #2 piston has a piece missing from the top at the edge of the piston. It is about 1/4 inch by 30 mils, and you can see the cylinder wall is scuffed a bit in this area. The top of the piston looks as though something bounced around on the top a few times, but nothing was laying loose on the piston when I removed the head. I am thinking that I might have broken a ring, but I won't know for sure until I pull the engine and remove the pistons. The #4 piston looks OK from the top, so I am still not sure why it was showing very low compression. Maybe something else happened there. I will post again when I have the pistons out.

Thanks

Grady

[CDN OpelNut]

Originally Posted by amishhacker Thanks everyone for the suggestions. I am thinking that I might have broken a ring, but I won't know for sure until I pull the engine and remove the pistons. The #4 piston looks OK from the top, so I am still not sure why it was showing very low compression. Maybe something else happened there. I will post again when I have the pistons out. Grady

FWIW, you can pull the pistons without removing the engine. You need to drop the front crossmember enough to remove the oilpan so you can remove the rod caps and push the pistons up from the bottom. I have replaced a broken ring this way in the past.

Corey