low compression

[Deleted member 220]

2 years ago i put a 440 in my 73 newport it had 155 psi last weekend i put a new carb on it thinking it would help with it being lazy on take off and after days of tuning i checked the compression and i only have 50 psi why the drop and does this mean the motors junk

 

[1978 NYB]

2 years ago i put a 440 in my 73 newport it had 155 psi last weekend i put a new carb on it thinking it would help with it being lazy on take off and after days of tuning i checked the compression and i only have 50 psi why the drop and does this mean the motors junk

Are all the cylinders 50 psi? If 50 psi is correct, its way too low. Did you try giving each cylinder a squirt or two of oil and checking compression again? It depends which 440 you have to determine what the cylinder compression psi should be. Depending on compression ratio it could be between 150-165 psi. All cylinders should be within 10 psi.

 

[Deleted member 220]

all eight 50-55 psi add oil 110 it had 155 2 years ago dont get why it did this

 

[has2bmopar]

First I would try another compression guage.

Was the engine overheated at any point?

 

[detmatt]

Or try the same gauge in another car, maybe your daily driver.

 

[1978 NYB]

BTW. Putting a new carb on doesn't have anything to do with cylinder compression psi.

 

[Wildaugust]

If your engine isn't using oil then the most likely cause of a gradual (2 year) decrease in compression would be valve seats IMO. But, that's assuming that there actually is something wrong with the engine. It could just be the guage. Does this guage thread into the spark plug hole or is it the push in type?

 

[1978 NYB]

I'm not questioning your ability to install or adjust a carb...but it might be the carb is not installed right or the adjustments are way off. If you still have the old carb, put it back on and see how it runs. If it runs as good as it did before you put the new carb on, then the new carb is the problem. If it still runs bad, I would double check that all the plug wires are on the right spark plugs and cap properly. I would also put a timing light on it and check the timing. It was sitting for 2 years? Are you sure the fuel filters/lines are not clogged?

 

[Wildaugust]

Wait. It lost 105 p.s.i. in one week? Then forget what I said about valve seats.

 

[has2bmopar]

If at one point you noticed the car was suddenly a pig one day, and now you have poor compression, you may consider that your timing chain jumped a tooth...

However if you have added oil to the cylinders with a drastic improvement, I will fall back on it being over heated at some point.

 

[Deleted member 220]

i checked the gauge with a friends i works fine. i lost 105 psi in 2 years of driving this car about 15000 mile in 2 years. the carb i used is brand new it was on my newyorker untill 2 weeks ago with the old carb the car still runs like ****. everything is new wires,plugs, disturbutor, had a new timing chain before the motor was put in.
when i loose the valves i can get a 10-13 psi increase but i should still have better compression even if theyre leaking a little

 

[Deleted member 220]

If at one point you noticed the car was suddenly a pig one day, and now you have poor compression, you may consider that your timing chain jumped a tooth...

However if you have added oil to the cylinders with a drastic improvement, I will fall back on it being over heated at some point.

if i add a shot of oil 110 psi i only checked one cylinder and the hottest the car ever got is 220 when the a/c pump locked up

 

[has2bmopar]

if i add a shot of oil 110 psi i only checked one cylinder and the hottest the car ever got is 220 when the a/c pump locked up

Probably not hot enough to score cylinders, but it may be enough to stick rings.

You could make an easy, inexpensive attempt to free them up, before tearing your engine down for rings, bearings etc. This will also clean up your valves and make dissasembly of the associated parts easier also, if you end up rebuilding your engine.

With the engine warmed up, and the air cleaner removed, run the throttle by hand and slowly pour some Marvel Mystery or ATF down the primaries adding a little throttle to keep it running until it is smoking pretty good... Then dump about half a cup in it, then in a couple seconds, let the throttle close and have a second person shut the ignition off. Leave it alone for a week or two, then start it and drive it a bit, then recheck your compression.

If you are in a residential area, do this in a different location, as it will make a large white cloud that will distress little old ladies and mosquitos.

 

[detmatt]

How about a leak down test on all the cylinders? It might give you a better read on just where your compression went.
http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/viewall.html

 

[Wildaugust]

If you have a vacuum guage I would suggest checking the vacuum with the engine at normal operating temperature. If it shows steady but low vacuum it could be the timing chain or even the rings but, IMO if you're losing that much compression past the rings, it should be burning oil. If the vacuum guage needle is jumping/fluctuating all over it's probably valve related.

 

[Deleted member 220]

thanks for the input guys im going to work on it more over the weekend. and for vacuum its a steady 15 on the vacuum guage. i will try a leak down test this weekend if i find nothing ill try the oil trick or i might say screw it and build a bigger motor for it over the winter

 

[detmatt]

That's the spirit!

 

[BillGrissom]

Try turning the crank over by hand. If you feel a good resistance from each cylinder (4 "air springs" per rev), then the cylinders are holding pressure and your rings, valves, and gasket are good. Even better if you hear the air leaking down in each cylinder for ~5 sec each. That would leave the camshaft and valve timing to explain the low compression. My money is on the chain having jumped a tooth or more. The original camshaft gear was made of nylon, at least in my 65 383. I wonder how brittle >40 yr old nylon is. One simple test before you tear open the front is to turn the engine while watching the distributor rotor. Reverse direction and note how many crank degrees before the rotor starts turning again. >10 deg is a sign of excessive slop in the timing chain. Even if it passes, doesn't mean it didn't skip. You could also pull some lifters up thru the valve cover to look at the bottoms. There is a tool for that. Scored bottoms means you "wiped the cam lobes" (bad lubrication, ...).

 

[detmatt]

I just tried to see if a lifter would fit up through the head on a 440 and was sure it wouldn't. I didn't follow through with trying because I didn't want to drop it and then have to play "get the lifter back in its bore with a magnet" game for an hour.

 

[Deleted member 220]

i found the cam that was in last a lobe or two and the valves are hanging open too much lift ? need a cam and adjustable rockers they where holding the valves open. so i know have more work to do on this car over the winter looks like ill have to wait on painting it.