Compression test

[Guest ant31]

I bought an engine that supposedly was a rebuilt several years back and minimal mileage.

I assembled everything and tried to start it and couldn't get more than 1 or 2 cylinders to fire.

Gas is getting there and good spark on all 4 plugs.

Soooooooo, I decided to do a compression test.

1 and 2 have 25lbs, and 3 & 4 have 45 lbs of compression.

What's next. I didn't see any signs of cracks when I assembled it.

Should I be pulling the head and the first 2 pistons?

TIA

[DavidAU]

Bit hard to know when you not telling us what sort of engine it is but at a guess if it has been rebuilt and you say you assembled it therefore the rings should be free and the piston clearances are correct, I would say either the valves are sticking or the valve settings are too tight or too loose.

[Larry Schramm]

Maybe a dumb question, but did you hear it run before you purchased it? If not are the spark plug wires in the right position? Just a thought.

[Rusty_OToole]

All cylinders are very low unless this is a pre 1925 engine. Did you oil the cylinders? Sometimes you have this problem (low compression) on a new engine if the rings are too dry, or if the cylinders have been flooded and the cylinders washed down by gas.

Compression test should be done with the throttle fully open and coil disconnected. Let the engine turn over 3 or 4 times to build up pressure.

Test all cylinders once, then squirt some oil down the spark plug holes and test again. 2 or 3 squirts from an oil can. This will oil up the rings.

If pressure rises significantly it could mean bad rings. If it does not rise, could mean bad valves or a hole in the piston. Of course this is on cylinders that read low on the first test.

[Guest]

I bought an engine that supposedly was a rebuilt several years back and minimal mileage.

I assembled everything and tried to start it and couldn't get more than 1 or 2 cylinders to fire.

Gas is getting there and good spark on all 4 plugs.

Soooooooo, I decided to do a compression test.

1 and 2 have 25lbs, and 3 & 4 have 45 lbs of compression.

What's next. I didn't see any signs of cracks when I assembled it.

Should I be pulling the head and the first 2 pistons?

TIA

Start over again. What type of engine are we talking about

Are you saying you bought a used engine- tore the engine down and then put it back together? If this is what you did were all the measurements and tolerances within specifications? Valves and seats ok? valve guides ok?,lifters ok? camshaft ok? piston to wall fit ok?, cylinder wall condition? piston ring gaps ok? piston pins good correct offset? checked all bearing clearances. VALVE TIMING correct? valves adjusted properly? Oil pump working, ignition timing set correctly?

D.

[Guest ant31]

Sorry, it is a Model A ford stock engine.

I just installed it, didn't assemble it except for the oil pump, pan, and water pump.

Plug wires are correct. Impossible to get wrong on a Model A.

No, didn't hear it run first.

Timing is correct.

[broker-len]

check the valve clearance may be your problem also vacumme leak

[broker-len]

In the mid 70s I had my 32 PA plymouth motor rebuilt by a reputable shop here in south jersey,,,,,,,,,only drove it a few hundred miles,,,,,last summer got the body done and began putting miles on,,,,,,big problems,,,,,,the shop had some one working for them that messed up my motor,,,,,,valves were bad,,guides were bad,,,rods were in backward,,,pistons were new but scored,,,,,had to have it done again they did the labor for free- but the new parts cost more that the entire job in 70s-----in this world,,,if you want a good job eather do it yourself or monitor the work as it is being done I had to pull the motor and now have to put it back--------what a pain ???

[Guest]

Sorry, it is a Model A ford stock engine.

I just installed it, didn't assemble it except for the oil pump, pan, and water pump.

Plug wires are correct. Impossible to get wrong on a Model A.

No, didn't hear it run first.

Timing is correct.

OK, valve timing correct? unlikely if it ran before and since it's a gear to gear deal there is no chance of skipping/jumping like on a timing chain engine---but check the valve timing anyway. Valves adjusted ? If these are OK do a compression test again, still low, then a wet compression test. If the numbers don't come up do a leak down test...this will tell you where the compression is going-i.e. intake valve, exhaust valve, head gasket,piston/rings.

D.

D.

Edited April 23, 2012 by helfen (see edit history)

[Guest]

in this world,,,if you want a good job eather do it yourself or monitor the work as it is being done I had to pull the motor and now have to put it back--------what a pain ???

I agree, unless you can trust someone it's best to do things yourself,but you better know what your doing. If you have someone do the work and this includes the work the machine shop does you better have all the measuring devises to check ALL their work. Everything gets checked from the machine shop. This is especially true today because on average young people in the trades don't have the proper math background and sometimes work ethic.

I'm building a car today and the rebuilt engine I have is from a friends wife who sold it to me after he passed away. It's never been run-still vacuum bagged---but I will completely disassemble it and MEASURE everything. It's just insurance against never really knowing or worse grief and money down the drain.

When you find someone you can trust... a word of advise. We used to have a sign in my old shop that read;

One hundred dollars a hour charge rate, or

Two hundred dollars a hour if you watch, or

Three hundred dollars a hour if you help.

Let the guy do his job, stay off his back, then check his work later.

D.

Edited April 23, 2012 by helfen (see edit history)

[Guest trainguy]

Cyl #1and #2 low, hmm ,sounds like the head gasket between the cylinders are leaking between each other.

[Guest ant31]

Came home and poured a little oil in 1 and 2. Re-tested the compression on both. It came up 10 lbs on each.

35lbs on both.

[Richard1]

On engines that have sat for a while, anything can happen. Could have been assembled wrong, or in a lot of cases the valves or rings stick. Sometimes just getting it running for a while with some extra lube in the combustion chamber helps. Some say seafoam, but I've never gone that route.

Sometimes things rust.

I like to assemble my own engines. That way I have no one to blame if something goes wrong.

[Guest Bill_N]

I agree on checking the machine work. It does mean, however, that you will need a complete set of micrometers and know how to use them. I had a very reputable engine rebuilder do the machine work on my engine a few years ago. I did the assembly, and checked everything along the way. Everything was dead on - except one connecting rod was much heavier than the other three (those three weight exactly the same). If you are trying to run a business, you can't do everything or check everything yourself - and good help is hard to find.

Bill

[Guest]

I agree on checking the machine work. It does mean, however, that you will need a complete set of micrometers and know how to use them. I had a very reputable engine rebuilder do the machine work on my engine a few years ago. I did the assembly, and checked everything along the way. Everything was dead on - except one connecting rod was much heavier than the other three (those three weight exactly the same). If you are trying to run a business, you can't do everything or check everything yourself - and good help is hard to find.

Bill

EXACTLY, and I forgot to mention:eek: that engine balancing is as much a part of the job as proper clearances, and even though you can't do this job you can always weigh pistons,pins, rings and rods with your digital gram scale. If you have found one bad apple, it's back to the machine shop to have the rotating mass redone.

D.