Compression test questions

[optimusprime8]

I did my compressions test and the results are:

1-85

2-85

3-85

4-90

5-80

6-85

The specs say it should be 120-150 with no greater than 10 psi difference between cylinders.

I'm wondering if maybe my compression tester is not correct or since the engine has been sitting so long (more than 20 years) it would raise the compression numbers up a little if I got it running and warmed up.

The engine was rebuilt prior to being parked 20 years ago, and probably driven less than 50 miles before getting parked.

I'm trying to determine whether I should chuck this motor and put something else in it or rebuild the carb, put in a new radiator, and give it a tune up and drive it as-is.

[aero3113]

Try it again with a little oil in the cylinders, if the PSI goes up you know it's your rings if not your valves need to be adjusted. You can also give another gauge a shot.

[aero3113]

Drive it as is for a while and it may free up the rings if that's the cause.

[Dave72dt]

Sitting that long will stick rings even in a "fresh " rebuild. Only fifty miles means the engine isn't broke in yet. Both will give you a low compression reading. Readings are even. That's good. Try the test again after 500 miles. Drive it like your daily. Don't abuse it but no need to baby it either.

[greg g]

Couple of valves might be hanging open a bit also, If that was a dry test, engine not runining before checking, it actually looks pretty good, average the high with the low and it all evens out to 85. Which will come up about 10 lbs with a shot of oil down the plug hole.

Lots of folks have reported readings as low as 60 or 70 and having what they categorized as good running motors. Down a bit on power from new but still running well.

Probably isn't even broken in yet with such low milles on a rebuild, rings aren't even scrubbed in yet.

get it running get it to temp a few times and then check it againn after shutting it down.

[Don Coatney]

If the engine is not in the car build an inexpensive live test stand and see how it runs. Then re-check the compression as has been recommended.

[austinsailor]

When I was getting ready to get my 48 B1B running after 30 years or so of sitting it checked about 30 lbs per cylinder. I was actually surprised it started. I've run it maybe 1000 miles now. It runs pretty good and burns only a slight amount of oil. It would certainly benefit from a rebuild, but doesn't do too bad. I've never re-checked the compression.

As others have said, I'd run it for a while and see how it does.

[NiftyFifty]

If the engine is not in the car build an inexpensive live test stand and see how it runs. Then re-check the compression as has been recommended.

WHAT!!! I CAN'T HEAR YOU DON??? LOL... LITTLE LOUD IN THE SHOP WITH NO EXHAUST

I checked the compression in my truck shortly after I got everything cleaned up and it read about 85 across the board...the problem was the oil pressure was VERY low...so it cooked a bearing and threw a rod..that's what you wanrt to be careful of. I would get some sea foam in that motor and run her up a few times and see where it at after that...UNLESS it's already out of the truck....then I would pull it apart now and check everything over...those motors were infamous for breaking rings and then you coook the piston and possibly the cyl.

[greg g]

why sea foam an engine with basically no miles on it???

[Dave72dt]

why sea foam an engine with basically no miles on it???

Help free up rings if they are sticking a bit.

[greg g]

I would just run it for another couple hudred miles then check it again. I'm betting he gains 10 to 15 #'s each cylinder by adding some oil, which would put the egine pretty much in the nomal range.

But there are lots of choices one could make.

[NiftyFifty]

I would just run it for another couple hudred miles then check it again. I'm betting he gains 10 to 15 #'s each cylinder by adding some oil, which would put the egine pretty much in the nomal range.

But there are lots of choices one could make.

Adding oil will boost numbers on almost any engine, but that doesn't help free up the rings, and yes running it may...but sometimes that gunk is hard to break free, so an additive like seafoam will help and it doesn't harm the engine at all. I would put a can in a fresh oil/filter change and a can in the gas..run it for about 2-300 miles, then change the oil again..filter will be fine

You don't have to change the oil, but I'm big on only leaving those additives in for a short time, as they pull a lot of crap down and best to drain that stuff away as soon as possible IMO

[William Davey]

They say the dumbest question is the one not asked, so here goes.

1) did you have the throttle wide open when testing?

2) try pulling the plugs in all cylinders so the engine cranks over faster?

3) buy, rent or borrow a leak-down tester? Test each cylinder at Top Dead Center on the compression stroke. Apply air and see if you hear noise in the carb or exhaust pipe. Also listen at the oil fill pipe. This will tell you whether you have leaky valves or leaky piston rings.

My opinion - your pressures do not vary widely, and they're almost what they should be. Drive it until something bad happens.

[Don Coatney]

They say the dumbest question is the one not asked, so here goes.

1) did you have the throttle wide open when testing?

2) try pulling the plugs in all cylinders so the engine cranks over faster?

3) buy, rent or borrow a leak-down tester? Test each cylinder at Top Dead Center on the compression stroke. Apply air and see if you hear noise in the carb or exhaust pipe. Also listen at the oil fill pipe. This will tell you whether you have leaky valves or leaky piston rings.

My opinion - your pressures do not vary widely, and they're almost what they should be. Drive it until something bad happens.

No need to have the throttle wide open. No need for a leak down device.

[optimusprime8]

Thanks for the tips guys. Looks like I will try to get it running and see what happens.

[dodgebrice]

If you do a leak down test the engine needs to be run a while and warmed up to be accurate. The cylinders will seal more as the metal expands when hot.

[Eric SW 48]

I have a 1969 industrial flathead Compresion / leak down yes results

The compression test was about 110 +-2 per cylinder which i think is not bad, the leak down test was horrible lots of air out the oil fill the engine runs pretty good , should i just forget it? I did not put oil in the cylinders and it was some what warm but at 40lps of pressure i could hear the air coming out of the oil fill. 

[kencombs]

Was the leak down results on all cylinders the same?  Are you using a leakdown tester that provides a % leakage or something else?

 

Some leakage is normal and the amount about the same at each cylinder. 

 

My tester specifies a test pressure of 15psi.

 

my wild guess is that your engine is ok, judging from the compression test.  Not perfect by any means, but serviceable.

[Tooljunkie]

8 hours ago, Eric SW 48 said:

I have a 1969 industrial flathead Compresion / leak down yes results

The compression test was about 110 +-2 per cylinder which i think is not bad, the leak down test was horrible lots of air out the oil fill the engine runs pretty good , should i just forget it? I did not put oil in the cylinders and it was some what warm but at 40lps of pressure i could hear the air coming out of the oil fill. 

Air will find its way past the rings. .010 end gap or whatever will allow air past. They arent o-rings like in a hydraulic cylinder. So a small amount of leakage will happen. 
thats why the crankcase is vented. 

[Tooljunkie]

My 1951 218 was siezed. Not bad but still stuck. I freed it up, cleaned up what i could with head off. Cleaned carb and points and tested compression. A couple were 45,others were 65. It started and ran. I run it regularily, into shop to work on and back to storage. Lots of warm up cycles. Should recheck compression but it runs great. 

your engine needs a good run in. 

[JBNeal]

10 hours ago, Eric SW 48 said:

I have a 1969 industrial flathead Compresion / leak down yes results

The compression test was about 110 +-2 per cylinder which i think is not bad, the leak down test was horrible lots of air out the oil fill the engine runs pretty good , should i just forget it?...

 

Sounds like good compression...it's metal on metal sealing in there so it will leak some.  Remember how fast the engine parts are moving so your leak sounds manageable.

 

I'm reminded of the leaky bucket I had to use as a teenager to draw water from a 30' well twice a day for several summers to water 20 head of cattle on the pasture by the house.  That vintage bucket had been around for a long time, but not rusted out, tho the bottom seam was giving up a tad.  The more I used it, the more it leaked...the last few years, it would dribble all over my feet if I didn't hustle from the well to the trough.  The faster I moved, the less water got on the ground to muddy up my boots.  So a good set of rings will leak a little; a worn set of rings will leak like a sieve...

 

You could always install a PCV system to deal with any blow-by smoke coming out of the draft tube system

[Eric SW 48]

Thank you all for your help the leak down test was done by my friend that had used it before i  do not know the type ie % or other  wise it was the same in all cylinders based on what you suggested i will runtime engine and think about a rebuild next summer.  

[Eric SW 48]

Another Question when i start my engine I have to crank it a lot i have new fuel pump and filters it almost sounds like as it cranks compression is keeping it from spinning freely is that a timing issue?  if so which way should i turn the distributer I have a new carb, converted to electronic ignition.  the timing is set to as best i can tell 10 deg advance but i do not have marks so i kind of am guessing.

Thanks in advance for your help

 

[Tooljunkie]

10 is probably too much to start. Turn distributor a little clockwise, shoulnt take much. 

[JBNeal]

how much throttle are ya applying at startup?  Depending on the weather and how warm the engine is, I have to pump the throttle while cranking a few times when cold, or floor the throttle when it's hot...