4A084 compression test

[cranetruck]

Checked the compression ratio on the 10KW genset engine this morning and measured 4.5:1 on both "instrument panel side" cylinders, will check the others later, but at 4.5:1, it may explain why the engine is hard to start...
According to the references I have, it's supposed to be 6.8:1.

 

[PeterD]

Compression ratio is basically mathematically based, not measured. It consists of the ratio of the volume of the cylinder with the piston fully down plus the volume of the head's pocket and the piston's pocket (which for a domed piston can be negative) to the same numbers with the piston fully top of cylinder.

Did you measure the compression itself, and if so what was it.

Did you follow established procedures for measuring the compression (number of turns of the crank, crank speed, etc.) That can make a big difference. Once you do a dry measurement, did you do a 'wet' one (inject some oil into the cylinders to seal the rings and see if the compression improves...)

 

[glcaines]

Well an update. The tank sat overnight with about a gallon of gas and a pint of acetone. This morning at -4 below, I was anxious to see the results. Well I shook it and shook as the length of chain was still in there. I emptied the tank and I could not believe the greenish/brown fluid came out of it. Then I sprayed a can of oven cleaner inside of it and emptied a lobster pot of boiling water in it. I let it sit for about 20 minutes and then shook it again. more crap came out of it, but what a difference. One more rinse with $3.05 a gallon gas and one hot water rinse and it looked great. Then I blew it out with my air compressor, and put my heat gun into the fuel inlet four about 1/2 hour. Clean and dry as a bone. I reconnected the fuel lines, filled thank with a pint of trany fluid, two onces of PRI-D and a few gallons of fresh diesel. Tomorrow, I will test the glow plugs before trying to starting him! Again, thanks for all the suggestions, what a project!!! You folks are great!

I think you meant to post this to: Fuel Tank Question - Status Report on my MEP-002A

 

[cranetruck]

Used a "compression" gauge, screwed into the sparkplug seat...the remaining cylinders showed 3:1 and 5.8:1, so it looks like it's time for new rings.

 

[DieselBob]

so it looks like it's time for new rings.

FYI, Saturn surplus has the ring sets listed under the 4A084 section.

 

[PeterD]

Used a "compression" gauge, screwed into the sparkplug seat...the remaining cylinders showed 3:1 and 5.8:1, so it looks like it's time for new rings.

Something is wrong here, a compression gauge gives reading in pressure, not ratio. Is there a PSI reading for this gauge, and if so, what were they reading in PSI. That will be more meaningful than a 3:1 or a 5.8:1 number since we don't know that those numbers relate to.

That said, your numbers seem to say that you have one good cylinder, two so-so cylinders and one that is likely almost completely dead.

This page has the steps to do a Compression Test though you will have to remember that yours is a carburetor and not fuel injected. You probably will have to do the wet test as well as the initial dry one, to isolate it to rings or valves. There are additional steps on the second page of this web page...

HTH, and maybe you can post good news. If not good news, maybe only needs a valve job.

 

[rickf]

How in the world do you come up with a compression ratio from a gauge? Can't happen.

Rick

 

[cranetruck]

Well guys, normal atmospheric pressure is 14.7 psi, so the pressure reading on the gauge is simply divided by 14.7 for the ratio. A ratio of 4.5 is derived from a pressure of 66 psi (from the gauge reading) divided by 14.7 and so on.

 

[PeterD]

Well guys, normal atmospheric pressure is 14.7 psi, so the pressure reading on the gauge is simply divided by 14.7 for the ratio. A ratio of 4.5 is derived from a pressure of 66 psi (from the gauge reading) divided by 14.7 and so on.

OK, it might be best to post them as PSI as that is the convention for a compression test.

That said, you should follow the steps described in the link I posted earlier, and these steps will help isolate whether it is rings or valves that are the problem.

If I multiply out your 'ratios', even your best cylinder is really low, and your worst is horrible. That lowest cylinder would not be contributing to the engine's performance at all, the two middle ones would be barely supplying power, and even the best would be marginal. So it is either do a more through compression test, both dry and wet, or just tear it down and rebuild the whole thing. Of course if the rings are OK, a fully rebuild is probably unjustified and an excessive expense--a valve job would be much cheaper.

 

[rickf]

Well, You may have me there, maybe, I think, Don't know.................

Compression ratio is a mechanical thing where compression pressure is a math thing. Compression ratio is a result of the stroke of the piston. Compression pressure is a result of the compression of the mixture minus any losses due to valve overlap or leakage and piston ring sealing. They are related but not the same. A lower pressure does not change the mechanical properties of the piston stroke.

Compression Ratio =
Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vol.

Note that this is one of several ways of figuring out ratios. I do not know how to enter all of the square root and Pi symbols to spell it out correctly.


Rick

 

[cranetruck]

Okay, I'll take another look at the procedure, in any case, something will have to be done.

The really low compression reading is for the cylinder, where a previous owner installed a threaded insert for the spark plug. The original threads must have been damaged and this brass insert installed after boring out the opening some...

 

[DieselBob]

If they let much shavings in the cylinder when the drilled out and taped the spark plug hole they may have scored the cylinder wall/rings causing the low compression.

 

[cranetruck]

If they let much shavings in the cylinder when the drilled out and taped the spark plug hole they may have scored the cylinder wall/rings causing the low compression.

A very good point, there were specs of aluminum in the intake manifold adjacent to this cylinder, which I noted with some concern when it was taken off last year for some magneto work.

 

[PeterD]

...
Compression ratio is a mechanical thing where compression pressure is a math thing. ...

Well, perhaps just the opposite, ratio is computed, and pressure is measured after the fact (and varies with the condition of the rings and valves, for example...)

I read your other posts about the damaged spark plug threads and the insert. I suspect your next step will be pulling the heads and seeing what's in there. But I think you've reached the point where a full rebuild may be needed... Ug, but it could be a great experience to learn more about these engines.

 

[hoop]

It always inspires me when students argue with the professor. Helps me keep hope that we are not just producing "Lemmings".
Cranetruck rocks.......

 

[cranetruck]

So I found this little beauty at a local junk yard for the cost of its scrap price (15c/lb).
I have noticed that most posts these days are on the newer diesel MEPs, but you gotto love this all aluminum 20 hp (basically 35 hp) aircooled engine. It's light enough for two people to handle and sits on the work bench here with the cowling removed.

Compression is still an issue, one cylinder has a thumping 125 psi (8.5:1) while another has zero psi.
I'm starting to take #1 cylinder head off to check the valves...

 

[Recovry4x4]

I would take the engine back to where you got it, seems that you have been ripped off! LOL. Sorry Bjorn, I know you have other irons in the fire, had I known I would have told you about it!

 

[cranetruck]

I would take the engine back to where you got it, seems that you have been ripped off! LOL. Sorry Bjorn, I know you have other irons in the fire, had I known I would have told you about it!

The restocking fee is a killer...

Never a prob, Kenny, it's when things don't work right when we learn and I'm enjoying this engine...need to find a better quality valve spring compression tool though, kind of a tight spot here. One spring must come out to get to a "hidden" head bolt.

 

[cranetruck]

So I got the spring compression tool to work and ended up removing the cylinder head and cylinder as one unit, not possible to take the head off by itself without damaging the cooling fins IMHO.
Anyway, the exhaust valve seat was loose and jammed the valve. From the TM, the seat can only be installed by heating the cylinder head to 500 degrees and cooling the seat to -65 °F in a procedure beyond my home shop capabilities, so I'll be taking parts from the MEP018 to make one good engine.

 

[cbvet]

Bjorn,
Keep the info coming please?
I really need to work on mine again when the weather gets better, & by then you should know EVERYTHING about these engines!
Eric