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Compression test; held, now doesn't hold at all

L

Laz

Forum Newbie
Spec is 1,000kpa for my 1979 GS550

Test was performed with cold engine as it will not currently run.

#4 DRY TEST @ 800kpa

#3 DRY TEST @ 725kpa

#2 DRY TEST @ 890kpa

#1 DRY TEST @ 910kpa

#1 WET TEST @ 1100kpa

#2 rose then immediately fell. Air now escapes out the other cylinder. Needle will rise, and air quickly escapes when test is performed on any cylinder. Will not hold any compression at all.

What happened? Did I add too much oil? I added four caps or so.

I'm pretty sure I messed up. What are the first steps I need to take to make a repair? Thanks
​
 
The purpose of running the engine in (I'm assuming new rings and a hone as the minimum work done) is to seat the rings so that they seal properly. An engine rebuild is like mixing a cake. Running it in is like baking the cake. It ain't finished until its out of the oven.


So, I'm interested in why you want compression pressure data on a cold engine prior to running it in? If it were me I'd (carefully*) run it in and then look at the compression pressures, and compare it with what compression was prior to the work being done.

The gauge should hold the maximum pressure obtained, as it should have a valve to trap the air compressed by the piston, until you release the valve. if the needle drops after you stop cranking its likely to be a gauge fault.

Oil poured into the cylinder is a test to differentiate the source of low compression, which will either be rings or valves (or both) in a worn-out engine. A significant increase in compression when oil is added suggests the issue it rings as the oil temporarily improves ring seal. A couple of caps of oil are unlikely to cause harm. Worse that will happen is that you'll get a lot of smoke on start up or oil foul a plug.

*running in = varied rpm and load using non-friction modified mineral running-in oil, in short rides of 30 minutes (initally) allowing the engine to cool down between rides, for the first 600 mi (1,000 km), then oil and filter change to whatever your usual is. Increase the peak loads and rpm limit (including the occasional trip to redline) during this period and for the next 600 mi (1,000 km). Done.

https://www.dailymotion.com/video/x8xa466
 
Last edited:
KiwiAlfa156 said:
The purpose of running the engine in (I'm assuming new rings and a hone as the minimum work done) is to seat the rings so that they seal properly. So, I'm interested in why you want compression pressure data on a cold engine prior to running it in? If it were me I'd (carefully*) run it in and then look at the compression pressures, and compare it with what compression was prior to the work being done.

*running in = varied rpm and load using non-friction modified mineral running-in oil, in short rides of 30 minutes (initally) allowing the engine to cool down between rides, for the first 600 mi (1,000 km), then oil and filter change to whatever your usual is. Increase the peak loads and rpm limit (including the occasional trip to redline) during this period and for the next 600 mi (1,000 km). Done.
Click to expand...

The motorcycle will not run. it will not start at all. I have never had a shop do work on it, and I have never opened the engine. this was my first bike I've been trying to get started. I finally am making decent money again so I can actually afford to work on it.
 
Laz said:
The motorcycle will not run. it will not start at all. I have never had a shop do work on it, and I have never opened the engine. this was my first bike I've been trying to get started. I finally am making decent money again so I can actually afford to work on it.
Click to expand...

OK.... sorry not 'running'. If you have compression (which you appear to have), its either fuel or ignition. So first step is diagnosis

How long has it been sitting? I'm assuming it was a runner previously?

Check that you have spark. Remove all 4 spark plugs so that the engine will spin freely on the starter.
Put one plug in the number 4 lead and hold the metal hex of the plug against the cylinder head. Hit the starter you should see a blue spark. If you don't, you have ignition issues.

If you have spark, its likely you carbs aren't flowing the correct amount of fuel to start. The carbs need to be removed (not easy if it has the stock air box) and cleaned (disassembled and washed surgically sterile clean).
 
Last edited:
I had compression, but after wettesting #2 I no longer have compression in any cylinder. The previous owner said it ran great, but when I bought it I noticed it ran extremely rough, wouldn't idle well, etc. I really want to get it back on the road because it is a cool bike for sure.
 
Laz said:
It's brand new, but when I put my finger over #1 I can feel the air pushing my finger as I pump air into #2. I'll double check the gauge, but I really don't think it's the guage.
Click to expand...

Sounds like a blown head gasket, then.
 
Valves tight and not closing? Hard to remotely diagnose. I would check them. I just worked on my son's '79 ATC110 and after adjusting valves the compression rose dramatically, up to 155, which isn't bad for a clapped out '79.
 
Put your finger over the plug holes and crank it. If theres compression on your finger then your guage is junk. Also you may have pumped all that excess oil into your guages internals. If it ran before i would first COMPLETELY tear down and rebuild the carbs, put a plug in each wire and hold against engine to check spark on each lead. Any lead that doesnt spark unscrew the cap and trim a 1/4 INCH off the lead and rescrew that cap on then retry it.
Once carbs are rebuilt start it with a fan blowing on the engine and let it run a while. Then stop the fan and spray water on each pipe to see if its good and hot and sizzles the water. There are manuals and rebuild tutorials here and PM NESSISM for a CV oring kit.

https://gsarchive.bwringer.com/
 
Just saw the post saying airs passing between 1 and 2. I agree with the head gasket diagnosis. But its an easy fix and maybe 4 or 5 hours labor. Its usually best advice to replace the base gasket too. Just be darn sure that none of the thin retainer rings for the oil ring do not get snagged resetting the cylinders. I actually keep the retainers from old oil control rings till ive reset the cylinders. They can be reused if needed. The wavy part of an oil ring is the crucial part.
 
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