Low Compression

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PaulVmax

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Hello Guys,

My front left cylinder is only showing 30 psi.

I have had the engine out blasted with soda and painted. When I restarted the bike only the rear 2 cylinders fired.

Carbs and spark is fine.

The front right started working after letting the bike run like if it had a sticky valve.

The front left cylinder has not recovered however any idea's?
 

Fire-medic

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Well one way is to bring that up onto TDC on the compression stroke, and to pressurize the cyl (leakdown test kit), and see if you hear air escaping from the carburetor, the oil filler port, or the exhaust.

Air leak
@ carb: intake valve/seat/stem/seal

@ oil filler port: piston rings

@ exhaust: exhaust valve/seat/stem

Yanking the valve cover to see the valve actuation, or the lack of it, might point to a broken spring or a stuck valve.
 

02GF74

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Interesting question but I would have thought even with low compression, providing fuel and air is being drawn in, the mixture would fire although there would be less power - comments?

When you measured the compression, you did it with all plugs removed and with throttle wide open? And you measured it for all cylinders?

For the cylinders with low compression, if dropping some engine oil into the cylinder and repeating the test gives a higher result, then piston rings are not sealing.

Engine was running fine before disassembly and did you do comp test before?

Any work done on the heads?
Where the valve shims moved or cams from their original location?

... actually I see you just mention engine was removed and no word of it being taken apart. How do you know no blast media entered he engine via inlet or outlet ports?
 

redneksoldier

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Interesting question but I would have thought even with low compression, providing fuel and air is being drawn in, the mixture would fire although there would be less power - comments?

When you measured the compression, you did it with all plugs removed and with throttle wide open? And you measured it for all cylinders?

For the cylinders with low compression, if dropping some engine oil into the cylinder and repeating the test gives a higher result, then piston rings are not sealing.

Engine was running fine before disassembly and did you do comp test before?

Any work done on the heads?
Where the valve shims moved or cams from their original location?

... actually I see you just mention engine was removed and no word of it being taken apart. How do you know no blast media entered he engine via inlet or outlet ports?
Media getting into the engine would be my first concern especially if the engine was running fine prior to removal. Best bet would be to pull the cam covers and see if the valves are moving properly and check valve lash. Also, a leak down test may help point you in the right direction before you start taking things apart.
 

02GF74

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I haven't used one but been thinking about getting an endoscope - these may be a quick and easy way to see what is going on at the valves.

If soda did get in, then it may have been melted over the valves so that they do not close. Sodium bicarbonate (baking soda) is what's used and turns into sodium carbonate (washing soda) when heated. But it dissolves in water so maybe pouring hot water to dissolve it may work. I have to say I wouldnt be overly happy to do this but water is a byproduct of combustion and as long as it is removed then it shouldn't cause any problems (draining the engine oil to check none has got into the sump would be advisable). Also note solution of washing soda is quite good at removing carbon but I seem to recall it eats at aluminium so don't keep it in for hours.

For around $10. 0.1 in in diameter so it probably can get in through spark plug hole. Screenshot_20210328-001208.jpg
 
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