evilaugust
Been Around the Block
I have a CB350F that is showing only 60 psi on all 4 cylinders. Is the head gasket the most likely cause?
Causes of low compression?
- Thread starter evilaugust
- Start date
danejurrous
Been Around the Block
there could be a number of causes that would make it have low compression from a head gasket to the valve timing being off where the timing chain could have skipped a tooth or something to that effect. Was the engine previously running, is this an initial compression test after it has been sitting for a long period of time?
evilaugust
Been Around the Block
It still runs. Needs about half choke to make it run right. However, this has been the case since I bought it. It sat for about 30 years. I have only rebuilt the carbs. No other engine work except a Dyna S ignition and coils.
Bozz
Coast to Coast
If anything has been done to it recently, like a valve adjustment, double check that.
It is possible to adjust with the cams in the wrong position and get them all uniformly off.
But yes, as mentioned, valve adjustment, valve timing, head gasket and rings are generally the things that control compression the most.
Were your carbs wide open when you did the check? They should be either wide open or removed for a compression check.
It is possible to adjust with the cams in the wrong position and get them all uniformly off.
But yes, as mentioned, valve adjustment, valve timing, head gasket and rings are generally the things that control compression the most.
Were your carbs wide open when you did the check? They should be either wide open or removed for a compression check.
evilaugust
Been Around the Block
Yes, choke and throttle were open. I haven't adjusted the valves.
HerrDeacon
Over 1,000 Posts
Some other possible reasons are bad rings, stuck rings, valve clearances out of spec, valves not sealing correctly, bore out of round, and also person not doing the test correctly. Not saying this is you, but thought I'd mention it in case. The test should be done with the throttle wide open and the choke opened as well. If you have CV carbs, taking them off is good as well.
evilaugust
Been Around the Block
Its was like this when I got it. It has 11k miles on it. What is he best way to do a leak down test?
danejurrous
Been Around the Block
The easiest way to do a leak down test on an engine is to get a leak down tester, which you can get from harbor freight for nothing too expensive. If you get one and do the test make sure the valves are closed on the specific cylinder you are testing.
http://www.harborfreight.com/cylinder-leak-down-tester-94190.html
I would thing it is something with the valve timing since it has such low mileage. Do you know if the PO had done a Valve adjustment on it?
http://www.harborfreight.com/cylinder-leak-down-tester-94190.html
I would thing it is something with the valve timing since it has such low mileage. Do you know if the PO had done a Valve adjustment on it?
A leak down test is basically putting compressed air into the cylinders via the spark plug whole. With the piston at TDC, if air escapes out the open oil filler, it would suggest bad rings (air getting past them down into the crank case and out the oil filler). If air comes out the exhaust, the exhaust valve isn't sealing properly (bad valve adjustment or carbon buildup etc). If air comes out the carbs, same deal on the intake valve.
If air comes out a neighboring cylinder spark plug hole, the head gasket is blown between the cylinders.
Honestly I've never done one myself, but I have the tool. You can measure the amount of pressure the cylinder is holding / loosing etc.
60PSI is pretty darn low for the bike to actually run. Can you test your tester on another bike?
If air comes out a neighboring cylinder spark plug hole, the head gasket is blown between the cylinders.
Honestly I've never done one myself, but I have the tool. You can measure the amount of pressure the cylinder is holding / loosing etc.
60PSI is pretty darn low for the bike to actually run. Can you test your tester on another bike?
60 PSI sounds like a Harbor Freight tester. If that's what you used, throw it out and borrow a good one from your local auto supply store - autozone or pep boys. A buddy of mine went through 3 of them before he told me and I lent him one that works.
I have a pro level leak down tester and to be honest, only use it on race bikes to watch the percentage leakdown as an indicator of change over time - like over a race season.
On street bikes I just use the compression tester hose and connect it to an air compressor and listen for leaks. A replacement hose is a few bucks if your compression tester only came with a solid connection. You want one with quick disconnect.
You remove one plug and rotate the motor to TDC with valves closed on that side. Screw in the adapter hose, put bike in gear and stand on the rear brake to stop the motor from rotating. Connect the hose to an air compressor with a regulator and open the regulator to increase pressure. Listen for the air escaping as Tim suggested. That will tell you what's leaking and the amount of air will indicate how bad the leak is.
But repeat the compression test first with a good compression test gauge.
I have a pro level leak down tester and to be honest, only use it on race bikes to watch the percentage leakdown as an indicator of change over time - like over a race season.
On street bikes I just use the compression tester hose and connect it to an air compressor and listen for leaks. A replacement hose is a few bucks if your compression tester only came with a solid connection. You want one with quick disconnect.
You remove one plug and rotate the motor to TDC with valves closed on that side. Screw in the adapter hose, put bike in gear and stand on the rear brake to stop the motor from rotating. Connect the hose to an air compressor with a regulator and open the regulator to increase pressure. Listen for the air escaping as Tim suggested. That will tell you what's leaking and the amount of air will indicate how bad the leak is.
But repeat the compression test first with a good compression test gauge.
evilaugust
Been Around the Block
Harbor Freight indeed.
carnivorous chicken
Over 1,000 Posts
Bike shouldn't be running at 60 psi across all four... I'd recheck. But if compression is low I'll second what most have said already: rings, valves (including blown valves), or gasket.
