Compression Test- 1975 CB750K

[jbird7262]

Well it's been a while since I messed with my project, finally got my 12mm adapter for the compression tester. since I don't have the throttle cables hooked up I had all four carbs wedged wide open, Had to kick all cylinders, plugs out, #1 117 1/2 psi, #2 117 psi, #3 118 psi and #4 118 psi and that's a lot of jumping to get that gauge moving up!! No battery so it was all leg. I think it's a good base line, and I'm rebuilding the another bank of carbs off the extra motor. I think with the starter she would pump up quicker. I will use the 120 jets but I also have 125 and 110 and a 40 slow jet, the bike came with 2 pairs of 2 into 1 pipes, don't no what route I'm going with air box or pods yet, but the bike came stock with 120's, I'll get my point/condenser sets soon, have the new battery and plugs. One day at a time.

 

[Travis]

How many times did you kick it for each cylinder? Like 5? Your numbers are a touch low, but at least they're all consistent! I wonder what the numbers will be after putting a few miles on the bike.

 

[jbird7262]

This bike has not been tagged since 1986 so it has not been run since then as well, thats a mule to kick up to 120psi, with elc. start it probably pumped up easier, it's the same condition as the burgandy 750 that you have. I just wanted to get an idea of the cylinder condition before I go further with points and the carbs, I have a second motor that came with the bike, so next will be one of the sets of carb banks to rebuild, I would like to fire it up just for shits and giggles.

 

[jbird7262]

those cylinders are dry, I might hand crank it from the lower end to move some oil to the cylinders and see if they come up a bit, I realize they are low, I kicked each around 10-12 times, #1 cylinder was only up to about 70 psi but I shot a bit of spray lube into the cylinder and got it up to match the others.

 

[Travis]

haha.. you cheated! That one probably has bad rings. I still need to check the compression on mine. I haven't done anything to it since I posted about it months ago.

 

[jbird7262]

haha.. you cheated! That one probably has bad rings. I still need to check the compression on mine. I haven't done anything to it since I posted about it months ago.

Hey, I gotta start somewhere, it was either the motor in the bike or the one on the floor

 

[dirtdigger]

I think once you get it running the compression will pick up. It is actually a good thing to get a little oil in the cylinders if the engine has been sitting for a long time. The oil will run out of the cylinder walls if it isn't run for a long time. I like some wd in the cylinders and let it set for a couple of days if possible. It lubes the cylinders but also works in around the rings and helps to make sure they are not stuck in the ring lands of the pistons. On an engine that has set for a long time I would expect to see lower than normal readings because of the lack of oil in the ring lands and on the cylinders. Your readings are still good enough for it to start so I think they will come up if you check them after you get the motor running. You are better of getting it running then cranking it by hand, you will never get oil onto the cylinders by hand cranking it. Cylinders are oil by the oil thrown off the rods from the side clearance between the rod and the crank. It needs to be running to get oil on the cylinders.

 

[jbird7262]

I think once you get it running the compression will pick up. It is actually a good thing to get a little oil in the cylinders if the engine has been sitting for a long time. The oil will run out of the cylinder walls if it isn't run for a long time. I like some wd in the cylinders and let it set for a couple of days if possible. It lubes the cylinders but also works in around the rings and helps to make sure they are not stuck in the ring lands of the pistons. On an engine that has set for a long time I would expect to see lower than normal readings because of the lack of oil in the ring lands and on the cylinders. Your readings are still good enough for it to start so I think they will come up if you check them after you get the motor running. You are better of getting it running then cranking it by hand, you will never get oil onto the cylinders by hand cranking it. Cylinders are oil by the oil thrown off the rods from the side clearance between the rod and the crank. It needs to be running to get oil on the cylinders.

Cool, good advise, I did blow some spray lubricant into the tops of the cylinders before I put the old plugs back, once I get the carbs done I will set or replace the points and plugs and see happens.