Compression question

I have in the shop a 1978 200 HP Johnson. I'm familiar with how compression works but am getting 83-85 lbs. of compression across all 6 cylinders. The number seems low. We charged the battery to be sure and got the same readings. On the other hand, even compression, even if low, makes me think its not blown. Worn rings, or is that a good reading on an engine this size? The same compression tester on a 3 cylinder that age would have me reading between 115 and 125 on a good engine so it's not just a low compression gauge. Any thoughts?

Re: Compression question

Do you have "all" of the spark plugs removed when checking compression? If not, do so.

That engine should have compression in the 100+ psi range BUT the fact that "all" of the cylinder compression readings are so close together indicates that perhaps you are not obtaining a proper reading due to cranking speed.

The psi readings will vary as the cranking speed changes, ie pulling the engine thru slowly via a starting cord might have the reading down to 50 psi or so. The engine should be cranking over at least 300 rpm in order to obtain a proper reading.

A psi difference of only 2 psi between all cylinders on a six cylinder engine has me assuming that the compression is fine and the low reading could be contributed to dry cylinders or a slow cranking engine.

Re: Compression question

I think it might be slow, Joe. We lubricate the cylinders beforehand and remove all the plugs. The battery is charged to 100% so I'm just thinking the starter may be a little weak. I'm usually working with smaller engines. I know that in the 90s, 95-100 psi on a motor this size is common and good but

Re: Compression question

Could be a weak starter BUT before going into that, remove and clean all cables/wires pertaining to the starting system, including the battry terminals, the starter solenoid, and the ground at the powerhead. Clean also the components that the cables/wires attach to.

The above is a common problem that should a connection be slightly loose, or tight but dirty, a voltage drop results which in turn results in a slow cranking engine.

Re: Compression question

I think we got it. Thank you for your support. We had the powerhead already loose from the engine and solved it in this case by hooking up a 24V system rather than the traditional 12, in order to get up to proper cranking speed. I'm not sure my battery was providing the necessary voltage to turn over this large of an engine. In any case, that got us up around the 115 psi marks and it appears that we have a good powerhead. Thank you again.

James

Re: Compression question

In good condition, the compression on that engine (assuming stock 456/457 heads) should be in the 110 lbs range. You are good to go.

Re: Compression question

Stock 456/457 heads ???????????