Honda BF90 Low Compression

[ryanw10]

I have a 1999 honda 4 stroke 90 hp with low compression on both cylinder 1 & 2.
Cylinder 1: 90 psi
Cylinder 2: 60 psi
Cylinder 3: 150 psi
Cylinder 4: 160 psi

When I do a leak down test on Cylinder 1 I have air coming out both intake and exhaust. When I do the leak down test on Cylinder 2 I have air coming out intake, exhaust, Spark plug hole #1, and I can hear bubbling from the oil fill.

I took the cylinder head off expecting to see a blown head gasket but that looked to be OK, and I didn't notice any valves sticking open either. I did notice however that the cylinder sleeves on 1 & 2 are raised up around .015" from the block surface, where cylinder 3 & 4 sleeves are flush. I do not know the history on this motor but is it possible that this motor got so hot that the pistons started seizing to the sleeves and pushed them up? And the sleeves being raised up are what's causing my low compression, and likely a warped head too?

 

[ahicks]

How do the valves look? When was the last time they were adjusted?

Keeping in mind we're working on an older motor, and attempting to move forward, trying to minimize analysis paralysis - There's not a lot you can do about the cylinder height. Not economically anyway -

I'd have a good look at the valves/clean those up as necessary, and reassemble with a new head gasket - while being very careful to torque the bolts to spec and in the correct order.

Adjust the valves, then see where you are at with a compression check.

 

[ryanw10]

I adjusted the valves after doing the first compression check, and before pulling the cylinder head. All valves for cylinder 1 & 2 needed adjustment. After adjusting them though cylinder 1 only came up 10 psi and cylinder 2 did not come up any. After that is when I did the leak down test and the pulled the cylinder head.

After pulling the cylinder head and not seeing anything seriously wrong, I got a new head gasket, and put it all back together. Still had the same compression results. I double checked the valve adjustment and it was in spec.

Just by looking at the valves/seats I can't see anything wrong, my next step was going to be disassembly of the cylinder head.

One other thing to note:
The first time I had the cylinder head off I disassembled the oil pump and found a pretty significant amount of metal shavings/chips

 

[ahicks]

Metal shavings not real encouraging, but not sure they are relevant to our low compression?

Wondering about stuck rings? Do you know the maintenance history of the motor going back quite a while? If it was operated with no thermostat for quite a while, the motor never gets hot enough to burn all the carbon from the rings, especially the bottom one. Once stuck, they may never pop loose on their own.

Haven't had that happen on a 90, but I have pulled down several 40-50 hp motors (same motor only shorter), freed the stuck bottom rings, cleaned them and the ring land up thoroughly and reassembled (very carefully matching the way everything was installed originally) using nothing but a new head gasket. Pretty risky move, but one I've had good success with.

From what you've said pulling it down for a look at the rings might be my next move. Or lacking the time/incentive for that, shopping for a used power head/short block..... -Al

 

[MattFL]

Quick and dirty check for leaky valves: set the head on its edge, intake side up, and fill the intake pockets with alcohol. Look at the valves on the combustion chamber side to see if any is leaking through. Flip and repeat on the exhaust side.

Since your leakdown test showed air crossing from cyl 2 to cyl 1, but the head gasket looked OK visually, it seems plausible that cyls 1 and 2 have some sort of bad piston or ring issue. You could repeat the leak down test, except with the valve cover removed to prevent the crank case from holding pressure, to see if that resolves the air crossing over. If so, then that's one more vote for air leaking around the pistons.

 

[ahicks]

Another thought, if this motor has been sitting awhile prior to all this, is a sticking valve. Don't see it often, but I've been into motors that have been sitting for 10 years that look terrible when the rocker cover has been removed, but amazingly enough it all cleans up really well. Others may have just been sitting a couple of years and clearly have an issue with a valve or 2 sticking. I've always blamed this on the quality of the oil in use, and frequency it's being changed. Just a thought....