96' XP Losing Compression After Carb Rebuild

[cmoore217]

I apologize in advance for the length of this post. I have done a significant amount of searching and have not found this specific problem yet...

I purchased the ski about two months ago and at that time I check compression, which was at 150psi on both cylinders.

Boat ran great prior to the carb rebuild with the exception of a low end hesitation. This prompted the carb rebuild. I completed the rebuild and crank it up, it started great out of the water and after a few quick throttle jabs I figured I was off and running again.

I put it in the water and could barely get it to run. Carb adjustments are set to 1.5 LS and 0 HS. I cranked and eventually the starter went out. I had figured it was on its way prior to this as well. Order the starter, slapped it on and cranked again. It started right up but didn't sound healthy at all. After a few more tries the boat quit running all together. It still cranks freely.

I then looked at the exhaust and found a decent amount of oil (sticky) in the exhaust pipe. This prompted a compression check which revealed that while cranking the compression was between 130 and 140 but when I stopped cranking the compression would fall to 0 immediately. I have tested the compression gauge and it is not faulty. I pulled the head off and inspected the tops of the cylinders and they were black but I did not see any visible where. However, from the sounds of things the wear is normally in the sides of the pistons.

Any thoughts? Would a boat loose compression on both cylinders like this at the exact same time? Any help at this point would be very much appreciated. Thanks!

Chris

 

[scot750]

if there was fluid in the cyl when the first test was done it would read higher since the fluid makes the rings seat better. it should run on 130 psi. did you check spark?

 

[kustomkarl]

This is the proper way to check the compression and some suggestions.

. To test compression, remove both spark plugs. Place spark plug caps on the plug cap studs near the cylinder head to ground the empty caps. This completes the circuit of the ignition electrical system and prevents any electrical problems from the caps being un grounded. Using the correct adapter for the threaded end of the tester,( same length of the spark plug threads length)screw in the tester in one plug hole. Hold the throttle wide open. Push the start button. Watch the compression gauge, when it peaks out at the most compression, let go the start button. Read the psi number. I would do it 3 times to be sure it is accurate. Check both cylinders the same way. The ideal compression is 150 psi per cylinder. If it is less, it's not a problem as long as they are close to being the same. If the psi is less than 90 it might need be time for a tear down and a rebuild. If the psi in 1 cylinder is say 140 psi and the other is 80 psi you need to tear down and repair. This difference is a lot then there is a problem. I hope this helps you.

Karl

 

[cmoore217]

Thanks for the suggestions fellas!

I do have spark and new plugs. I checked the compression as per your instructions with the same results. The compression gauge reads between 135 and 145 during cranking but as soon as you stop cranking the pressure falls to zero. I am thinking it could be a bad compression gauge too. Is there anything else that would cause it to have good compression during cranking and loose compression when the cranking stops?

Also, I noticed that there is oil (similar to what comes out of the jug) sitting in the exhaust. Is this normal. Is it in there because it is not being burned up because the engine will not run? I was curious because I wondered if this was a sign that something else was wrong.

Also, along with a carb rebuild and new starter, I also replaced all of the fuel lines with black hose from autozone. I did not buy fuel injection hose as I was told it was not needed. The fuel lines would not be collapsing would they?

Sorry for all the questions, I am just dying to get this ski back on the water. Thanks!

 

[nswillin]

Defiantly your gauge is bad. Check the alignment on your oil pump and make sure your not dumping too much oil in with the gas.
Your cool on the fuel lines.

 

[BC3]

replace the needle release valve..on some guages they are the same style and size as a tire valve stem. I have had this same situation happen to me... Perhaps the oil ingested up into the guage itself causing it to malfunction. bc3

 

[cmoore217]

So compression should be good. Fuel lines good. Carb rebuild should be fine. What do you think the possibilities are that I bought bad gas from the station the last time? The gas is a freakish orange color. It is very odd. I guess that is my next step, I am going to drain the gas and refill with new.

I will also check the alignment on the oil pump. Thanks again for all the help. This forum is great!

 

[cmoore217]

I am also going to go premix...is the following all I have to do on the 787 motor?

"Easiest way to block off the oil pump is to just remove the oil pump, take out the oil pump shaft, put oil pump back on and leave everything else connected. Keep a little oil in your injection tank so the rotary still gets a little oil.
Mix your gas 40:1 and take off."
Quote Per nswillin (thanks)