Low compression rear piston

[sony224422]

So, I had finally rebuilt my top end and got everything buttoned up and running. I broke it in using SBT's new engine break in period. I premixed and used the oil injection. I also kept the RPM low through out the first tank of gas. She ran beautifully. Now, on about 2-3 hours, I lost power after jumping a couple wakes and barely limped back in. I took the plugs out, and they didn't look horrible, but it wouldn't start unless the throttle was about halfway and alot of cranking happened. So, i took it home, took the plugs out again, and got 120 PSI on the first cylinder and 40 on the rear. The top end rebuild was from SBT, it was the top end kit. New pistons, head and everything. I took the dome off and The first piston looks beautiful, like it should. The rear, no visible damage, no gouges on the wall.. and i can turn the engine over with little resistance. There is however, like a gray matter on my dome and on the walls and piston itself. I still can read the .05mm on the piston and see the arrow as well on both pistons. Def has no time on it. Now, i am thinking theres something wrong with the rings.. SBT didn't send directions and i assumed the rings were the same. Is this not the case? Or am i having other problems. I will host pics in a second as well.. but i would like to get this figured out. I've never seen anything like it, but i've only really rebuilt seized motors. Original cause of the rebuild was because the previous owner broke the drive shaft and let water in and sucked it through the carbs and let it sit on the pistons seizing them to the walls.. no other damage. I cleaned the carbs well so i thought, but now im not so sure.

Pics http://s11.photobucket.com/albums/a200/sony224422/jetski/

 

[sony224422]

is there a way to test the carbs out put of fuel too?

 

[robin savell lloyd]

So, I had finally rebuilt my top end and got everything buttoned up and running. I broke it in using SBT's new engine break in period. I premixed and used the oil injection. I also kept the RPM low through out the first tank of gas. She ran beautifully. Now, on about 2-3 hours, I lost power after jumping a couple wakes and barely limped back in. I took the plugs out, and they didn't look horrible, but it wouldn't start unless the throttle was about halfway and alot of cranking happened. So, i took it home, took the plugs out again, and got 120 PSI on the first cylinder and 40 on the rear. The top end rebuild was from SBT, it was the top end kit. New pistons, head and everything. I took the dome off and The first piston looks beautiful, like it should. The rear, no visible damage, no gouges on the wall.. and i can turn the engine over with little resistance. There is however, like a gray matter on my dome and on the walls and piston itself. I still can read the .05mm on the piston and see the arrow as well on both pistons. Def has no time on it. Now, i am thinking theres something wrong with the rings.. SBT didn't send directions and i assumed the rings were the same. Is this not the case? Or am i having other problems. I will host pics in a second as well.. but i would like to get this figured out. I've never seen anything like it, but i've only really rebuilt seized motors. Original cause of the rebuild was because the previous owner broke the drive shaft and let water in and sucked it through the carbs and let it sit on the pistons seizing them to the walls.. no other damage. I cleaned the carbs well so i thought, but now im not so sure.

Pics http://s11.photobucket.com/albums/a200/sony224422/jetski/

What do you mean both rings are the same? Do you mean that you did not see a differance between the 2 and that thear is a top and botom for the rings or what?:cheers:

 

[sony224422]

That both rings are the same in every aspect.. although i think i figured it out. I haven't removed the head but theres signs of a very very lean burn.. no oil.. one of the oil lines from the pump was dry

 

[timmyboy76]

you gonna stick w/ injection, or convert to premix??

 

[sony224422]

both lol. no i'll probably go to premix

 

[derek]

did you put the pistons on the right way? there are little roll pins in the ring grooves to keep the rings from spinning. they must be on the side away from the exhaust port. they keep the ring gap in an area where they do not cross a port opening on the intake side. this is a 787 motor? if so, the carbs and exhaust are on the same side of the outside of the motor. that is the EXHAUST side. the intake side is the other side. i don't understand the top and bottom rings being the same. if so, they are SBT only style pistons. which means you have to go to them for new rings, but the rings NEVER are the problem. (at least not all of it). there is always piston damage if compression is low, especially if it dropped fast on new stuff.

 

[seadoosnipe]

Lean.......

Your pictures show definite signs of leaning out. I don't think it was the carb, I think it was your oil line coming from the injection pump. It is either clogged up or broken, at which point, you shoul have seen a bit of oil in the hull.

Did you do a volume test on the oil pump prior to rebuilding the top end? In any work that requires to go into the engine, the root cause needs to be found before you continue. Here, I think you only said it was looked up due to flooding........

If you've made no changes to the carbs, screw in your high speed screws on both carbs. They should be at zero, but double check. Do the same with your low speed. Although, the low speed probably wouldn't cause this, check them anyway.

I think this is caused from an oil pump failure of some sort. Bad line, clogged oil injector in the carb.............somewhere along that line.

Take the jugs off and inspect. If you have a set of inside and outside mics, you may be able to measure and get enough clearance for honing and replacing the rings........if they are what let go. The rings also have these little pins built into the cylinder, to make sure you have the proper stagger.

You can also think about boring over.

I'd also go to premix..........no since taking another chance.

 

[sony224422]

Your pictures show definite signs of leaning out. I don't think it was the carb, I think it was your oil line coming from the injection pump. It is either clogged up or broken, at which point, you shoul have seen a bit of oil in the hull.

Did you do a volume test on the oil pump prior to rebuilding the top end? In any work that requires to go into the engine, the root cause needs to be found before you continue. Here, I think you only said it was looked up due to flooding........

If you've made no changes to the carbs, screw in your high speed screws on both carbs. They should be at zero, but double check. Do the same with your low speed. Although, the low speed probably wouldn't cause this, check them anyway.

I think this is caused from an oil pump failure of some sort. Bad line, clogged oil injector in the carb.............somewhere along that line.

Take the jugs off and inspect. If you have a set of inside and outside mics, you may be able to measure and get enough clearance for honing and replacing the rings........if they are what let go. The rings also have these little pins built into the cylinder, to make sure you have the proper stagger.

You can also think about boring over.

I'd also go to premix..........no since taking another chance.

Yep, the top end was replaced due to water sitting ontop of the pistons. Oil pump was not checked... and there is a broken/clogged line on the line going to that carb. When i was running premix while breaking it in it obviously was getting oil.. therefore was able to live. These are SBT .05mm pistons, not even 2 hours on them. The rings are not in pieces, i am assuming they melted to the piston. I will see when i get a chance to pop the head. I will probably do a very light rehone and then new rings with the pistons, and then run premix lol. I don't think the pump is bad, im pretty sure its just the one line. The block off kit looks too complicated for me, unless theres a write up just for the 947/951 motor.. the directions of course only show the 747 motor.