Sportjet Gas/oil mix

[ProbeGT]

I have a 120 Sportjet with the oil injection cancelled.

Did a compression test last week, it's not very good. 135-150-125-150.

Plugs were kinda blackened from oil, my mechanic told me to put less oil, i'm running 50:1 and he told me to go 75:1 since i use synthetic oil. I tought it was a bit to steppy so I ran a full tank of 65:1

Gained some RPM, engine had a lot of trouble topping 5000RPM before, now it runs 5200 and even 5400RPM sometimes (well, that's what the tach tells me but I don't think it's very reliable.)

I held it 2 mins at WOT then stopped it, checked 2 plugs, very nice brown chocolate.

Am I ok to run the engine with a 65:1 mixture? should i go to 75:1, go back to 50:1 or keep it to 65:1 since it ran great like that? Engine didn't overheat or whatever.

forgot to tell it's a sportjet 120XR in a '96 Sea-rayder F16. Jet pump has been rebuilted 1 month ago.

 

[JB]

Re: Sportjet Gas/oil mix

First, do a decarbon treatment. Recheck compression. Then go back to 50:1 (actually 48:1). Let us know what you find.

 

[ProbeGT]

Re: Sportjet Gas/oil mix

How do I do that and with what product?

I've done this on a car before by pouring seafoam in the throttle body but in a boat?

I think the compression reading is good because the engine has moderate vibrations (more than usual on an old 2 stroke) and I bought it with the injection cancelled so I think that what happened is the pump failed, the engine overheated but not enough to blow, they cancelled the pump then added oil in gas, engine was still running good so they sold it like that.

Boat runs 37MPH WOT on a flat river @ 5000RPM with new wearing, new solas impeller and new rideplate/gasket.

Is it true that i can put less oil if I use synthetic oil?

 

[JB]

Re: Sportjet Gas/oil mix

See decarbon procedures in the Engine FAQs forum, Probe.

NO. It is not true that you can use less synthetic oil than regular oil safely.

 

[ProbeGT]

Re: Sportjet Gas/oil mix

ok so basically it's the same procedure as a car.

I'll buy the stuff, then buy new plugs and do this next time I go in the water.

One question, if my engine was full of carbon, wouldn't all of my cylinders low on compression?

since it's a jet unit, it basically always run at WOT or nearly WOT because under 4500RPM, there's very little thrust.

I'll try to do it tomorrow and give you some feedback. Thanx!

 

[ProbeGT]

Re: Sportjet Gas/oil mix

ok I did the thing today.

There are a few things to know

1st, I used another comp tester, a brand new Equus with braided line.

2nd, I tested the engine HOT

3rd, Tested with boat in water but it shouldn't change a thing.



ok so I went to put some gas in the boat, there was still 1/8 of the tank left with the 65:1 mix. I added 25 liters of gaz, 500ml of oil, and a full can of seafoam to clean everything. (on the can it's writted that it can clean mix to a 30liters tank of gaz or up to 90)

Then i bought the Deep clean seafoam stuff. Ran the boat, let it get hot, then removed the intake box. Opened the engine to 1800RPM, then sprayed the stuff. Bizzare thing is that when i sprayed it trough the top carb, nothing happened, when sprayed to the bottom carb, i could clearly hear the engine struggling. so i sprayed about 1/3 of the can to make sure product penetrate everywhere, then stopped the engine and waited 10 minutes. Started it and it smoked. There was a good smoke but not as much as i would've think of. There was a lot more when i did it on my car a few years ago.

Did the procedure twice, the 2nd time it barely smoked.

Took a new compression test.

1st : 145
2nd : 135-137
3rd : 145
4th : 145

i could reach alone in the boat 38MPH and engine revs about 5000-5200 depending on algue or water conditions.

plugs were a little more black with the 50:1 mix and seafoam but not clogged in oil so I'll follow your suggestion to stick to the 50:1