Problem with New SBT Motor

[RadioDave]

I just put a new SBT motor into an '87 JS550. I used the supplied oil, 32:1. According to the instructions, I allowed it to idle on the trailer for twenty minutes while I dialed in the pilot jet. After the warm up, I ran it up and down the river from near idle to around half-throttle in 30-second or so cycles. All was well, motor felt great. After about 30 minutes, the motor started losing power and began backfiring with any substantial throttle. This sounds bad, huh? Both cyliners are showing around 100PSI on a SunPro automotive compression tester.

[Bryan Glynn]

Well, assuming your gauge is correct, it sounds like all the symtoms of low-compression. As to the cause, without inspecting the pistons and cylinders we would just be guessing. Most of the time this is due to improper carburation, if it's detonating.

Obviously we will need to get the motor back for warranty inspection, if that is what you wish to do. We will inspect it as soon as it arrives and call you with the results. We also take pictures of the motor and internals when they arrive and post them on the webserver if you wish to view them, at that time.

[ June 15, 2001: Message edited by: Bryan Glynn - SBT Webmaster ]

[RadioDave]

Bryan,

Check your email.

Thank you.

[RadioDave]

I have since bought a new gauge and read around 130 PSI on both cylinders. Seems the motor is OK. I completely disassembled the carb and cleaned it and replaced the plugs (again again). I'm off to the river to give it another try.

Dave

[RadioDave]

Well, basically, the ski is fouling out plugs after a very short run on the river. The last set I fouled on the water was accompanied by intake side backfiring. Now, back at the shop, I can't even get the things started without fouling plugs, rough running and backfires. When I manage to start it, the plugs quickly get a thick black coating. I now have the pilot as lean as I can set it without stopping the motor, but the plugs still coat right up with thick black soot, a cylinder misses and the ski stops due to fouled plugs. If I clean the plugs or put in fresh ones, the cycle starts all over again.

Could the motor have a load of oil somewhere below the cylinders that keeps burning bit by bit?

Is it possible that the 32:1 break in oil mixture is just too thick?

Bryan, I need help. It shouldn't be this difficult.

Thanks,

Dave

[jalaycock]

did you rebuild carbs? did you check pop-off pressure on your carbs. Low pop-off pressure can cause your ski to flood. I guess if you are still breaking in the engine a good wide-open throttle plug reading will be hard to do. It is aggravating trying to set carbs on a new engine, because you cannot run wide open to see if problems exhist there as well. What was the cause of engine failure? And could this be a ignition problem? Do you have a good strong spark on the plugs? And are you using the plugs kawasaki recommends for your ski, look in the owners manual. Just a thought. I just put in a sbt kawasaki motor and it runs great. I have had great luck with all of sbt's engines and so far any problems were with the ski or components, not the engine itself. You should always keep in mind when changing a engine, that there was a problem that caused the engine failure in the first place, so you should find and fix the problem before trying to install a new engine.

[RadioDave]

I have not checked the pop-off pressure. The carb is only two years old. I did disasseble it and clean it though. After putting in the rebuilt motor and cleaned carb it ran fine until the first set of plugs fouled.

The old engine did not die a rough death. The head and cylinders needed decking and the seals were getting shot, so I just opted to swap and core it out.

I do not beleive any longer that my current problem is in the motor.

I have just never seen plugs get so black so quick! The only logical thing would be the pop-off pressure. Could that be causing this?

Please help!

Dave

[Bryan Glynn]

The plug fouling may be caused by too large fo a gap, or the wrong plugs being used, helped by the extra oil for break-in - what gap are you setting?

The intake backfiring is usually due to an ignition problem, like the timing being off (how did you set your stator?) or too lean a mixture.

[RadioDave]

I'm using B7ES plugs at .030" as specified in the manual.

I set the stator by marking the center of the mounting screws on the stator, where they were on the old motor and lined them back up the new one.

Like I said before, the motor has run fine since putting it all back together, it just fouls out really quickly.

The backfires occur once the ski starts to miss on a cylinder.

Can I try using the special oil at 40:1?

Dave

[Bryan Glynn]

Yes, try 40:1.

[RadioDave]

Well, I tried 40:1 here at the shop on the trailer with the garden hose.

After changing the fuel, I ran the old plugs for a few minutes then put in fresh ones.

The motor will idle smoothly for a short time then begins to backfire, which will stall the engine. Revving the motor clears the backfire for a short time.

My only clue is that the insulators around the electrode are turning black almost instantly. On my other boats, they stay white.

Keep in mind, that the motor, as reassembled has run quite well for several hours on the water.

I really need some help!

Thank you.

[Bryan Glynn]

Now it's kind of sounding back to low-compression...do you have any other gauge you can use?

[RadioDave]

Well I just purchased a new one which is giving me 135 PSI on both cylinders, engine warm. That same guage gives me 136-138 cold on my perfectly running 900ZXi with the cool head, which makes sense.

Should I take it to the water again for some real-world revving after all of this fouling and plug changing?

[RadioDave]

Remember, that after the first episode of fouling and backfiring on the water, the ski did another hour on the water running fine before this started happening again.

I have not had the boat on the water since going to 40:1, only the trailer and hose.

[RadioDave]

Another thing,

Years back, I used to have a restrictor in the fuel return line with my round-body Mikuni 44. The ski has run fine without it for a few years, but someone told me to check it.

I also just checked the fuel tank check valve which is allowing air to flow in both directions. I do suspect that it has been that way for a while, but I thought it may be worth mentioning. I'll get a new asap.