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RSRBOB, Technician
Category: Boat
Satisfied Customers: 1078
Experience:  Former Yamaha Factory Service Rep, Dlr Line Tech, Serv Mgr, G/M
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Got an 06 stx 900 just installed an sbt motor it wont rev

Customer Question

got an 06 stx 900 just installed an sbt motor it wont rev over an idle
Submitted: 2 years ago.
Category: Boat
Customer: replied 2 years ago.
I installed an SBT motor in a 2006 Kawasaki STX 900 that had an oiling problem. I fixed the oiling problem and completely disassembled the carbs and cleaned them. It start and runs fine at idle, it wont rev above an idle it seems like it is missing. The plugs are soaking wet.
Expert:  RSRBOB replied 2 years ago.

Hi and thank you for your question.

I haven't seen Kenny on here for awhile so if it is ok with you, I will attempt to help you.

First, I would like to know what the spark plugs were soaking wet with when you pulled them out. Was it gas, oil or water?

Typically if an engine is getting too much gas, it has the opposite symptoms you are experiencing. It needs lots of air (more throttle) to keep it running.

My next question would be, did you replace any components in the carbs, and what did they look like when you took them apart?

What was the oiling problem that you fixed?

Where do you have your high speed and low speed screws set on all 3 carbs?

If you could provide those details, we can move forward with your repairs.



Customer: replied 2 years ago.
The throttle cable broke where it attaches to the oil pump, and the oil tank outlet was plugged so the motor was run with no oil and lost compression (wiped out the rings) on two cylinders. When it came in it ran but had no power (running on 1.5 cylinders). I installed a brand new SBT motor. I completely disassembled the carbs and cleaned them and put them back together. they are CV carbs so there are no high and low settings.The plugs are soaked with a mix of gas and oil.It starts right up and idles fine, as soon as you give it any throttle it just starts missing and never revs about an idle.
Customer: replied 2 years ago.
The carbs were mostly clean and they needles and seats looked great and sealed up, I did not have a gauge to check pop-off I think that is is electrical in nature
Expert:  RSRBOB replied 2 years ago.

Thank you for the additional information. Sorry about the misinformation on the Hi and low speed screws.

That notwithstanding, this still sounds like a fuel starvation problem. However, the excess fuel could be a result of insufficient pop off. It definitely is not too high to cause the starvation however. The reason I believe it is starvation is because when you open the throttle plates, you are adding more air and if the engine is not responding, it is not able to add the correct ratio of fuel to go with it. That would cause it to want to quit as you open the throttle. As a test, have you tried easing the choke on to see if you can find a happy medium where it will take the throttle? Since you are choking off incoming air, the engine is very sensitive to that and you have to move the choke in the tiniest of increments to avoid going from too lean to too rich without realizing it. You may have to try it a few times to get a valid test result.

Have you run it without the top on the flame arrestor? I am curious if the slides are trying to lift as you try to rev it.

Let me know what you find



Customer: replied 2 years ago.
The slides operate smoothly when I apply throttle but the ski just doesnt rev. It doesn't die it just starts missing.
Expert:  RSRBOB replied 2 years ago.

This sounds like a very lean condition.

Did you try easing the choke on to see if you could find a spot where some choke does allow it to rev?


Customer: replied 2 years ago.
I have followed the fuel starvation thread... Checked the pulse line, checked the fuel selector switch, the lines to and from the tank... It certaily seems like a fuel problem... The chokes did not help much...
Expert:  RSRBOB replied 2 years ago.

I see in one of your previous posts you mentioned the plugs were soaking wet. My question would be, are the tips of the plugs coming out white or dark? Having asked that, if it doesn't run more than a minute, they may not show any color.

If the spark plugs are literally dripping gas when you remove them, then it is getting too much fuel. The symptoms, only revving to half throttle, would indicate the opposite however. We both know it can't be both so we have to determine which it is then run down all the possibilities that could create that scenario.

Dripping in gas would sound like maybe the needle and seat are leaking, or the float arm spring is too light. However, it sounds like you had this problem before and that is exactly why you cleaned the carbs. Just to be sure, did you say cleaning the carbs did not affect the problem at all? Or, did it not have this problem when you took the carbs apart and this showed up after going through the carbs?

If you think the problem is electrical in nature, you can use an automotive style inductive timing light to check the ignition. First, you can put the inductive lead on each plug wire and try to rev the engine. If you loose spark (flash) on any cylinder, then it is definitely electrical. If you don't loose spark, put the #1 cyl @ TDC and make a mark on the coupler. Start the ski back up and try to rev it to see if the ignition wants to advance or not.

Let me know,