merc sport jet 120xr poor idle/high speed

[slummer]

I have a 1998 sport jet 120xr. the boat has been running great until today. The engine would not idle and I lost approx. 6 mph top end. I drained the carb bowls , cleaned the fuel filter, and changed the plugs. I took it for another ride but the engine would still not idle and had a slight miss at top end. I then checked the plugs and they were all white.
I think I may have a timing problem but have no clue what to look for or how to set the timing.
Any help would be appreciated.

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

WHITE PLUGS ARE NOT GOOD !! Do not run your boat without finding the problem unless you would rather learn how to rebuild a motor . I do not know the XR motors as well as I know the early jets but you are running way lean . Let me do some research and in the mean time pull the carbs and clean and rebuild them , Look for the cause being varnish , sluggy muck . Timing will generally produce spark knock . If you can post a pic of the bottom of the plugs so i an get an idea of what you are dealing with . Also did you attempt to change the timing or try to adjust the carbs prior to this happening ?

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

The carb bowls were very clean (no varnish or sludge). I had read a few other older posts about the stators on these motors were a problem. I don't know if a bad stator would cause white plugs but you are correct that it is running way too lean. All four plugs look the same.
I didn't make any carb adjustments or touched the timing. The problem came on all of a sudden.

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

Do a fuel flow test , pull the main feed line to the carbs off , put it into a glass jar and crank the engine , Does it pulse or is it a consistent flow ? .. Pulse indicates a fuel line blockage or a bad fuel pump .

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

I did the fuel flow test.
The fuel was definitely pulsing (spitting) out the hose. I pulled the hose off at the pump to make sure the line wasn't plugged and got the same result. I dissassembled the pump and all the gaskets (diaphrams) seemed ok.
There is a primer ball going to the fuel filter/pump block. I squeezed the ball and fuel passes through the pump ok.
I did some other tests while I was looking over the engine.
engine ser #OE 316065
red stator - (did not see a switch box)
compression test cyl#1 = 150, cyl#2 = 165, cyl#3 = 160, cyl#4 = 150
battery voltage = 12.62
engine had spark after disconnecting black/yellow wire
engine had spark after disconnecting yellow wires from voltage reg
OHM the stator = .700 on the 2K scale
OHM the trigger could not find a white/black wire for ground

After all the tests I tried to start the boat and it still would not idle. I shut it down and tried to restart it but could not get it to fire up.

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

Spitting or pulsing fuel is an indication of the diaphragm being stretched and you have low fuel pressure . Get a fuel pump rebuild kit that includes the diaphragm and the valves , should be about 18.00 or so .

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

BTW there is no trigger ohm test for a 120 CDM system , it is done using a test harness to each CDM to test trigger output but if you have spark on all 4 cylinders spark is not the issue .

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

The fuel pump diaphram is definitely stretched so I will go to the boat shop tommorow to get a kit. Also I tore the carbs down to clean them and I noticed the idle air screw on the top carb was backed out 1 turn and the bottom carb idle screw was backed out 1-1/2 turns. Do these screws have to be backed out the same amount?

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

I start all my idle screws at 1 1/2 turns out and adjust from there but all are adjusted the same . they usually end up around 1 1/4 turns out total . If you do not want to "precision tune" the idle , set them at 1 1/2 turns out and forget about them.

 

[Daley]

Re: merc sport jet 120xr poor idle/high speed

Kend301 has helped me (and many others here) with these boats - his advice has been pretty spot-on.

That said, I'd ask his advice about that primer bulb. I don't think it has any business (read: legal issues) in an enclosed area.

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

I changed the fuel diaphram and the fuel now flows as opposed to pulsing but the flow was not that much. I put my finger over the hose to see if the fuel would build up pressure but I was able to hold the fuel back (not sure if this is ok or not). Primed the carbs and the engine fired once (rough) for about 5 secs and then would not restart. I pulled each plug one at a time to check spark. The spark on all plugs were a thin blue spark.

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

The flow should be a consistent 6 psi , You have good compression , you have spark . What color are the fuel lines and how old are they ?

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

fuel lines are black and look original. I am not sure how to check the pressure but I do know that the fuel did pump up to the carbs and filled the bowels while cranking.

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

Here is what I would do , Be sure you have spark , replace all the fuel lines from tank to carbs , E85 fuel can mess them up . Also pull the fuel pick up and check for blockage . if you still have the problems, pull the carbs and intake and check the fuel reeds . See if they are broken or chipped if they are replace them .

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

Bought a fuel pump gauge and new fuel line.
I was getting 3 psi coming out of fuel pump. I put the new fuel line from the intake of the fuel pump into a can of gas and then got 3.5 psi fuel pressure.
I tore down the fuel pump again. I only replaced the diaphram originally so I vaccum tested the 3 check valves in the pump and they seem fine. The pump still has the same spring in it also but it looks ok.
I there a way of bench testing the fuel pump?

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

Is your fuel pump square or figure 8 shaped ? .. It should be square and there are valves that stretch just like a diaphragm that could need to be replaced , they are holding vacuum but are not in spec .

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

The pump is the figure 8 design with three check valves. Does this style deliver the 6 psi fuel pressure?

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

NO , 2.5 to 3 PSI at idle and 6 psi at over 1800 rpm for the figure 8 style.. did you use a knife to hold the spring down when you installed the diaphragm ? but if you got 3psi for idle it should be enough . going to do some research and will post up more later tonight .

 

[kend301]

Re: merc sport jet 120xr poor idle/high speed

Do this , pull the plugs , GROUND THE PLUG WIRES so they will not spark at all , crank the boat with the plugs out and see if a fuel mist sprays out of the cylinders . Like I said GROUND THE PLUG WIRES SO THERE IS NO SPARK ! do not want to tell you how to blow your boat up

 

[slummer]

Re: merc sport jet 120xr poor idle/high speed

Reinstalled fuel pump diapham using flat blade (worked much better). Made sure fuel pumped to carbs (checked carb bowels to ensure full of fuel).
Pulled all plugs and grounded. Cranked engine and held a peice of paper to spark plug holes but paper was dry (no fuel going through engine).