Throttle Issue
Submitted by tcarroll on Wed, 2013-12-18 11:24
Hi there,
I have just purchased my first boat, a 4.75m Qunitrex Bayhunter. The last couple of weekends i have taken it out for a spin just to give it a run around and to get used to. There seems to be an issue with the throttle. I can cruise steady at 8 knots, but if i try to give it a bit extra to get to around 12 knots, it continues to increase in speed, gets to about 17 knots for a minute or so and then loses power and drops back to about 5 knots.
Does anybody know what this might be?
Rig
Posts: 2925
Date Joined: 27/12/06
motor?
maybe let us know what type and size motor you have, might help
tcarroll
Posts: 265
Date Joined: 17/12/13
50hp Johnson 2 Stroke
Sorry, the outboard is a 50hp Johnson 2 stroke
sunshine
Posts: 2601
Date Joined: 03/03/09
?? Portable red poly fuel tank
Breathers not blocked is it ?
tcarroll
Posts: 265
Date Joined: 17/12/13
It is a 70lt underfloor fuel
It is a 70lt underfloor fuel tank. that was one of my first thoughts however i thought that would cause it to occur more intermittently than it does. it always seems to occur at the same advance and pressure on the throttle which makes me think it is something more mechanical? I will check anyhow!
Thanks
Swompa
Posts: 3885
Date Joined: 14/10/12
Could it be some sort of
Could it be some sort of safety on the motor. Is there enough oil? Does it only happen in water or can you run at high revs out or the water?
tcarroll
Posts: 265
Date Joined: 17/12/13
I ran it out of water last
I ran it out of water last night and seemed to be able to hold 4000 revs no problem and then backed it off to around 3000 revs and after a short time it dropped back to around 1700 revs
I grabbed this list of potential problems off a troubleshooting page so i will work through these!
11 Runs well, idles well for a short period, then slows down and stops.
A Weeds or other debris on lower unit or propeller.
B Insufficient cooling water (especially probable with inline).
C Carburetor, fuel pump filter or screens dirty.
D Bleeder valves or passages plugged.
E Lower unit binding (lack of lubrication or bent).
F Gas tank air vent not open.
G Not enough oil in gas.
H Combustion chambers and spark plugs fouled, causing pre-ignition.
I Spark plug heat range too high or too low.
J Wrong propeller (PreIgnition).
K Slow speed adjustment too rich or too lean.
Thanks Swompa
sstevee
Posts: 472
Date Joined: 15/11/11
IMO it sounds like a fuel
IMO it sounds like a fuel issue... If you've ever shut the fuel tap off on anything you'll notice how the revs rise just before it conks out.
Id try looking for a blockage or restriction in the fuel system/breather llines. If its like my bayhunter there will be a silver 90 degree breather which exits just under the gunnel on both port and starboard side. Check to see if the fine mesh is blocked, also check for kinks in the breather lines which may stop the tank from sucking in air.
There should also be a main filter/water seperator on the inside of the boat at the transom and maybe another snall in-line filter under the cowling of the outboard. Wouldn't hurt to replace both to eliminate them as a problem, they need to be done as running maintenance anyway.
Only other thing may be fuel pump is struggling. It might be delivering enough fuel at low rpm but when the carbs open up there's not enough fuel being delivered, causing the motor to run lean and increase revs until it reduces back to a slow steady rate.
tcarroll
Posts: 265
Date Joined: 17/12/13
Thats great help thanks
Thats great help thanks steve.
I will get on to that stuff tonight and over the weekend and let you know how I go!
crasny1
Posts: 7003
Date Joined: 16/10/08
Spot on IMO
That running lean increase revs then fall back to an idle = fuel delivery issue.
Not much else does that. A classic non boating example is the immobaliser on a WRX. When the fuel is cut off the engines revs up first because of the lean mix, and then dies.
Now as others have said follow the fuel delivery line from start to finish, and you should find the problem.
"I would like to die on Mars. Just not on impact!!" _ Elon Musk
carnarvonite
Posts: 8669
Date Joined: 24/07/07
Rubbish in tank
Check and see if the inspection plate on the fuel tank has been sealed with silicon.
If so ,there could be little balls of it running round the bottom of the tank, one gets lifted in to the suction pipe then as the revs drop and vacuum decreases it drops back to the bottom of the tank
Rig
Posts: 2925
Date Joined: 27/12/06
Throttle
Comments about the fuel are probably on the money. You should pull your floor up and have a look at the fuel line and if its in bad nick replace it plus check for any sinkers or loose screws for electrolysis. The primer bulb is a spot that can fail and also the fuel line attachment on to the motor. You may need to source this from a boat shop as the omc ones they sell at some boat shops are no good (I used to have a 40hp johno) boat city got the proper one for me.
another thing to check I'd your throttle adjustment on your forward controls. Your manual should explain it but there is a little toggle under your key switch that us adjustable.
My diagnosis is I think it could be your carby maybe semi blocked jets limiting the fuel that can get through. Depends how much time you have up your sleeve but this could be one for a mechanic. Replacing your fuel lines yourself is easy and could fix a blockage of leak and will give you piece of mind going forward
Olliej
Posts: 155
Date Joined: 27/08/12
is the throttle simply
is the throttle simply moving back towards neutral as you bounce over the waves? My old johno use to do it and only took a small movement to drop the rpm and fall off the plane. I'd guess that's why it's going 8-12-17kts as you get on the plane you pick up speed for the same rpm