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Discussion Starter · #1 ·
When I start my motor it will only run just a bit over idol speed for 10-15 minutes until it eventually will get up to full speed. This has happened the past 3 times Ive had the boat out. I thought it might have been bad gas so I got new gas, didnt help. After the initial time getting the motor "warmed up" it works better but still takes time to get up to full speed. It will be running slow then kind of starts surgeing a little bit then starts running fast.

Any ideas?

7,537 Posts
Fuel lines from tank to carb(s).
Airleaks, blocked filters, bad fuel pump, sealed fuel tank (vent not open)

My troubleshooting checklist from a previous post:
When an outboard acts up, it can be the most frustrating thing to deal with.
But the rules are simple...Air, fuel, spark, timing, compression, cooling, lubrication.
Any of the items listed above can affect how your outboard runs.

My outboards are carbed and my trouble shooting routine is this:

Check the kill switch...make sure it's not activated.
Verify the vent is open on the tank.
Quick visual check for spark at plugs, condition of plug tips. Clean if covered in crud.
Drain fuel tank, check for contamination by dirt or water.
Verify interior of fuel tank is clean, including pickup screen.
Air leaks between the tank and pump have to be eliminated.
That means every fitting from pickup tube to the fuel pump has to be inspected and tested.
Brittle or cracking fuel hose and primer bulb means time to replace.
O-rings in connectors may need replacing.
Verify hose integrity from connector to fuel pump, including last-chance filter.
Clean or replace fuel filters, check water separator.
Pressure test fuel pump to verify function.
Verify hose integrity between pump and carb.
Hoses under the cowl are exposed to heat-vibration and get brittle. Replace if needed.
Check intake vacuum pressure.
Check integrity of scavenger tube from crankcase.
A cracked or broken tube changes the fuel/air ratio.
Rebuild carb, verify every passage is clean and clear.
Sometimes bits of metal from the machining process make it into the carb.
Solvents won't remove those bits from the idle passageways.
A manual/visual inspection has to be done to ensure a true and complete cleaning has been done.
Replace the carb float, don't keep using the old one, they do leak and lose buoyancy.
Replace float valve and valve seat, vibrations will change the shape of the tip and seat, they won't seal properly.
After reassembling carb and installing, check linkages and throttle sychronization.
Verify choke settings and idle settings.
Run compression test.
Replace spark plugs, one of the easiest things to do to make an outboard run better.
Plugs don't last forever, recommended replacement is after 300 hours of use or 3 years, max.
Replace spark plug wires at 5 years due to resistance build up and cracking of insulation.
Run outboard at night with cowl off, check for visible sparks, including from under flywheel.
Check spark with a spark gap tester, easy to buy or make one.
Obtain a volt-ohm meter and learn to use it.
Use the meter to test the ignition components, specs are in the OEM manual.
Coils, CDI controllers, sensors, stators have specific resistance readings at each wire connection.
if readings are outside OEM manual parameters, replace.
On older outboards, check ignition timing.
Check throttle cables for fit. Any play or looseness can result in low rpms on the water.
Ensure proper oil levels are maintained in 4 strokes, proper mix in 2 strokes.
Check water pressure exiting block, check thermostat, poppet valve.
Check engine overheat sensor. (again, info in OEM manual )

Lots of steps there, but it's what I have to do sometimes
in order to find the problem...or in some cases, problems.


New outboards have brain boxes that record engine data
and that info can be read by a computer connection.
I'm not looking forward to having to buy that software.

Brandon, FL
12,583 Posts
I would not be looking for an obstruction because you mention that it eventually gains rpm's and it takes less time when warm. My guess is that the little gasket around the needle vlave in the float is rotting and turned to a gummy substance and is sticking.

Take the bowl off and remove the float and the needle valve and remove the o-ring and reassemble and start. If you get rpm's right away that was your problem. Your motor will not idle down or run right with this piece out, but it may give you a starting point. Some carbs have a rubber tip on the valve - if this is yours then clean it well and spray some cleaner where it seats and reassemble and test. Either way it is a 10 minute experiment.

The o-rings are made from two types of material; neoprene and rubber. Neoprene rings are not long lived with gas.
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