Yamaha 250hp 2 Cycle - Bogging Down?

jeff6645 · #1 06-13-2010, 11:02 AM

Looking for some help on solving a problem I am having with my 1993 Yamaha 250hp 2 cycle engine, thought someone on here may have had the same problem or knows what can cause it. I tried searching other sites and the answers ranged from trash the motor and start over to bad fuel.....

Here's what happing, it starts fine, idles find but when you give it fuel is starts out as normal then about 10 second later it start slowing down (bogging) sometimes to a idle sometimes just drops a few RPM. After a few minutes it clears up and runs fine. This is what I have tried, new gas/water separator filter (no water found in gas). New gas lines, removed an after market auto filter that was in line before the carburetors, new plugs. OEM bulb primer pump (old one had smaller entry & exit ports & was sucking flat). Seafoam, Deep Creep and another fuel product that they advertise on Sea Shape TV(can't remember the name).

When I pull the plugs they all look the same in color and wetness. I really think it's a fuel problem. If all else fail the next step is to rebuild the six carburetors. But if it is the carburetors wouldn't one or two plugs look different and the problem consistent?

Thank you for your suggestions.

jwclbi · #2 06-13-2010, 11:23 AM

Jeff, if you think you have suspect fuel try running off a seperate 6 or 12 gallon tank. I just had similar problems with my engine and when I ran on six gallon tank the engine ran great. Also I have had luck with a quicksilver product called power tune in an carburated engine a couple years ago. Hope this helps and good lick.

castalot · #3 06-13-2010, 11:26 AM

i had a simmilar problem with my two engines started fine then when they warmed up the would basically die down to about 1200 rpm found out the gas i bought with the boat was bad when the engine warmed up there wasn't enough kick in the gas get a 6 gallon tank put fresh gas in it and see what happens if the same thing you will know it's not the gas that's what i did and it ran fine pumped out the gas put in startron and filled up and it hasn't done anything except purr

Old'sCool · #4 06-13-2010, 02:11 PM

The bulb sucking flat could indicate a blocked (or partially blocked) vent. If the vent hose isn't alcohol resistant it may be the problem.

BigLew · #5 06-13-2010, 05:54 PM

I had a similiar pfoblem with an I/O years ago. It would idle fine and then either shut down or refuse to accelerate like it should. It kept getting worse as the days went by. Finally talked to a mechanic and he said to check the float chambers of the carb for water. Sure enough. There was enough water in the chambers to preclude anything more than a trickle of fuel across the water's surface and into the engine. I suspected that when it shut down it actually sucked enough waster to simply kill any combustion.

Good luck.

jeff6645 · #6 06-13-2010, 06:31 PM

Thanks guys, I am going to try the 5 gallon fuel tank idea if I can get off work early enough one day this week. Also going to drain the bowls again and put in another set of plugs.

Just got back from running it the bay, it bogged down a couple times when we 1st got there. Coming back in it ran like a new engine until we had to slow down for a "NO WAKE" zone, that was it for the day it would never even get enougn speed to get back up plane again... if I had extra $20k setting around I would re-power it tomorrow.... would save enough in fuel to pay for in a couple of years...

Thanks again for your help! I will let you know what I find.

eggsuckindog · #7 06-13-2010, 11:01 PM

all I can say is isolate and eliminate - it works - try the different tank that isolates and eliminates the tank and fuel lines as well as the vent - save your money on another set of plugs that ain't it. Does sound like a fuel issue though but if you have drained the carb bowls and found no water anywhere I wouldn/t bother doing that again either - you eliminated that unless you found trash in them.

rebuilding carbs is nothing more than cleaning the bowls out - you did that - and new gaskets and a new needle and seat, I would pull the air box and make sure the carbs are not overflowing fuel which is eliminating the needles and seat. Do not run WOT without the air box on it - it will lean it out too much, just look for flooding carbs

lost2a6 · #8 06-15-2010, 01:57 PM

Pull the carbs down completely and blow/clean every nook and crany out with carb cleaner and compressed air.

ReelToy · #9 06-18-2010, 12:38 PM

Check your pickup in tank ethanol is breaking loose a lot of old sludge in gas tanks. Screen in tank could be sucking up trash intermittently. Also gas hoses have been separating inside and collapsing another ethanol dilemma. Especially the gray hose. After that have carbs rebuilt by a competent mechanic.

jeff6645 · #10 06-27-2010, 11:42 AM

I am back... work has kept me tied up.

Ok made some progress; after trying the "fuel can" idea was able to rule out a fuel delivery issue.

Pulled the carburetors, cleaned and set the pilot screws to 5/8 as the manual stated (they were set @ 1/4 to 1 turn). Cleaning the carburetors made a world of difference no more bogging down. But after idling back through the no wake zone developed a miss and lots of smoke and gas smell. Pulled the plugs, 3 of them were soaking wet. Changed them and adjusted the two pilot screws I could reach to a 1/4 turn from a soft seat. Ran much better. Got home and adjusted the other carburetor to a 1/4 turn and took her out again. Almost back to normal but still not 100%, pulled the plugs and they look pretty consistent in color and wettness... scared to tweak the plot screws to much.

Also checked the plug wires getting 7.2 - 8.4 ohms resistance on them....

Now running around 4800 RPM tops (41mph)... was around 5200 RPM (45 mph) before the bogging down issue started. But not complaining.