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Mariner 45 Slowly Loses RPM/Surging

C

christopher425

Member
Hey Everyone! My'87 Mariner 2-stroke 4cyl (model# OB113850) runs idles fantastic ang great on startup, but after about 5 minutes at WOT the rpm's slowly drop and it begins surging causing the motor to shake. If I let off the throttle slightly it will smooth out and stop surging. It will keep doing this gradually reducing the rpm's. Primer bulb stays firm and gas tanks are vented properly.
After this happened on the lake today, we stopped and swam for about 30-45 minutes, then she fired right up and ran great again....but, the same thing happened like clockwork after about 5 minutes (rpm loss, throttle down, repeat cycle). It even stalled on me right when I finished trailering it.
I know this points to a fuel starvation issue. Carbs have been rebuilt with all OEM parts, cleaned thoroughly, and floats are set to spec out of the manual. This all started when I took it on the water a month ago to test out a new prop and I kept tilting the outboard into the highest tilt/trailering position so I could move adjust the manual tilt to test which position was best b/c I was by myself. It started doing it that day and has been losing rpm/surging ever since. I assumed the carbs got flooded. I thought by letting it sit, the excess gas would slowly clear out and it would run right again.
Thoughts??? I know it was dumb for me to keep tilting the motor over and over (hindsight is 20/20 🤦🏻‍♂️). This seems like a simple fix, just keeps eluding me. I need y'all's expert help here fellas. I've finally got this boat to run great and this is the only thing left for me to fix. What's some quick troubleshooting that could help me out? Throw out anything y'all think would help me out. Thanks so much!
 
Outboards need a new water pump impeller about every 5 years.-----Inspect exhaust tuner for blockage while installing new impeller.
 
racerone said:
Outboards need a new water pump impeller about every 5 years.-----Inspect exhaust tuner for blockage while installing new impeller.
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Took a quick look just now and saw gas on both carbs. Hooked up the fuel line and pumped the bulb. Fuel came pouring out of the bottom carb throat. So, probably the needle isn't sealing. Crazy thing is, these are brand new needles installed last year. Float should be level where the carb meets the bowl when upside down, correct?
 
Check the fuel supply line as the liner in it may have degraded. This would let debris get under needle and hold it open and cause a supply issue
 
faztbullet said:
Check the fuel supply line as the liner in it may have degraded. This would let debris get under needle and hold it open and cause a supply issue
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Yes sir, I sure will. I'm gonna take the carbs off and break them down tomorrow. I plan on polishing the seats and needle valves to make sure they glide smoothly. I checked them out last week and I remember the needles being a little tight when I disassembled them. I just dismissed it at the time. Also, I replaced all the old gray fuel lines with automotive fuel lines last year as suggested by others on here. Will be on the lookout for trash when I take it apart. Really appreciate help fatzbullet!
 
Alright, quick update...
Polished the carb seats and needles really well. Got the floats to spec (replaced one that looked suspect). Took it on the lake today and it still was surging at WOT. Not as bad this time and it took longer for it to first occur. Ran great otherwise. Got home just now and checked the carbs. Top one is sealing off correctly and looked clean. Bottom one still is having float/needle/seat issue. Fuel game running out of the throat when I barely primed the bulb. Just like it did before.
My next step would be to get another needle valve and replace the bottom one, but I put new ones in them not last year. What else could I do? These seats are pressed in and not replaceable. A new carb is next to impossible to find.
I checked the max timing and it's still in spec and the plugs looked good. Bottom 2 were fouled b/c of the excess fuel dumping, but that made sense after I saw the gas running out of the bottom carb.
Should I lower the bottom float some so it will shut off the fuel supply a little sooner? Kind of like an overcompensation procedure? Just replace the needle valve (again)?
What advice do you guys have for me?
 
I just wanted to post a follow-up to this thread b/c I know that step can sometimes be forgotten. Long story short, I bit the bullet and bought a used carb body for my lower one and cleaned it really good. The motor has never run better! No misses, surging, or loss of rpm. I actually gained 4 mph in speed and my rpm also went up to within the acceptable range as it should be. I really appreciate all the help from everyone on here and sometimes I suppose we just have to go step by step until we get our issue resolved. Slow down...think it through...and use your gut. That helped me out tons. Can't wait to get it out on the water in the spring. Happy & safe boating everyone!
 
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