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New 20hp 4-stroke will not run above idle

R

ruralcompanion

New member
MFS20C 4-stroke brand new out of the box. Took her out for a very disappointing maiden voyage today. I was all set to putt putt along for a good few hours of the break in period.

Motor starts easily but idles somewhat rough, although I've never had a 4-stroke so I'm not exactly sure how smooth it ought to be. Will idle in N and continue to idle in F or R gear, but engine will not increase RPM when throttle is applied. Many times when I tried to apply throttle it would just cut out and die. Sometimes it will keep running but only the very slightest increase in RPM. I idled up river for a little while but then decided it would be the smarter to just shut it off and ran the trolling motor back to the ramp.

When setting the engine up on the boat I installed a Sierra fuel/water separator between the tank and the primer bulb. The bulb primed up the filter and all the way up to the motor with no problem, the motor started on the second pull. So I don't think it's a fuel feed issue.

I bought the motor online so it hasn't had the pre-delivery inspection done. I'm thinking I should just take it in to my local Tohatsu dealer and have that done. But I wonder if there is something simple I'm overlooking so I thought I'd post here and seek advice. At the least maybe someone could help me with what the possible issues might be.

Thanks for any thoughts and advice. -James
 
James,

It could be almost anything, even a poorly-grounded or defective CD... but it's likely a fuel issue. If it's less than 60 days since your purchase, you can take the motor to any Tohatsu/Nissan dealer for a free PDI. There may possibly be a bit of leftover manufacturing junk in a carb passage, causing you to have restricted fuel delivery to the intake, even though the fuel is getting to the carb properly. The PDI should detect that. Regardless, you are under the 3-year warranty, so unless the problem was caused by "abuse", the service should be at no charge. If the nearest dealer is too long a trip to be convenient, you could pull the carb, disassemble it, and inspect for any tiny fragment of metal, etc., that might be partially clogging a passage... but again, you shouldn't need to do that -- that's why you have the warranty.

It's best to have any water separator as close to the motor as possible. Always best to have it between the primer bulb and the motor. Any filter in the line will present some flow restriction, and you don't want a vacuum in the fuel line to cause the primer bulb to collapse (which would effectively shut off all the fuel flow). Make sure that you lubricated the seal at the top of the filter cartridge, and snugged it up firmly. Otherwise, you could get a slight vacuum leak, allowing air into the fuel line. That could limit the available fuel, (contributing to poor high speed operation), and might make the motor run lean enough to burn a piston. Also confirm that you have the filter head IN and OUT connections properly set up.
 
Hi Paul,
Thanks a lot for the advice. I was thinking I might try bypassing the fuel water separator just to see if that had caused any issues. I probably should have run it without the filter first just to have a baseline. I guess it's off off to the dealer for PDI. I hope next weekend is as beautiful as this one was. With the luck I've been having lately I'll be running my break in hours in the rain! -James
 
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