6 HP bogging down from half throttle onwards

Hello everyone,

Earlier this year, we purchased a Porta-Bote, a folding plastic fishing skiff type of thing. You can look them up online if you are interested - cool concept especially for those looking for car-top portability. We bought ours thinking we would tow it behind our 25' sailboat as a dinghy but we haven't used it like this yet.

Anyway we got a great deal on the Porta-Bote which included a 2005 model year Nissan 6 HP 4-stroke, standard shaft. The thing looked new, and the owner ran it in a big trash barrel full of water, putting it into forward and reverse.

He did mention that he'd never serviced the water pump / impeller. That was not exactly confidence-inspiring in terms of overall maintenance, but he did say that it had never been run in salt water, and that it had provided many years of reliable service, though it had only been used once or twice a year in the ten years he'd owned it.

Anyway we took our Bote out a few times and found that the motor started and idled well, and ran great in forward and reverse at lower speeds - say, up to about half-throttle. When we finally did have occasion to take it out where we could open it up, at half-throttle, the engine RPMs dropped way off, as if I had twisted the throttle grip closed, rather than open.

I experimented a bit with that and managed to actually kill the motor at one point. We were in the Long Beach harbor (Long Beach, California), and it was a little intimidating since we had to make a choice, row it all the way back in the channel into Alamitos Bay, or to the beach and then split up for one of us to stay with the boat while the other set out on foot to go get the car. Fortunately we were able to restart the motor and get back in.

Finally this week I received my carburator gaskets and I removed, disassembled, spray-cleaned (did NOT use true "carb dip" solvent), blew canned air through, and reassembled and installed the carb. I haven't had a chance to take it for a test run yet so perhaps my problem is now solved.


My brother, who is a mechanic and a boater, suggested I look at the timing advancement if the cleaning didn't do the trick. Not sure about all that!


I read through many posts in this Tohatsu/Nissan forum, and there seems to be a consensus regarding varnishing from E10 gasoline, which may be the culprit here since there was a tiny bit at the bottom of the float chamber. The float chamber gasket/O ring, which I also replaced, was a bit varnishy/gunky. I suspect the previous owner was not running the gas out at the end of his days, and certainly he had never disassembled it.


Any other thoughts here? I also read here about suggestions around the fuel tank line, how the line has O rings and check valves near the priming bulb that can dry out and allow air to enter the fuel line under higher RPMs, thus introducing air into the fuel.


Thanks in advance,


-Mark

The Porta-bote can be folded and then lashed to the lifeline stanchions. That way you don't have the liability of towing it. Many long distance cruisers do that. Many dinghies are lost when the painter parts or comes uncleated.

Most problems like yours are due to carb or fuel delivery issues. If you did not do a 4-hour soak in real carb dip, you have not properly cleaned the carb; however it may be somewhat improved. Canned air is usually much too wimpy to blow any organic crud from those carb passages. If you suspect fuel starvation, get the malfunction happening, and add a little choke to richen the mix. If the running improves, you have verified that you are too lean, and you can work to remedy that.

If there is a vacuum leak in the fuel line, allowing air into the fuel, that usually can be diagnosed by either the bulb not firming up, or "running out of gas" at higher RPMs, even though you have plenty in the tank.

There is no such thing as a "2005 model" Tohatsu/Nissan outboard. You probably read that date from an EPA compliance sticker. Odds are you have either an NSF6A2 or 6B.

Spark advance is not adjustable on these; it is controlled by the CD (or ignitor on the newer-style magneto). If your CD is retarding (instead of advancing) the ignition as speeds go up, then the CD is bad. That can be checked with a timing light. Very very unusual, but I am aware of one case where that did happen... however, it was on a new motor.

Also, since the service records aren't stellar for that motor, it's possible that the crankcase is over full (due to too few oil changes, etc.). That can cause your symptoms.

Probably a good idea to do the wp kit, as the pump is definitely starting to dry rot after more than 10 years.

Hi Paul, thanks for the reply! Yes, I did service the motor myself once I brought it home. It's now had a new impeller installed, plus a change of the lower unit gear oil and the engine oil up top. I'm beginning to suspect the fuel line as it is very stiff and seems dried out. I'll look at replacing that if my carb clean did not do the trick.

We use the Porta-Bote infrequently but I will post an update here once I have something else to share. Thanks again for the reply.

P.S. - re: model year, I have read that about Nissan motors on this forum. I guess it must be an EPA tag on the side of my motor that has the "MY 2005" but indeed, the Owner's Manual shows a printing date but doesn't explicitly state that it is for a "model year xxxx" motor. Is that because the design so rarely changes or is updated?

Unlike autos, Nissan/Tohatsu motors are identified and differentiated by model/serial. The same model could be manufactured for a decade, or several changes could be made in a single year.

Ah, good to know, thanks Paul. Model number is NSF6A2S - 3R4-1; serial is 050109XD

The 6A2 is the best model you can get, as long as you don't require front-mounted shifter; the carb idles well, and the cd is very reliable. Your serial number was manufactured in 2004.