Nissan NS40D2 Excess fuel consumption, flooding, and rough running.

[siddobson]

40HP on a pontoon. Recently purchased and trying to get it running good. It would start immediately when cold without choke. First indications were max rpms about 4200 but every once in a while would jump up to 5000 wide open. When it did this it sounded good and had good speed. Usually it would drop back down to 4200 shortly. After experiencing the 5000 rpm mode that's what I wanted and expected. Idling and slow speed running was not smooth. Initially I thought it was running on two cylinders and sometimes the third cylinder would kick in. Turned it in to the shop and they found a few problems. Filter leak repaired, plugs replaced and the carburetors had kits put in them. Reed valves looked good when apart. It uses an excessive amount of fuel, like a gallon in about 8 minutes. Out of the shop and symptoms are the same. The mechanic took a ride and adjusted things and now believes the culprit is the fuel pump. He said the carburetor kits he put in did not have anything for the fuel pump. He's ordering a pump or kit now. He says excess fuel is in the carburetors. Is this possible for the fuel pump to force too much fuel? I hoping he's right cause I want to go fishing without worrying about getting home.

 

[Sea Rider]

Welcome to Iboats,

Could be anything from badly adjusted float heights, worn float valve tip, ignition coil shorting when heating up, bad spark plug cable or plug cap. That engine revs 5000-5700 at wot. At 5000, you're lugging the engine. Once it's ailment has been found a prop maximization will be mandatory to be revving near 5500-5700 rpm for best engine performance.

The only fuel pump component that deteorates with time and use is the rubber diaphragm in which case will need to be constantly priming in order for engine not to drop dead while speeding. Is the fuel filter correctly screwed in onto base, that's with no fuel spills outing through cap ?

Being a 3 cylinder engine must have all 3 carbs spot on clean & adjusted for a smooth operation throughout the whole throttle range provided that there aren't electrical related issues.

Happy Boating

 

[pvanv]

Revisit the carbs and the whole fuel system. Then understand that your motor consumes more than 4 gallons an hour at speed.
http://tohatsu.com/tech_info/fuel_consumpt.html

 

[Sea Rider]

You have a tech issue besides engine's ailments, that 40D2 is powering a pontoon boat which has more hull drag compared to other hull shapes. That engines consumes 4.5 gal per hour at wot. This fuel consumption data is with a boat/engine combo running on flat no wind water cond at it's max stated rpm, in your case 5700 rpm. Running full wot at only 5-5.2 K rpm expect more fuel consumption.

If you dial a proper prop pitch that will rev up your lightly loaded combo to its max 5700 rpm, will consume less fuel with reduced throttle at same speed compared to what you're achieving right now at wot. Well propped, your engine will run much better and safer throughout the whole throttle range. Once there, 1/2 to 3/4 throttle gives the best fuel rate consumption while enjoying the ride, wot is $$ guzzler.

Before disassembling carbs again, try interchanging first or second cylinder small ignition coil to third cylinder if you assume it's not working properly. If it works flawlessly, that's your culprit. I'm a firm believer that you should always stock a spare new ignition coil, you never know when will stop working efficiently.

Happy Boating