Yamaha 40 running poorly

liebman · Sep 19, 2011

Im new to Iboats, but could use some help. I have a 98 yamaha 40 that I have owned a few months. Out on the water the engine seems to start dying while going about 3000 rpms. Changed plugs, cleaned carbs, cleaned fuel pump, and changed primer bulb. Now engine starts fine (as usual) and runs for about 30 seconds and cuts off. Starts right back up and runs about the same and cuts off. Plugs look a little damp, but not saturated. I do notice that it seems to me to have a good bit of smoke, maybe normal though. If anyone has any ideas, will be glad to get the info, thanks

robert graham · Sep 19, 2011

Re: Yamaha 40 running poorly

May have a bad fuel pump diaphragm causing raw fuel to get sucked into that cylinder causing very rich mixture, wet-fouling plug. Remove pump from motor, leave lines hooked up, sqeeze primer, check for fuel coming out back of pump. New pump is $50, very easy 2 screw replacement. In fact, I replaced mine this year just because it was 12 years old...now I have a spare! Good luck and let us know what you find!

liebman · Sep 19, 2011

Re: Yamaha 40 running poorly

Thanks, I went and checked that and there is no fuel leaking out the back side, is leaking a little around the gasket but I think that is where I took the two mounting bolts out. It was not leaking there before. I have had this fuel pump apart before and the diaphram looks free of holes but is a little stiff.

robert graham · Sep 19, 2011

Re: Yamaha 40 running poorly

liebman said:
Im new to Iboats, but could use some help. I have a 98 yamaha 40 that I have owned a few months. Out on the water the engine seems to start dying while going about 3000 rpms. Changed plugs, cleaned carbs, cleaned fuel pump, and changed primer bulb. Now engine starts fine (as usual) and runs for about 30 seconds and cuts off. Starts right back up and runs about the same and cuts off. Plugs look a little damp, but not saturated. I do notice that it seems to me to have a good bit of smoke, maybe normal though. If anyone has any ideas, will be glad to get the info, thanks

Is your fuel tank/system venting properly? When motor starts to cut back, try pumping the primer bulb, if it improves running then maybe a weak fuel pump? Is your fuel mixture fresh, not left over from last year? Do you have a 50:1 fuel/oil mix? Are your carbs really clean on the insides/jets, etc.?

liebman · Sep 19, 2011

Re: Yamaha 40 running poorly

I checked the tank vent and it is open(plastic 6 gal tank). Is suppose to be a fresh tank of gas (per previous owner). I did try pumping the bulb and it continued to run, but rough. Carbs have been removed twice and were clean both times. Removed jets and float seat and cleaned with carb cleaner and air pressure. I removed what I think is called the choke solenoid, which is on the center carb. Hooked it to 12 volts and the stem part gradually became longer, which is the way I think it works? Not sure if there is a choke problem. Didnt want to keep running it and cause more problems, thanks

robert graham · Sep 19, 2011

Re: Yamaha 40 running poorly

The "runs about 30 seconds and cuts off" sounds like a fuel delivery, venting or restriction issue. If motor continued to run while pumping bulb, that would suggest a bad or weak fuel pump. After running motor check that #2 spark plug for very wet condition caused by bad fuel pump. There's no choke, just the Prime Start unit on #2 carb. Maybe start checking for healthy spark and compression on each cylinder.

liebman · Sep 19, 2011

Re: Yamaha 40 running poorly

I will check it out some more, thanks

99yam40 · Sep 19, 2011

Re: Yamaha 40 running poorly

sounds like you need to clean carbs better and do the link and sync like manual says after reinstalling. pilot jet/passages are plugged

liebman · Sep 20, 2011

Re: Yamaha 40 running poorly

Do you have a link that shows how to sync the carbs? Found a loose pickup in the fuel tank today, replaced pick up tube in tank and installed a new fuel pump. Tried to sync carbs and engine seemed to run better, just not sure I am doing this right.

99yam40 · Sep 21, 2011

Re: Yamaha 40 running poorly

I believe Rodbolt gave me some info on how he does it when I asked about clarification
could have been on this web site do a search of the threads I started to see if you can find it here first

liebman · Sep 22, 2011

Re: Yamaha 40 running poorly

I just wanted to say that I am new to this site but I see a lot of family atmosphere here. Very impressed (thanks). I think I may just sell my boat and start playing tennis or another hobby, (want to at times). Well I put a new fuel pump on my yamaha 40, fixed a pick up tube problem on the tank and tried to sync the carbs. 99yam40 suggested the link and sync(think this may be my problem) and after trying this it ran good. Tried to set idle and made it run worse. Just wondering if I may be doing this wrong.Thanks..

99yam40 · Sep 23, 2011

Re: Yamaha 40 running poorly

I do not know if you did it wrong because we have no idea how you did it.
Did you look at the info from Rodbolt I talked about ?
http://forums.iboats.com/showthread.php?t=366637

I had a problem with the idle timing on mine being out of spec (too retarded 14 ATDC). Had to replace CDI to get it into spec.
It would be a good idea to check you timing with a induction timing light just to make sure

liebman · Sep 24, 2011

Re: Yamaha 40 running poorly

Tried to do like Rodbolt explained. I still have it running rough and popping. Kinda like timing may be off. Got any ideas on setting the timing or do you think it may be something else. Thanks

liebman · Sep 24, 2011

Re: Yamaha 40 running poorly

I removed the carbs again today. Looked great inside,cleaned with carb cleaner and air pressure, no deposits or trash in carbs. Reistalled, link and sync carbs, and set idle. Ran good with some light rough idle but choke does not seem to work like it did before. It would run a litle bit faster rpms and then go to idle before. Thanks...

liebman · Sep 28, 2011

Re: Yamaha 40 running poorly

Thanks, I think I may have figured it out.

99yam40 · Sep 28, 2011

Re: Yamaha 40 running poorly

would you like to share what you figured out?

liebman · Oct 1, 2011

Re: Yamaha 40 running poorly

I guess the main problem was the link and sync. Still have a rough idle though, Thank you all for your advice and info.

Yamaha 40 running poorly

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