1995 Mercury 30/40 HP Troubleshoot

[aselle]

I bought my first boat this month. It has a 1995 merc 30/40 hp, auto start, tiller. The guy who sold it to me bought it as a 30, said he upgraded it to a 40 hp by changing out the carb. The engine runs ok in idle, a little jerky instead of smooth, but when I get up on plane it runs good but cuts out every 5 seconds or so, the boat slows down then picks right back up to speed. Sometimes its more frequent than every 5 sec, other times its less. So I thought maybe fuel prob, but I am getting good fuel into carbuerator, which was just rebuilt 2x in 5/06 and 11/06 (according to the previous owner) so doesn't seem to be the problem. The top spark plug had a little oil on it when I removed it, the bottom plug was completely oiled up indicating a rich mixture. Anybody have some ideas where to start troubleshooting? Is there something in converting up from a 30 hp besides just tossing on a new carb that might be causing this? Thanks in advance for replies.

 

[jimmbo]

Re: 1995 Mercury 30/40 HP Troubleshoot

Did the previous owner give you the original carb? There might be additional parts that need changing to attain a 33% power increase. Quite often things like manifolds, reed assemblies, engine blocks and cylinder heads, exhaust tuners also need changing. Since the bottom cylinder is badly fouled I suspect a very high fuel level, or a very weak spark on that cylinder.

 

[aselle]

Re: 1995 Mercury 30/40 HP Troubleshoot

No he didn't give me the original carb. I called the place that serviced it last and got a record of the work they had done to it, thats when I found out about the carb rebuild in May and Nov of last year. He said he called mercury and checked everything out that he needed to upgrade to the 40 hp, but you know how that goes. Maybe I should just put a 30 hp carb on there and see what happens.

As for the spark issue, I can adjust the mixture in the carb, but since it isn't really affectiong both cylinders, it may be the spark. how do I test taht?
Thanks again for replies.

 

[hammyloz]

Re: 1995 Mercury 30/40 HP Troubleshoot

asselle get a spark tester i got one off of ebay for 3 english pound inc delivery very handy to have

 

[blimkie]

Re: 1995 Mercury 30/40 HP Troubleshoot

No he didn't give me the original carb. I called the place that serviced it last and got a record of the work they had done to it, thats when I found out about the carb rebuild in May and Nov of last year. He said he called mercury and checked everything out that he needed to upgrade to the 40 hp, but you know how that goes. Maybe I should just put a 30 hp carb on there and see what happens.

As for the spark issue, I can adjust the mixture in the carb, but since it isn't really affectiong both cylinders, it may be the spark. how do I test taht?
Thanks again for replies.

I have a 95 30/40 two stroke and did the upgrade myself after research confirmed by Mercury rep. Replaced carb for 40hp version which has larger jets. You also have to remove the "reed stops" at the base of the carb. A must or you gain nothing except burn more fuel.