90 HP Merc - runs then dies after throttle

[user0211]

I have a 1992/3 Mercury 90HP - (s/n 0D049608 m/n 1090412LD) - it sat up for about 2 years before I got it. Local shop has done the following:

-removed and cleaned carbs
-adjusted and sync'd carbs
-replaced fuel pump diaphragm

I have:

-drained all old gas
-refilled the tank with fresh gas and topped off the oil resevior
-replaced full line from tank including primer bulp and the fittting that snaps to the motor

Here's the problem:

The motor seems to run okay (maybe a little rough) but runs okay at idle or at trolling type rpms - when I give it throttle - gets up on plane eventually and then after about 30 seconds - just dies completely. I'll get it started again and it will idle and troll fine...but when I try to get her up on plane....about 20/30 seconds...then dies again. The motor also seems like i should have more power, but just not sure which direction to go at this point. The motor is on a pretty small 15 1/2 foot fiberglass bass boat. It also seems to run best after I've let it sit for a day or so....runs and idles okay, then when I try to get her going, she'll get up and then 30 seconds later, die. The main cranking battery needs to be replaced, but since carb and not efi, it seems this shouldn't be the cause of the problem - but please advise if so.

Can you guys give me a track to go down to find the problem? One last thing - I noticed it seems that there was some fuel / oil making it's way into the water at times - is this normal?

Any help is appreciated. Thanks.

 

[user0211]

Re: 90 HP Merc - runs then dies after throttle

Just checking in...anyone have any ideas...?

Thanks.

 

[Plainsman]

Re: 90 HP Merc - runs then dies after throttle

Welcome to iboats!! I'd start by checking spark, compression and fuel.

Spark should jump a 7/16th gap and be a hot blue
Compression should be over 100 on all cylinders and within 10% of each other.
Just before it dies, try priming the bulb and see if that helps.

 

[user0211]

Re: 90 HP Merc - runs then dies after throttle

Thanks for the reply. I don't really have the tools, etc to test compression, but I'll get my shop to do it. I'll try the other suggestions and report back.

Are you thinking fuel pump / carb or some other problem?

Thanks

 

[Plainsman]

Re: 90 HP Merc - runs then dies after throttle

Could be. sounds like when you go to take off it could be fuel starved. Others will be along with ideas as well.

 

[Shylock]

Re: 90 HP Merc - runs then dies after throttle

I just got my first boat with a 115 1980 merc and it's doing the same thing. Seems to idle fine and and even rev up in neutral, but starts a slow stall shortly after getting to plane.

When I pulled the fuel filter i found yellow crystals there (almost like sand). I opened the tank and found a rusting fuel sender and after draining it some more yellow stuff on the tank bottom.

I'm hoping after I clean this up things will be better. I'll keep ya posted.

 

[user0211]

Re: 90 HP Merc - runs then dies after throttle

Thanks guys. I'll check my fuel sender and filter as well. I'll report back.

 

[mwmotorsports]

Re: 90 HP Merc - runs then dies after throttle

I had the same issue with my 1985 Mariner 75 horse last fall. We took a trip to Canada and it started acting up while on vacation but kept running all week with a little feathering of the throttle. After I got it back home I checked out some of the basics and replaced the fuel line and primer bulb. No success. Took it out and it was worse. Got to the point of what you are stating about yours. As soon as the boat planed out it would slowly run out of fuel and die. Plenty of power getting up to speed but then simply nothing. So I was thinking the worst since I bought the boat used and thought some gremlin I wasn't warned about showed it's ugly head. So I asked around and was given a few tips to try, but no success without digging into the motor too deep. I found that on one outing when this was occurring that if I had my wife sqeeze the primer bulb in the midst of stalling it would come back to life. So I knew it had to be fuel related. I spent a few bux and bought a repair manual. Then one Sunday afternoon I attacked the entire fuel system and everything checked out ok until I drained the carbs and found there was quite an amount of water settled in the bowls of the carbs. I took off all the lines and blew them out with air, drained my fuel tank of all gas completely and reassembled everything with fresh fuel/oil mix and clean, dry carbs. Primed the bulb a few times and to my amazement it has been running better than ever since the cleanout. So after all I have said, hopefully if you spend some time to do the same it will also cure your problem. I saved myself a whole lot of money by doing it myself(I am pretty mechanical and this is my first boat) and felt good about the job well done. Mind you, I did not do anything extreme when checking everything out. Simply disconnected all lines at the point of connections, blew them out with compressed air and drained and flushed the carb bowls with a spray cleaner into the fuel intake where the hose connects to. If you're up to it, give it a try. I spent only about an hour of time on it and hooray for me it runs like new again.

I hope your problam is as simple and not being overlooked by your mechanic as some major issue.
GOOD LUCK!

 

[Shylock]

Re: 90 HP Merc - runs then dies after throttle

So I tried running the motor on a jerry can with clean gas as the tank is going to need a lot of cleaning. Seems to run well now however I dont know if there will be a difference on the water since testing was not under load.

But now I noticed the telltale was only sputtering water after running it WOT when it was really streaming out before. Is this a major issue or does it not really matter as long as water is coming out. I turned the ignition off and restarted at idle hoping the flow would resume but no dice. Just kind of trickling out now.

One thing fixed and now another acting up?

 

[Shylock]

Re: 90 HP Merc - runs then dies after throttle

Got the carbs cleaned out and she is running like a champ now. Changed out the original factory plugs from '79! Running clean gas from a jerry can and we had a great day on the lake. In fact the mechanic offered to buy the boat on the spot. (Only 75 hrs on the boat and motor) He said the GlasPlys are especially well built.

It looks like the sediment in the tank was sea salt that came in through the breather hose. This was then sucked into the carbs. The fuel filter was also plugged with it. Installed an after market fuel/water filter to avoid further issues.

The mechanic also said you have to becareful with fuel on the old Mers. Said they need a high octane due to the high compression of the engine (similar to old cars like cameros) however they dont make gas like they used to. Alcohol is used to arificially boost the octane number. That in turn can wash the oil off the cyclider walls. So he said to go with an 89. He also said to run the carbs dry by disconnecting the fuel line after each use. Said that fuel shouldnt be sitting for more than a month as well or you can get into issues with that.

Now i just need some sun on my days off!

Thanks everyone and good luck.

 

[wetdrive]

Re: 90 HP Merc - runs then dies after throttle

I have a 115 Merc that did the same thing. I found that while I was at full throttle and pumped the fuel bulb that it helped. I later found out that the check valve coming out of the gas tank was sticking. I took the check valve out and took a punch and knocked the ball and spring out. (there is already 2 check valves in the fuel bulb). Problem solved. The valve was sticking enough to not allow fuel flow at high RPM's.