1988 merc 90hp can't break 4000rpm or plane

[k_loy]

I've adjusted timing, mixture screws, checked compression, fuel flow, and spark and running out of options here. The best I've had so far was about 3800 rpm. It won't plane at all. I have a service manual to refer to but still can't pinpoint the culprit. Also it has some minor carbon build up but nothing that I would think could cause it to run like it does. Either way I'm in the process of burning it out. Can someone help please it's driving me crazy.

 

[Mi duckdown]

check compression. what type boat? more info.

 

[k_loy]

I've checked compression. All 3 cylinders are 100+. It's a 16ft bass boat.

 

[racerone]

Timimg normally does not need adjustment on these engines.--------Did you just install it on your boat ?---Have the correct prop ?--------Perhaps remove the exhaust cover to look at pistons , rings and cylinder sleeve.

 

[Silvertip]

Is it actually running all three cylinders?? If one is dead, it won't perform. What do the plugs look like? Have you changed them? What's the history? New engine to you or did this situation just pop up? Carbs opening fully??

 

[k_loy]

I just got the boat on a trade. As far as I can tell it's running on all cylinders as it sounds to run smoothly with an occasional miss when accelerating or even idling in forward gear. I haven't changed the plugs but they don't seem very dirty or even very old but I suppose it wouldn't hurt. As for the prop it meets factory specs minus 2 pitch. Carbs are opening fully I've synced and adjusted all throttle linkage and timing. They were supposedly recently taken apart and cleaned according to the guy I got the boat from. I just took ok it out today and sucked something into the water pump making my way thru some shallows and it's not spitting any water out so now I have to tear into that as well.

 

[k_loy]

I'm honestly just losing hope and can't afford to pay $90 an hour for someone to work on it for me.

 

[racerone]

You might want to remove the exhaust cover to inspect as suggested.--If nothing wrong it costs you a few gaskets and you will learn a lot about the motor.---------Checked the flywheel key ?

 

[k_loy]

Not sure what the flywheel key is. Completely new to working on boat motors and just kno the basics of working on cars.

 

[racerone]

A small woodruff key that positions the flywheel for proper timing.-You say that you adjusted the timing and how much was the timing out.---Timing does not normally change on these motors.---Inspection and trouble shooting is required here.

 

[k_loy]

It was just slightly off but I set it per the service manual

 

[Mi duckdown]

fiberglass boat ? DRY? is boat water logged?

 

[k_loy]

It's fiberglass and to my knowledge not waterlogged....tho not sure what u mean or how to tell

 

[Silvertip]

prop with too much pitch. You guess it's running on all three cylinders???.

 

[quicktach]

Tried changing the trim angle?

 

[bullbat]

Make sure that the Timing is advancing when you give it throttle if the time doesn't advance all the way it will run like crap

 

[k_loy]

Timing is advancing. #1 is dead It would seem. Got the boat to plane though, gets to 4200 rpm on plane but my question now is why do I have to put all the weight in the front of the boat to get it to plane? Related to running on 2 cylinders or a different problem?

 

[enginepower]

I have a 90HP on my 15ft fiberglass. Lost a cylinder due to a switch box failure and could not get it to plain no matter what I did. You could put a timing light on each cylinder to make sure they are all sparking and as asked, what do the plugs look like?

 

[k_loy]

Well they are all firing but #1 has no compression. It will plane I just have to put all the weight towards the front to get it to do so. When it finally starts to pick up the engine pitch changes almost like it kicked into gear....it's weird.

 

[k_loy]

Oh and I just put new plugs in it last weekend. Also I have tried playing with the trim angle and it doesn't help anything. If I just level the motor out and stand in the front to where I can just hold the wheel it will plane slowly