2003 Mercury 90hp (3 cyl) - Won't plane

[kaysdad]

Help!! I have this motor on a Nitro 750. Was running along great and just fell off plane. It revs up fine out in neutral but under load will not rev over 2800 even at full throttle position. Since this I have replaced the bulb, fuel filter, fuel lines, intake gaskets, inspected and blown out the carbs, run cleaner thru carbs while running under load, chocked out each carb while running, switched out power packs, checked spark at the plugs, checked compression (low to mid 120's), and installed a new stator. Still no joy!! Next I am going to refit the fuel pump. Does anyone have any other thoughts?? I was SURE of the stator!

 

[Silvertip]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

What gave you the idea it was the stator? If you have good spark at all three cylinders you don't have a bad stator. I didn't see where you replaced the plugs. I didn't see where you checked for water in the fuel. I didn't see where you checked the plugs to see if they all looked the same or if one or two were different looking than the others. I didn't see if you checked the carb butterflies to see if they were perfectly horizontal in the carb bores when the throttle was wide open. If one was in a different position than the others, that would indicate a linkage problem. These are no-cost checks. Only if you didn't have spark on one or more cylinders would one suspect a stator. And just so you know, any engine will rev to the moon with no load on it and running on half it's cylinders. Do diagnostics, -- don't just throw parts at the problem. If all you did was spray carb cleaner into the carb with the engine running, all you did was clean the bore on the carb. The cleaner didn't get into the inner workings of the carb. Looking at carbs when disassembled means nothing. They need to be totally disassembled and soaked, then blown out to ensure the small passages are clean.

 

[DJ]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Your spark plugs can tell you a story. Do they all look the same?

Do this:

With the engine running, spray some pre mix (50:1) ratio fuel/oil into each carb throat, one at a time. That may tell you which cylinder is not pulling its weight.

Use an empty spray bottle to do this. Use one with good atomization.

 

[Texasmark]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Before you change out a possibly good fuel pump, you can insure that the squeeze bulb is firm as you are attempting to get on plane. You can use that in lieu of the fuel pump to feed gas to the engine and determine if it is doing it's job.

I have the same engine. I have had a nagging problem for some time where the first planeout of the day was a slobbering mess. Also when idling out to the end of the no wake zone there seemed to be a lot of vibration. And lastly, starting was time consuming, the colder the weather the worse (course I expected some of the weather related problems). On attempting to planeout from the zone, the engine staggered and stuttered and then finally cleaned out and went about it's business for the rest of the day.

I found that I had lost spark to the lower cylinder till the engine warmed up. Replacing the CDI module solved the problem. Point here is that engine does not run well on 2 cylinders.

Mark

 

[Willyclay]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

You mentioned reving the engine in neutral. You need to limit that to no more than 1500RPM's or you might destroy it since there is no load on the prop. Been there, done that! Also, just went through a similar sounding problem with a 2004 Mercury 60HP 3-cylinder. It was clogged main jets. Was able to isolate the problem by making an "in the water test". After warm-up, try WOT in forward and activate the Choke/Primer switch. If engine catches and runs close to normal, then one or more of your main jets are probably clogged. Did NOT have to remove the carbs on the 60 due to the side location of the jets. We used carb cleaner and a needle to open the jets. CAUTION: wear eye protection if you use aerosol spray because it will vent out the mouth of the carb. Good luck!

 

[kaysdad]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Well, I guess I should say thanks...for putting me in my place!! I feel like such a ...a....a...oh yeah! a Cadet.

Anyhows.....When you say to check the butterflies at WOT, is that with the engine warm? I just checked the butterflies and while they are all in the same position, they are not horizontal. But the engine is stone cold. Is this because of the cold engine?

Again thanks for the advice.

 

[Silvertip]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Nobody intends to "put you in your place" -- just trying to keep you from spending money that doesn't need to be spent until you prove that it needs to be spent. When you visit the Doc the first thing they do is check your weight, height, pulse, temperature and blood pressure. Motors are sort of the same way. They require fuel, air, compression and spark and everything needs to happen at the right time. It takes a mere couple of minutes to pull the spark plugs to see what they look like. They should all look the same -- or at least very nearly the same. They tell a story. So you see, it didn't cost a dime and can point you in the direction you need to go. The other tests lead one in other directions depending on results.

The position of the carb butterflies has nothing to do with engine heat. You open the throttle wide open (engine off) and the butterflies should be wide open -- as in perfectly horizontal in the bore of the carbs. And they should all be that way. If they go over center they begin to close off air. If they don't open all the way the engine can't make full power either. If they aren't right the engine needs a link & sync (linkage adjustment) and you need a service manual to perform it correctly.

 

[MikDee]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

By the way, the engine has to be shifted into forward gear to get the throttles (carb butterfly's) to open completely, with the engine shut off. It doesn't matter hot, or cold.

 

[kaysdad]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Thanks for all of the advice. It looks like a link 'n sync is in the forecast. Are the "Seloc" manuals a good bet? Or would you recommend something else? For vehicles I have found that nothing is quite as good as the "Haynes" manuals, is there a marine equivalent?

Again, thanks and I will keep you all posted on my progress.

 

[Silvertip]

Re: 2003 Mercury 90hp (3 cyl) - Won't plane

Clymers and Seloc seem to be the generic versions but nothing beats the factory manuals which are specific to your engine. Order them on-line at Mercury Parts Express. Or check E-bay. You want a manual "specific" to your engine. Remember, you need to start the process at the beginning and follow it through. Can't start in the middle and expect it to be right. Just make sure you've done the other trouble shooting before launching into something that may not be the problem. Are you sure you have spark to all three cylinders? What do the plugs look like? Have you squeezed the primer bulb to see if that helps get on plane? Give us some information and we can help. Without it we can't and this thread only gets longer and longer with "what if's".