1985 merc 90 hp 4800 rpm @ wot

[jake69442]

Ive got a 1985 merc 90 that only gets to 4900 @ wot, ive fixed all electrical problems ( all new parts ) have great compression, new electric fuel pump rated @ 7 psi. the motor is on a 1970 aquasport 19.6, the prop ia a 14.75 x 15. ive checked it with an inductive tach and reading is true, boat gets up on plane and cruses @ 31 mph. till i synked the carbs i was only getting about 4600 rpm. i still think i should be able to get to 5500 any one got any ideas

 

[swordfish25]

Re: 1985 merc 90 hp 4800 rpm @ wot

Need more info. Is the boat new to you ?, is it slow getting on plane ? if so it may need a different prop. See if you can borrow a smaller prop and try that. That is the best place to start.

 

[daveswaves]

Re: 1985 merc 90 hp 4800 rpm @ wot

Ive got a 1985 merc 90 that only gets to 4900 @ wot, ive fixed all electrical problems ( all new parts ) have great compression, new electric fuel pump rated @ 7 psi. the motor is on a 1970 aquasport 19.6, the prop ia a 14.75 x 15. ive checked it with an inductive tach and reading is true, boat gets up on plane and cruses @ 31 mph. till i synked the carbs i was only getting about 4600 rpm. i still think i should be able to get to 5500 any one got any ideas

With a 15p prop you should be able to spin it up to 5500, the diameter sounds off, I did not think a 14.75 would fit on that lower end, interference with cavitation plate. Its also possible that your fuel pressure is too high at 7psi and you are flooding the carbs. I run an electric fuel pump on my personal engine but also run a pressure regulator set at 5psi.

 

[jake69442]

Re: 1985 merc 90 hp 4800 rpm @ wot

its a 13.75 x 15 stamped on prop..ive only had the boat for 2 months. it gets right up on plane and goes well at almost 32 on gps ..i fixed quite a few other issues, wiring etc. all new electronics. im only going by what was on package for fuel pump psi i will check with a gauge, doesnt seem to be flooding out is it possible not enough volume @ wot..

jake

 

[daveswaves]

Re: 1985 merc 90 hp 4800 rpm @ wot

its a 13.75 x 15 stamped on prop..ive only had the boat for 2 months. it gets right up on plane and goes well at almost 32 on gps ..i fixed quite a few other issues, wiring etc. all new electronics. im only going by what was on package for fuel pump psi i will check with a gauge, doesnt seem to be flooding out is it possible not enough volume @ wot..

jake

You will need 4.5 to 5 psi to feed it at WOT and your fuel lines need to be large enough. Try running it on a portable tank and see if it makes a difference, you may have restrictions in the boats fuel system. 3/8 fuel lines are needed, especially if you have long runs of line.

 

[Laddies]

Re: 1985 merc 90 hp 4800 rpm @ wot

At 4900 RPM your right in the ballpark a inline 90 HP is rated at 4500 to 5000 Max. RPM and is ported to run in that range.

 

[daveswaves]

Re: 1985 merc 90 hp 4800 rpm @ wot

At 4900 RPM your right in the ballpark a inline 90 HP is rated at 4500 to 5000 Max. RPM and is ported to run in that range.

Thanks for chiming in Laddies, I agree completely, I am turning an 86 90 block at 5200 with a 17p and 4900 with a 20P, I just thought he should get a little more with the 15P. Thanks for the voice of sanity

 

[jake69442]

Re: 1985 merc 90 hp 4800 rpm @ wot

thanks very much for the help.. i just thought it should buzz up around 5500 rpm looks like im all set the motor runs great just thought i could squeez a little more

jake

 

[Crater]

Re: 1985 merc 90 hp 4800 rpm @ wot

agree with Laddies. Clymer Man. says range is from 4800-5200 @WOT so prop it accordingly

 

[sschefer]

Re: 1985 merc 90 hp 4800 rpm @ wot

It's hard to call your achieved max RPM correct without knowing a ton of information. It is important to stay within max RPM range but if there's a chance that you're under prop'd you should easily hit the top limit with throttle left over if the engine is in good running order.

There is one check that is often missed. Place your throttle control (at the helm) in wide open then go back to the engine and make sure that the max throttle stop is against the stop on the engine and the throttle plates on the carb are within .015 of wide open. If they are and the engine is running well, then you are properly prop'd and getting the best RPM's you can.

If not, check the throttle cable to see if it's prematurely bottoming out against the retainer on/near the attenuator plate mounting point. If it is, you can try adujsting it to get full throttle or if that doesn't work then you can trim some of the plastic off with a tubing cutter and small banjo hack saw. Typically you'll need to remove about 3/8". If you do this you really need to be sure that your WOT stop is set correctly or you could jamb damage your throttle plates. Under normal conditions you have to be sure of that anyway.

You might also find that your fuel line routed in such a way that it is binding with the carb linkage in some way. That can cause this type of symptom also.