1999 50hp 4stroke only 4000 rpms

[jimdd810]

I am working on a 1999 50 elpto 4 stroke 4 cylinder motor. Customer stated that it ran like s$#%. So I took the pontoon boat for a ride and discovered it idled fine but mid range (2000 to 2500 rpms) ran rough and only ran up to 4000 rpms. Customer changed his 19x22 prop for a 16x22 prop with no gain in rpms. I rebuilt all for carbs and test ran the boat today. Seems to be running real smooth all the way up to the 4000 rpm range. I ran it in my test tank (actual test tank) got it to nearly 6000 rpms witch the sticker on the motor said was max. It didnt matter if it was in gear or in neutral with the charging handle. It still went to nearly 6000 rpms. So i put it in the lake and only 4000 rpms. I did notice that shifting and control box was tight so i adjusted the friction on the control box and found it was all the way loose. Customer bought the boat and motor used and never was able to get it past 4000 rpms. He took it back to the dealer and they resynced the carbs 2 times and still no more than the 4000 rpms. I checked compression and spark both very good. I am leaning towards stretched control cables, do to the tight controls and louder than usual clunking when shifting. Oh yeah idols around 1000 rpm smooth.
Any ideas would be great.

 

[jimdd810]

Re: 1999 50hp 4stroke only 4000 rpms

any ideas?

 

[jimdd810]

Re: 1999 50hp 4stroke only 4000 rpms

bump

 

[Silvertip]

Re: 1999 50hp 4stroke only 4000 rpms

Geez -- take a look at the prop numbers. I doubt you can even install a 19 inch diameter prop on a 50 HP 4-stroke Merc without serious surgery. Moreover a 22P prop is about double the pitch that should be on that motor (16 or 19 diameter). Since RPM changes about 150 - 200 RPM per inch of pitch up or down, at 2000 RPM below spec you would need to drop prop pitch by at least five inches which puts you at 17P. On a pontoon that motor should have about a 14 x 11 prop, maybe 12 or 13P max depending on boat size. The reason you were able to get more rpm in the tank is because of ventilation. Stretched control cables are not likely either and are easy to check. Look at the throttle plates in the carb throat. At wide open throttle they should be perfectly parallel horizontal in the throat.

 

[jimdd810]

Re: 1999 50hp 4stroke only 4000 rpms

Silvertip after further investigation the sticker on the lower unit says 12x22 prop i discovered that this is actually the gear ratio which i think comes out at 1.83 to one. I went to the merc prop calculator and with this boat size and engine it should have a 10.5x12 or 10.5x13 prop. I happen to have one of these props in my shop and will put it on the customers boat and try it. He did say that the boat has never been over 4000 rpms since he bought it 2 years ago.
I feel stupid for not realizing it sooner but am learning each day. I also noticed that the motor seems a little low in the water and motor is mounted with the bolts in the top holes and is plowing quit a bit. I will check it tomorrow and see if it needs to be raised.
thanks for the help and I love this site.

 

[Silvertip]

Re: 1999 50hp 4stroke only 4000 rpms

What you also need to know is that a 1.83 gearcase Merc should not be used on a pontoon. Yes -- it is done all the time but that is exactly why Merc sells a Big-Foot motor in the 40/50/60 HP range. Dealers are not doing their customers a service by packaging pontoons with that motor. They do it strictly for the price point. The Big Foot engine has a bigger lower unit with a 2.33:1 gearset (in two strokes) and 2:1 on current four strokes and therefore can twist a bigger prop with much more blade area. A 13P prop on a 1.83 gearcase will very likely NOT help much. That combination is used on "planing hulls. I'm guessing you will neet to get down into the 9 or 10P range.

 

[jimdd810]

Re: 1999 50hp 4stroke only 4000 rpms

I agree. Ill get it figured out. I was working on a yamaha last week and its basicly the same motor. The prop was a 10p.