70hp Johnson high RPMs need advice

[nim713]

Have a 1999 70hp Johnson on a 2000 16ft Sylvan Expedition. Replaced the spark plugs and cleaned out the carbs with carb cleaner. Took it out today and starts and idles great. It will not slowly accelerate, it will delay and then speed up. If you punch full throttle it will get on plane fast but will only go to around 27 and RPMs are off the scale, like around 8000. I had no alarms going off. When I got home i pulled the plugs and they look odd, so Im just looking for thoughts or info. Im trying to trouble shoot before getting killed by the mechanic.

 

[emdsapmgr]

First off, you need to make sure your tach is set on the correct setting. The movable arrow on the back of the tach must be set to the correct number of poles for the alternator on that engine. If it's on the wrong setting, you will get these unusually high readings. Also possible you don't have enough pitch on the prop on that pontoon. What pitch prop are you running currently?

 

[jakedaawg]

Is this a new boat and motor to you or did it previously run properly?

 

[oldboat1]

Said in an earlier thread you were doing a manual mix -- appears to be too lean.

 

[nim713]

Its acutally a deep V boat emdsapmgr but I did think of the pitch. The prop is a 14x11. After I wrote the post last night I did realize that the tach has to be adjusted, my previous motor was a 2 cylinder 60hp. So I am going to adjust that. It may be to lean, but I have noticed that the lower to spark plugs look good, it is possible the top isnt getting enough fuel?

 

[oldboat1]

Believe you were working on conversion of VRO to manual mix, and replacement of wiring harness(es). Can't address the conversion (others can), but could in effect have a fuel pump problem. You were also replacing a warning horn(?). Is that complete and working? What mix are you running?

 

[nim713]

Yeah I disconnected the VRO and went with a manual mix. I put in all new controls and horn with a new system check and I wasnt getting any warning sounds. I was running a 50:1 mix but I was told by someone when you disconnect the VRO you need to go to a 25:1 mix or it would be to lean? Truth to that?

 

[oldboat1]

all VRO related questions. If the warning system is related to the VRO (low oil), it would be irrelevant. If an engine temp horn, should check it independently to make sure it's going to work if needed. And I really doubt that advice -- makes no sense if the system is not metering the mix. 25:1 is going to be too rich if that is what goes to the engine, although the plugs look to me like a lean condition. Is the water pump working? Engine temp OK? You should measure that independently, as the gauge and harness could be suspect until readings are confirmed -- assuming you have a temp gauge.

You took a compression check earlier, so have a benchmark. I would check it again now after running and see if there is a difference.

 

[racerone]

Well an 11" pitch prop on a 16' boat is the wrong prop !!!!!!!

 

[oldboat1]

^^^ha! over revving.

(edit.: nim713 -- agree with Racerone and Emdsapmgr, although understand you are working on the tach. Think a more standard prop would be something like 13 1/4 by 15 or 17. This mechanic you mention -- chance of borrowing a prop to test?)

 

[nim713]

oldboat1 the water pump is working great, has a strong stream coming out. Engine temp is normal as well. I did change the tach gauge because I didnt realize it had to be readjusted. It idles now from 1500 down to 700 hooked to hose, so I dont know what top end will do just yet. I rechecked the plugs to and they actually look good now after running it for awhile. Im starting to look at it just needing to be tuned up properly. racerone, what would be the ideal prop for mine. I just know that was the prop that came with the motor.

 

[oldboat1]

sorry -- think I was editing while your were posting. Maybe you can borrow a prop to test -- would think rpms at top end would be something like 5500 or 6000, probably with the same or better speed. Be good if you could try out a different prop first.

 

[nim713]

oldboat1 yeah id like to try a test prop for the pitch. Which may explain a few things about why the top end was only 27mph on GPS. I contacted the person I bought the motor from that the motor was originally on a pontoon. Something he neglected to mention and I honestly didnt look at the prop. Im looking to gain more speed, I fish a big bay at times and have to get off the water quick.

 

[oldboat1]

Maybe the PO has an earlier prop (assuming it was repropped for the pontoon). Probably unlikely, but never know. Sounds like you are making progress.

 

[emdsapmgr]

I run a 14 X 11 prop on my 96 70 hp. It's on a 17.5' pontoon boat. On a regular 16' boat, I'd think you should be running something closer to a 17 pitch prop. Getting the correct prop and setup for your boat should drop your rpm's to a more normal rpm range. The factory recommended rpm range is 5000-6000 rpm's. I'd suggest you get the correct prop to run right at the max rpm's at WOT- 6000 rpm's.