Prop versus RPM help.....

[ssportsmfg]

I have a 2001 Johnson 50hp 2 stroke 2 cyl. on a Lowe 17 SE bass crappie combo. I purchased it used late last summer. When I got it the tach was not working, and it had a Michigan wheel 11.75 x 17 prop on it.

Being that it was not a Johnson prop, I am guessing that it was a replacement prop. The Johnson book lists a 12.25 x 15 as the proper prop. I used a Solas prop calculator ( it listed my exact boat) to purchase a new prop before I got the tach replaced it selected the 12 x 17 as the proper prop, so I purchased the Solas 12 x 17 Amita 3 prop and installed it.

A few weeks later I found, purchased and installed a new Faria system tach. While running at WOT it only runs at 4000 RPM. This is about 700 rpm slower than the Johnson book lists as full power at 4500-5500 rpm at WOT. I do know that a higher pitch will slow the RPM down.

I can tell that the motor has been messed with as all the orange lock tight has been broken off all the throttle linkage screws, including the WOT stop. The roller pin on the first plate with the curved slot is not at the two arrows. which I am guessing is where it should be at idle?

So two questions, did the Solas calculator pick the wrong prop? Should I be running higher RPM at WOT ??? It seems to run fine, no hesitation or missing at WOT. Should I just leave it alone, or is it running too slow and causing strain at the lower RPM? It does run rough at idle. And should I adjust the throttle linkage to the two arrows on the plate?

Thanks for any and all help !

 

[bob johnson]

I woulkd fix the run rough at idle....no matter what...

that has nothing to do with the prop of course.

the prop estimator is a guide... your boat could be loaded with wet foam! who knows.... you want to be at 5,000 rpm at WOT....so you :

need to make sure the motor is running in the best shape possible, you need to make sure you can adjust trim during your test runs.. you need to make sure the motor is mounted correctly... then you take any prop you already have and try to get it to its highest rpm on flat water...

then what ever that rpm, will tell you what to do next

I would definatley not run the combo you have now...a well running motor might carbon itself to death running such a steep prop....if you changed nothing but the prop right NOW...id bet you need to go to a 13 pitch prop.

bob

 

[F_R]

So the full advance stop has been monkeyed with? First order of business is to make sure that is set correctly. Too much advance will cause pre-ignition, which will destroy the pistons and rings. Then if at all possible, find a dealer that has the correct test prop and check the WOT RPM on that. That will tell if the motor is developing it's rated horsepower or not. Only after you know these things should you be investing in props. Putting on a flatter prop to get the RPMs up on a sick motor is not a cure.

 

[ssportsmfg]

Thanks everyone, I am running Seafoam in it, and I don't think that it is carboned up, I don't have a book other than the owners manual and that does not tell you how to work on it at all. I am clueless as to the proper settings for all the various linkages.

My compression is good, and within 10 psi on both cylinders.

Around here finding a mechanic is near impossible everybody is a Mercury mechanic or if they work on Johnson's they are swamped, have been trying to find someone since last year when I got it.

When I de-winterized it a few days ago, (blew it out etc and fired it up, the water pump went out, I already dropped the lower unit and replaced that and what seals I could on the lower unit without the specialized pullers you need. Changed the lower unit oil, put it back together and it is running fine rough idle but other than that fine.

And I don't understand the wet foam statement, is the foam used in boats absorbent? That doesn't make a lot of sense, course I am new to boats so a lot of this makes no sense lol.

Thanks for all the suggestions!

 

[bob johnson]

I woulkd fix the run rough at idle....no matter what...

that has nothing to do with the prop of course.

the prop estimator is a guide... your boat could be loaded with wet foam! who knows.... you want to be at 5,000 rpm at WOT....so you :

need to make sure the motor is running in the best shape possible, you need to make sure you can adjust trim during your test runs.. you need to make sure the motor is mounted correctly... then you take any prop you already have and try to get it to its highest rpm on flat water...

then what ever that rpm, will tell you what to do next

I would definatley not run the combo you have now...a well running motor might carbon itself to death running such a steep prop....if you changed nothing but the prop right NOW...id bet you need to go to a 13 pitch prop.

bob

 

[boobie]

Is the new tach set on the correct position for your motor ??

 

[ssportsmfg]

Hey guys, thanks for all the great tips!!

I purchased a Seloc book last night and it is on the way. I am very skilled mechanically, journeyman machinist and welder and other things, but I am clueless about outboard motors. This is my fist boat, but I will try to answer some of the questions you posed.

This is a factory combo original boat, motor and trailer, when I purchased it late last summer the seller had all the books and paperwork.

The new Faria tach I set to 6 on the back before installing, it is a OMC system tach for my Johnson 50; it is a 2 cylinder. This is the setting I was told to use. It reads 900 at idle in a barrel of water with exhaust submerged so I am guessing this is close to normal. And my water barrel is 6 inches above my cat plate and no problems with enough water to feed the pump.

I have no idea if the foam under the floor of the boat is wet, everything seems to be fine in that regard, there are a few things that were loose and I put new stainless screws of a larger size in and tied everything back down good when I got it.

I have a induction timing light, and have the Seloc book coming so hopefully I can go through from A-Z in the proper order to tune it up properly and set all the linkage properly. I have read some of these posts on here and the terminology used, I have no idea what part they are talking about. The part names are totally foreign to me. lol.

I have good compression in both Cylinders, about 10 lbs lower on the top which is from what I have read pretty common as this cylinder gets hotter.

I have a induction timing light, this motor is very clean, the timing setting is on a sticker on the air cover, and two white marks on the flywheel.

It has almost new plugs, maybe 2 hours on them.

I purchased a new 12 gal Moller tank and replaced the original 6 gal. tank that was factory so the tank and fuel line is all new.

It comes off idle and gets on plane great, and runs wide open without missing or hesitation. Just starts a little hard and rough idle, it doesn't die once it gets warm and once it is started that day it starts fine the rest of the day the rough idle remains all the time, it just misses and jerks around some, it is not horrible, just not as smooth as I think it should be.

 

[boobie]

Just for the heck of it, put the tach on position 5.

 

[ssportsmfg]

Boobie.....If you saw how hard it is to get to you would understand that it would require taking it out of the dash, which is a upside down undertaking all in itself lol. It is so crowded under the console with all the wires and the bracing for the steering that you can barely even get to the nuts to tighten them up on the back of the tack let alone see the little position indicator to change it.

If you don't mind, why are you asking this? It seems to be reading normal at 900 on idle and 4000 at WOT. If it was on the wrong position would this not be even close to correct? Something tells me you know more than "just for the heck of it" lol.

 

[F_R]

I was wondering why the suggestion also. Perhaps he is wondering if it has a 3-phase alternator. I wondered that too. I don't think it does, but I could be wrong. Anyhoo, putting it on 5 pulses when it should be on 6 certainly would get the indicated RPM reading up. But it wouldn't be a true reading.

 

[boobie]

Most two cylinder motors I ever worked on had a 10 pole flywheel and therefore you would set the tach to position 5. Was just a thought.

 

[ssportsmfg]

I also got my Seloc book today,and started reading and started at ground zero again. I went to the tune up section as everyone suggested that I first get the motor right running wise before I look to props to get the RPM up.

So far I today I de-carboned it. (In great shape you can see the silver of the alum on the piston faces, and plugs are clean) Ran warm compression test on both cylinders. Top is 145 psi bottom is 140 psi. So although the book says that Johnson does not publish compression numbers, and basically said it should be at least 100 then this is great. Clean and good compression, and very close to the same both cylinders.

I am going to sit down tonight and start reading on the exact ordI looked my Johnson 50 up and it has a 12 pole alternator, so I set it at 6 which Faria says is correct for this motor.er to set up all the linkage for sync on the advance versus carb. I started reading it but got a call and distracted.

I have also in reading the beginning of the sync section that I need to put it in the water under load to really accomplish anything.

I did have one Johnson mechanic that said he would tune it for me here at my house, but he got too busy. I asked him because I had heard that you have to have the boat in the water or a test tank to set everything properly as I have heard that many times. He said that he could do it in my driveway in my water barrel, but alas, he is too busy to help me.

Is he correct that it can be done without being under load at WOT? The book says no. He says yes. I don't know what to think. So far when looking at when the carb opens and the spark advance begins to move it looks like it is not electrical or timing. I am beginning to lean toward a fuel pump being weak and the culprit.

Thanks !!

 

[boobie]

If it's got a 12 pole alternator, then position 6 is correct.

 

[ssportsmfg]

Yes it has a 12 pole according to a chart I have on this motor.

 

[steelespike]

Get us the prop size, do a wot lightly loaded test run.get us the rpm and max speed by gps.
This will tell us how well your setup works.And should tell us if your numbers make sense.

 

[V153]

re Tach. Might wanna pop the flywheel and count the poles. Just to be sure.

 

[ssportsmfg]

Why would I think that the stator doesn't have 12 poles? All putting it on 5 would do is up the RPM and at idle as well. I guarantee you that it is not idling at more then 900. It is idling at 900 which is idle speed for this motor just roughly. It has an overly aggressive prop on it which is slowing the RPM's at WOT to 4000. The WOT recommendation is 4500-5500 per the Johnson owners manual. The Johnson book states the prop is to be a 12.25 x 15. It is (my fault) running a 12 x 17.

I should have read the book before I bought this prop, or replaced the tach with the old 12 x 17 that was on here and I would have known this was too much prop.

But alas I am a newbie to boats, but I am learning.