25 hp johnson/prop question.

[carltonwebb]

If anyone has any suggestions on a prop. I have a really wide lowe 14 foot flat bottom jon boat. Its older and someone painted over the decals so I cannot tell you the exact model number but the rails are 5 ft apart side to side. I will have to take the prop back off to find the current size/pitch but it matches the rest of lower section of motor so I am going to assume its the original one.

In reverse & idle, the motor revs up nice and high. In forward, it moves the boat but not easily by any means. I can see where the throttle linkage is for the piece under the flywheel and it is full stroke as well as I took the rod and tried to give more and could not get anymore. It just seems to be that much load. I also adjusted the manual tilt some (one hole at a time) but did not help any. It will get me through this week and then I can address this. I had a yamaha mariner 15 hp and it would move the boat a little better but I wanted a little more so when the Yamaha died, I got the 25 hp johnson.

This week we are running it in the tidal creek so sometimes we are going against the tide but even with the current it does not really pick up any rpm. This was me in the boat and two batteries today. The motor had good compression when i bought, all new fuel system to the carb and recently cleaned carb, New points/plugs and one new coil.

I was reading some previous posts for johnson and props and saw someone mention that both plugs only fire at certain times? Can anyone elaborate on that a little more?

I am also ignorant as to the proper way to set the tilt (?). Its manual tiller so I have tried it in a couple different holes with no real improvement.

Thanks in advance for all advice.

 

[steelespike]

Re: 25 hp johnson/prop question.

Jon boats are usually classified by length and bottom width i.e.: 1448 etc.
What year is the 25.I suspect that it is running on one cylinder.
Many people don't realize it except for the lack of power.Unhook one plug wire at a time and test run.
Prop size should be near the base of the blades or under the prop nut if its a thru hub exhaust.
What is the total load?

 

[carltonwebb]

Re: 25 hp johnson/prop question.

Steelspike....I read this earlier this morning and thought about it during breakfast. While we were out, I decided to get new plugs for it. Its a 1975 johnson 25 hp model 25e75b. I wanted the Champion J6c but they did not have it. SO...I ended up with an autolite equivalent gapped at .030. They will come out soon but I tried them. It had almost new NGK but i forgot to look at which ones. They were in them when i bought the motor. I put the in the tackle box for now. While i had it apart, I noticed that one of the coil leads where it attaches to the plug was pretty corroded. I cleaned up with a battery brush and put it back together. The boat has a lot more power now. I would say it may need a prop to get any faster but even fighting the current it would plane out which it would not do yesterday. I still have not read the prop but will when i take it out of the water.

Looking at the newer boats, I would say this is basically an older 1448 lowe model. Today, we had the two batteries, johnson 25, my wife and son, and me which the 3 people probably avg 550 lbs so I pretty impressed with how it did now. This week I have done the shear pin ( my fault), plugs, the fuel pump, shifter linkage, and fuel lines to finally get it where it was dependable but it was overall a good ride. I have to work on the low speed setting as it wants to cough and cutoff if you turn the handle all the way low throttle but just slightly off, it does fine.

 

[steelespike]

Re: 25 hp johnson/prop question.

The speeds printed on the tiller are just reference points so you may not be on slow when t it is going slow.
If it's coughing or sneezing its probably too lean.
When you find the prop size get us a lightly loaded wot rpm and speed.
I'll bet the motor is too low. Antivent plate(just above the prop should be about even with or slightly above the bottom of the transom.
If you don't know the condition of the impeller be watchful for an over heat or change it.

 

[carltonwebb]

Re: 25 hp johnson/prop question.

When i bought it was supposed to have a new water pump in it. I pulled the prop today when i got it out of the water while I was flushing the motor with fresh water. The markings on the prop were too eroded too see under the nut. I could only make out what looked like 31.5. I measured from the center of the hub to the tip of blade and came up with 4.5 " so I think it is a 9"prop. I know that standard props for the motor were 9x10. Its worn pretty good. I did not have photo tach here but top speed according to iphone app was 7 mph with the current. It was little less than 5 against the current. That was with the load mentioned in the previous post. I then ran it without the wife and son but did not pick up much speed but the current was terribly strong by that point b/c of the storm but the speeds were essentially the same.

I know more HP uses more gas but it seems to use a lot more gas. On the yamaha mariner that I had, we rode all week last year and still had gas left over. This year I basically used it about 6 hrs and used 5 gallon of gas. I had to get more to load it. Maybe though it has to do with all the issues I had earlier in the week with fuel pump/lines etc and all the cranking of it.

I think the antivent plate is pretty much even with the bottom of the transom. right about the drain. I had planned on working with the tilt again some but there was a nasty storm that came up with large lightning bursts so I just wanted to get it out of water.

 

[Mi duckdown]

Re: 25 hp johnson/prop question.

Check spark and compression on both cylinders. A 25 hp on that size boat should run 25 MPH. Something does not add up.

 

[steelespike]

Re: 25 hp johnson/prop question.

On the 76 it has the old style coils and they are the weak point of the motor.
If they are original; they very likely are starting to fail. Its probably running on one cylinder.
Pull one plug wire at a time and see if it will run.
A 25 will use "about" 2.5 gallons per hour(gph) at wot(wide open throttle) The 15 would use "about" 1.5 gph at wot.
Your lack of performance could be fuel system related or a combination of problems.
As suggested speeds should be in the 25 range lightly loaded.