35 HP Johnson only run 4200 rpm at WOT.

[ianferron]

I got a 1976 Johnson 35 HP. I have 2 propeller. One is 10-1/8 X 13 and I run 4200rpm max, the other one is 10-1/2 X 11 and run 4700rpm max. My engine seem to work fine, have 120psi in both cylinder. Points, condensers and coils are new. Carburetor has been cleaned 3 time and lat time got a rebuilt kit on it. Point has been adjusted to .020. Not sure why I can run faster than that. Ideal rpm suppose to be about 5500 as per the engine specs. In order to get that I would have to go with a 8 or 9 pitch I guess witch doesn't make sense.

 

[hidef]

Re: 35 HP Johnson only run 4200 rpm at WOT.

What is the size and type of your boat you are trying to push? Also how many people and how much gear do you have in it?

 

[jbjennings]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I agree with hidef about the boat size, weight question.
If it's a lightweight boat situation, you may check to see that the armature plate is making it's full turn and giving full spark advance, and that the throttle plate is completely open at full throttle position. Other than that, I'm thinking that since it's getting up to 4700 rpm's, it's probably doing all it can for your boat. How about the position it's running at? Is it plowing or is the bow too high? Is it a longshaft motor on a 15" transom?
Just a few things to think about.....
JBJ

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

The boat is a 14 feet fiberglass basic V-hull shape. They call it a Cadorette Fury. Not too heavy I think with 2 person and gas on board. The motor is a short shaft and it's position at the right height. I try different angle setting and came out with these rpm # as the best i could get. I run on plane on calm water at about 22 mph on the GPS. Not much difference of speed between the propeller, better take off with the 11 pitch, that's it. The throttle plate is fully open at WOT. Not sure about the timing tho. Would the gap of the points would be off and bit and trowing timing off?

 

[F_R]

Re: 35 HP Johnson only run 4200 rpm at WOT.

Does your boat have a floatation chamber under the floor? Maybe it is full of water. We used to see an awful lot of that.

Is there ANY marine growth or fouling at all on the bottom? That is like putting the brakes on.

Is the boat bottom perfectly straight with no deformations or "hook"? I've seen boats with terribly crooked bottoms from sitting on an improperly fitting or improperly adjusted trailer.

I don't think you have a RPM problem, but that is as fast as the motor can push the boat for some reason. You probably have a boat problem, not a motor problem. It won't rev up unless the boat moves out of the way as it does. For an extreme example, tie it to the dock and see how many rpm's you get---very low.

If you want to find out for sure, drop by a dealer and have him run the motor at the dock with a test prop & tachometer. We used to call that a "poor man's dynamometer". That will tell if the motor is doing what it is supposed to do.

 

[Tim Frank]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I got a 1976 Johnson 35 HP. I have 2 propeller. One is 10-1/8 X 13 and I run 4200rpm max, the other one is 10-1/2 X 11 and run 4700rpm max. My engine seem to work fine, have 120psi in both cylinder. Points, condensers and coils are new. Carburetor has been cleaned 3 time and lat time got a rebuilt kit on it. Point has been adjusted to .020. Not sure why I can run faster than that. Ideal rpm suppose to be about 5500 as per the engine specs. In order to get that I would have to go with a 8 or 9 pitch I guess witch doesn't make sense.

Is this motor new to you, or have you had it for a while?

i.e. ~ did this problem just happen, or is this the performance since you have had the boat/motor?

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I just got that motor this summer. I've been play with it tune up for a while. My boat hull is in very good shape. The flotation chamber is empty. There is no deformation of any type on the boat. I always take the boat out of the water so the hull stay pretty cleaned.

 

[Tim Frank]

Re: 35 HP Johnson only run 4200 rpm at WOT.

Is this motor new to you, or have you had it for a while?

i.e. ~ did this problem just happen, or is this the performance since you have owned this boat/motor?

Has this boat ever performed as you would like?

 

[F_R]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I just got that motor this summer. I've been play with it tune up for a while. My boat hull is in very good shape. The flotation chamber is empty. There is no deformation of any type on the boat. I always take the boat out of the water so the hull stay pretty cleaned.

Well then, do as I suggested--run the motor with a test prop. You need to narrow the problem down--boat or motor. Or, keep on guessing.

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I found the motor hard on gas and expected a little more for a 35 HP. I had a 18 hp on a 12 ft john boat and it was giving me closed to 40 mph and was running about 5600rpm.
That 35 HP doesn't sound like it giving all what it got. I suspect my leaf valve no closing properly. Would that be the cause of my problem?

 

[F_R]

Re: 35 HP Johnson only run 4200 rpm at WOT.

As I said, keep on guessing. But that is a lousy guess.

 

[Tim Frank]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I have 2 propeller. One is 10-1/8 X 13 and I run 4200rpm max, the other one is 10-1/2 X 11 and run 4700rpm max. Ideal rpm suppose to be about 5500 as per the engine specs. In order to get that I would have to go with a 8 or 9 pitch I guess witch doesn't make sense.

The "rule of thumb" says that for each 1" you drop the pitch you should get 150-200 extra RPM.
That is about exactly what you got.

1) Why doesn't a 10 or 9 pitch make sense if that is what it needs? They are available, and probably would not be if there was never a requirement.
2) Re-read F-R's post/suggestion. That is the best advice you are going to get.

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

11 pitch is the lowest I could find. With a 14ft boat I should see at least 5200rpm but for some reason the motor is not powerfull enough to reach that speed. That's what I am trying to find out.

 

[Tim Frank]

Re: 35 HP Johnson only run 4200 rpm at WOT.

These guys do a 9"...at a good price.

http://www.deepblueyachtsupply.com/PropGuideResults.aspx?egid=2

 

[ukmember]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I have the same problem, I have a 16.6ft boat that will not plane.

I am currently running 11x 13 which I belive is the standard prop supplied with the motor from the factory.

My outfit wieghs in at 1200lbs laden but has a water line width of 6ft dead.

When I get my tacho I can verify my RPM and work out which prop will het her up on the plane.

Regards

Daren

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

Daren, I can understand ur set up, U have a heavier and bigger boat than mine and u run a 13 pitch. Mine weight less than 500 pounds and with a 11 pitch I cannot run higher than 4700 rpm.

 

[ukmember]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I have used a programme called castle marine prop calc which is a free download, now they recommend a prop of 10 x 11 not my current 11 x 13 I cant verify my RPM and I have posted a question re fitting an aftermarket tacho but need to verify connections before purchasing, as I cant justify ?100 almost for a plug in unit.

I know that the engine is not reaching its wot rpm to get it up and over the hump to plane, I know when it does I will hopefully reach 18knts ish.


Regards

Daren

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

My 14 ft boat only go to plane around 16-18 MPH so expect at least 20 MPH to get on plane with your 16 ft.

 

[jbjennings]

Re: 35 HP Johnson only run 4200 rpm at WOT.

I found the motor hard on gas and expected a little more for a 35 HP. I had a 18 hp on a 12 ft john boat and it was giving me closed to 40 mph and was running about 5600rpm.
That 35 HP doesn't sound like it giving all what it got. I suspect my leaf valve no closing properly. Would that be the cause of my problem?

1) Are you absolutely certain you were going over 30mph with an 18hp? Do you have a tach on the 18? I personally have never seen an 18hp motor on any boat go 40mph. Especially not a 12 foot john boat. I own one and my 12 footer is slower than a 14 footer. I'm just saying that without a gps, boat speed is hard to judge. My '57 18hp will go about 24-25 mph on my 14' boat, a little less on my flatbottom 12foot johnboat.

2) Leaf valves are absolutely not your problem.

Good luck with your motor,
JBJ

 

[ianferron]

Re: 35 HP Johnson only run 4200 rpm at WOT.

My 12 ft boat is very light, maybe 150lbs and with that evinrude it was pretty scary!!! Only the back of the boat was touching the water lol. So you think the leaf valve has nothing to do with my problem? I know it make my idle bumpy a bit but seem to run great at higher RPM. I ask a few boat store and mechanic and they all say that the 13 pitch is the standard and should work decent. Well I can try a 9 pitch and see how that work. It's the end of the season here in Est Canada so it's getting cold. So I might keep working on it next summer.