1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

[danthemanx07]

I have a 1991 140hp Johnson. It runs great at idle, starts easily, and otherwise is a great running motor.

Its problem is that it won't make more than 4100rpms. Here is what I have checked so far:

1. Even compression, 120-125 across all 4 cylinders.
2. Carbs were rebuilt last season when the problem developed. The jets are clean and clear. Low jet= 37, Med = 54 and high = 63D.
3. I had a new control box and cables installed last year. Throttle cables were adjusted by a qualified mechanic at that time.
4. All coils and spark plug wires were replaced last season.
5. I have been trouble shooting with both a 6 gallon portable tank and the main tank to isolate any fuel supply issues. No change between the two.

My course of action for further investigation is:

1. Picked up a spark tester last night. Will be checking on that asap.
2. Will be picking up new spark plugs tonight. The ones I pulled looked a little dirty. I will get a picture up.

My question is where to go from there? What should I look at next? All of the threads I have read seem to go dead shortly after the OP tests spark. Maybe they get it fixed and just don't report back? Not sure, but either way, I havent been able to find much beyond that.

Thanks for help.
Daniel

 

[racerone]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

There have been issues with the seal rings on the crankshaft.---This results in a loss of crankcase compression and poor scavenging of the cylinder.--Try running with the cowling off.-----Other thing to do is run with a timing light on each plug wire and see if all cylinders are still firing at full throttle.-------Run with carburetor silencer off and shine a strong flashlight into each bore to see how fuel comes up the main jets.---

 

[danthemanx07]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

There have been issues with the seal rings on the crankshaft.---This results in a loss of crankcase compression and poor scavenging of the cylinder.--Try running with the cowling off.-----Other thing to do is run with a timing light on each plug wire and see if all cylinders are still firing at full throttle.-------Run with carburetor silencer off and shine a strong flashlight into each bore to see how fuel comes up the main jets.---

I assume you mean to do all these tests in the water at full throttle, correct?

Thanks for the tips! Will see about getting it in the water tomorrow.

 

[Chris1956]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

What kind of boat and what prop are your running? Did the motor ever rev above 4100RPM, since you have owned it?

 

[danthemanx07]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

What kind of boat and what prop are your running? Did the motor ever rev above 4100RPM, since you have owned it?

Prop is a 13.25" x 17 pitch Stiletto.

Will check the diameter when I get home, but I am fairly certain that it is 17p. The boat is a 1972 Mako 19. Shallow deadrise boat. Dry weight should be 1400lbs (without motor).

Yes, it did when I first put the motor on a few years ago. Unfortunately, i have been in school for the past three years and the boat has only seen intermittent use. I can't recall when exactly the trouble started, but it did not always do this.

It also ran once last year at 5400-5600rpms for a brief period. Maybe 20 minutes. Not sure why. I was out in it, on a usual fishing trip in shallow waters down here east of New Orleans. The boat ran as I suspected it would, around 4100rpms tops. I stoppped for a while, cutting the engine off to fish. I cranked it back up to head up and throttled up and climbed all the way up to the proper rpms. I was stoked and rode it that way all the way back to the dock. Never happened again.

 

[danthemanx07]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

Well - right off the bat. Low to no spark on all cylinders for the most part. Unfortunately, the stupid POS spark tester I got from AutoZone broke while I was doing a second round of testing. I am going to get a second tester to double check my findings asap.

What would be the cause of the weak spark? Was cranking on two fully charged batteries with new cables. The engine fired right up though - this was the first time it has been cranked since last year. Are those two things mutually exclusive? Ie - would the engine start at all if the spark was in fact weak?

 

[danthemanx07]

Re: 1991 Johnson 140: RPMS will not reach higher than 4100 - Help Please

Bump. Anyone got any thoughts?

I can't get the boat in the water until its registration is updated, and the the LDWF is backlogged by 4 weeks. Plenty of time to trouble shoot.