'78 Johnson 115--Low Power

[grw5]

I have been trying to debug a '78 Johnson 115 for the past several months and haven't overcome these problems yet. Listed below: (I had previous posts)

1. The main problem is that it has low power (or torque) to get the boat (17 ft. Champion Bass boat) out of the hole. The engine bogs down at about 3500 rpm with full throttle. If I redistribute the load (passengers) to the front it will gradually get out of the hole and plane out, with the engine rpm increasing up to rated speed of about 5500 rpm.

2. When trolling, slightly above idle, after a few minutes, the engine will start smoking, badly and running rough. I have to take it up to a higher speed for a few minutes to blow it out, then I can resume trolling. It is like one of the cylinders may be flooding.

3. The tachometer quit working. Could this be the rectifier?

Here are some of the things I have done.

1. Checked the ignition--per recommended spark check, and per all of the recommended tests in the SELOC manual. I used an ohmmeter for the peak voltage measurement. I also inspected the stator coil.

2. Overhauled the carbs. Checked all the high speed jets.

3. Overhauled fuel pump.

4. Installed all new fuel lines and eliminated air leak from tank.

5. Gave it a Sea Foam treatment. Spraying it in the cylinders, letting it soak, in the intake, and in the fuel.

6. Replaced the spark plugs.

The compression checks out at about 95 to 100 on all cylinders. Could this be the problem? Am I going to have to tear this thing down and overhaul it completely? I have not pulled the heads to inspect the cylinders.

Your help and advise is much appreciated!

GRW5

 

[tashasdaddy]

Re: '78 Johnson 115--Low Power

did you do a link n sinc, this synchronizes the spark and carbs, so the work together, efficiently.

 

[grw5]

Re: '78 Johnson 115--Low Power

Yes I did. I even tried adjusting it some each way and there was no improvement.

grw5

 

[CharlieB]

Re: '78 Johnson 115--Low Power

The symptoms you describe sounds like slight carb flooding, loww RPM power loss and smoking, but at higher RPM its not so much as to affect operation.

Test by removing the airbox from the front of the carbs. Make sure that you have the engine trimmed such that the carbs are level.

With the engine OFF, pump the primer bulb while looking down the carb throats for ANY fuel, there should be none.

If carb float level is set too high that will need to be readjusted.

A secoond possibility is a pinhole in the fuel pump diaphragm allowing fuel to reach that cyl for the fuel pump vacuum/pulse hose. Check by removing the hose, swab inside with a Q-Tip looking for wet fuel.

 

[ezeke]

Re: '78 Johnson 115--Low Power

If the engine is in reasonably good operating condition, the problem may be that you are over-propped. You want to be propped to reach your WOT RPM under conditions that are normal for you. Shoot for 5800 RPM max under normal conditions.

The carbon problem is common in the 1970's motors. Good results have been found by using high grade synthetic or synthetic blend oils combined with de-carbon additives; you can find both qualities in BRP XD-50. Use Champion QL77JC4 spark plugs at .040 rather than .030 for trolling. Finally, it is essential that the correct operating temperature be maintained when trolling a lot so check the temperature and correct the thermostats if it is off.

A thorough decarb following Captain Bob Dunkelberger's plan will get you off to a good start: http://forums.iboats.com/showthread.php?t=158076






.

 

[grw5]

Re: '78 Johnson 115--Low Power

Thanks for your replies and advice. I will run the tests for flooding the carbs. I will take ezeke's advice on decarbing. I will repeat the decarb treatment. I did not do the SeaFoam treatment with the boat in the water. I should not be over-propped. It had a 13.5 X 18 and I put a 13.5 X 17 on it, which seemed to help. On the link about decarbing, it mentioned problems with broken piston rings. How do you know if one is broken, by low compression?

Also, why would by tack quit working.

Thanks,
grw5

 

[ezeke]

Re: '78 Johnson 115--Low Power

The compression test will usually indicate that a part of a ring is missing by that cylinder's reading being lower than the others. Broken rings are often found when replacing head gaskets because of the scoring. You can partially observe the actual rings on your crossflow by removing the intake bypass covers, but you will need new gaskets if you do.

Rings usually break because they are forced into the ports by coking, and that is often caused by lugging the engine, which is usually caused by being over-propped.

 

[emdsapmgr]

Re: '78 Johnson 115--Low Power

Normal compression on this engine when just new or just overhauled would be 125-130 lbs per cyl with stock heads. Your compression is low.

 

[grw5]

Re: '78 Johnson 115--Low Power

Well I think I fixed the problem. It was in a carb. I was chasing my tail all along. Here's the story.

In trying to follow some of your advice, I rechecked the compression, after doing another treatment of SeaFoam in the cylinders, and I still got 95 to 100 psi. But, I noticed that the top right spark plug looked brand new, with no carbon color like the others. I checked the spark--it was over 7/16''. I ran it a while to see how fast that cylinder warmed up--it was slower. So, with the compression (even though it is low) and good spark, the only thing missing was fuel.

I must have gone through those carbs 2 or 3 times, but I had to pull that top carb one more time. It looked clean. I poked a welding tip cleaner in the high speed jets. They seamed open, but then I looked again, and as I poked the rod through the jet, it pushed out a piece of Silicone RTV! In my earlier days of checking the carbs, I used some RTV to repair the gasket that was falling apart, until I could get the carb kit and fix it right. A chunk of it must have come off and was hidden behind the jet.

I took the boat out yesterday to test it and Wow what a difference having a 4th cylinder working makes. It shot right out of the hole with plenty of power. I am considering going to a higher pitch prop now, because it seems to be revving too high on the top end. My tach is not working, so I don't know my actual rpm's. It also idles much better and doesn't start smoking when trolling.

Well, thanks again for all of your help. Any suggestions on where to look at fixing the tach would be appreciated.

 

[aclmvh]

Re: '78 Johnson 115--Low Power

Congrats on finding your problem grw5! Hopefully that fuel/oil starved cylinder wasn't damaged while running dry.

Sounds like you have an unregulated charging system- my 98' 115SPL has a 6amp non-regulated with a rectifier and my Teleflex tach stopped working a few years ago. Here are my check-notes that may help.

I believe the tach for my '98 Johnson 115 is fried- it worked before, but stopped working with no prior notice- Here is what I have done to test, so please, anyone make suggestions. It's a Teleflex 8k RPM International Series and my 115 has a rectifier....no regulator.
1. Tach zeroes when I power on the ignition- my VOM says I have over 12.5 volts DC at the tach IGN and NEG poles.
2. I have a good ground- no resistance when I check the tach NEG pole to a good ground on the boat.
3. With my VOM connected to the tach NEG and SND poles and the motor running, I get good and correct variable AC voltage based on motor speed.
4. I have checked the rectifier based on the shop manual instructions and it checks out.
5. I shorted the tach SND pole to the tach NEG pole- according to Teleflex the needle should go to the max position, which it did.
6. Calibration setting on the tach is 6P....which is correct for a 2 stroke Johnson V4.
Conclusion: Tach was fried. Got a new one identical to the old and it worked like a champ.

mvh
Jax/Fl