1978 70 HP Johnson Low RPM Limit

[karl3419]

I have a 1978 70 HP johnson that will not go above 1100 rpm. It starts easily and runs fine but when the throttle is advanced above 1100 rpm it dies as if the ignition was turned off. I replaced all fuel lines and tested the fuel pump pressure and it was good. A mechanic replaced the timer because #2 plug lost spark as the throttle was moved during testing and he thought it had a bad wire. Put the boat back in the water and it ran fine without a rpm limit for 20 minutes then the 1100 rpm limit started again. When I advanced the throttle above 1100 rpm the engine dies. When I test for a spark with a home made point tester I do not seem to get the 7/16" spark that everyone talks about but I have a hard time seeing the spark. A timing light shows all plugs firing. I just put it back in the water tied to the dock and in gear the same thing happens, above 1100 rpm the engine dies as if the ignition is turned off. Out of gear it will easily go to 3000 rpm. Is it the power pack or stator?

 

[boobie]

Re: 1978 70 HP Johnson Low RPM Limit

What's the compression on it ??

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

What's the compression on it ??

110, 105 and 105

 

[BonairII]

Re: 1978 70 HP Johnson Low RPM Limit

I'd be pissed if a mechanic charged me for a new timer base/labor and it WASN'T the problem. I'd make him take it off and reinstall the old one.

Prob gonna need a dva adapter to test whether it's the stator or PP. Or you can flip a coin and change the stator or PP and hope for the best.

I almost did the same thing just yesterday. Was going to order a new PP.....only to find out that I was running the wrong spark plugs. :facepalm:

Throwing parts at a motor in hopes it fixes the problem can get expensive quickly.

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

What's the compression on it ??

The compression I gave you was reading from a distance as I did not have a started switch. I just remesured them and reading up close I get 130, 125 and 125 so it is not compression.

 

[BonairII]

Re: 1978 70 HP Johnson Low RPM Limit

I have a 1978 70 HP johnson that will not go above 1100 rpm. It starts easily and runs fine but when the throttle is advanced above 1100 rpm it dies as if the ignition was turned off. \

Have you checked all wires from stator and timer base to make sure that you don't have a short? Is it possible that when the timer base advances, it is hitting something and shorting the system out?

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

Have you checked all wires from stator and timer base to make sure that you don't have a short? Is it possible that when the timer base advances, it is hitting something and shorting the system out?

The mechanic thought there was a broken wire in the timer since #2 would stop firing when the throttle was moved so he installed a new timer and checked all of the wires. It ran fine for 20 minutes after that but then the problem developed again. I have ordered a new power pack and will check to see if I can get the 7/16" spark when that is installed. Could a weak spark show this type problem?

 

[BonairII]

Re: 1978 70 HP Johnson Low RPM Limit

Weak spark could cause the motor to bog and die quickly when you throttle up, but you describe the motor turning off like ignition was turned off, so I still suspect a short.

I hope the PP is the issue, for your sake cause this must be getting expensive for you.

If I were you, I'd get yourself a DVA adapter and multimeter. You can use them to test your system, instead of just randomly throwing new parts at your motor.

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

[I agree with you that it sounds like a short but all of the wires have been checked several times. The boat is set up with a single lever for throttle and shifting. You move the lever outward for just throttle. If it in not in gear the engine will run up to normal rpm. There is a switch in the throttle that keeps you from starting in gear. But this should only effect the starting circuit.

I have a shop manual and it describes testing of the ignition but only in terms of settings on a particular meter and not in terms of AC or DC or voltage. I do not understand what a DVA adapter is or what it is used for. Thanks for your comments.

 

[Big Bubba]

Re: 1978 70 HP Johnson Low RPM Limit

I was thinking maybe the carbs needs to be cleaned out because the high speed jets maybe clogged up, what about the fuel is it fresh? Is the fuel pump putting out at high rpm's? If the carbs haven't been cleaned in a while I would clean them really good and see if that fixes your high speed issues.

 

[BonairII]

Re: 1978 70 HP Johnson Low RPM Limit

I have a shop manual and it describes testing of the ignition but only in terms of settings on a particular meter and not in terms of AC or DC or voltage. I do not understand what a DVA adapter is or what it is used for. Thanks for your comments.

Go to the CDI ignition website. They have some good info on troubleshooting with a DVA adapter

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

I cleaned out the main jets when this first occured and checked the fuel pump pressure which was OK on the high side. I finally received my new Power Pack from CDI and installed it. I set my spark tester at 7/16" and had good spark on all cylnders. Tomorrow I will put the test wheel on and lower it into the canal and see how it runs.

 

[BonairII]

Re: 1978 70 HP Johnson Low RPM Limit

That's good news. Keep us posted on the test results.

 

[karl3419]

Re: 1978 70 HP Johnson Low RPM Limit

That's good news. Keep us posted on the test results.

The results, I took the boat out after I had secured the 7/16" spark and while it ran fairly well it still had hesitation and would not take full throttle so I thought that the fuel supply was all that was left. I cleaned out the carb main jets again by removing the plugs and the main jets, cleaning out the jets and then pumping some gas out the plug hole. Today it ran fine so the problem is solved. Something must have been plugging one or more of the main jets. Thank you for all of your help.

 

[Gideon_70]

Re: 1978 70 HP Johnson Low RPM Limit

I have a 1978 70 HP johnson that will not go above 1100 rpm. It starts easily and runs fine but when the throttle is advanced above 1100 rpm it dies as if the ignition was turned off. I replaced all fuel lines and tested the fuel pump pressure and it was good. A mechanic replaced the timer because #2 plug lost spark as the throttle was moved during testing and he thought it had a bad wire. Put the boat back in the water and it ran fine without a rpm limit for 20 minutes then the 1100 rpm limit started again. When I advanced the throttle above 1100 rpm the engine dies. When I test for a spark with a home made point tester I do not seem to get the 7/16" spark that everyone talks about but I have a hard time seeing the spark. A timing light shows all plugs firing. I just put it back in the water tied to the dock and in gear the same thing happens, above 1100 rpm the engine dies as if the ignition is turned off. Out of gear it will easily go to 3000 rpm. Is it the power pack or stator?

Check your spark advance lever to see if it is moving freely. If it doesn't make it to the forward stop when you push the throttle forward and is sticking, then there is your issue.