140TL78C Missing for the first 50 yards

[jwracca]

Hi all,

Thanks for taking the time to read this. I have a 140 HP 1978 Johnson with a couple of problems.

1. When the motor is running, and i turn the key to the off position, the motor keeps running. I thought this may have been a grounding issue, so i ran a ground wire from the battery negative post to the engine itself. That didn't seem to help. When the motor is running and i turn the key off, the motor sounds like it goes to running on just a couple cylinders, just gets quieter.

2. The motor starts up just fine, but when i push the throttle all the way to take off. The motor sounds like it's missing as it gradually builds up rpm's. Once the rpms finally gets up to around 3000 rpms, the motor takes off and runs great. It acts like there is some restriction at takeoff until i reach about 50 yards, then it runs great. I have done a carburetor job, cleaned the fuel pump screens. Have great compression on all 4 cylinders (120's) and good spark on all 4 plugs. Any ideas on this?

Thank You

 

[WrenchHead]

When the motor keeps running without an ignition spark it is called "dieseling". The red hot carbon buildup in the cylinder head, on top of the piston or the spark plugs are hot enough to ignite the fuel mixture in the cylinder. The missing under load sounds like carboned-up plugs. Pull the plugs and inspect them. Cleaning and re-gaping might help but you may need new plugs. New plugs may solve both of your problems you mentioned..

 

[jwracca]

Thank You WrenchHead, i will try that when i get off this afternoon. The plugs that are installed don't have the tip to set the gap, its just flat. I am wondering if i have the correct plugs installed?

Thanks for your reply

 

[Bosunsmate]

Try a spark tester to test you have good spark on all plugs.
You can also use the spark tester after you turn the key off to check if its still sparking- it shouldnt be. If it isnt sparking on ANY cylinders then its dieseling
I have champion ul 77V listed for that engine which are flat

 

[emdsapmgr]

You may have a defective power pack. That engine was one of the first with the new dual packs, back in 78. They get a kill (ground signal) through the black wire with the yellow tracer, from the key switch when it is turned to off. The pack will normally respond to that ground signal and shut off the pack(s). [Check to be sure that wire feeds both packs.] It is also possible that one of the packs is internally defective and won't shut off as a result of the ground signal. I've had that happen on a dual pack V6 crossflow before. The engine kept running, but the tone of the engine changed. (the pack was defective and it's replacement cured the problem.) You can test by: determining which pack may not be shutting off (one pack for each cyl head.) Once you know, swap them. When the packs are switched, the problem should move to the other head. If so, likely a single pack replacement should cure your problem.

 

[jwracca]

I went and purchased new UL77V spark plugs and installed them yesterday, after installation the motor is still performing the same with both issues. I didn't find a spark tester to see if i am getting spark with the key off, but i will try and find one today and test that part out.

Emdsapmgr, i just replaced both power packs a couple months ago, but maybe one of them went bad on me already. I have spare power packs and will try that this evening.

Thank You for your expert help

 

[Big Fish Billy]

Please post your fix once you're there.......have you done a decarb lately?

 

[jwracca]

So i replaced the new power pack i purchased 2 months ago with a spare i had. That did the trick on the issue of the engine not turning off when i turned the key to the off position. Thank You emdsapmgr.

#2 problem still exist. Its like when it starts (starts easy) the motor idles really nice. When i push the throttle on the carbs just a little, between 1000-2000 rpms, it almost sounds like its missing. It doesn't backfire, but it just sounds rough and i can hear air puffs or short burst of air coming from the carb area. On the water, it does that until the motor plains off. Then once plained off it has full power and runs like a spotted ape. As i mentioned before, i just did a complete carb cleaning. 2 new power packs, 4 new coils, 115-120 compression on all 4 cyls. and great spark on all 4 plugs. Only thing i can think is it may be a carb adjustment, but i dont see any adjustments to try.

Big Fish Billy, Thanks for your response. I run Sea Foam through a 6 gallon tank about once a year to decarb, i am no pro but i just assumed that was efficient enough. What you think?

Thank Yall

 

[Bosunsmate]

it could be worn reeds or link and sync is out (lever timing)
Just pressing the carb throttle can on some engines cause a weird effect as thats only increasing fuel and it might be that the engine needed spark advancement then too which wouldnt happen if just moving carb levers.
But since you say that happens out on the water to it is suspicious. Try doing it with the main throttle and see if it does it too.
If reeds get worn you do get a rough idle but the engine goes fine at fast rpm.
To test them you can hold a dollar bill in front of each carb, check it gets blown in towards each carb wit even pressure and doesnt get blown back out by any.
Dont let go

 

[emdsapmgr]

Bosun has a good suggestion. Perform the sync and link adjustment (from the factory service manual.) Possible that the carbs are not opening at the correct timing advance..once you get to full throttle, it's all max throttle and max timing. Up to that point, they need to be in sync with each other to run well.

 

[jwracca]

Thank Yall, i am going to pull out my service manual and verify the link and sync adjustments. I will post update

Thanks again