'74 40hp only runs on one

[fisherman007]

Hello, I have a 74 40hp johnson that I just got. It seemed that it didn't have close to 40hp. I checked for spark by holding the plug to engine block, both have spark. With the engine running I touched the "shorting"(the ones that kill the engine when grounded) wires to ground, one makes the engine sputter and die, the other changes nothing. I am a diesel mechanic, but no experience with two stroke, no less marine engines. Any help?
EDIT: Sorry it's a 40el74s electric start.

 

[Daviet]

Re: '74 40hp only runs on one

Will the spark jump 1/4 inch and is it bright blue? Is the compression OK? Has the engine been sitting for a long period of time, maybe time for a carb overhaul. If you don't know the history of the engine, might want to replace the water pump before using it to much.

I think the shorting switch works by connecting both sets of points togeather and killing the engine. I don't have a manual in front of me right now, will try and check it out for you later.

 

[mikesea]

Re: '74 40hp only runs on one

if your talking about the 2 wires to the switch,the switch just connects the 2 together,so what is happining is supposed to happen,only one wire goes back to short the points.The first thing you should do is a compression test,then get a spart tester and make sure you have good spark that jumps blue in color 7/16 of an in.you may have coils going bad,very common for those yrs to have coils that are under the flywheel to breakdown,you may get weak spark,but once it goes thru a sparkplug under compression u lose it

 

[fisherman007]

Re: '74 40hp only runs on one

Ok the shorting wire thing makes sense now, but they do work just by grounding them as well. I have replaced the water pump and lower unit oil. I haven't checked spark color or distance but I will tomorrow. Mikesea, you mentioned the coils being under the flywheel, am I wrong in thinking that the two boxes that the spark plug wires come out of are the coils? I haven't done a compression test either but that will be next. BTW, how does the shorting switch work? I see that it has a vacuum line to it.
Thanks for the replies guys I really appreciate it.

 

[tashasdaddy]

Re: '74 40hp only runs on one

you have a coil under the fly wheel, that creates the electricity for the spark, you also have 2 sets of points, one for each cylinder coil. shorting switch, kill one cylinder, incase of a run away engine. do some reading here, good stuff >>> http://forums.iboats.com/showthread.php?t=299680

 

[fisherman007]

Re: '74 40hp only runs on one

Ok, I tested compression. Bottom cylinder(The one that runs good) has 65psi. Top cylinder(The one that doesn't run) has 80 psi. I assume that compression isn't the problem. I checked the spark with an adjustable tester. Both cylinders sparked blue across at least a 7/16" gap. Now I'm really confused. Spark plugs?

 

[mikesea]

Re: '74 40hp only runs on one

sorry to confuse you,your engine has external coils,the older models had the coils under the flywheel,sorry.Basically,you havea set of breaker points for each cyl.and a condensor located under the flywheel,YOUR engine has the coils with the spark plug attached.Any one of those components could cause a no spark issue.Points and condensor are a maint.item.A once a yr thing.I suggest you get new points and condenser to start.Easy to change.You set points with a feeler guage to .020.Its still possible you have a bad coil.Once you change the components,check you spark,if you happen to still have a dead cyl.SWAP the coils,likely the same coil wont work.Get a replacement.There is equiptment to test those components,but u and I likely dont own it.Again ,soory for my mistake

 

[fisherman007]

Re: '74 40hp only runs on one

No problem Mikesea, I appreciate your help. I see on the parts breakdown that there are some "driver coils" under the flywheel. I don't know what they are or what they do I just thought you were referring to them. Thanks again for all the help guys. Any insight into the testing results?

 

[fisherman007]

Re: '74 40hp only runs on one

I changed the spark plugs and it seems to run a little better and there is an actual difference in performance when I pull the plug wire off of either of them. But it still seems that one cylinder is definately doing more than the other. Maybe because of the compression difference?

 

[fisherman007]

Re: '74 40hp only runs on one

Any thoughts?

 

[Daviet]

Re: '74 40hp only runs on one

I changed the spark plugs and it seems to run a little better and there is an actual difference in performance when I pull the plug wire off of either of them. But it still seems that one cylinder is definately doing more than the other. Maybe because of the compression difference?

Try swaping the coils and see if the poor preforming cylinder moves. If maybe you have a weak coil.

 

[fisherman007]

Re: '74 40hp only runs on one

I tried that but it's hard to tell if it changes without being able to get it under power. I'm heading out this weekend, so I'll take some tools and do some experimenting. Thanks for all the help guys, I really appreciate it.