Jump to content
Sign in to follow this  
Joseph Krach

40 Hp 1975 Johnson Missing

Recommended Posts

I have a1975 40 hp 40el75 that runs but is missing an has no power under load ive tuned it up with pts an condeners rebuild carb checked compression both at 120

checked fuel filter an is clean I put new plugs in an it ran great an had power but it went back to missing after a short while I checked the plugs after that run an they were black one was wet do I need a hotter plug im using ngk B6S an I have also checked the spark an it is strong on both plugs should I try re gapping pts ? if you have anything let me know

Share this post


Link to post
Share on other sites

There no way to say exactly what's causing the miss fire. As a minimum, you need a peak volt meter and make sure both coils have approximately 250 to 275 volts peak to them. This will tell you if it's before the coil or the coil/plug wire. It MUST be a peak reading meter or an adapter plugged into the meter to read peak voltage. The adapter is made up of a diode, capacitory and resistor, but there is no way to check it with a standard VOM, it's going to read a whole lot less.

If the peak voltage is good, on the coil input then the coil or coil wire might be bad. If one of the peak inputs is low, then it could be the power pac, timer base or could even be a weak magnet in the flywheel. It requires a manual and know how to trouble shoot those parts.

This is just checking the electrics side, it still could be a fuel problem. Spray a fine mist of fuel in each carb and see if it stops skipping. If it does, then you have a fuel delivery issue.

The plug that's wet is the one it's skipping on. No, do not install hotter than factory plugs. On carb motors, the plugs are always going to be black when it idles. The way you check the plugs and jetting for proper mixture settings is run the motor at WOT for about five minutes, then while still at full throttle, cut the ignition switch off. Then when you pull the plugs out, they should be a paper bag brown color.

Share this post


Link to post
Share on other sites

I have a digtal volt meter could I use this to check the peak voltage? i also have the service manual

Share this post


Link to post
Share on other sites

Missing, and has no power under load. If memory serves, going back 30 plus years, the timing changes and butterflies open when throttle is advanced. If it doesn't miss/skip, at idle, but it starts to miss under load when you advance the throttle, my guess would be you have a spark plug wire or another wire that is shorting out when the timing is changed. I've seen it with a pigtail in an automobile distributor and a spark plug wire on an outboard.

Share this post


Link to post
Share on other sites

Ok, I did a search and if you go here, there is a post that shows you how to make a DVA. This will keep me from having to reinvent the wheel. http://forums.iboats...ad.php?t=371144 This is a basic DVA and will not let you check the voltage from the timer base but you could probably find one that uses op amps for that, but that one might be a little over your head.

Here's a ready made DVA you can buy. http://www.maxrules.com/fixtools640.html

Share this post


Link to post
Share on other sites

its missing at idle to it one of the coil packs looks fairly new the othe one looks original to the motor im wondering if this one may have low voltage?

Share this post


Link to post
Share on other sites

No, you can not use a standard DVM (digital volt meter). They read rms and will not show enough voltage. For instance, you plug you DVM into a wall outlet and it reads 120 volts AC. Use a peak meter to read the wall outlet and it will show approx 160 volts. Plus the pulse is going to be too fast for most digitals and make the reading even lower, so you can't do the mathematical conversion from RMS to Peak.

Switch them around and see if the skip changes cylinders. The wet plug should be the one skipping but just to double check, pull a plug wire, if it keeps running with no change, then that's the one. Swap the coils and try it again. If the same one is still skipping, then you can feel fairly certain it's not the coil or wire.

It could be getting too much fuel to that cylinder and wet fouling the plug. Like I said, it's a matter of determining if it's fire or fuel. Either will cause a skip.

since it only has one carb, I that would be my last suspect, but it does run twin reed plates and it could be a broken reed, I've never seen one break but nothings impossible. Like I said, you just have to go through a logical sequence of troubleshooting. Easy enough for me to do but trying to quess and explain is rough.

Share this post


Link to post
Share on other sites

ok got a new coil replaced the lower one found that the old one had 2 cracks in it fired up the motor seems to be running better wont know until I find a drum to put it in or put it in the lake to see if it has power will see soon I hope

Share this post


Link to post
Share on other sites

Ok took the boat to the lake today an it took off like its rear end was on fire almost lost my favorite hat couldnt be happier thanks for your help now I can fish yeahhhhh!!!!!!!

Share this post


Link to post
Share on other sites

your compression is good 

Move the mageto plate (armature plate) i.e. the plate points condeser and (driver) coil and stator coil is bolted to

back and forth lateral if any play at all makes points have wild irratic clearances if take off flywheel you can see

point setting change when you move back and forth .the plate has a brass bushing that has excess wear you

can buy new one $125 I got lucky I made a shim out aluminum beer can and it worked perfect

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

  • fishing

    fishing forum

    fishing rods

    fishing reels

    fishing

    bass fish

    fish for bass
    fish

×