Phantom short

[Wrench]

I was having a problem with my Whizzer NE-5 M/B misfiring every once in a while. At Idle, mid range and full throttle but not always only once in a while.
Sometimes it would start missing at idle and sounded like the fuel air mix screw was off a little. (too rich or too lean) Adjusting the A/F screw did nothing.
Next I cleaned the Carb That seemed to fix it but the misfire soon returned.
So having a 4 stroke I checked the valve lash. The lash was OK. I ruled out valve float Because it acted up at idle besides full throttle.
I check the S/P and wire Check for a compression leak Check the trigger P/U and wire to the CDI Checked wires to coil
The CDI is in question now. It's old and I figure it must be weak and it's time for a new one. I put in a new 5 pin AC CDI.
The bike ruins great no missing and I think I have fixed the problem only to have the bike start acting up a few hundred miles later.
I'm thinking the new CDI is failing, but why so soon? I decide to start wiggling wires and the wiring harness to see if I can duplicate the problem. I can
I chase the problem to the CDI plug. It turns out one of the contact pin connectors was dirty, loose and hardly making contact. So I took all 5 pin connectors out of the plug cleaned and tightened up the little slots where they slide together. put everything back together, Now my bike runs great again NO missing.


I tell this story because it has taken me 5 weeks to figure out a plug contact was dirty and kinda loose.
While working on the bike or just riding the bike I would bump the wires and plug from the CDI by accident and that was making it better or worse without me knowing it.
The 5 pin CDI plug looks good and the wires can't be pulled out. Looking at the contact pins and connectors from inside the plug they looked good too. But one contact pin connector inside was dirty and loose making a weak connection. A Phantom Short

 

[Chainlube]

90% of all carb issues are electrical, so all to the noobs out there, check your electrics before you replace a carb.

 

[Sidewinder Jerry]

Another culprit for misfires can be the bad kill switches. Also don't forget about bent keys in flywheels either.

 

[Midwest Rider]

Another culprit for misfires can be the bad kill switches. Also don't forget about bent keys in flywheels either.

So true Jerry. I got mad at mine and yanked the kill wire out. Problem solved. Makes it kind of interesting to kill the engine now

 

[Sidewinder Jerry]

So true Jerry. I got mad at mine and yanked the kill wire out. Problem solved. Makes it kind of interesting to kill the engine now

A bent key in the flywheel is most common on mowers. Which is why if I'm buying a used mower (push or riding) I always check to see if it has a bent blade on it. If a used mower has a bent blade don't buy it. There's no telling what kind of internal damage it caused inside of the engine.

In our world a bent key in the flywheel could be the result of a burnt out clutch.

 

[Frogslayer]

A bent key in the flywheel is most common on mowers. Which is why if I'm buying a used mower (push or riding) I always check to see if it has a bent blade on it. If a used mower has a bent blade don't buy it. There's no telling what kind of internal damage it caused inside of the engine.

In our world a bent key in the flywheel could be the result of a burnt out clutch.

I've run into bent/ sheared keys mostly on outboard boat engines but i bet you're right, someone probably hit something with the prop and the damn shear pin didn't do its job and let go.