Weak spark on one side

[The1Chef]

Need some help on this one guys.
I've got spark on both cylinders but one isnt strong enough to start the cylinder. If I plug the coil that is firing into the harness wires for the second cylinder, the cylinder that was running no longer runs. I have switched to 2 other cdi units and the problem persists. I've switched coils and plugs as well. I know the problem is coming from the harness, just not sure what to check next. Would the stator cause this? Or maybe a broken or loose wire coming from the stator?

[Lechy]

first fit NGK DP8EA9 plugs, you already have a resistor in the plug cap so a resistor plug is not really required (it was an import requirement), next would be to dismantle the innards of the plug caps and give the parts a good cleaning, then cut 1/2" off the HT lead. Turn on the ignition and check for 12 volts or more on the coil tabs, both wires connected. Remove the trigger wire from the problem coil (o/w is power the other 2 are triggers), ground the plug and short the trigger side of the coil to ground, this should give a good spark at the plug each time. Closely inspect the plastic connectors and wires .
Try to inspect the wires going into the alternator cover, I had a couple break at this point.

[The1Chef]

Hahahaha sorry Letchy you're going to have to dumb that down a little for me.

Dismantle plugs? Stator plug?
Ground the stator plug and the trigger wire to short it out?

I'm lost bud

[The1Chef]

Also, I just took this harness from a donor bike that I assumed was running rich and sputtering. I am now assuming that it was also getting intermittent spark so I actually believe the problem is in the harness. Again not q00% what to look for. I assume that I need to chase the power through the switches etc and back to the coil wires and cdi

[navigator]

Hahahaha sorry Letchy you're going to have to dumb that down a little for me.

Dismantle plugs? Stator plug?
Ground the stator plug and the trigger wire to short it out?

I'm lost bud

The easiest place to start is at the spark plug boot.
Unscrew the plastic boot from the spark plug wire. then cut off about 3/8" to 1/2" from the end of the wire and screw the boot back on. There is a small spike in the boot that pierces the center of the plug wire for a good connection...clean that spike too. This often improves spark quality.

[The1Chef]

Thanks, I know it's not in the boot. It's either in the harness or in the stator. The spark is fine when plugged into the working wire in the harness

[Lechy]

Trace the wires from the CDI back to the alternator. There should be 2 plastic connectors 1 near the CDI and another near the rectifier, check the wires are not broken where they enter the connectors. check that the brass connectors are clean. Check where they enter the transit block in the alternator cover as per the pic I posted, they tend to get hard and break inside the sheath, happened to me so I replaced all the wires coming from the stater.

Plug cap innards:

[The1Chef]

Perfect, thank you. Hopefully it's a quick find. I. Lucky the bike the harness came from only had 6000 miles.

[The1Chef]

So I have narrowed it down to the white trigger wire that runs from a coil to the cdi. It's not the cdi or the plug, I have checked 2 other cdi and same problem on all 3. The plug is perfectly clean and I added some dielectric grease.
My plan now is to splice into the white wire near the cdi and run a new wire from there to see what I get.

I do have spark, it's just not firing up a cylinder and I can see visibly it's not as strong as the other plug. Even if I start and warm the engine then switch coils, it still won't fire up the cylinder that was just running

[Herb]

From my experience there are only 4 things that can cause a weak spark. Coil, spark plug, spark plug wire or low power to the coil. If there is spark then the trigger wire is good or you would have no spark at all.

keep in mind that a plug that has a weak spark may not be sparking at all under compression.

When I worked in a gas station with an auto repair shop we had a spark plug tester that allowed us to watch the plug fire. We could control rpms and cylinder pressure up to 10,000 rpms and 300lbs pressure. I have seen plugs that would fire fine with no pressure and get them up to, as low as, 80 psi and they would start to misfire and 90psi they would quit completly.

[The1Chef]

So.....I'm now guessing it's not a spark issue. I have spark so maybe it's not that it weak.

I have fuel, the bowl is filling and front cylinder runs nicely. What I am getting is this.

When the front cylinder is at idle I have vacuum at the rear but the rear won't catch. If I accelerate even slightly the rear cylinder spit the fuel back out through the carb. I've cleaned the carb several times, removed all jets air and fuel, taken the choke assembly off and blown compressed air through all holes. The bowl is filling, I know because I've drained it 10 times lol. I'm going to take the carb off again tonight and blow compressed air again through all holes to be a million % certain.

When I pull the plug it has a slight coating of oil on it but running the engine with the plug pulled and only air comes out the plug hole. No fuel from what I can tell. Also if I plug the spark plug hole with my finger it blows it off the hole.

Also, I get nothing from the exhaust, no even cold,warm air....nothing.exhaust valve stuck shut? Would that stop it from starting?

[The1Chef]

keep in mind that a plug that has a weak spark may not be sparking at all under compression.

When I worked in a gas station with an auto repair shop we had a spark plug tester that allowed us to watch the plug fire. We could control rpms and cylinder pressure up to 10,000 rpms and 300lbs pressure. I have seen plugs that would fire fine with no pressure and get them up to, as low as, 80 psi and they would start to misfire and 90psi they would quit completly.

Hmmmmmm but it would still run at idle

[navigator]

A. you need a strong battery, have yours load tested at an auto parts store.
B. did you sync the carbs after removal and cleaning? if not, do so.

[Herb]

keep in mind that a plug that has a weak spark may not be sparking at all under compression.

When I worked in a gas station with an auto repair shop we had a spark plug tester that allowed us to watch the plug fire. We could control rpms and cylinder pressure up to 10,000 rpms and 300lbs pressure. I have seen plugs that would fire fine with no pressure and get them up to, as low as, 80 psi and they would start to misfire and 90psi they would quit completly.

Hmmmmmm but it would still run at idle

Nope you have more than 90 psi while cranking. and at idle.

[The1Chef]

A. you need a strong battery, have yours load tested at an auto parts store.
B. did you sync the carbs after removal and cleaning? if not, do so.

I can't sync the carbs if the rear cylinder won't run

[The1Chef]

Well it turns out that I have an exhaust port either blocked or balve not able to open it. Luckily I had another motor to throw in and I am up and running Ning. Thanks for the input all.

[Herb]

Well it turns out that I have an exhaust port either blocked or balve not able to open it. Luckily I had another motor to throw in and I am up and running Ning. Thanks for the input all.

Real curious about exactly what was wrong. If you tear it down and find the problem please let us know what was wrong.

[The1Chef]

Well it turns out that I have an exhaust port either blocked or balve not able to open it. Luckily I had another motor to throw in and I am up and running Ning. Thanks for the input all.

Real curious about exactly what was wrong. If you tear it down and find the problem please let us know what was wrong.1

Definitely going to find out. That's MY motor with the je pistons and hot cams. Ran but leaked up top due to a stripped bolt. Fixed that and it sat a year and a half. Never tested it after I fixed it and broke up with the spouse, hence the delay. It's not making noises so I don't feel anything is broken. I'm kind of assuming it's a nest lmao

[Herb]

Well it turns out that I have an exhaust port either blocked or balve not able to open it. Luckily I had another motor to throw in and I am up and running Ning. Thanks for the input all.

Real curious about exactly what was wrong. If you tear it down and find the problem please let us know what was wrong.1

Definitely going to find out. That's MY motor with the je pistons and hot cams. Ran but leaked up top due to a stripped bolt. Fixed that and it sat a year and a half. Never tested it after I fixed it and broke up with the spouse, hence the delay. It's not making noises so I don't feel anything is broken. I'm kind of assuming it's a nest lmao

Thanks for the update, it is appreciated. Let us know what you find. When you tear it down, pics would be nice too.