KZ400 has weak/sporatic spark!...

Since no one's mentioned it yet, look at wiredgeorges' website ( www.wgcarbs.com )under the tech section and check out the coil mod section. He's got some troubleshooting tips and if it's appropriate a modification to feed the coils directly from the battery. Definitely worth reading

Thanks for the input guys.

I'm going to double check all of the wiring and check for voltage (or lack there of in some cases ) very soon. I bid on a $10 coil on ebay so that will be nice to get. I am also going to tear into and check the points.

I'm thinking that I may end up just ripping out the ignition and swap to electronic, but that'll be awhile.

I'd still love any other input if you guys have it.

valvesprung wrote:

...I bid on a $10 coil on ebay so that will be nice to get...

Hmm. I don't know. If you're troubleshooting, and you're not certain the coil is the problem, you probably shouldn't replace an unknown variable with another unknown variable. I'd just pop the extra money for a reasonably sure thing. Maybe even the $40+ one from JCWhitney, if you don't want to spend the $60+ for a Dyna or other name brand....

But definitely do the "coil relay power" mod no matter what... it can only help, and is cheap to do.

The shop rejetted your carb? Hmmm

If you are an automotive tech, it makes sense to verify things on a bike, just like in an auto, that will effect performance and need to be adjusted prior to working on the carbs... Why sync the carbs if there is no compression in a cylinder to produce engine vacuum? (just saying this to make a point).

First thing to do is to check your valve clearances and check compression. Make sure that the cylinders are the same compression. Make sure you hold the throttle open when checking compression.

Next, if compression is good is to figure out why you are getting poor spark. If you held the plug electrode near ground and saw weak spark, I would FIRST replace plug points and condensor. Set the points gap using a dwell meter as specified in a manual and set the timing using an inductive timing light. Replace the plug wires and boots and use some new NGK spark plugs because the application charts used at most autoparts stores where they hand you a set of Autolite plugs are not accurate. Use the correct plugs! It wouldn't do any harm to read the tech article I wrote on COIL REPOWERING linked off our website under "wg's Tech Stuff Index". Most older bikes get poor voltage at the coils due to the path that power takes to get there and subsequently the coil(s) don't fire as effectively as they should.

OK... the carbs... cleaning visible varnish is OK but doesn't help the carbs work properly. The issue is varnish in the internal passageways and jets. If you had varnish, the carbs need to be removed, disassembled, soaked in dip then cleaned ultrasonically. The rejetting comment keeps coming back to me... Most bike shops won't know where to get jets for a KZ400 carb... slow jets are not available and the small headed secondary and main jets are a problem for them generally. Why would they rejet? Are you using pod filters or aftermarket exhaust?

Compression is deffinately good and equal, it has the proper NGK plugs (which now need replaced due to fouling), the wire are integrated into the coil, but they aren;t on the new unit (the one on the way), so I will upgrad the wires.

Next as I stated will be checking for burn marks and proper spec on the point and double checking voltage throughout.

BTW I do have pod filters.

So here is my help or attemtp at help. I have a '79 400.
I did the WiredGeorge coil mod, and it helped a lot with cold starting.

The 400 requires that both sides of the coil be corredtly connected in order to fire correctly. So if one plug is bad the motor runs funny if at all.

Also check your timing advancer. Located behind the points plate. It should open and close freely. My bike wouldn't rev past 5k, because the advancer was rusted in solid.

That may prove very helpful thanks

Thanks for all of the help guys.

She runs, and runs like a champ.

I'm not sure if it was the worn wire insulation I repaired, the cleaning of the carb, the new spark plugs, or the filling of the battery (it was out of water, but mind you it was being tested on a charger), but it runs.

I took it out for it's maiden voyage yesterday now that I have my endorsment and had a blast.

Thanks again for all of the words of wisdom.

Its still not a bad idea to check your points to make sure there is no pitting. If there is I would suggest replacing them instead of filing them down and making sure that you replace the condensor too since that is usually what makes the points pitted in the first place. I just replaced the points on mine and it was like 20 bucks for the points and condensor. Also I had a snowmobile with a similar spark problem and all i did was take a business card and turned the engine til the points were open and stick the card inbetween them and then close them and pull the card out so that they are clean. I did that a few times and it ran great. of course that was a two stroke so i have no idea if that relates back to this.

onuLine57 wrote:

clean and gap the points. I am having the same problem with my 650 and I seems my points are bad and are burning up quickley and are causing me to loose spark to the 1/4 cylinders.

Points that fry quickly may have a bad condenser next to them. That condesner (capacitor) reduces the arcing at the points if it is doing it's job.

valvesprung wrote:

I kicked her off and she fired right up...then died. A few more kicks and no luck so I gave full throttle (clear flood) and she fired but was running on one cylinder or both very poorly. It would barely hit 2k RPM at full trottle. I pulled the plugs and found soaked, cleaned and tried again.

I would have guessed a float sticking open flooding one of the carbs from those symptoms.