A friend sent me a screenshot of a bike for sale at a local shop. It was a Free Spirit with a ZA50 swap, piston port 70cc kit, 15 bing, boss pipe, and some nice paint and details. I had no history with the bike but it seemed nice, she liked it and that was that.
Once she got it though it seemed to only do 30mph. No worries she thought. She had a 70cc treats reed kit, malossi 4-petal reeds, PHBG, and techno circuit so he swapped it over. Making the assumption the bike was slow because of simply not having fast enough parts. Alas, it still did 30. So she called me.
She brought the bike by the house and it was time to do some basic problem solving. Now I had never seen this bike before but she rode it to the house so it had to be close to running right. Right? You can tell by the article length clearly that is not what happened.
I am going to try to write this in a way that you can follow my logic progression as we worked through the issues. I am probably going to fail at that. What I knew at the start was this was a bike I had never seen before so I should make no assumptions as to the quality of the work done even though it came from a shop. I also know that it ran 30mph, the whole top end was replaced and it still did 30. So I planned to start with checking every single major part of the motor.
The first test ride, before I checked anything, left me thinking that it ran, the carb settings were a bit off, and it had no real power, but it idled, ran, and shifted. I knew the top end parts combination should be an easy 45mph setup so I knew something was up. Although the difference between 30 and 45mph is not usually a jetting issue, it’s simply too large of a gap.
I needed to establish a baseline of settings. I checked the ignition time with a dial indicator and timing light, and while it was a little off (too close to tdc) it was not far enough away to account for the low power alone. Timing was then set with a timing light to 18dbtdc, a nice easy starting spot. Next was a compression test. Too low or too high compression can be a big problem, but it came in right around 140psi and honestly that’s about perfect. Next was a basic external spray test looking for air leaks around the main gaskets but it had no issues at those locations. I then popped the reed block off to check the reeds are good and that the piston was installed correctly. And again, everything was perfectly fine. So far everything is ok, and that was suspicious because it ran terrible. Next I popped the carb apart to see what was going on in there.
The carb was setup by me for one of her other bikes, a piston port k-star. She swapped it over with no adjustments assuming the setting would be the same. While they would be close there would be some minor adjustments between jetting. The first thing I noticed was the idle rpm adjustment was set really far in and the fuel adjustment screw made no difference. It would idle but only because the slide was being held open too far. We have a lead. Nonresponsive air adjust and too high idle screw would normally indicate an air leak or a tragically setup carb. I took the carb apart and needle was on top of the retainer cap so the needle was floating around. An easy fix but no change in how it ran. Ok, we can do this, just have to be methodical. I know there are no air leaks on the outside gaskets but that leaves two crank seals to test. This ZA50 is a maxi which means no oil injector to fail leaving only two options, the crank seals (I know the case gasket can be bad too but that’s much less likely). I removed the carb and attached a rubber cap I built with a tire valve in it. Using a bike pump on the adapter, I pumped air into the crankcase and sprayed soapy water onto the trans breather hole. It bubbled, there we go. Air is leaking from the crank into the trans. It explains the idle issue and the large amount of smoke from the exhaust. So I told her it needs a new crank seal and to send it back and have them fix it. This was a positive test that indicated a failure on one of the seals and while it would solve two of the problems (idle settings and lots of smoke) I didn’t feel it would fix the main issue of low power, but maybe it was just burning lots of trans oil and making carb jetting impossible. We called it for the day, feeling ok with the diagnosis and progress made.
A week later she messages me they had changed the seal out, and rejetted the carb. Good news! Then she rode it and it still did 30mph. Bad news! Ok but what does it mean? Well the idle circuit adjustments actually started to respond correctly and the slide was down at a reasonable level. So the new seal did fix what I expected it to do. Now I think that I can adjust the carb to see if I can get the rpms I was expecting.
There are a couple of other things to pay attention to on a ZA50. The rear gearing is different from an E50 and this bike has 19:42 which is kinda short. Another thing is there are two different sets of internal gears as well. A 38T was ordered to get the gearing a bit more on track but my gut feeling was the motor wasn’t making enough power yet to even pull that taller gearing. I also didn’t open the trans to count gear teeth so I just assumed it was a 2hp trans and called it a day. The correct thing to do is to put a tachometer on and see what rpm the motor is actually hitting but mine was on loan at the time so I was just going on feel that day.
She brought the bike back by and we started with carb jetting. Upon the first run I noticed it felt really rich on the main jet. Simple enough, jet down, move the needle down (up one clip), try again. It runs the same. Hmm… ok, well that’s odd. Normally when you make carb adjustments you generally have an idea what the difference will feel like and when the bike does not respond that way it can be very confusing. Right here is a place where a lot of people get trapped. Commonly it is assumed that it is just a jetting issue and they will continue to attempt carb adjustments. This can be wrong and leave you in a loop. I noticed that when I made small changes in the jetting (2 points on the main or one clip on the needle), the bike responded with a very drastic change in how it ran. This clued me in to the carb not being the problem and I should look at the ignition.
So a common basic test on an ignition I will do is attach a timing light and then rev the motor on the stand. If you have a bike that misfires you will notice the light will not light in time with the motor misfiring. That tells you the ignition is missing and needs to be addressed. The limitation of this test is typically is it done with no load on the engine. Now in this specific case I was feeling that I was having an issue with ignition flame out under load, which will not show up under the no load rpm sweep test. Basically what happens is the ignition is not producing enough voltage or amperage to fire correctly when the motor starts to produce more power. The ignition on this bike was the treats internal Wat Yong points coil, a notoriously weak ignition coil. So the next step was to replace that stator plate for a stock points plate. (You can actually test the peak voltage of the ignition coil and here is a handy guide!)
A couple days later she texted me back they had swapped a new internal coil stator plate and it immediately started running 40mph. Finally! The temp sensor during this whole adventure has never really read over 300F which is very low for a running bike. I gave it another test ride and it ran really smooth through the throttle. No dead spots with the exact same setting, which was a constant issue before. At this point some minor main jet adjustments (too rich but responding correctly when changed) and different rear gearing and it became a reliable 45mph+ bike.
The main takeaways from this journey is having a plan for diagnosing. It’s very easy to fall into pits and waste a lot of time on trying to fix things that are not actual problems. You can also have multiple problems and after fixing the first one not realize there other issues still to fix. Repairing and tuning bikes is a series of problem solving issues and without the understanding of the systems involved it’s very easy to lose your way. Stay focused, don’t redo tests once they are complete, and be methodical. Find your basic settings and recheck everything you might have taken for granted when hit with a hard problem. It sometimes feels easy to get lost and I still do it. But with patience and care you can sort any bike out in a reasonable amount of time. Good luck on your repairs!