Extreme Troubleshooting : a more complex exmaple of getting a two stroke engine to run

A simple job

After picking up the bike at the Telford show, on a Sunday I started to prep the bike, including putting oil in the crankcase and the forks. Also noticed the the stator wasn’t wire to the ignition coil and only one connector was present. Motoplat electronic ignition is a standard replacement for early 4 speed motocross Huskys, as they became standard in the early 90’s. Also took a look at the Bing 54 carb, which was of a type which wasn’t OEM to the Husky and like the Mikuni mods you see needed a rubber joiner to the inlet manifold.

Looks great, but not running

Statement 1 : You need a spark

Statement 2 : You need fuel

Fitted a connector to stator wires and put some fuel in the tank. Got a spark and got fuel in the carb, time to kick if over. Quite a few kicks and a quick push and attempted bump down this hill and nothing. Check the plug and it was wet, so fuel getting to the cylinder. In debugging bikes you then move to statement 3

Statement 3 : You need spark and fuel at the right time

So next step was to check the ignition time. There are a number of types of Motoplat ignitions found on Husqvarna’s but common for the 4-speed model with the smaller crankshaft is one with internal stator and external rotor. To set the ignition time it couldn’t be simpler. Set the bike to Top Dead Centre (TDC) and then line up the flywheel / rotor with the stator via the small holes and a pin (opened up split pin in this case). One thing of note, is that the crankshaft has a woodruff key and the rotor lines up with this, so you need to move the stator around to get it in place. As with many Femsa conversions, they’ve made up a back plate, which ensures that the stator is in approximately the right position and the original adjustment slots can then be used to line up the rotor.

Now in some cases (and I have this on the Bolt Up Husky, the woodruff key location for the original Femsa rotor is completely different to the one for the Motoplat rotor, that as setting up an aftermarket ignition like PVL or PowerDynamo you have to ditch the woodruff key completely.

Set up the ignition as per the Clymer Manual, that is the holes lining up on TDC. Fellow bike and shed enthusiast Paul then comes up in the evening and checks through what I’ve done and kicks it over. We agree that this is down to timing and there is the odd kickback and puff of smoke from the exhaust and that it must be out. Paul also rightly pointed out that for most Motoplat set-ups, like with KTMs and Gori’s the holes are lined up at the point of firing, which for a 250cc is usually 2mm Before Top Dead Centre (BTDC).


So Sunday evening was consumed by timing adjustment by Paul and myself and quite a few kicks of the kickstarter and a couple pushes down the hill. Still fueling (wet plug) and still a really nice spark. I’d had a problem with a Montesa 123 a few years ago, see the article , which was another bike where I had fuel and a spark but it didn’t run. This came down to plug gap and rating. Some gapping (0.70mm) and swapping of Bosch and NGK plugs took place. Still nothing so Sunday night ended with a couple of beers (the excellent Blanche de Bruxelles).

The CR250 ready for further inspection


During work on Monday was thinking about next steps and one obvious course was to take a carb and ignition system off a running bike. So by the time Paul came round the garage in the evening, I’d swapped the ignition from the Bolt Up Husky (which had the same crank) and taken a relatively new Bing 54 from another bike, also with working provenance. This did involve changing the inlet manifold because of the rubber fitment. About an hour timing changes, checking plugs and fueling with the new components still nothing from the bike. So, off the came and we decided to go back to the carb and ignition from the bike, so why I put them back on the donor’s Paul started going through the Bing 54. This wasn’t one normally fitted to Huskys, but reckoned to be from a Sachs or KTM as it an open choke slot on the slide. Some another 54 was found from the extensive Bing carb collection, with float height and jetting (185 main, 45 pilot) changed to meet known working type. Ignition was set back to the initial TDC mark before another round of kicking was done. Further adjustment of timing, all the while with spark and fuel, with copious amounts of bike kicking. Another fruitless evening and adjourned for a beer or two, with Hopus and Jupiler being the choices. This was well earned after 6+ push start attempts down the hill, with the engine trying to cut it, but where it seemed to be choked up.


Spent a day working with a lunch break to finish the Yamaha TT500 after getting it running over the weekend and hit the garage again at 5.30pm. First job was a modification to the back plate to give it some extra holes so we could try some different timing positions. With the alignment right on TDC, and with a wet surface on the road decided to push the bike up to the pub car park before launching Paul down Eddie-the-Eagle style. Still needed me to push, we resulted in my emergency stop into the wall and the bike still spluttering and not starting correctly.

Ready to rumble

Next thought was to check the earth on the coil as the frame had recently been powder coated. Though it was sparking, it might be weak or failing under load, so stripped off some of the powder coat to get a better earth. Also turned the bike upside down to ensure there was no additional petrol in the crankcases, which there wasn’t. Time for a cup of tea and some further discussion. Though the bike was sparking whilst in the kitchen waiting for the kettle to boil thought I’d check some forums/pages on similar issues. I came across this thread on a failing Motoplat : and with tea and biscuits in hand return to the shed. Running the tests on the coil showed it was operating in tolerance (20-30 Ohms between black and blue and ~200 ohms between blue and the frame, earth). However noticed there were a couple of small colour marks on the coil, and that the blue and black cables from the stator were swapped. The coil has a 6mm and a 10mm male spade connection, but as the stator wires didn’t have the original connectors, then they might be wrong.

Motoplat Ignition
A swap of the cables and another kicking session. Nothing, but a quick squirt from Paul of the evil that is EasyStart into the carb and some response. Next kick the bike fired up. A quick dark run up the lane and seems like the bike is 99% right. Complete failure to success in about 15 minutes.

Retired to the house for a Westmalle Trippel, a worthy beer with which to celebrate success.

Lessons Learnt

Never trust an ignition
I’d installed the PowerDynamo ignition on the TT500 over the weekend as well and up and running first time. The big thing that threw us was the we had a spark, even though the coil was connected to the stator incorrectly. Also, swapping the complete unit from the other Husky also failed to get the bike running, including using the new carb. That was wired correctly, but maybe didn’t spend long enough on it.

It’s usually something you’ve done
Rather than part failure it’s usually how these have been together that is the problem if its not obvious. There are exceptions but as a general rule of thumb you can usually be sure that this is a factor.

Whether it’s computer systems, or 2 stroke engines, you need to be systematic in your approach. This potentially needs to start as you are building or rebuilding, not just when you are searching for the problem. Checking the connectivity of the stator cables rather than going with the connector that was present and trusting a spark seems great in hindsight but not something you always do. When you make a change you need to repeat the test and starting process you would do normally. Trying to kick the bike over 5-6 times might not be enough, as experience with other 4-speeders is that there is a specific starting sequence depending if you have the side-float or centre-float Bing carbs [[Side floats you tap the tickle 4-5 times and turn the fuel off, don’t flood completely, and then 3-4 kicks and it starts, Bing 54’s need flooding, turn the fuel off and then a few kicks before the kickback takes off your ankle. It will fire next kick.]] .


Always easier to have more than one brain looking at the problem as collectively Paul and I looked at the issues with different points of views. Sometimes you disagree but also you can work through ideas before you dive into a fix. Also there is an element of competition as you both try to be the person looking to solve the problem. Paul’s offer of buying the bike for £500 about an hour before we found the issue was becoming increasingly attractive but also an incentive to finding the problem.

Tea and biscuits

No coincidence that the eureka moment came whilst I was making the tea.

Statement 4 : Always take the time out to put the kettle on.

Many thanks to Paul for his assistance, but it has renewed my faith in the garage / shed as a place for men (and women) to embark on journeys of inner discovery. And swearing.

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