Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Well after all hard work , somehow either the CDI or Hall sensor gave up. Luckily I used PayPal to claim refund and maybe Microcosm learns to put more effort and quality in their products

Just curious how you return something that you did so many modifications to.

Time for an update, I haven't given up on this yet.

One of the biggest downside of this engine , in my opinion, is the carburetor. The 2 in and the overall design of it, while simple and functional, is not the most adequate in this application.

It's nearly impossible to match both cylinders intake opening identical as I noticed the crank shaft springed push mechanism in each cylinder is set differently. So the pushrod size needs to be different, however due to the nature of the carb, when the intake happens, the mixture takes the most direct path ( right cylinder viewed from the back ) and it leaves the right cylinder with a lean mixture.

You can  see in some videos that the right exhaust pipe is black and has a liquid soot which is unburnt fuel. Upon inspection of spark plugs, the right plug is shows the sign of rich mixture when the carb is opened enough to give enough mixture to the left cylinder.

You might think why not shorten the push rod on the intake valve on the right cylinder ? Well there is a very limited room for change there, otherwise the push rod will slip our of the valve rockers. This is a poor design which does not follow the proper pushrod architecture. Allowing you to adjust using a screw or similar mechanism.

Now, another issue which others have experienced is the CDI and hall sensor failing. 3 reasons based on my experience.
1- electrical "leak" through poor design of the ignition leads, this was also visible inside the CDI unit when running it in pitch black. This has mostly resulted in the fault which causing misfires continously. Tested with a another microcosm CDI ( used ) and confirmed this. I'm waiting to receive RCEXL CDI and put it to test.

 2- Poor ground connection. For whatever reason the way CDI is grounded is not efficient and this has lead to firing the Hall  sensor twice already. Attaching a wire and bring to the bottom of wooden base to touch the ground sorts this out

3- tenperture of hall sensor , the 44E type comes in 2 temperture resistant categories. Entry and Industry. The type used in this engine is the base which withhold upto 85C . The Industry type can stand upto 115C.  It also comes down to the build quality of the sensors. I beleive this has an impact on the longevity of these sensors inside the engine case when it's run for a while. A good cooling/ direct air circulation is necessary if you run these engines more than 5-10 minutes.


Lots of learning here for me, from such a simple engine, yet challenging. Makes me appreciate the marvel of automakers when I took it for granted

Glad to hear things are moving in a suitable direction, I certainly believe the grounding is a big source of issue on these, on the H75 it goes through a bolt into the base, where the cdi connects to the end of the bolt. I can't say that was all the issue, but improving the ground seemed to help.

Totally agree with you, since I modified the grounding with the replacement CDI, noticed almost no misfires which can result in arching else where in the CDI or hall sensor.
The extebded round wire is now fixed under one of the foam legs on the base. It's discreet and stays in place with a tiny bit of superglue.

Thank you for those great tips!!!

I connected a cable to the gorund screw where CDI connected and brought it to the bottom of one of the foam legs

Just wanted to give an update on this engine. Since I made the necessary changes it has been running great. Predictable and very satisfying to run. If you are planning to get this engine be mindful of mechanical knowledge needed to tackle issues and short comings of the factory assembly.

It has gone through 30 tanks of fuel. Nothing broke or worn off so far.