I tried this in the Gas Engines Forum and was pointed here...

From past experience with 2 stroke gas engines (motorcycle related), I know there is usually a lot of clean up and fine tuning available on mass produced engines. Things like matching ports, cleaning up casting marks, matching gaskets to the ports, setting an accurate squish band, etc. Are these things still an issue on our model engines or are they pretty well taken care of in manufacturing? The two specific motors I am thinking about are the Brillelli 40cc and the ZDZ 40. How much improvment is usually realized by sorting these out on an otherwise stock engine?


Thanks,
Mark

I have never had either of the engines mentioned apart to check them out, but basically the answer is yes. There is almost always room for improvement by blueprinting and "cleaning up" any mass produced engine. In the case of our conversion engines, there is generally a lot of room for improvement, because they were designed to be "idiot proof" in a machine that would work for hours at a time at high throttle settings in tough environments. Hence they are in a low state of tune to begin with in order to make them last.

I believe the Brilleli engines are all based on such engines, and I don't know how much they do to tweak them. I don't imagine it would be too much, as to do so would raise the price of the engine significantly. (Anyone that has direct experience with the Brillelis feel free to correct me here if I'm off base.)

AV8TOR

That's what I thought, too. The Brillelli engines are conversions and I expected that an industrial motor would be made for manufacturing convenience, not max power. They do pretty well as is, but I figured there would be a lot left over even after Scott has worked them over. Add another 10-15% from a good clean up and they would be fantastic values.

Anybody have any before/after comparison numbers on an engine they worked over? Does anybody re-cut combustion chambers to set the squish properly on these engines? This was always a big improvement on the bike 2 strokes.


Thanks for the reply,
Mark

Mark,
The Brillelli would be the one which would benifits most, because it is a less expensive engine. Any gain would be small, compared to the effort. Since both are purpose built engines, the port sizes and timing are already optimized for our use. Experimenting with props and carb tuning will give a greater return for your time.

Dave

Dave how about the Gt25...is it optimized already? Capt,n

Well, the gains would be for free except my time. This is a sort of winter project to keep me from going stir crazy and it really isn't that hard to pull the cylinder and do a simple clean up/match with the dremel and trim the gaskets to match. An easy weekend would get it all done with no sweat. Re-cutting the head is a much more involved chore and I would only do it if I had lots of time on my hands or it was WAY out when I checked it. I agree on the prop/carb/exhaust tuning, it definitely pays the biggest dividends for the time. But I hate to leave something on the table when I know it can be better with a bit of effort....


Mark

A Homelite 25 does really well with minimal mods, 8400 with a Mejzlik 18-6..
$50 for a "remanufactured" weedeater..Or about $75 for a new one from Home Depot...

RCIGN1: Here is my Homelite I did today. RPM= 8300 on 16x6. It was a freebie engine except for velocity stack. Run real nice...stock ignition. Capt,n

Captain,
The GT 25 is a converted Toro timmer, I beleive. He mentioned the ZDZ 40 and Brillelli 40cc which isn't a conversion as far as I know. "The two specific motors I am thinking about are the Brillelli 40cc and the ZDZ 40"

Dave


.

The Toro (Gt25) we did took a ton of work. We increased intake, exahust and transfer ports. We advanced the port timing on the exahust and increased the duration on the intake. We increased and smoothed the transfer ports. The squish band was allready so close there was nothing to do there. Remove the gasket and the piston would hit the top. It was that tight allready. That engine needed a lot of work to get the power, but the end result was very positive. The engine quality was very high so it was well worth it.

Yes any engine can bennifit from little things like smoothing the ports. It is well worth it on your own engine. Just in production it is too time consuming to do. The Toro was the engine we did the most to. Other engines like the 40 allready have huge ports. There is no gain to be had there. Smoothing them may help some. I have found that getting an engine ballanced well will pick up a few hundered RPM. Ballance is very important in these little engines. There is more power to be made there than polishing the ports. Id does depend on the engine though. Some are ballanced well to begin with. The 40 is one that is very bad stock. It is way too piston heavy.

Homelite 25's are good running little engines and can make power with some mods, but too heavy for the size. The 30cc is a better choice to convert. It is the same block, but larger displacement to make up for the weight. They are little pigs

How do you balance them?

I know it's asking a lot, but if you wouldn't mind, could you give a detailed description of how you go about balancing these single cylinder engines?

When you say "piston heavy" I'm pretty sure sure mean that the piston is too heavy for the counterweight. So, are you drilling holes in the piston or taking material off somewhere to lighten it up?

How exactly do you know if you have it balanced or not? You got a fancy little machine or something?

I've always been fascinated by the balancing act with a single cylinder engine. I see some engines vibrate more and some are so smooth is's almost imperceptable.

Thanks

YES....I too would like to hear how you (Scott) balance these engines. If you do not want to tell everyone....you can PM the imfo to me. I have heard of guys using a high point balancer to do crankshaft and flywheel. Heli boys use the same balancer. Thanks Capt,n

The actual act of balancing is not that complicated. Do a search on engine balancing on the net and you will find lots of info. The big trick (especially with a single cylinder engine) is picking the correct balance factor for your use. The balance factor is the amount of piston and connecting rod weight balanced out by the crankshaft counterweight, usually expressed as a percentage. The problem is that the balance factor changes according to the rpm range and and useage the engine will see. Essentially, you can only be really smooth in one rpm range and the rest will be worse (sometimes a LOT worse). This means you must set a compromise to get the best overall running, balancing high rpm smothness against low rpm roughness. It can be a can of worms and very tricky to sort out.


Mark

My guess.....when using a high-point balancer....a clamp-on bob weight equal to the weight of rod and piston would be a good start for crank....may be wrong! May want to do fly-wheel seperate....with no bob weight. Capt,n

It is very easy to do. Anyone who wants instructions just email me at [email protected] It is easier to send in an email. It does require a gram scale, a ballancer, and some lead.

Actually what you want is a bob weight equal to 1/2 the weight of the piston, bearing, wrist pin, and the small end of the rod. If the counter weight is equal to the piston, then you eliminate the up and down vibration, but get all of the side to side vibration. I go for a 50/50 ballance. There are other ratios people use but 50/50 will give you half the up and down and half the side to side vibration. There is no way to eliminate all of both, so you have to comprimise. If you get it to 50/50 the two tend to cancel each other out some, so the all around vibration is less.