Here is my new mower for the 2017 season of 13 HP Super-Standard class according to www.ERWM.de-rules. It nearly runs.

Specs
Chassis: 20x30x1.5 mm and 20x20x2 mm steel profiles for structural beams, 20x20x1.5 mm for support frame parts
Engine: 13 HP GX390 clone - a lot of mods done to it, now about 20+ HP, diy exhaust system
Transmission: Comet Torque Converter system (ATV/snowmobile), axles with keyways custom-made by a gokart specialist
Rear Axle: taken from a chinese quad (Bashan or alike), stock hydraulic breaking system, very large sprocket (water jetted cut-aways for weight reduction)
Front axle: pendulum axle,  quad wheel mounts, diy pitman arm & steering links, 8B-1 chain with a 1:3 steering ratio - wheels to steering wheel - by means of sprockets, 15° caster
Steering Wheel: Alloy RAID, throttle paddle for right thump, hydraulic brake cylinder to be activated by left hand, digital rev-meter, oil temp. gauge to come.
Foot rests: pegs talen from quad, aluminium sheets







More pictures and information at www.WurstExpress.tk , have fun!  (also in English)

Cheers,
Fred

Dear Sirs,

maybe someone is interested in seeing what I did to my Oppy so far.

      --> http://wp.me/p7g7mv-Du

Happy Browsing,
Ferd

Hey guys,
a new week - a new question.


Reagrding intake manifold and carb. bolted to it:

What the heck does this spacer-thingy do? Shall it create turbulance, direct flow to somewhere, ...? Could I cut out the fins or does this do something negative?
In my eyes, the fins restrict the Diameter. The fins not even have ponty tops but extent blunt into the open diameter. I cannot understand how this shall be of any good use to free flow.



Unfortunately, this topic does not really give any sbstantial answers: http://www.heymow.com/index.php/topic,10359.0.html
It only says: Let the fins in, they are supposed to keep the air-fuel mixture atomized.
  How?

Sorry guys, three questions in a row... 
But for better citing options I kept each question to one post 

No on the red circle,it does nothing for intake flow,these heads flow plenty on the exhaust side as is

Any suggestions regarding the intake side of the head? Should be grinded to look the same as the exhaust side?

Ignition timing
 1 tooth on the cam 10-12 degrees on the ignition

i.e. advance the cam only by 1 tooth which results in 10-12 degrees advanced timing of the ignition. Correct?

yup.
What else are you doing to your engine?  Unless you're running a cam and other mods the heads are not likely to be the bottleneck.
I use the small chamber heads on my pullers and cut off the ridge around the combustion chamber to raise the compression-we're
not allowed to run cams so this helps them out at the lower rpm levels we run (4000 or so under load).

I only do porting, head works and tweaking/ tuning the carb. Ah, sorry, and 'oil mods' of course!

I do not plan to install a racing cam, billet rods or forged pistons.
These components shall stay stock to keep (a) costs down since this engine will only race in 'farmer's fun races' and not competing in a national championship, (b) it is very hard (impossible) to come by such high-performance components here in Germany which means ordering them from the USA -> leading to very high costs, (c) max. RPMs will be kept under 4000 revs; I am looking for more grunt from below (to better get out of turns on muddy soil) rather than high-flying horses , yeah, that's it I guess.

The stock flywheel should be changed to a selfmade steel or (heavy, i.e. thick) aluminium one with bolted-on platic fins - this is the ultimate plan. I have some fear spinning the stock flywheel near and constantly about 4000 RPMs...
By the way, does anyone have dimensions as a drawing of the stock flywheel? This would save me a lot of work measuring everthing myself ;-)

Oppy mechanics manual says:



However, do you think, re-facing the valves and seats both to 30° is advantageous? Because of:




Moreover, the exhaust valve head's upper most part is rather thick compared to the intake valve head. I guess, this is due to better heat transfer enhanced by more material. On the other hand, I definetly believe this thick valve head reduces gas flow. See lower valve on this Picture; it really sticks out:


What do you think, should I "flatten" the valve head by grinding off some material?

The Oppy mechanics manual states minimum thickness of valve head(s) to 1/32" - this would be at least half of what it measures right now.