I'm in the process of converting a 2-cilinder 2.5 Cu/In Subaru-Robin weedeater engine from gas to nitro.
What I've done so far is remove the top of the heads an replace them with head buttons (the compression ratio should be 11:1 now) I've also made a new 1-piece heatsink which fits over both cilinders. Furthermore I have replaced the original carburettors with a pair of Walbro WT-499's and made some adapters for them to fit the engine.

Yesterday I started the engine for the first time, using a 20x10" propeller and open exhaust.
All in all it ran quite well, with a max. Rpm of 7600. The highest engine temperature measured was 205 degrees Fahrenheit (measured after the engine had been shut off and just above the exhaust port)

The thing I don't get though, is that the engine barely reacted to changes I made. I changed the plugs (started with Enya#4, then tried OS#8 ), changed the fuel (first 16% nitro, then 25%) and tried different settings on the high speed needle (anywhere from 1.5 to 4 turns), but couldn't get the engine to run lean or over-rich Everytime I got a peak of 7500-7600 RpM without any noticable change in performance. Does anybody have any idea what could be causing this?

Here's some pictures of the engine before testing:



Thanks in advance,

Cyril

Great job..[8D]
Check to see that the low speed needles are lean enough, the WT499s flow a LOT of fuel..

Beautiful job on the conversion. Appears that you gave it a lot of thought.

Thanks for the kind words! This is my first conversion, so I had no idea what to expect.

@w8ye; you're right, lots of reading and calculating was done before the first chips were made

@RCIGN1; do you think that could be the problem? The lowspeed needles were turned out 2 turns on all attempts, is that too much? I have little experience with pumped carbs, so I went with what I'm used to do with "normal" nitro carbs.

2 turns on a WT499 is too far...Lean both out until the engine won't transition and try again...

Any pictures of the assembly and such? Looks way cool.

Ok, I tried again today. I finally got the engine to run lean. Had to turn the lowspeed needle in to 3/4 turn and the highspeed to 1 turn to accomplish this.
Got a maximum Rpm of 7800 with the lowspeed at 1 turn and the highspeed at 1 1/4 turn. Idle is about 1900-2000 RpM with the butterflies completely closed. Thanks for the tips!

Next will be fabbing up some tuned pipes. I did some calculations and arrived at a pipe that´ll be way to long for the application this engine is going to be used for. I´m thinking of doing a little experiment and try to make a pipe that´s tuned for half the frequency of the engine (so the pressure wave has to travel op and down the pipe twice)
Has anybody got any experience with this?

@tim220225: I expect you mean some shots of the internals? We'll maybe when I start fiddling with the porting. I've assembled and dissasembled this engine a couple of times too many to get to this stage. in the mean time, here's an exploded view from the weedwacker's manual:

I thought you machined the case. What is the model number of the weedeater?

Tim

It's a Robin NBT-415.

Here's a short movie I made this afternoon of the engine running: [link]http://home.hetnet.nl/~cyril-wichers/images/Robin5.wmv[/link]

Looks and sounds great. What does it weigh? Tried to find it here but it appears that the engine is only available in Europe.

New test-session tomorrow:

Looks great, and interesting. I've read several times that silicone tuned pipe connectors are not for use with gas engines as they won't take the higher EGT's (exhaust gas temperatures) of a gas engine. I hope they work ok for you.

AV8TOR

Sorry to say that Robin states they do not sell that engine here in the states. A friend of mine and I tried but nothing. He found the brush cutter it comes from in Ireland. They want over $500 plus shipping for the machine. Anyone know where to get one here?

The pipe connectors may work with glow fuel....I am sure this engine is set-up for glow fuel. I wonder how it would run on Glow/Gas Mix? best regards Capt,n

Wov, what a great job - do tell us when it flies

When using 2 carbs be carefull one does not go to lean while pulling power. It may be better and more easy to tune with one carb....but you would have to build a intake manifold. Keep the fuel very clean and test each cylinder temp so they match as close as possible. Good luck with your very nice looking engine. Capt,n

This is very good advice Captin.

You need to tune so both exhaust pipes are running the same tempature.

This pretty well assures balance between cylinders.

Micropuller,

Wow....that is really beautiful work.

I'm very impressed as well.. any new videos with the new exhaust on it?