Larry882

Check your needle valves. They might need to be opened. On some of my conversion engines 4 to 5 turns are needed.
Larry

Jim.Thompson

Ok. However, this engine will not even fire now after we charge the carburettor with fuel. About 3 - 4 mls from a syringe.

Jim.Thompson

I received a new CDI unit from CH in America today. It arrived with no hall sensor. I fitted it to the engine and tried it with the Pegasus hall sensor. However, the engine still will not start. There is a spark from the unit; I tried it with the plug removed before fitting it to the engine.
I'm beginning to wonder if the magnet is too small. It is a 3 mm x 2 mm one. However, my understanding is that the hall sensor is either on or off, there is no proportional output. I wiped a large magnet past the sensor, and the very meagre spark was no different to that obtained using the fitted 3 mm magnet.
Can anyone clarify this?

spaceworm

Have you tried fresh fuel, maybe from a new source?

Maybe put a glow plug in an adapter with glow fuel and try it that way. You likely have to open up the needles a lot. Just to see if the thing will run. Maybe even a gas-glow fuel mix. My Echo runs great on glow fuel, may have to try it on gas-glow mix. Good luck.

Gizmo-RCU

My Feedback: (27)

Jim,

I know you have no doubt considered this, however I wonder if the timing is right? My guess is it is some little minor problem..........those big Stihl's are about as good as there is. Do you have a dealer nearby who you might discuss
your situation with? Often when then find out what you are doing they are more than willing to help.

spaceworm

I have gone to several Stihl service shops to see if they have any "returns" or "not worth rebuilding to the customer" engines. They said it was against company policy to encourage use of their engines for other than their intended use. I guess it is about liability and image. I had hoped to get a 4-MIX engine to play with. Good luck with your engine, I hope they will help you out.

Jim.Thompson

Yes, I mixed a fresh can of 50:1 stihl 2 stroke fuel.
I don't have any glow plug fuel and would have to research what the mix is etc. Perhaps a local flying buddy might have some, not sure.
I guess I could machine up and tap an adapter and buy a glow plug from the local hobby store.
It could be a way of making sure it will run, as you say.

When I talked with the spare parts attendant at the front counter of the local Stihl agents, they would not let me speak to a mechanic. When I mentioned what I was doing, the attitude turned to almost hostile. I don't think I will get any help there.
However, there is a small engines fellow I know, does general mowers and some chainsaws. He might be worth talking to.
I will check the timing again this morning. The supplier of the CH ignition, Adrian, recommends it to be set at 30 deg BTDC. I now have it set at 28 deg.

Gizmo-RCU

My Feedback: (27)

We have lot's of chainsaws here due to large scale logging. There lot's of folks who work on most of their own saws and I know several really sharp guys when It comes to such. Still think it's some little thing that
is causing the problem, that's how it usually is. 2 degrees should be in the run range and I would think the engine should run at that spot.

Jim.Thompson

I agree Gizmo, especially after my attempts this afternoon to start the engine.
I resorted to a tiny squirt of "Easy Start" spray into the intake. I got it to fire and run a short burst, less than a second. After that, I could flick the prop with the chicken stick and get it to fire almost every flick. Sometimes running a few revolutions then stopping again. Went on like this for some time, short bursts long enough to warm the cylinder, but it still would not run.
I had to stop, as my arm was getting tired.
The battery drill starter system will only work with the decompression valve pressed in, lowering the compression. As soon as it fires, the valve pops out, and the drill will no longer crank it smoothly. I have difficulty holding it anyway.
I need to walk away from it for a few days I think..............

Gizmo-RCU

My Feedback: (27)

Sounds as if it's a fuel flow thing? And yes walking away for a while will give a more clear view of whats going on........

Jim.Thompson

Success! I have it running and it's starting easily and running very well.
The problem was the ignition timing. I had it about 3 degree out, as I was using the point at which the magnet passed under the edge of the hall sensor sleeve/bracket. I moved it to the point where the magnet was under the centre of the hall sensor.
It started almost immediately.
Now it's looking like I can choke/prime with the ignition off and the decompression valve depressed. A certain number of times that will be found after some practice. Then, pull the decomp valve out, turn the ignition on, and flick with a chicken stick. Two flicks and it goes. Very happy about this, I've been walking around laughing about how good it is to get it going.
The engine sounds like it is loading up suitably with the 26 x 10 prop, as far as can be assessed on the bench. I was only game to give it about 1/4 throttle, as it has so much power and my bench mounting set up is a bit primitive.
Vimeo video here:

https://vimeo.com/167048008

spaceworm

Good on you mate. Congratulations!

Jim.Thompson

Thanks Spaceworm.
I started the engine after dark tonight with a light to see what we doing. I was showing a buddy who has been very helpful along the way with the conversion.
We noticed that at low RPM, there was a noticeable amount of fuel visibly spraying out of the carburettor and into the propwash.
It seems like a velocity stack would be of some advantage to reduce this.
I have no experience of working out the length of such a tube. We were wondering if 50 mm would suit this engine.
Anyone able to offer some recommendations about this?

Gizmo-RCU

My Feedback: (27)

Good deal!!!!! You stuck with it, what a beast, What are you going to use it on? Bet you are feeling really good (as you should) right now.

Jim.Thompson

Yes, I was walking around chuckling to myself yesterday after getting it running so nice. Build thread for the 30% scale plane is here:

http://www.rcgroups.com/forums/showt...6#post34804285

Jim.Thompson

I did a quick search on these forums and found a post from av8tor1977 (which I thought I would!) clarifying the carb fuel blowing phenomenon that we witnessed on the Stihl last night. It's called reversion and is explained well in his post #12 here:

http://www.rcuniverse.com/forum/gas-...l#post11663317

I'm glad it is a normal phemonemon. I think I will go for the air filter option over the velocity stack after reading this and understanding better just what is happening. As he indicates, the filter satisfies both needs better then the stack; keeps dust and debris out and reduces or eliminates this "reversion", otherwise known as "stand off".
Now I will just have to find or make a suitable filter and some bolts long enough to go through it, the carb and the insulating gaskets.

Gizmo-RCU

My Feedback: (27)

Jim,

The spray issue is very common, perhaps a little more on your application as normally that engine is really tightly enclosed in a saw case with an air filter. There are a huge number of small engines with all types of filters out there. Some of the chainsaws have a mesh type filter. Very light yet very effective. Is there a chance you can check out a few saws? I think all you need is to stabilize the air flow a little? Trial and error.

spaceworm

IMHO, this has gotten too complicated, unless you are flying over dirt or fine abrasive debris, you do not need an air filter. The blowback "reversion" issue is very common and is typically cured by fitting an intake velocity stack. They are commercially available from numerous sources in metal or plastic or can easily be made. This my thought on the issue. Good luck.

Here is a link to one, but a search for intake velocity stack should bring up a lot of options

http://www.ebay.com/itm/Zenoah-Stock...AAAOSwNNxWFRdb

Jim.Thompson

Thanks for the link with the picture of the Zenoah stack spaceworm. I was planning on how to machine one up out of brass or aluminium, but could not figure out how to get the bolts in. The Zenoah one solved that with the half round recess's removed to take the bolts.
Also, the trumpet shaped flare does not need to be so pronounced.
Where I will be flying is often very dry and dusty, so I will likely fit a stainless screen to it. If I do it in brass, the stainless screen can be silver soldered to the intake.

av8tor1977

My Feedback: (6)

Hobby King sells velocity stacks for $2.00 US. If you fly from a dusty or sandy area, a filter is not a bad idea. I have a large piece of old household carpet I use for starting and running up my engines, but was still seeing some cylinder scoring from grit injested during take offs and landings so I have fitted air filters to some of my engines. (Very sandy where I most often fly.)

https://adaracing.com/parts/hobby-rc/air-filter-kits

AV8TOR

spaceworm

Hi, I have several gas engines for which I need stacks. In looking at current HK stacks, I see some CNC anodized ones for $6.75 plus shipping. What stock number were those that you said were for $2.00? Thanks

Sincerely, Richard

Jim.Thompson

Thanks for the reply AV8TOR. I was hoping to hear from you.
the HobbyKing velocity stacks are too small for my carburettor. Mine has a 22 mm bore. All the ones I could find listed were only 18 mm.
I do like the configuration of this one which has the stack integrated into the filter mounting. :

https://adaracing.com/hobby-rc/air-f...cheap/gpafk-rc

I'm even thinking I might be able to make something similar big enough for my engine. What kind of foam is suitable for a filter, do you happen to know?

av8tor1977

My Feedback: (6)

Here ya go:

http://www.hobbyking.com/hobbyking/s..._Part_034.html

AV8TOR

Jim.Thompson

Yes, but the bore is only 16.3 mm.!
That is what I was saying above; they are all too small for my 22 mm carb.

Jim.Thompson

I have made up a new inlet manifold and adapters for the carburettor to take an air filter. It is a cheap motorbike one from ebay.

Also, this is a video of the throttle linkage operating. https://vimeo.com/168889537