Jim,
To change the subject a little, I Googled your location and found you live in a beautiful part of the world, love the river and landscape!

Yes Gizmo, nice part of the world I live in. I'm about 15 mins out of town in the "bush" as it is called here in Australia. In the US, it would be "the woods" I suppose.
You don't indicate where you are on your profile. Keeps us guessing I suppose!

I live in N. Idaho, the Rocky Mts. and large clean lakes are normal here. We are a vacation destination for many folks! I appreciate natural beauty having been born in the area and serving as a Game Warden for many years.

Actually my location is stated on my post:D, Athol, Id (Idaho), I am about 80 miles South of Canada.

That is also a wonderful part of the world. I never could interpret US address abbreviation. I'm sure it is clear to you!

We are both lucky!
Good luck on the engine project.

I machined the last part this morning; the centreing spigot for the prop hub.
I'm now working on timing the ignition and fitting the hall sensor magnet.

I now have the engine assembled and mounted on the bench for test running.
However, it will not run. It will fire and sometimes run for a second or so. But won't keep running.
This behavior seems to suggest loss of crankcase compression. However, the front seal is new and is doubled up with the front bearing seal. The two halves of the case were joined up with "three bond" sealant. I also applied this sealant to both sides of the cylinder to crankcase gasket.
I am wondering if the next step is to do a crankcase pressure or vacuum test.
Any suggestions?

Also, I might have to consider a starter for this engine as my right arm is getting a bit painful from swinging the prop!

I'll post a picture later when the site decides to respond in "Go Advanced"!

I received this reply on another thread about electric starters.

http://www.rcuniverse.com/forum/begi...l#post12206881

I have to figure out how to fit a spinner or something else on my prop to provide a drive attachment for a starter.
Any ideas?

There are inserts for electric starters that fit over the prop,no spinner needed. I have not used one and they seem dangerous in that they may not disengage as easily as a cone on a spinner. A cone shaped prop nut may work, although I don't know if their OD is large enough to develop enough torque to turn a gas engine over. Good luck.

Jim,
Any chance of a gasket leak between the carb and port......recently had a problem with a Zenoah G-26 where the previous owner had installed the wrong gasket and and the results were similar?

I suspected that and added some three bond sealant to the contact surfaces of the double thickness insulating spacer between the carb and the intake manifold. However, I am still not sure that the "O" ring inside the manifold which seals between the manifold and the intake port (or should do so), is doing to job. I will check that somehow.

Speaking of gaskets, have you checked your vacuum hole from the engine to te carb. Sometimes the gasket gets turned so the holes don't line up or, sealant gets in the hole and clogs it. No vacuum to the carb, equals no carb pump action, equals no sustained running. The engine may be starting on the prime and then quitting due to no fuel. Just an idea. Good luck.

Yes, all clear. The pump is pumping, as I can see the fuel move along the line when it is only part full and I turn the engine over.

I am in the process of organising the parts for an electric starter. Also, my mechanic buddy said that we should make sure the spark is working when cranked at slightly higher revs than I have been able to achieve.

Discussion here: http://www.rcuniverse.com/forum/begi...-starters.html

I have made a steel clamping disc to replace the aluminium one on the front of the prop for an electric drive. My little cordless drill will crank it with the compression relief knob pushed in, but it is not easy to hold while doing so. Really needs a larger motor to do it.
I checked the spark with the engine cranked with the drill and it is good.
I am starting to suspect that it is not getting sufficient crankcase scavenging. So, I'm planning to do a simple crankcase pressure test. I figure it will be easy to make a blank to seal off the exhaust and a plug of some sort to block off the intake. Then all I need to do is to make a threaded plug with a hose tail to screw into the spark plug hole. I can regulate my small compressor down to below 10 psi and apply some pressure with the case partly immersed in water.
I've read that sprayed WD40 on all seams will show bubbles if there is a leak too, but the water will be more positive.

My mechanic buddy and I made more attempts to get the engine checked out and running. Still no go. It will not even fire now. We tried two different spark plugs and different spark gap settings, all to no avail.
We suspect the HobbyKing CDI unit. I thought right from the start that the spark looked a bit weak.
Can an automotive type CDI unit be wired up to trigger with a hall sensor I wonder? Just for bench testing?
It will take several weeks to obtain a replacement Rexel CDI from America, and I would like to have the engine tried and tested soon.

The reviews on the HobbyKing site about this CDI are all good:

https://www.hobbyking.com/hobbyking/...s_Engines.html

What is the definitive test for a CDI unit? Use a CRO and see what voltage output it produces?

I don't suppose that there's any chance that you have the wrong direction of rotation?

Stranger things have happened.

Logical question!
Just checking, once again:
1. I have the propellor mounted on the end of the shaft that had the flywheel on in the original saw.
2. The flywheel takes, or rather took, the hand pull start.
3. The pull start pawls engage in the flywheel to drive it in a counterclockwise direction.
4. The prop runs in a counterclockwise direction when looking at it from the front.

Conclusion:
It is in the correct direction of rotation.

Talking with a flying buddy who has several converted engine powered planes this afternoon and the possibility of fitting the original magneto ignition was explored.
However, just inspecting the flywheel out of the original chainsaw, it does not look like it is straight forward. The flywheel is a combination of a metal hub, plastic surrounding that and a magnetic steel rim on the outside. It might not machine down to a suitable flat flange on which to mount a propellor hub.
Furthermore, it has a printed warning on it about modifying it; could cause it to burst (disintegrate).

edit:
I've just had another look at it and I think it can be done safely by removing a minimal amount of material from the alloy cylinder where the original starter pawls used to engage.

Probably a good idea to retain the original. You can be sure that the motor will run well in that configuration.

Yep, it's bulky. Yep, it's quite heavy ... but, you need the nose-weight, don't you?

I had a 40-odd cc Kawasaki that was "intact". Bloody lovely engine. And, of course, the Zenoahs are all great with their OEM ignition.

Maybe you'll happen across a more elegant engine in the future, and you will be able to swap.

Yes. I will wait until I receive another CDI unit that a flying buddy will post to me next week. All things going to plan, that would mean that I could receive it the following week. I will try that one on the engine before tackling the modification and fitting of the magneto ignition.
I've even been thinking that a fully alloy flywheel off another engine might be more suitable to re-bore the taper and keyway to suit the shaft of the Stihl. That way, I would have no doubt about the integrity of the part plastic, part alloy standard one after it has been machined to take a suitable prop hub.
The retaining bolts that would be required to fix the prop hub to this standard one would have to go through plastic. Perhaps a solution to the structural problem, is to fit an alloy clamping disc to the back of the plastic, through which all the bolts would pass.
I'm considering all options.
However, we will try the alternative CDI before anything else.

To test an ignition I recommend one of these.

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

They work great.

I say a must have for hall sensor type ignition work.

Yes, I have one of them. That is what I used to time and position the magnet in the prop hub.
It only indicates if the hall sensor works or not. It does not measure spark voltage.

I am confused about optional spark plug sizes. I've done an extensive google search to no avail. Can someone tell me what size is the plug in the Stihl?
Apologies for the ignorance.
I need to know this before ordering a new CDI unit.

You have one of those testers (the new one that that does spark 400-18000 rpm and hall sensor), and used it to run the ignition? You got spark out of it. Then it is good.

Soo..... unless the air pressure there has gone to 0, you have a fuel problem. I just reread post 107.You do not have consistent fuel. you have air and spark.

Air, fuel and spark, keep it simple. everything else you are looking at should be run, but, run badly..

Rebuild or clean the carb , check the pump and pressure pulse, the regulator and needle seat. The needle settings high and low. flow through the passages.

this is my 2 cents.

Yes, we have done all that. Checked, rebuilt and re-checked. The engine is definately getting fuel, as the plug is wet.