Jim.Thompson

I have a 26 cc industrial two stroke engine converted to aero application.
I started it on the bench on a many occasions, without problems.
However, now it will start when cold ok. But not when hot. It fires, but stumbles and stops.
It has CDI ignition.
I have tightened all bolts up including the head retaining bolts.
I have refaced the carb insulating block (it was not flat) and made and fitted new gaskets.
I also swapped the carb for a new on. It is a WT668 (which I use the same type successfully on a 40 cc two stroke as well).
The problem still occurs.

Any suggestion will be most welcome.

Jim.

1967brutus

That kind of thing usually is caused by bad atomisation.

For the ignitability of a fuel/air mixture, only the vaporied part of the fuel counts. The liquid part of the fuel does NOT take part in the "ignitability game" but it DOES take part in the combustion, although that combustion will be incomplete.
Gasoline has an ignitable range spanning roughly 12,5:1 to 15:1 (mass air/mass vaporised fuel).
If atomisation is bad, the absolute ratio (liquid fuel and vapor together) has to be significantly richer than that,to ensure sufficient fuel has evaporated to create an ignitable mixture. This is also the reason why a cold engine needs to be fairly wet in order to catch when handstarting.
It basically means that the engine is running "wet", with liquid fuel present all through the intake tract and up to the combustion chamber.
Now when the engine is stopped, the hot parts cause the liquid fuel to evaporate, and now the fuel/air mixture becomes too rich to ignite.
Ventilating the engine by cranking it over with a starter expells the too rich vapour/air mixture, and the engine should start within a few seconds of cranking.
It's a known thing and the reason why recoil starters were invented: a few pulls in quick repetition usually vents the engine sufficiently to receive a "wet" mixture again which allows ignition again.

A leaner idle setting sometimes helps, but will make cold starting more finicky. Stopping the engine by piching the fuel might also help, leaving a dryer engine before the next start.
Opening the throttle a bit more could help with ventilating the engine (more air and less fuel draw during flippping),

Jim.Thompson

Thanks for the reply Bert.
That all makes good sense to me.
I should add that the engine fires when hot, but stumbles and stops after a few feeble strokes.
Does that still fit with what you have explained? I assume it does.

I have two weeks before an event which I preparing this engine and plane for.
There is a chance that I may be able to borrow an electric starter for this occasion.
It hurts my pride a bit to use a starter though; my bigger 40 cc engine tug will start one flip between re-fuel stops! Something that I am rather proud of. Same carb on that engine too, by the way.
(I am prevented from flying the 1/4 scale Spacewalker with the 40 cc engine due to insurance complications and the rules of the venue - it is over the 7kg AUW! requiring registration). Long complicated boring story.

If I am able to borrow the starter, I will have to machine up a spinner or suitable drive point of connection for the smaller 26 cc engine on the smaller plane that this thread is about. That should not be too much trouble.
More about that when I hear back from my buddy who owns an electric starter.

In the meantime, I will try pinching the fuel line to stop it then trying to restart.

Jim.

1967brutus

Yes, it does... The bulk of the fuel charge is in the crankcase. Depending on the exhaust system, the part of the charge that is in the cylinder at standstill, CAN be ignitable, but as soon as the first ignition occurs, the engine will start scavenging, and the next charge is that overly rich one from the crankcase, which won't ignite. If the throttle is open wide enough, and the first ignition strong enough, the engine might catch at the second or third attempt.

A few good alternatives to an electric starter could be a springstarter, or a small pulley instead of a big spinner cone, allowing for a ripcord (buddy of mine used that very succesfully on a Zenoah 26 that refused to light up by hand either hot or cold), or if you want to go all out luxurious, an onboard starter from for example FEMA.... priceless coolfactor, you'd be the star of the event

Jim.Thompson

After thinking about it overnight, I fully understand.

I am unable to visualise these alternatives (early morning, old age etc.!)
Please elaborate. Pictures if possible?

If this engine is as successful flying as it shows promise of being, I will look again into fitting it with the electronic injection valve system, as described by you and others on a dedicated thread. But only after out flying festival next month.
What forum was that thread on again? Was it this one, or over on rcgroups.com?

Thanks again for the follow up. Good stuff!

Edit: I have been loaned a choice of two electric starters and a collection of suitable spinners.
I am in the process of modifying one of the spinners, so all requirements will be covered.
So, don't worry too much about pictures etc.
I figure I am on track to sort it all out.

Jim.

mgnostic

Often the starter that I use with my glow engines doesn't have quite enough torque unless the battery is freshly charged. For the bigger gas engines I use a cordless drill. I mounted a starter cone on a 1/4 inch bolt and chucked it up in the drill. The drill is geared down enough to spin over my G-26. It will spin the engine fast enough to fire even a magneto ignition.

1967brutus

Provided you turn the engine backwards against compression before engaging the starter, the Align self contained starter has ample torque and will even start a Zenoah 36.
It is a very compact and easy to operate starter. and works a couple of months on a single charge.

Jim.Thompson

I have a powerful 12 volt starter which a flying buddy has loaned me.
The engine now starts fine, hot or cold.

Thanks again for the replies,

Jim.