OS GT33

After Pennzoil's dead-stick and landing short and mostly surviving. Don't know why she cut out on approach on fast idle. Found the engine bolts had worked loose on the last flight. Today I sorted that out and went to start her up again and - struggling to start, firing but not running. It wasn't until I had gradually increased the throttle position that it finally started. The trim was nearly up and so had look in sub-trim. This made a little difference but here is a strange thing. moving from idle it will die. Moving from idle quickly to 1/3rd throttle or more and it runs just fine. Bring the stick back to idle and it seems as if I have gone idle and it starts to rev up again. Feels like the throttle flap has gone over centre and opening up again.

Throttle lever is a plastic lever held on with a small screw. doesn't appear to have splines either:unsure: Has it slipped? Anyone had aprevious experience with these OS engines?

Hi Phil, are the high and low speed needle settings OK? Throttle stop? Is the pump line leaking? What's the state of the dreaded Walbro diaphragm? Don't have a GT33 but I do have a temperamental two stroke with a Walbro carb. At least it was a right little winkle until I stripped the carb and cleaned it.

The engine is newish. About 6 flights I think. From what I have gathered these engines take time to bed in fully but last for years. The throttle lever is plastic and is held on with a small screw, no splines. On checking it just sits on the throttle spindle. Strange idea! Anyway I have reset the TX trims to neutral and reset the arm. Tomorrow, weather permitting I'll have another go. I also think the low jet was a tad too lean. Which is why the motor would try to run backwards on starting. Information gleaning tells me to start with the throttle at a fast idle to reduce the chances of reverse running on start up.

Check the reed valves. I had a similar problem after shunting my OS 22. It turned out that a small piece of debris had found its way through the carb and lodged itself into the reed valve. The engine would run but throttling was all over the place.

Thanks Alan. I'll check that out while the cowl is off as well. I also noticed that the digital servo was buzzing as if it was under load so I have moved the actual travel on the TX so it is quiet at both ends. Will find out tomorrow if I have discovered the issue.

BTW - I fitted the 20 inch prop but ground clearance is minute so will stick with 18.

As was suggested I contacted Ripmax regarding the Pepsi engine anomally.
I wrote:
"I recently purchased two OS GT33 engines a few months apart late 2016. These engines were finally fitted into two identical aircraft with identical set-ups and servos. They have to date an equal amount of running time.
The first plane which I will call Pennzoil has a nice controlled throttle response albiet with some throttle curve added.
The second plane called Pepsi is hard to control on the throttle. From idle, the slightest movement on the stick and the power really kicks in. Obviously this makes it very interesting when trying to land. The present throttle curve on my JR radio is virtually nil for the first two points, around 3.0% at point 3 and followed by point 4 at 16%. I have tried servo changes and arm throws but to no avail.

Are you aware of any reports of problem carburettors on these OS engines?"

Peter at Ripmax replied: "Hi Phill

I have not heard of anyone with this problem, the only way I can evaluate the engines is by bench testing both engines. A bench test will give me data comparison.

Please return them to
Peter Halman (Ripmax)"

Does anyone think this is the best course of action?

You say both engines have approx the same run-time, does this mean that the Pepsi's engine went flaky recently? Can't think why they want to see both engines, after all they know exactly how a GT33 performs, so sending back the good engine is a waste of time. Have you explored every possible glitch in the carburettor?

Based on your description (2 identical setups) It is highly probable that one of your engines has a fault and as Ripmax are offering to bench test the suspect unit, its got to be worth the postage to let them deal with it. If you investigate yourself and subsequently decide to send it to Ripmax they may say you've messed with it and deny warranty.

3 screws hold the inlet manifold onto the cylinder head. Why not just swap them over?

Hi Phil - In my opinion I would take up Ripmax's offer & return BOTH engines so they can undertake
the comparison suggested.
Colin.A