Comment: The list below captures known problems which can lead to erratic running. The list is organised in priority of ease of implementation.
It's recommended to start at 1. Tuning, and work through the list, testing the engine after each item is covered.
The harder fixes come last.
Note: If you are unfamiliar with engines and/or do not have the necessary skill sets, do enlist the help of an aeromodeller at your club who does. There is no better way of picking stuff up.
| Cause | Fix | Comment | |
| 1 | Engine off tune. | Tune the engine. | IC Engine tuning protocol |
| 2 | Glow plug loose. | Tighten glow plug. | |
| 3 | Glow plug deterioration or failure. | Replace glowplug | |
| 4 | Debris in fuel. | Filter fuel. | |
| 5 | Stale fuel. | Replace fuel. | |
| 6 | Cylinder head bolts loose. | Tighten cylinder head bolts. | |
| 7 | Engine backplate loose. | Tighten engine backplate. | |
| 8 | Debris in fuel tank. | Flush fuel tank. | |
| 9 | In line filters clogged with debris. | Clean in line filters. | |
| 10 | Fuel tank damaged. | Replace fuel tank. | |
| 11 | Fuel tubing damaged. | Replace fuel tubing. | |
| 12 | Gasket failure. | Replace gaskets. | |
| 13 | O'ring failure. | Replace O'rings | |
| 14 | Debris in carburettor. | Blow through carburettor nipple and high speed needle valve with carburettor cleaner. | |
| 15 | Carburettor internal problem. | Dismantle and clean carburettor. | |
| 16 | Engine internal problem. | Strip down engine to locate and correct faults. | |
| 17 | Low speed needle valve's O'Ring seal failing. Normal engine vibration then causes the the Low Speed Needle Valve to migrate forward into the high speed needle valve's venturi, eventally blocking it and cutting off the fuel supply. | Check the low speed needle valve's o'ring for wear and damage. | Although rare, this fault can occur. The manifest is that tuning the engine will become increasingly difficult and in the limit the engine will only run on primed fuel. This last point has two visible manifests. i.) When priming fuel is introduced from a priming bottle into the carburettor's air intake directly, then the engine will run briefy at full throttle for about 1 second as the priming fuel is burnt. ii.) Attempts to prime the engine by blocking the the air intake with an index fingertip whilst turning over the engine with the other hand, fail and the fuel which moves up the needle valve's fuel line quickly returns to the tank emptying the fuel line. This return occurs because the needle valve is blocked. Fortunately there is an easy definitive test for this condition as follows: Open the low speed needle valve completely so that with an open barrel, (full throttle), the low speed needle valve can be seen to be clear of the high speed needle valve's venturi, then try to run the engine at full throttle. If the engine can be run at full throttle and tuned so that running is continuous, then the condition is identified. At this point the the low speed needle valve should be checked. A second way of staying on top of this issue is to to carefully note the high speed and low speed needle valve settings as the number of turns back from the fully closed position. To do this well a right angle key must be made for the low speed needle valve, and a right angle key must be added to the high speed needle valve, via it's grub screw. |
| 18 | Loose or missing carburettor barrel retaining screw. This is sometimes called the throttle stop screw and is the screw which runs in the helix groove in the carburettor barrel to control the sideways motion of the barrel which controls the idle mixture.) | Tighten/replace carburettor throttle stop screw as needed. | If the screw is loose or absent then engine surges as described in the comment below can occur as variable extra air will enter the mixture. |
Comments
Well, I took the cowling off and looked over the innards only to notice that the carb barrel retaining screw was missing, however the felt washer was still in position. Diagnosis; this screw has been coming undone for ages and it was only when it finally came out that it became obvious. An easy item to check but so eay to overlook.
Replaced the screw and flew it yesterday, sweet as a nut.
24th. September 2016
Added point 18 in line with Ian Jones' comment.