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When a glow engine will not start, the fastest fix comes from working in order rather than changing parts at random. Swap the plug, the fuel, the lines and half a carburetor all at once and you may well get a running engine, but you will not know why, and the same trouble will come back next Saturday.
This page is a researched guide, not a record of our own bench time. It leans on two O.S. Engines instruction manuals (hosted by Horizon Hobby), a Model Airplane News cold-start procedure, a Model Aviation column on tuning and weather, and Du-Bro's fuel-system product pages. The manual of your own engine always outranks any article. If your engine is gas rather than glow, our gas engine no-start checklist is the better page, and for the fuel and plug basics see the fuel guide, the glow plug guide and the gas versus nitro comparison.
The four things an engine needs
The O.S. MAX-55AX manual opens its troubleshooting chapter with four key points: "For quick, reliable starting, the following four conditions are required: good compression, adequate glow at the glowplug, correct mixture, and sufficient electric starter rotating speed." It also marks the most common causes in its chart with three asterisks, and those common causes are mostly cheap: a weak starter battery, a discharged glow battery, a burned-out plug, flooding from too much priming, or too little priming.
Glow engines have no timing to adjust. O.S. explains that ignition timing is automatic: under reduced load the plug becomes hotter and fires the charge earlier, and at reduced rpm it becomes cooler and ignition is retarded. So "timing" is not a user fix here, and the useful checks are the four above.
Start here: the quick turn-over check
Remove the glow clip and turn the engine over once or twice by hand, or listen to how the starter spins it.
| What you notice | What it can mean | Where to go |
|---|---|---|
| The prop will barely move, or the starter stalls | Liquid fuel in the cylinder (hydraulic lock), or a weak starter battery | Clear the flood (below) and charge the starter battery |
| The engine turns with a normal pinch at the top | Compression is plausible | Go to the glow checks |
| Very little resistance anywhere | Possible compression problem | Ask the maker or a dealer to check it |
Do not mistake a normal pinch for trouble. O.S. says the piston "will feel tight at the top of its stroke (TDC) when the engine is cold. This is normal." The bore has a slight taper, and the piston and cylinder reach their intended running clearance at operating temperature. A brand-new engine can also be stiff and reluctant until it is run in.
Step 1: Check the glow system first
The O.S. chart puts a discharged glow battery and a burned-out plug at the top of the "engine fails to fire" list. Per O.S., an unused or almost unused dry battery "may sometimes be of insufficient capacity if it is 'old stock'," so a fresh battery is not automatically a good one.
Connect the igniter to the plug outside the engine and look at the element. The O.S. starting procedure says to make sure the element glows red before you install the plug. Hold the plug with pliers, not your fingers, because fuel remaining in the filament can burn you.
A bench glow is not a full test. A plug can glow in the open and still fail under compression and heat, so if the engine will not start and everything else checks out, a known-good plug of the right type is the real test. O.S. says plugs are expendable items and lists the signs a plug needs replacing: a filament that has roughened and turned white, a distorted coil, adhered foreign matter or corrosion, cutting out at idle, and starting qualities that deteriorate. O.S. recommends its No.8 and A5 plugs for the 55AX and ships the No.8 with that engine. The FS-95V four-stroke ships with a Type F plug. Our glow plug guide covers matching plugs and reach to an engine, which is something to read from your manual, not assume.
O.S. Type F glow plug (four-stroke)
Ignition begins with a 1.5-volt power source per O.S. If the plug glows dimly on a clip, suspect the battery or the leads before you suspect the plug. O.S. also says to check that the applied voltage is not too high when a plug burns out.
A glow driver with an indicator saves guessing. Horizon lists the Dynamite LiPo Glow Driver as in stock, and says its LED shows whether the plug is good or the unit needs recharging. A simple AA-powered igniter is also sold, with the batteries bought separately.
Step 2: Clear a flooded engine
Both O.S. manuals say the same thing about electric starters: do not over-prime. Too much fuel in the cylinder can cause a hydraulic lock and damage the engine when the starter is applied. O.S. also says not to choke the carburetor air intake while applying the starter, because that can draw excessive fuel into the cylinder.
The maker's cure is:
- Close the needle valve fully.
- Remove the glow plug.
- Apply the starter (or flip the prop) to pump out the excess fuel. O.S. adds: invert the engine if possible.
- Cover the head with a rag so pumped-out fuel does not reach your eyes.
- Reinstall the plug, and restart without priming.
O.S. also gives a repeat-attempt rule. If the engine does not start within 10 repeat applications of the starter, remove the plug, check that it glows brightly, and check that the cylinder is not flooded. If the engine fires intermittently but will not run, O.S. says to try up to four starter applications, then disconnect the glow current for a few minutes, re-energize it and try again, and finally remove the plug and pump out the excess fuel.
Never keep forcing a locked engine. Stop and clear it first.
Step 3: Priming and starting technique
O.S. 55AX starting steps, in summary:
- Fill the tank. Use a fuel stopper to keep fuel from flowing into the carburetor while you fill, and release it before starting.
- Make sure the plug element glows, then install it.
- Check that the needle valve is closed (do not overtighten), then open it 2 to 2.5 turns for the starting setting.
- Open the throttle about one-quarter, which keeps rpm down when the engine fires.
- Press the starter for 5 to 6 seconds to prime.
- Connect the glow battery leads to the plug.
- Touch the starter to the spinner for one or two seconds, repeating if necessary. When the engine starts, withdraw the starter immediately.
The FS-95V four-stroke manual differs in useful ways: it starts with the needle open 2.5 to 3 turns, turns the prop backwards by hand until it is against compression so that the prop's inertia helps the starter through the compression stroke, and says that if the starter fails to turn the engine completely, "this may be due to the cylinder being over-primed, or to the starter battery being insufficiently charged." These numbers come from those engines' manuals. Yours may differ, which is the point: use your own manual's starting setting.
Starting by hand is a different skill. Model Airplane News says to always use a chicken stick or an electric starter, and O.S. warns that a warm glow engine can abruptly restart if the propeller is casually flipped over compression without the glow battery being reconnected.
Step 4: Fuel delivery
If the engine fires and then dies, or fires only briefly, suspect fuel. The O.S. chart is specific: "Engine starts, rpm increases and engine cuts out" points to fuel not reaching the engine. Check that the tank is full, that the fuel line is not kinked or split, and that the carburetor is not clogged with dirt.
Fuel. O.S. says to use a methanol-based fuel containing not less than 18 percent castor oil or a quality synthetic lubricant (or a mixture), plus a small percentage, 5 to 20 percent, of nitromethane. The 55AX carburetor is set at the factory a little rich for a fuel with 20 percent lubricant and 15 percent nitromethane. O.S. also says to keep the fuel container closed to prevent foreign matter getting in, and that when you switch to a different brand or formula, it is wise to revert temporarily to in-flight running-in procedures until you are sure the engine runs well. Our fuel guide goes deeper.
Lines and clips. Any air leak between tank and carburetor can starve the engine. Du-Bro makes Nitro Line silicone fuel tubing in a 3/32 inch inside diameter (.080 in ID, .200 in OD) that it says stays soft and does not kink, and sells fuel line clips for 3/32 inch and 1/8 inch tubing, four per pack, to stop tubing slipping off fittings, which it notes can lead to air leaks and fuel starvation.
Du-Bro Nitro Line silicone fuel tubing
Filter and clunk. O.S. says to fit an in-line fuel filter between tank and carburetor and clean it periodically, because restricted fuel flow can make the engine cut out or run too lean. Du-Bro's in-line filter unscrews for cleaning and has a 130-micron mesh screen; its standard size works with 3/32 inch silicone tubing for glow fuel. Du-Bro's clunks are nickel plated, in standard (7.5 g) and large (13.6 g) sizes, and the clunk has to stay submerged in fuel.
Step 5: Needles and mixture
The O.S. chart separates "too rich" from "too lean" by symptom, which helps you stop turning needles at random:
| Symptom | O.S. says it is | Correction in the O.S. chart |
|---|---|---|
| Starts, rpm decreases, eventually stops | Mixture too rich | Close the needle valve half a turn and wait a few minutes before restarting |
| Stops when the glow current is disconnected | Mixture too rich, or plug and fuel mismatched | Close the needle a little before disconnecting; change fuel or plug |
| Starts, rpm increases, cuts out | Fuel not reaching the engine | Check tank, line and carburetor for dirt |
Starting needle positions are engine-specific: 2 to 2.5 turns for the 55AX, 2.5 to 3 for the FS-95V. The 55AX carburetor has a separate Mixture Control Valve for part-throttle and idle that is factory set, and O.S. says to run the engine as received and readjust it only if necessary. The FS-95V manual adds: turn the needle in small steps after starting, because abrupt changes can make the engine stall, and do not leave the glow battery connected while adjusting.
For idle, O.S. gives a test on the FS-95V: if the engine responds sluggishly and emits excess white smoke after idling about 5 seconds, the idle mixture is too rich; if it hesitates or stops, it is probably too lean. Model Aviation's Dean Pappas offers a similar rule of thumb: an engine that "sags straight away" is too lean at idle, and one that "speeds up immediately" is too rich.
Do not chase a perfect tune on a cold engine. Get it running, warm it up, and tune afterward.
Step 6: Cold and hot starts
Weather changes matter. Pappas writes that glow engines "respond dramatically to substantial weather changes," and that when cool weather returns the engine "will need to be richened, both at idle and top-end."
Model Airplane News published a cold-weather procedure for hand-started engines that includes these steps: rotate the prop until fuel reaches the carburetor (and "Don't flip it!"), prime with two turns if it is warmer than 50 degrees F or three if it is colder, flip the engine over six to eight times with the glow plug disconnected, set the throttle about a quarter open, reconnect the plug just before the final try, and let the engine warm up at least a minute before final adjustments. For a hot engine it advises drawing fuel to the carburetor without choking or priming. That article is for hand starting a carbureted engine. Follow your own engine's manual first, particularly with an electric starter.
When nothing helps
If you have worked through the four conditions, check the basics once more: starter battery charged, glow battery proven good, plug fresh, needle at the manual's starting position, fuel flowing, no kinks, filter clean. A stubborn case is better handled by the maker or a dealer than by taking the engine apart on the field bench.
Two more habits from the O.S. manuals are worth adopting because they prevent next month's no-start. After flying, cut the fuel supply with the engine still running at full throttle, expel residue by turning the engine over for 5 to 10 seconds with the starter, and then inject after-run oil through the glow plug hole and turn the engine over by hand several times. O.S. says this matters because modern alcohol-based fuels leave acid by-products that cause corrosion. For storage across seasons, O.S. describes washing the engine exterior with alcohol (not gasoline or kerosene), removing the carburetor and plug and silicone tubing, immersing the engine in alcohol while turning the crankshaft, then drying it, injecting after-run oil and storing it in a vinyl bag in a dry place.
If you are matching a prop to the engine, our propeller guide covers that.
Common mistakes
- Forcing a flooded engine. Clear it with the plug out and the needle closed.
- Choking the intake while cranking. O.S. warns this can cause hydraulic lock.
- Trusting a bench glow. Test with a plug known to be good.
- Over-priming. More fuel does not mean more fire.
- Turning needles at random. Use the symptom chart and the manual's starting setting.
- Leaving the glow battery connected while tuning. O.S. says not to.
- Skipping the filter. O.S. says restricted flow can make the engine cut out.
- Leaving fuel in the engine after a day's flying. O.S. recommends expelling it and using after-run oil.
Frequently asked questions
Q: Do I need a new glow plug every time the engine won't start?
No. You need a plug known to be good. If it glows brightly on a charged igniter and the filament looks normal, it can be reused. Replace it when the element does not glow, is burned out or distorted, has roughened and turned white, or when starting and idling deteriorate.
Q: How do I clear a flooded engine with an electric starter?
Close the needle valve fully, remove the glow plug, and apply the starter to pump out the excess fuel. Cover the head with a rag, invert the engine if possible, then reinstall the plug and restart without priming.
Q: Can I use a four-stroke plug in a two-stroke engine?
Use the plug type your engine's manual names. O.S. recommends its No.8 and A5 for the 55AX two-stroke, and ships a Type F plug with the FS-95V four-stroke. Mixing types can change reach and heat range, so match the plug to the engine.
Q: Why does my engine start but stop as soon as I disconnect the glow igniter?
Per the O.S. chart, the likely causes are a mixture that is too rich, so close the needle a little before disconnecting, or a mismatch between plug and fuel, in which case change the plug or the fuel.
