O.S. engine MAX-91SZ-Hring

Questions and Answers

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Frank MillsAug 8, 2025
What to do if O.S. Engine fires intermittently but does not run?
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  wericksonAug 8, 2025
  If your O.S. Engine fires intermittently but doesn't run, it could be due to several reasons. First, make sure you have enough fuel in the tank by filling it up. A deteriorated glowplug can also cause this issue, so consider replacing it. Check for a clogged fuel filter or a dirty silencer; clean or replace the fuel filter and clean inside the silencer. If the engine has overheated, allow it to cool down. Ensure you're not disconnecting the plug battery too soon; wait until the RPM becomes stable. Finally, check for air bubbles in the fuel and fit O rings to the tank screws to prevent them.
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Heather DennisAug 13, 2025
Why my O.S. engine is not reaching expected peak r.p.m.?
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  Jean FrostAug 13, 2025
  If your O.S. Engine isn't reaching the expected peak RPM, make sure you've warmed it up sufficiently and completed the running-in process before setting the needle. Also, verify that the silencer or manifold is securely connected; check the connections and secure them if necessary. Finally, inspect the fuel tubing from the tank for splits or breaks and replace it if damaged.
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John StoutAug 18, 2025
How to fix poor response of O.S. Engine?
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  Laura ShahAug 18, 2025
  If you're experiencing poor response with your O.S. Engine, it could be due to a deteriorated glowplug, which you should replace. Incorrect carburetor settings can also cause this; readjust the low RPM range using the metering needle and mixture control valve. Finally, check for inaccurate linkage and recheck the throttle opening against the pitch angle.
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Jasmin CraigAug 23, 2025
Why does my O.S. Engine have an unstable idle?
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  Keith WilliamsAug 23, 2025
  If your O.S. Engine has an unstable idle, consider these potential causes: * The glowplug might be unsuitable; use the suggested glowplug from the instructions. * The fuel might be unsuitable; avoid using extremely high nitro or low oil fuel. * The silencer might be disconnected or loose; install the silencer securely.
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Jennifer HillAug 28, 2025
What to do if O.S. Engine has poor r.p.m. drop?
- K
  Kelsey PhillipsAug 28, 2025
  If your O.S. Engine exhibits a poor RPM drop, it could be due to a few reasons. There might be too much throttle opening at idle, so lower the throttle trim to an adequate position to reduce idling RPM. An incorrectly fitted carburetor could also be the issue; ensure the carburetor is securely fitted. Lastly, the needle valve might be closed too far; try opening the needle valve slightly.
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Dr. Scott JacksonNov 2, 2025
What to do if my O.S. engine fails to fire?
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  rogersjacobNov 2, 2025
  If the O.S. engine fails to fire, several issues could be the cause. First, ensure the fuel tank isn't empty and that fuel is actually reaching the engine; if needed, refill the tank and repeat the priming procedure. A burnt-out glowplug element or a discharged glowplug battery can also prevent firing, so replace the glowplug or recharge/replace the battery. Other potential causes include a clogged fuel filter or a dirty silencer, which require cleaning or replacement, over-priming (remove the glowplug and pump excess fuel), disconnected or damaged fuel tubing (connect securely or replace), incorrect carburetor settings (return to basic position), incorrect servo linkage (re-link after setting servo at neutral), or the electric starter rotating in the wrong direction (ensure counter-clockw...

Summary

SAFETY INSTRUCTIONS AND WARNINGS ABOUT YOUR O.S. ENGINE

WARNINGS

Covers events that might involve serious (even fatal) injury.

NOTES

Covers less obvious dangers that may cause damage or injury.

Notes on installing cooling-fan and clutch

NOTES WHEN APPLYING AN ELECTRIC STARTER

BEFORE STARTING

Note on heating the glow plug

INTRODUCTION

FACTS ABOUT GLOWPLUGS

Fuel

Fuel Pump

Alternative for transferring fuel directly from container to tank.

O.S. Super Filter (Fuel Can Filter)

Filter for refueling container outlet to prevent foreign matter entry.

Fuel Filter

Recommended inline filter between fuel tank and carburetor.

Silicone Fuel Line

Heatproof tubing required for connection between fuel tank and engine.

TOOLS

Hex Wrench

Necessary for engine installation, various sizes.

Phillips Screwdriver

Required for carburetor adjustments.

Screwdriver

Necessary for carburetor adjustments.

Socket Wrenches

Various sizes for engine installation and adjustments.

Long Socket Wrench With Plug Grip

For easy removal/replacement of angled glowplugs.

End Wrenches

Various sizes for adjustments and assembly.

Needle Nose Pliers

Useful tool for gripping small parts.

ENGINE PARTS NAME

INSTALLATION OF THE STANDARD ACCESSORIES

INSTALLATION OF THE CARBURETOR

Steps for installing the carburetor and its retainer.

INSTALLING THE GLOWPLUG

Procedure for installing the glowplug correctly.

ABOUT THE HEAD GASKET

Information on adjusting head gasket thickness.

ABOUT THE WOODRUFF KEY

Guidance on using the woodruff key for fan pulley.

INSTALLATION OF THE ENGINE

When to replace the glowplug

CARBURETOR CONTROLS

A The High-Speed (Main) Needle Valve

Sets basic fuel/air mixture strength at full throttle.

B The Idle Mixture Control Screw

Manually adjusts the mixture control valve for idling.

C The Medium Speed Needle Valve

Fine-tunes mixture strength for hovering mode.

BASIC POSITION OF MIXTURE CONTROL SCREW

GRADUATIONS ON THE CARBURETOR BODY

Explains the graduation marks on the carburetor for throttle opening.

STARTING

1. Opening and closing of the Needle-Valve

Instructions on how to operate the needle valve.

2. High Speed needle-valve

Initial setting for the high-speed needle valve.

Idle Mixture Control Screw

Initial setting for the idle mixture control screw.

Medium Speed needle-valve

Initial setting for the medium-speed needle valve.

3. Preparation of the starter

Correct installation of the starting shaft to the starter.

4. Checking the rotating direction of the starter

Ensuring the starter rotates in the correct direction.

5. Inserting the starter shaft

Proper insertion of the starting shaft into the shaft cup.

6. Priming

Procedure for priming the engine before starting.

7. Preparation of starting

Checks before initiating the starting sequence.

8. Starting

Step-by-step instructions for starting the engine.

9. Stopping the engine

Methods for safely stopping the engine.

RUNNING-IN (Breaking-in)

ADJUSTMENT

High-Speed Needle Valve

Guidance on adjusting the high-speed needle valve.

Medium-Speed Needle Valve

Guidance on adjusting the medium-speed needle valve.

Idle Mixture Control Screw

Guidance on adjusting the idle mixture control screw.

ADJUSTMENT PROCEDURE

SUBSEQUENT READJUSTMENTS

CARBURETOR CLEANLINESS

ADJUSTING CHART

INSTALLATION OF THROTTLE SERVO

CARE AND MAINTENANCE

TROUBLE SHOOTING

Engine fails to fire.

Corrective actions for when the engine does not start.

Engine fires intermittently but does not run.

Corrective actions for intermittent engine operation.

Unstable idle

Causes and solutions for an unstable idle.

Not reaching expected peak r.p.m.

Troubleshooting for failure to achieve maximum engine speed.

Poor response

Corrective actions for sluggish engine response.

Poor r.p.m. drop

Troubleshooting for sudden engine speed reduction.

ENGINE EXPLODED VIEW

ENGINE PARTS LIST

CARBURETOR EXPLODED VIEW

CARBURETOR PARTS LIST

O.S. GENUINE PARTS & ACCESSORIES

THREE VIEW DRAWING

SPECIFICATIONS

MEMO