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The OS 50 Hyper Max carburettor assembly (60L)

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Removing the carburettor

Loosen the pinch bolt underneath the carburettor. You don't need to remove it completely, just a few turns should be enough to allow the carburettor to be removed.

Remove the main needle

Before winding it out to remove it, wind it all the way in to make a note of it's current position counting full turns and then clicks until it stops. This is not really necessary for a new engine, but if it's an engine in use, this step makes re-tuning it when refitting to the heli much quicker.

Once you've made a note of the current position, wind it all the way out and also remove the clip.

Remove the carburettor barrel

The carburettor barrel is held in place by a short screw that doesn't actually hold anything down, but is instead used as a stop point. The carburettor barrel has a groove cut into it that this screw sits in and the screw prevents the carburettor arm from moving beyond the two ends of the groove. That groove determines the fully open and fully closed points of the carburettor.

Remove the retainer screw and the carburettor barrel will slide freely out of the carburettor body.

The mixture control valve

This mixture control valve is simply push fitted into the end of the carburettor barrel. It can only fit in one way around as there is a slot in the end of it that does not go all the way across. The cutaway must be lined up with a small pin in the mounting point in order to push fit all the way in.

It also has a rubber ring which should be checked to ensure it is in good condition. They tend to last for a long time so you shouldn't find any issues with that ring.

Make sure the mixture control valve and the barrel end into which it is push fitted are both clean and muck free.

The base needle valve assembly

The first thing to mention here is that you can deconstruct the base needle valve area without touching the base needle setting screw, so there is no fear of altering the engine tuning when cleaning this area.

The base needle valve assembly is made up of a cam bolt (the adjustment screw itself) which holds the copper plate in a particular position. The cam bolt can be turned through 180 degrees which is the full range of adjustment available on the base needle. The copper plate is actually part of the base needle barrel and moving the screw turns the barrel a small amount by moving the plate position, which affects the tuning of the engine.

The 5mm bolt opposite the base needle screw is the one that holds the assembly together. Removing this bolt will allow the base needle barrel to be removed. There is a small black plastic spacer washer below the edge of the copper plate which keeps it spaced away from the carburettor housing, allowing it to move.

Remove the screw being mindful not to lose the black shim washer.

With the screw removed, the base needle valve can be carefully prised from the carburettor housing using a flat bladed screwdriver (or similar tool). Once it is out enough to be able to grab the sides with your fingers, simply pull it out of the carburettor housing.

The base needle valve has two rubber rings on it which should be clean. Also, make sure the rest of the base needle valve is clean.

To re-install, line up the cutaway in the plate with the base needle screw and then push the base needle valve back into the carburettor body, but not all the way in. With it partially fitted, slot the black plastic spacer washer into position over the hole and slightly under the edge of the copper plate. Fit the bolt through the black plastic spacer washer and then tighten the bolt down firmly.

The plate should be firmly held in place with the black plastic spacer washer spacing it away from the carburettor body.

The fuel inlet

The fuel inlet is a screw fitted part. You need a 5mm spanner to undo it.

You shouldn't need to remove this part unless you think you may have a blockage in it. If you do remove it, make sure you don't lose the white plastic washer which sits behind it against the carburettor body.

When refitting, as with all bolts in the engine, no threadlock should be used.

Refitting the carburettor barrel

With the carburettor barrel ready to be fitted into the carburettor body, line up the retaining groove in the barrel with the retaining screw hole in the carburettor body and then slide the barrel all the way in. It will become snug once it's almost fitted and needs to be pushed in all the way.

Fit the retaining screw and tighten it down, ensuring that the barrel has completely free movement and stops at the ends.

Refitting the main needle

Fit the clip to the main needle tube, then wind the main needle all the way in, being careful when you get near the end stop so as to not cause any internal damage.

With the main needle screwed all the way in, wind it back out to the previously noted position (from when it was first removed).

Refitting the carburettor to the engine body

The carburettor is clamped into the engine housing with a pinch bolt.

There are two shaped sleeves that are mounted on the pinch bolt in the carburettor hole and it is these that hold the carburettor in the engine body and secured in place so it doesn't move.

When the pinch bolt is tightened up it pulls the sleeves together to clamp it in place. When re-fitting the carburettor, these two sleeve halves need to be manoeuvred into position so that the inside of the barrel surface feels smooth all the way around. This will allow the carburettor to slide into place quite easily. If these sleeves are not in the right place, and the carburettor is then forced past them into the barrel, it will be difficult to fit and will likely cause some damage to the part of the carburettor being fitted into the engine body.

Position the retaining sleeves flush with the carburettor hole, I use my finger which seems to work well.

Make sure the rubber ring is in good condition and pushed up to the base of the carburettor barrel, then slide the carburettor into the hole. It should slide in quite easily.

Push the carburettor all the way in, then position it so it is set correctly before pinching up the pinch bolt to secure it in place. There are markings on both sides of the carburettor and matching markings on each side of the engine body where the carburettor fits. These markings should be lined up.