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Carburetor throttle shaft play

by Had Robinson

Having the engine stall just when you have the wing safely overhead and are ready to go is a nuisance.  You squeeze the paramotor throttle and the engine promptly dies.  What happened?  You checked the ignition system, fuel system, and maybe even the reed valve.  You have rebuilt the carburetor and are using fresh fuel.  The pop-off rest pressure is good and you have correctly adjusted the low speed system on the carburetor.  When the motor was new it ran fine.  What is wrong?

More often than not, the pilot is at fault.  There is no 2 cycle engine with a diaphragm-type carburetor e.g. the Walbro WG-8 that will not stall if full throttle is applied with great speed.  This is because these carburetors have no fuel enrichment system that kicks in when when full throttle is applied quickly, as in automotive carburetors or in electronic fuel injection systems.  When the throttle plate is opened too fast, the air rushes into the engine before the fuel can get moving through the various passageways and jets in the carburetor and then into the engine.  Consequently, the engine leans out and stalls.  Pilots must be careful to always apply throttle slowly and deliberately when launching.  Attention overload is very common when launching and pilots will jam the throttle unless they consciously tell themselves not to.

On the other hand, the cause might be excessive throttle shaft play.  The important distinction between this and attention overload is that sudden did not used to happen until recently.  A sign is the presence of copious amounts of 2-cycle oil on or about the center of the carburetor.  The oil will not be black but have the same color is had before it was mixed with fuel.  It is normal to have some oil on the carburetor but it should not be forming a lot of drips.  However, if the connection to the airbox is not tight, you may see a lot of oil at the joint and even some drips.

Why is this?  All 2 cycle engines "spit" some fuel out of the carburetor.  As the reed valve closes after the intake stroke, the air fuel mixture that is in the carburetor throat will "bounce back" out of the carburetor.  If the joint between the airbox and the carburetor, the fuel leakage will be minimal.

After a few hundred hours, the constant rotation of the throttle shaft begins to cause serious wear of the soft metal that makes up the bearing surfaces in the carburetor body.  On a WG-8 with 250 hours, I measured over .010" (0.25mm) of play in the throttle shaft.  Such wear and the air leak can cause changes in the air/fuel mixture at the low and medium ends of the power band by causing a lean-out condition.  The most annoying effect can be stalling under a sudden throttle increase from idle.

Measuring throttle shaft bore wear on a Walbro WG-8

Throttle Shaft Play  Throttle Shaft Play

This is not a problem that you can easily fix.

An expert machine shop could bore out and then fit bronze bushings in the old body but the expense would be more than a new carburetor  The throttle shaft has to be perfectly aligned for the throttle plate to work correctly, especially at near closed positions.

Your average home mechanic could probably do a repair using new bushings and epoxy.  He would first have to drill oversized holes in the carburetor body with a drill press.  The replacement bushings would be about 1 mm smaller than the new holes in the carburetor body.  Next, the carburetor would be completely re-assembled with the throttle plate fully closed.  Using (30) min epoxy, inject it with a needle around each of the new bushings.  This would ensure that the bushings are perfectly aligned.  It would take some skill to keep the epoxy out of the tiny holes in the throat of the carburetor.  The question would be:  would the epoxy hold up to the severe vibrations generated by the Top 80 engine?  It would be fun to try this when time permits.

Purchasing a new carburetor is the easiest thing to do.

Minimizing throttle shaft bore wear

I have developed a quick and inexpensive modification for the throttle cable return spring system that puts almost no lateral load on the throttle shaft.  In this setup, the carburetor only needs the weakest spring to close the throttle plate.  The less lateral pressure there is on the shaft due to return springs pushing them to one side, the better, as it will minimize wearing of the bore.

It is unfortunate that parts other than those of the main engine wear out sooner on the Top 80.  Peripheral parts should be designed to wear out at about the same time as the piston or crankshaft bearings.  This should include engine and redrive seals which should never go bad like the seals in the old Volvos that were made in Sweden.  Those seals would last 30+ years.

vulture