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How to OVERHAUL the YS F91AC
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Be Patient! This is a very LARGE File!

Every YS engine is engineered to operate at peak performance thus not allowing much room for excessive part wear or part failure. What this means is, every part in the engine must function perfectly within its intended limits in order to run at its best.  

My engine has a fair number of flights on it and was beginning to show its age. The compression was getting low, the idle was irratic, and the bearings were starting to get a little rough.

The engine was carefully disassembled and thoroughly cleaned. I use Berryman Chemdip for the first phase of cleaning. Caution, Chemdip will destroy seals and gaskets so be sure there's nothing but metal parts in the cleaner. After it has soaked for a day or so, I rinse the parts in hot soapy water and blow dry them with a hot-air gun. After a close inspection, if the parts check out OK I then put the parts in a tumbler using walnut media and a small amount of polishing rouge to finish the cleaning process and shine up the parts. This is purely optional but the engine will have that brand new look & feel.

You may find the bearings difficult to remove, so I will detail how to do this WITHOUT damaging your engine. A propane torch is all that's needed. Simply heat the aluminum area surrounding the bearing and tap the crankcase on a block of wood and the bearing will fall out. Avoid heating the bearing as much as possible.

NOTE: If you have an older YS engine, you may want to Gas Port the piston while you have it dis-assembled. Here is how it's done:  Early YS Engine Gas porting

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That brownish color is corrosion

You can easily remove the front crankshaft bearing using a drift punch or similar tool. However, the rear bearing will need the heating method as well as the cam bearings. Check the bearings for smoothness and side-play. If they show any signs of wear, replace them. My bearings were totally shot, and one of the cam bearings was siezed. Most people choose to ignore the cam bearings because they are tough to get out. Do not over-look these bearings. Use heat, and they'll drop right out.

There's lots of bearing options available for the YS engine! You can go from OEM (factory) bearings all the way up to CERAMIC highspeed bearings. I chose to use Stainless Steel High Performance bearings. They are affordable, won't corrode, and rated at 30,000 RPM. Boca is the place to shop for bearings.

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Here is the crankcase with the new bearings installed. In order to insure PERFECT bearing alignment, use the crankshaft to set the new bearings.You may need to heat up the crankcase to get the bearings to drop in. DO NOT FORCE THE BEARINGS into the crankcase, this will ruin the alignment and destroy the bearings!  this is true with the cam bearings also. Put the bearings on the cam, and install the bearings in the crankcase & outer support. Heat up the bearing supports as needed to make a gentile installation.

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The Valves were removed, cleaned & polished. They were also lapped to get a perfect seal in the combustion chamber. If you want to check the seal, install the valves and put alcohol in the combustion chamber. Let it sit for a few minutes and check the ports for alcohol leaks. If there is, lap the valve(s) again. I use super-fine lapping compound, tooth paste works OK but it's a lot slower.

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Of all the parts in a YS engine, those little KEEPERS are the easiest to get lost. They also get flung if you are not careful when installing them. I assemble the valves in a big baggie so the keepers don't fly across the room if they decide to take a ride. Notice the KEEPERS are tapered, the small side goes towards the valve spring and locks in the Retainer.

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The crankshaft has been measured with a precision micrometer to check for wear. It shows minimal wear so it's ready for final installation

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Set the CRANKSHAFT to TDC (Top Dead Center) as depicted in the photo below. Do not let it move during the cam installation! Stuff a paper towel in the crankcase to help hold it in place during cam installation.

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Insert the CAM with the TIMING MARK visible to the outside. Rotate the cam slightly to synchronize it to the crankshaft and have the TIMING MARK in alignment with the centerline of the screw holes as illustrated below. 

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When the cam is installed properly, the CRANKSHAFT will be at Top Dead Center, and the CAMSHAFT will align as seen in the photo above. When the timing is confirmed install the cam cover with a new (red) O-Ring and a pair of countersink screws. ROTATE the Crankshaft to check for smoothness. It should be perfectly smooth with the cam installed.

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Install the tappets (lifters) face-down in to the crankcase. Be sure to lubricate them with Techniplate.

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Assemble the piston with the illustrated parts.

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Insert NEW O-RINGS as shown, wet them with Techniplate. Make sure they don't get cut or pinched when installing the pushrods & guide tubes.

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Carefully Insert the Piston into the Cylinder as SHOWN here.

Carefully install the cylinder with piston assy, pushrods & pushrod guides on the crankcase and snug it down with two capscrews. Make sure you have your cylinder gasket is in place!

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Make sure the O-RINGS are properly seated & sealed! Loosely lock the cylinder down with 2 capscrews as shown. Rotate the guide tubes and be sure they fit properly. Also, make sure the o-rings are not cut or damaged in any way.

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Lubricate all the moving parts with Techniplate, especially the bearings and rod.

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There is a cut-out in the ROTARY VALVE that fits over the crankpin. This must be properly set and checked. The entire engine function depends on this timing. If its not right, the engine won't run... PERIOD.

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Check the alignment through the port by rotating the crankshaft back & fourth to be sure you didn't miss the slot in the Rotary Valve. The Rotary Valve should stay in sync with the crankshaft at all times. If you see any slop in the movement between the crank & rotary valve, it is not set properly!

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Loosely install the carb assy with all four screws & gasket to the backplate as shown here.Keep it a little loose for now. 

To install the AIR CHAMBER, put ONE screw in the corner as shown (with gasket) and keep it loose. Now rotate the AIR CHAMBER down over the carb outlet & O-RING. Insert another screw on the bottom. Check it for alignment and gasket position, then install the remaining screws as shown below...

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  You'll find this to be the easiest way to install the Air Chamber AND get a perfect seal every time! You can SNUG up all the screws at the rear of the engine. DO NOT over tighten these, as it will cause leaks.

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Place the new diaphram on the base as illustrated, then set the Regulator Housing on top the diaphram with the NOTCH facing forward! THIS IS CRITICAL.

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Adjust the brass screw even with the housing. This will be your "factory" setting for your FIRST START and initial Tune-up.


Now is a good time to go over the engine and look for loose screws, bad seated gaskets & o-rings. Also rotate the engine many times and feel motion of the parts. Listen for unusual notching, or binding.

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FACTORY recommended SETTING is 0.04 to 0.1 MM for both valves

Carefully adjust your valves! The settings illustrated are my own recommendation that I have found to run best on the 91. The Exhaust valve should be a little looser setting.

When you are satisfied with the valve adjustments, install a new gasket on the valve cover and snug it down.

Install your remaining pieces such as oneway valve, fuel line to the carb, muffler, ect. The engine is "officially" ready to fire up!

Your YS engine is new again and it will likely be a little erratic at mid-throttle until the ring seats. I do not recommend attempting to fine tune it until it is completly broken in. Just keep it running on the rich side. Once broken in, it will settle down to a good tune and stay tuned indefinitely. I prefer not to bench-run YS engines! They don't get adequate cooling and they never un-load properly. I've found that they break-in faster and run much better in actual flight. Last but not least, don't change fuels if at all possible.


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NOTE the opposite directions in RICH/LEAN settings in the highspeed and lowspeed needles.

If you notice your idle screw doesn't have much, or any effect, you need to close the throttle barrel more to make the air-bleed function and cut off the FUEL INJECTION!



Start the engine in idle position. Let it warm up, and build up tank pressure. Throttle it (slowly) up once to clear the crankcase, and finilize tank pressure. Bring it  back down to idle at 2000 RPM, it should be idling steady with slight smoking. As you throttle up again (a little faster) you should see more smoke as the RPM increases. It shouldn't bog, but smoking nicely If it bogs and there's a LACK of smoke, the regulator is LEAN. If it bogs with lots of smoke, the regulator is RICH.. --CAUTION: make very small adjustments.-- As you speed up more, you will see a more smoke all the way up to full throttle. At full throttle you should observe a little less smoke than the transition. If there is the same amount of smoke, or more smoke at full throttle it's a little too rich on the top-end needle. If the smoke substantially disappears, and/or it bogs slightly, it's too lean on the top-end needle. There is an interaction between the  regulator and the top-end needle since the regulator supplies the fuel. Do not adjust the top-end with the regulator.

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The ONEWAY valve needs to be installed correctly. It pressurizes the fuel tank. Be sure it is correct and working properly.

(1) The YS 91 will not run with the valve cover off or even loose. After making your valve adjustments, promptly install the valvecover and tighten it down. If there's any leaks at all, the engine will not run steady except at full throttle. Double check all gaskets and o-rings.

(1a) Do not re-use your old gaskets, o-rings & regulator diaphram.  It only takes one leaky gasket or o-ring to mess up a good overhaul.

(2) You may happen to notice air bubbles in your fuel line between the regulator and carb at lower RPM. This is NORMAL. Do not attempt to eliminate these bubbles. YS has started using a dark blue fuel line to make it difficult to see the air bubbles but they are there. Look at the very first photo (at the top of this page) and you'll see the dark blue fuel line.  

(3) Do not assemble the engine dry and do not use petroleum based lubricants. You may have seen where I mention Techniplate. This is Klotz 100% synthetic oil. You may use castor or a castor blend for assembly. Petroleum lubricants will destroy your seals and o-rings.

(4) Always use a fuel filter with YS engines and always relieve the tank pressure when you're not running the engine.

(5) The most likely place to encounter an air leak is at the Airchamber. This is a long thin gasket only held by four screws. The perfect way to insure it seals correctly is to spray the gasket with 3M Number 77 Contact Cement. Spray each side and let it completely dry. This will make the gasket just tacky enough to stay in its place and it adds a little more thickness to the gasket as well. I've done this for years, and never had an Airchamber leak. 

ABOUT AFTER-RUN OIL: I don't like using it in YS engines. The best thing to do is run the engine completely dry when you're done flying. If you insist on using it, use only non-petrolium based lubrication and use it sparingly. Adding oil to the crankcase really changes the tuning properties until it's completely gone. 

If your engine goes excessively lean it will likely throw your prop EVEN if it's double nutted.. Please stay behind the engine while running.

EDITOR'S NOTES regarding the YS F91AC Engine

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Copyright 2006 M. B. Fuess