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YS91AC Carb cleaning

Old 06-01-2021, 04:40 PM
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Default YS91AC Carb cleaning

Back in the hobby after hiatus of about 16 years. Never owned a 4 cycle always a two cycle and gas runner. My YS lost power the other day and when I landed it it wouldn't start afterwards. I have been advised to clean the carb and regulator. Is there a process to this since its my first 4 cycle rodeo I could use some direction.
Thanks in advance
Old 06-06-2021, 03:38 PM
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Go purchase a can or two aerosol brake parts cleaner. I get mine at wally world as it's first ingredient is methanol, unless they have changed the formula. I would suggest ordering two or three diaphragms and a couple plunger assemblies for the regulator. I suggest these numbers as you might as well get you shipping monies worth and I consider these parts a consumable items just like a glow plug.

CK Areo is the new US distributor for YS and also services YS engines. Another source for parts is Central Hobbies. Parts diagrams are available on both web sites and are a good reference along with part numbers.

So, did the engine flood out? Was fuel running out of the carb? Was there a hissing sound from the carb? Did you try a new glow plug? Was there still pressure in the tank after you landed?

Assuming the glow plug is OK, Usually the regulator is the problem. Disassemble the regulator and clean the parts with the brake clean. Make sure your work surface is squeaky clean as the regulator is not very tolerant of dirt. Getting the little stopper on the pin can be a little tricky but I find a pointy tweezers helps. After the new plunger is assembled to the regulator housing give it one more spray of break clean to make sure any dirt is flushed out. Let it dry well then install the spring and adjustment screw. Install the housing with a new diaphragm making sure the line on the housing is facing the drive washer. Yes it is important! The adjustment screw shout be set flush as a starting point. Turning in is rich, out is lean.

If you disassemble the carb the barrel will only come out on the left side of the engine. The barrel has a rubber lip style seal on it. Pushing it out to the right will damage the seal. Take a look at the parts break down to see the parts layout. Clean well and reassemble.

DO run a GOOD fuel filter to keep any debris out of the fuel system. When adjusting the carb, make an adjustment and let the system stabilize for a few seconds before making another adjustment. The low speed adjustment is an air bleed so in is rich, out is lean. Adjust the high speed first then the low. If the midrange is rich or lean then adjust the regulator. This will mean you will need to readjust the high and low speed needles again. It's a little more fussy a procedure that a "normal" engine but once set will be the same forever.

Hope this helps.


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