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Makotosun

yamaha gt50 1974 no throttle (help)

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Hi guys, been working on my father gt50 for the last 8 months, she was left alone for 24 years +-. She had no spark so i checked, she had broken and bent ignition points and the magnet i replaced those with ones from a pw80 and it has a bright blue consistent spark.

Had to buy a new air filter the old one was crumbling like dust, and the air boot is clean and has no crack or damage.

Engine compression, she has compression it blows your finger off the spark plug hole with no issue. Also did a leak down test, and found one behind the stator so replaced that crankcase seal.

Fuel. Cleaned the tank and the petcock works fine fuel flows freely. And now the issue starts the carb. Carburetor was cleaned about 3 times. 1st time was gummed up and pilot jet was clogged so cleaned it all evey nook and crany the main and pilot circuit, like everything.
She started to iddle not great iddle but iddle and she leaked from the overflow line and everything but no throttle. After this i decided to buy a kit from amazon (waste of money don't buy those kits) replaced the main and pilot jet with those provided (same numbers) and did 0 difference and she kept leaking.
After reading and reading float height and float valve seemed like the most likely culprit so i put the float height to 23mm and it stopped leaking doesn't leak anymore, the iddle now is good but still no throttle with a bit of throttle she falls on her face and dies. I moved the needle to the 2nd position and she stopped idling BUT she would throttle up to 40%.
Ordered a kit from japan from keyster waiting for it to arrive (the float needle has a tiny crack and i'm not happy with it even if its not leaking now).  

Point is i don't know what else to check, need some advice/help because after 8months i'm very frustrated.

 
21 May 2024 09:16 #1

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Replied by RT325 on topic yamaha gt50 1974 no throttle (help)

Welcome from NZ. I may not have read the story thoroughly but did you screw out the centre emulsion tube (call it the needle jet) because they get built up in there & I think has fine cross holes to poke (with my acupuncture needles lol). Also an airway to 'that' from carb mouth. Float setting if 23mm is correct, should be set with float tang just touching the spring loaded float needle otherwise if bottomed out, compressed, you'd end up with not much fuel in the bowl when it floats up to the needle valve Spring pin. Also, does operating the choke help it get through the leanness. Is choke working because they get blocked in the fixed jet down in the bowl cavity. Cheers.
21 May 2024 15:07 #2

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Replied by muxus on topic yamaha gt50 1974 no throttle (help)

I took the main jet and the emulsion tube from the hole and from eachother(they are screwd together), the emulsion tube is not the most clean if i have to admit, but i cleaned the tiny holes with a needle there are 3 in a side 2 in the other. The airway is tiny so i got a toothpick and glued on it a super fine sandpaper to clean the inside and then acetone and wd40 to finish. The float height i measured without the gasket from the gasket crevice to the highest point of the float with it barely touching the needle.

I've got to admit i never tried to run with the choke on because i kick the kickstart with the choke on until i start to hear a rumble or pop in the engine then i take the choke out and start after one or two kicks
21 May 2024 16:21 #3

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Replied by RT325 on topic yamaha gt50 1974 no throttle (help)

I'd say you're onto all the trick so not much more i can add at the mo.
With choke on if its doing its job it should richen it up plenty if idling or even stop the motor from over-rich.
Could lift the needle fully [clip down] can't remember if those had 3 grooves of 5.
Is it a Mikuni or one of those TK brands i think they might've run.
They're likely slightly restricted by the air cleaner-not sure-but if running minus the filter hose connected it might be a bit lean to get up through thje needle.
Also idle mixture needs to be set 'not out too far' as those little motors & carbs can be quite affected by pilot screw setting up through the trtansition from pilot to needle.
Easy to test & just go lightly right home with the screw for full rich.
21 May 2024 17:01 #4

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Replied by Ht1kid on topic yamaha gt50 1974 no throttle (help)

Greetings from Tennessee 

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21 May 2024 17:48 #5

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Replied by muxus on topic yamaha gt50 1974 no throttle (help)

Greetings. Well i will get proper carb cleaner and clean everything again spotless, check the float height might have missed something. Its a Tk the needle has 5 positions. Some people sugested to just buy a new carb thing is i want to keep it as closer to original as possible. And this carb is fine i'm just a bad mechanic xD(first time working with motorcycles) and a used one in terrible conditions is 140$+. So i'm not giving up
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21 May 2024 19:33 #6

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Replied by RT325 on topic yamaha gt50 1974 no throttle (help)

For the minimal cost of experimenting you could go here & sift through vaious options.
nl.aliexpress.com/item/1005005729711677....arch%7Cquery_from%3A
21 May 2024 19:50 #7

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Replied by RT325 on topic yamaha gt50 1974 no throttle (help)

Or here. I go nowhere asking for GT80 or GT50 but MX80 got somewhere & same motor as GT80 so closely related to GT50.
nl.aliexpress.com/w/wholesale-yamaha-mx8...=a2g0o.home.search.0
21 May 2024 19:52 #8

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Replied by muxus on topic yamaha gt50 1974 no throttle (help)

Tomorrow my keyster carb kit FY-5245N for the gt50 is arriving from japan, will take the chance to clean it again wish me luck
22 May 2024 10:00 #9

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Replied by muxus on topic yamaha gt50 1974 no throttle (help)

Well. I'm starting to believe its not a carb issue, no matter what its running the same, idles but throttle kills it. I can increase the idle speed with the air screw and the fuel screw but no throttle. I'm out of ideas :( can anyone suggest something. I'm getting ready to call a mechanic at this point xD.
23 May 2024 12:53 #10

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