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Carb issues(only will run on choke)

  • Billy Mild
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Replied by Billy Mild on topic Carb issues(only will run on choke)

1971 Yamaha AT-1 with a CT-2 top end, sleeper pipe. 

youtube.com/shorts/n4q1_EbNd7o?feature=share
Bike runs on Choke without an issue. The carb is clean, even threw in a new pilot jet. I also sprayed pilot jet and air idle screw passages before reassembly. 

​​​​​While riding when the bike drops below 25-3000 RPM it feels as if the engine is dead and you have to crack the throttle more than 30% for it to start running again. It is not an ignition problem but more fuel related. 

Should I try buying a new carb or what?
03 May 2021 18:26 #11

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Replied by apex on topic Carb issues(only will run on choke)

It's just tough to buy new flange carbs anymore genuine Mikuni. I have a chinese VM replacement flange on my kid's AT2 w/CT2 top end, and it works OK, you just can't really tune it without modifying jet passages, and, well, when you go WOT the slide comes up over the nubbin that keeps the slide pointing the right direction, and...yeah.

Your better off with the carb you have. Question asked above is was the pilot jet taken out to clean? That is a must. And you should be able to see through the bore the pilot fits into directly into the venturi area if I remember correctly. You can use a twist tie with the paper taken off to clear the passage (Guerrero Tortilla Shell packaging uses them if you need one in a pinch). I use 120 PSI compressed air with a nozzle, the twist tie, and brake clean to clear all passages with all pieces removed, to include the nozzle, air screw, choke plunger...etc...get all the pieces off and blast away. Use the brake clean red nozzle, put it into the bore you want to see if it's clear, and blast away. Wear safety glasses.

Your description of the issue is classic plugged pilot.

 
Allrighty Then
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03 May 2021 20:20 #12

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Replied by Mothersbaugh on topic Carb issues(only will run on choke)

+1 on Apex, and then I'd add 'new pilot jet.' Is that a new Mikuni pilot jet of the same size? Or some POS from a Keister kit?

You say "I also sprayed pilot jet and air idle screw passages." With what? Carb cleaner and/or brake parts cleaner, and then compressed air?
I'd clean the carb again.
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03 May 2021 20:55 #13

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  • RT325
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Replied by RT325 on topic Carb issues(only will run on choke)

Crank seal on one side or other is a possibility. With pilot jet out can you see straight up through where it screws in. I'm not 100% sure that's possible as there's a hole on an angle as well for the pilot progression just above idle. Also the air bleed leading to the needle jet from the mouth of the carb that can get old air filter stuff in it. All pretty unlikely though if you've been through the carb & sounds like you've given it a good go. If all else fails, check the left crank seal. If it;s smoking lots more than normal then go to the right side seal as they do get forced out then throttle goes all hollow--no power & gasping--.All a guess though but been there & had similar.
04 May 2021 01:51 #14

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  • Billy Mild
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Replied by Billy Mild on topic Carb issues(only will run on choke)

Just to clarify these are the steps I have taken on this carb and fuel system. 

1. Drained gas and refilled with clean 91 octane with seafoam mixed at 1 oz/Gal
2. Taken the carb apart and used carb cleaner and sprayed it in all passenges with Jets removed. I also removed the air/idle screw and cleaned passage for that. 
3. Taken pilot jets and let them soak in B12 Chem Tool then cleaned them out with Carb cleaner and using a small wire to poke through all the holes. 
4. Tried a brand new pilot jet. 

The only thing I haven't tried is doing a ultrasonic cleanser for the carb body. 
04 May 2021 08:16 #15

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Replied by Mothersbaugh on topic Carb issues(only will run on choke)

One thing Apex mentioned and I seconded you do not mention: blow out all passageways with compressed air. Often times the starter jet will be clogged even after repeated soakings with cleaner. Have you blown air from a standard sized air compressor through that particular passageway?
Also, what are the sizes of the pilot jet that you are trying?
What are the settings that you are using for the idle screw?
And for the air screw?
04 May 2021 11:21 #16

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Replied by apex on topic Carb issues(only will run on choke)

One other thing I just thought about- check that the reed valve petals seat to the reed cage. If they are lifted, the bike engine won't want to idle.
Allrighty Then
04 May 2021 12:05 #17

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  • Billy Mild
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Replied by Billy Mild on topic Carb issues(only will run on choke)

Have you blown air from a standard sized air compressor through that particular passageway?
-Not recently
Also, what are the sizes of the pilot jet that you are trying?
-25-30-35
What are the settings that you are using for the idle screw?
-lots of combos
And for the air screw?
-a lot of combos. 1.5 usually. I have tried all the way open and all the way closed.
04 May 2021 20:05 #18

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  • msavitt
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Replied by msavitt on topic Carb issues(only will run on choke)

Pretty hard to add to the great advice already shared. But I had a same issue after removing “jet, needle jet” and pushing it back in without aligning slot with pin, thereby putting air scoop thing in an improper alignment which obviously impeded has siphoning. Once a figured out my mistake prob solved. 

not suggesting your as incompetent as I am but this what I did!
04 May 2021 20:29 #19

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Replied by MarkT on topic Carb issues(only will run on choke)

In my experience, complete disassembly and thorough cleaning is the most important thing you can do.  Spraying a little carb cleaner and/or air might work sometimes...  but even soaking in the good Berryman's doesn't always work.  On my MG1T I had soaked carb a few times...  even found another carb on ebay...  while waiting for carb I got a little Harbor Freight ultrasonic cleaner and couldn't believe the amount of crap that came out of the carb.  Ran fine after that.  Wouldn't idle without choke before that.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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04 May 2021 20:56 #20

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