Moderator: chasthor

Carb parts (main jet)

dmgeurts
Junior Fizzy Nut
dmgeurts
Junior Fizzy Nut
Joined: June 3rd, 2008, 9:40 pm

June 23rd, 2018, 9:50 pm #1

Just managed to get my 2RU FS1 (Dutch import, like me) on the road after 10 years. Lucky for me HMRC did not fine me when officially importing the bike 10 years late...

MOT passed but the bike shop didn't test the carb settings so it ran (badly) with the choke on but would stall with choke off. Swapped the 150 jet with what looks like a 170, no markings (assuming immi). And now it's better but still not great. No longer stalls after raising the jet needle one ring, but it now revs uncontrollably rather than dropping back to idle when releasing the throttle and gas at the same time when slowing down. Also, mid throttle is I think too lean, as it's got no torque and makes a loud hollow sound (sorry no idea how to describe this better). The spark plug is also the colour of a milky builders tea (too light I think). I took the 150 main jet out as it needed the choke on half to run reasonably well and respond to the throttle as I'd expect. Turning the choke off would result in no issues at full throttle but slowing down would mean the engine would cut out rather than idle. I'd again need the choke to start the engine, which is silly for a hot engine.

Based on the carb setup
1) main jet (170?)
2) needle (raised 1 ring), I think this is a 3G9 (is this a standard needle for a 16mm carb?)
3) air screw (set to 1,5)

Some more background. The cylinder says FS1, no idea about size so assuming this is a plain 50cc. Also no idea about the alu head. If I remember well it used to run fine although a little hot, got used to being quick on the clutch when it would cease up and would be fine once cooled down. The Mrs surprised me by getting the local bike shop to get it running and MOT'ed, ready for V5 registration last month. Top speed at the moment is 65kph (40mph), changing the main jet has had no impact on this.

My assumptions, and please correct me as I'm likely to be wrong:
1) Find the right main jet so the spark plug ends up coffee coloured
2) See if mid-throttle power is still an issue then

Oh, and the throttle valve cutaway is 2.5mm. No idea if this default either.

Is it normal to need the choke to start the engine on a warm day 24C today? Bike shop says no, but I can't remember what I used to do 20 years ago. Rebuilt the bike back then but the carb has always been a mystery to me...
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chasthor
Fizzmeister Extrodinaire
chasthor
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Joined: June 29th, 2005, 7:40 pm

June 24th, 2018, 9:07 pm #2

Ok, so, running issues.
Based on what you've written, Im going to say a 150 jet, with the needle raised by 1 notch and a throttle slide with a 1,25 slope if my memory serves me with anything. 1 3/4 turns out for the mixture screw is the starting point, but depending on the engine, it can be up to 1/2 a turn in either direction, to satisfy it.
The choke issue again could be what the engine likes or needs.
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dmgeurts
Junior Fizzy Nut
dmgeurts
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Joined: June 3rd, 2008, 9:40 pm

June 25th, 2018, 11:22 am #3

chasthor wrote:Ok, so, running issues.
Based on what you've written, Im going to say a 150 jet, with the needle raised by 1 notch and a throttle slide with a 1,25 slope if my memory serves me with anything. 1 3/4 turns out for the mixture screw is the starting point, but depending on the engine, it can be up to 1/2 a turn in either direction, to satisfy it.
The choke issue again could be what the engine likes or needs.
Thank you.

It had a 150 main jet and would only run with choke at half or it would stall, but it doesn't stick there and drops down. Meaning stopping in traffic, results in choke and kickstart to get going again. So assumed the mixture was too lean. I can put the 150 back in and leave the jet raised as it is now to see.
With the 170 in it's happier once warm, but revs will stay high in neutral when rolling off the throttle (once the engine is properly warmed up).

Not happy with the 40mph top speed, but I don't fancy taking the engine apart just to check the disk. So for now I'll just aim for a well running engine, I don't really need to do 50mph cruising around town.

EDIT: completely missed the throttle slide remark. 2.5 vs 1.25mm, Googling for one now.
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dmgeurts
Junior Fizzy Nut
dmgeurts
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Joined: June 3rd, 2008, 9:40 pm

June 28th, 2018, 12:26 pm #4

Any advice on where I could get hex short mikuni main jets? And a throttle slide...
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chasthor
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chasthor
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Joined: June 29th, 2005, 7:40 pm

June 28th, 2018, 7:26 pm #5

Yamaha should still do them
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chasthor
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chasthor
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Joined: June 29th, 2005, 7:40 pm

June 28th, 2018, 7:29 pm #6

just checked, and they do, but only on back order, and quite expensive too!
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dmgeurts
Junior Fizzy Nut
dmgeurts
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Joined: June 3rd, 2008, 9:40 pm

June 28th, 2018, 10:58 pm #7

I thought a 2.5mm throttle valve was standard for 16mm carbs?

Kicking myself now for getting rid of some of my old spares and parts list years ago...
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chasthor
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chasthor
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Joined: June 29th, 2005, 7:40 pm

June 29th, 2018, 4:07 am #8

Just log onto Fowlers or CMSNL and the parts lists are there.

As for a 2.5mm on the slide, I can not confirm.
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kcela
The 65mph Club Member
kcela
The 65mph Club Member
Joined: December 3rd, 2007, 8:03 pm

July 4th, 2018, 1:05 am #9

Check your points and make sure the 5 little holes in the needle jet are clear.
Your symptoms sound similar to what I had before I checked these
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dmgeurts
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dmgeurts
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Joined: June 3rd, 2008, 9:40 pm

July 9th, 2018, 1:09 pm #10

kcela, bought a feeler tool to check the points. And when riding last week I followed the advice I saw on another thread to turn off the fuel while riding. If I do, then mid range power is (temporarily) solved so contrary to what I've been thinking the mixture is too rich. So I changed the fuel jet back to the 150.

I've looked at the holes before and they seemed clear, but will do so again. Like you suggest, I need to ensure the points / timing are good before going back over the carb set-up.

Slowly things are starting to make sense, when I was younger these bits never made sense to me. I know what they did but couldn't grasp what difference each part makes. Very grateful to the internet and all on this forum who've asked similar questions and received great answers and help. Thank you!
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