31k lc2 strange jetting

[doggs]

some background first
bought a 31k ypvs - pretty tidy,ran ok but occasionally at large throttle openings in higher gears-never lower gears - it stuttered held back and miss-fired.close the throttle and reopened all is well again - strange - felt like fuel starvation but could be numerous issues.anyway decided to open up the carbs(which are marked 31k) - do the cheap things first and check float heights - they were pretty much spot on but noticed the mains were 280 - now my question - got there eventually - these seem a bit big to me with standard air box and pipes so anyone know why you would put 280 in - is anyone running anything similar and could this rich in my opinion environment cause the mis fire under high load
i shall await the forums wisdom

[jessy03]

240 is std, might of been running modded airbox before you got it

The CDI’s on these are not particularly reliable either, change the jets and see how you go.

Jess

[midlifecrisisrd]

It has had non standard exhausts most likely

Drop them back to 240

This happens when people take the spannies off and put the standards back on and can't be arsed changing the carbs back

Steve

[Yogi]

I agree with the above
Probably had allspeeds or microns back in the day

[doggs]

thanks everyone
pulled the air box apart to make sure std - it was but looking down the reed blocks seems to have had some mild work done in there so
have decided to check the needle position and bang some 250 mains in - a bit bigger than standard but way smaller than whats in there now - erring on the side of caution - bit bigger in case its had a mild tune - i bet the bloody thing blows up - will do a plug chop to see whats going on
- yes officer the reason its flat out in 4th was i was checking the fuelling - oh thats ok then,on your way sonny
- ha sonny - haven't had that condescending comment for quite a few years now

[mceoni]

Hello Everyone !

I´m trying to set my carbs by myself, but it´s been a litte tricky.

I have a RD350 YPVS (1YH - Brazilian) (it has 1UA barrels and 31K head...I thinks it´s the same set as 1WW, right ? ). But it has Mikunis VM26 (no PWK)
So...I´ve got separate air filters (UniPod) + JL exhausts.

The original settings on the manual says it uses 250 on main, 27.5 on pilot jet, and clip on the 4th position.

My bike was running with 1/2 turn on the air speed screw.
It was "OK" on low/medium revs, but when I get on 6,5Krpm...lost a lot power / seizes. ( I understood as poor mixture )

Then...I got some 270 main, moved the clip from the 4th to the 5th position ( more rich )...opened more 1/2 turn on the air screw...and now:

Low revs: screwed!
Medium revs: almost screwed!
High revs ( from 7 to 10Krpm ): "OK"

After warm, when I release the throtlle it still stays on the 4Krpm...like if I were still holding ! ( Can it be "pre-ignition" ??)

So, I´m wondering what to do.
Does anybody have a clue ?

Tks!!

[midlifecrisisrd]

May 9, 2018 3:44:29 GMT 1 mceoni said:

Hello Everyone !

I´m trying to set my carbs by myself, but it´s been a litte tricky.

I have a RD350 YPVS (1YH - Brazilian) (it has 1UA barrels and 31K head...I thinks it´s the same set as 1WW, right ? ). But it has Mikunis VM26 (no PWK)
So...I´ve got separate air filters (UniPod) + JL exhausts.

The original settings on the manual says it uses 250 on main, 27.5 on pilot jet, and clip on the 4th position.

My bike was running with 1/2 turn on the air speed screw.
It was "OK" on low/medium revs, but when I get on 6,5Krpm...lost a lot power / seizes. ( I understood as poor mixture )

Then...I got some 270 main, moved the clip from the 4th to the 5th position ( more rich )...opened more 1/2 turn on the air screw...and now:

Low revs: screwed!
Medium revs: almost screwed!
High revs ( from 7 to 10Krpm ): "OK"

After warm, when I release the throtlle it still stays on the 4Krpm...like if I were still holding ! ( Can it be "pre-ignition" ??)

So, I´m wondering what to do.
Does anybody have a clue ?

Tks!!

Are the carbs marked 31k or 1ua

1ua has powerjets which affect things but not best with pod filters

I'd say you are way lean

Standard 31k is 240 so with pods and jl pipes you will be in the 3-400 range

Also I think 27.5 pilots are standard so need to go up there too

Turning the airscrew out will have leaned out the low running hence it being worse and holding revs

Steve

[mceoni]

Thanks for your answer Steve.

My carbs are marked as "1YH". ( it doesn´t have a "Power jet", and I think it´s very close to the 31K )

The original manual says 250 main, 27.5 pilot jet, 4th position on needle and 1 1/2 on pilot air.
On my first scenario I was almost like the manual says, except on the pilot air..I left just 1/2 turn open.
I had the feeling of being a little rich on low revs and really lean on highs (seizing at 6,5krpm )

On the 2nd scenario, I just switched the 250 to 270, moved the clip from 4th to the 5th position, and pilot air with 1 turn open
( actually, I made a 1mm washer to put under the clip, simulating a 6th position )
Spark plugs show dark and wet ( which tells me it´s rich, right ? )

But now, I´m trying to understand which factor is more relevant in the a:f ratio, and where ( rev range ) they act...

What I understand is: when I move the clip from 4th to 5th position, the main jet "starts to act" earlier...so perhaps I´m running rich because main jet is acting to early ( still on low revs ).
So my next attempt will be to move it back from my "created" 6th position to the 4th...maybe 3rd and check what happens.

But how about the pilot air screw ? Does it impact both low and high revs the same ?

Tks!

[midlifecrisisrd]

Air screw won't have much affect on top end

A little air passage compared to a wide open carb is nothing

If it's holding revs it either lean or you have an air leak

I've never heard of 1YH carbs

The main jet size is mainly wide open throttle but is affected on slide position by the needle position

So raising the needle means that it will richer the mixture at a given throttle position

Steve

[doggs]

did a plug chop - plugs were bit darker than light tan colour but still a mid brown - so happy with that just on the rich side i think but on the safe side - was flat in 4th for the chop so hope thats good enough.still mis fired on the way home though - bol@oks. have to try something else.runs really well nearly all the time so it must be something daft - hopefully - will recheck carbs again then maybe new coils
found it hard to find a suitable place for the chop - empty road,straight and somewhere safe to cruise to a halt and dis assemble bike - riding along had a great idea - theres a long straight leading to biker cafe at fridaythorpe - will nail it along there kill the motor and cruise majestically into the car park silently and have a tea while motor cools down so i dont burn my pinkies removing plugs.imagine my horror when just as i killed the motor a jam sandwich pulled out of said cafe - as i was then in stealth mode i got away with it - but really ! midweek mid afternoon country road and still nearly get into bother