Hi i have a 1983 Yamaha Rd125lc...

[MARTIN RD125LC]

Hi i have a yamaha rd125lc.. It has plenty of power, however, the guy i got it off (works for tds motorcycles in barnsley) has fitted a k&n filter and a all speed exhaust which he said is jetted to suit. My bikes fast afte 4500rpm but up till then you can hear it dragging air in.. do i 1) tighten the air screw so it dont get too much air 2) find a standard air box?

Which would be best with this exhaust?

Thanks in advance!

Martin
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RIDE : 1983 - Yamaha Rd125lc

[Blau Zedong]

Standard airbox (if you can find one). K&N's are shite unless you can find a suitable spacer and the time (and box of jets) to get it to run well. Then you'll find it's still slower than standard.
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[MARTIN RD125LC]

[MarJay]

[evoboy]

Its difficult to tell what needs doing without actually hearing/seeing/riding it.

Are you sure your just not asking too much from a 125cc 2stroke and trying to make it pull a high gear at a low speed?

I cant see it making much power below 4k revs.

My 250 twin 2 stroke has K&N's and you can hear the air rushing in at low revs, but it still pulls clean without bogging/hessitating.
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[MARTIN RD125LC]

[MARTIN RD125LC]

[MinhDinh]

Have you tried phoning a good mechanic on what he thinks the problem is and how much it will cost? Then do it yourself!
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[MARTIN RD125LC]

I have he did not say what could be causing it he just said, standard is the only and best way to keep a bike at its best
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RIDE : 1983 - Yamaha Rd125lc

[evoboy]

Does it run any better low down if you use the choke?
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[MARTIN RD125LC]

[evoboy]

Are you adding the choke a small amount while riding the bike?

Air screw is only gonna help at small amounts of throttle input.

It could be the needle height, but try the choke idea first.
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[MARTIN RD125LC]

[Old Git Racing]

That combination of exhaust, filter, and a bigger main jet will move the powerband up the rev range. Try raising the needle 1 notch and screwing the air screw in a quarter of a turn at a time and see if its better. It wont harm it. Only do one thing at a time and make sure the engine is at operating temperature before trying it.
Just mess about with these settings until you make it worse or better, if worse do the opposite half way.
I suspect returning it to standard would make it more rideable but they were never 'bottom end bikes' anyway.


OGR
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[Vincent]

Deffo get rid of the filter, the Allspeeds are designed to run on standard set up, maybe just use the next jet up at the most. Unless the cylinder has been ported that is. I've got a spare standard airbox but it's missing the lid. If you can't find a lid or another complete box in the UK, I may be able to get you one from Japan. LC Yamahas are reknowned for poor running with K&N filters.

[stevo as b4]

If you can find and want a std airbox to fit then great. If not, then you can get 2strokes to run fine on a large open filter if it's spaced at least the same distance back as the airbox would have been from the carb.

You could get it to run good with a nice belmouth and no filter at all if you wanted, It's just a case of having to over every jetting circuit several times in a circle to get the ideal fuel slope and thus ensure clean carburation. There is no magical power adding device in a std airbox, it's just that in most cases it's far easier to make the bike run well and carburate cleanly all through the rev range with the minimum amount of re-jetting, if you fit a std airbox.

[MARTIN RD125LC]

Thanks for the advice guys.. I just want to be careful as its in the running in stage and dont want it revving to high until run in!.

I'll try the air screw, (if im right) its the one near where the filter fits on to the rear of the carb?.. When i got it i was told it was upjetted to suit exhaust and filter etc.. But this aint my version of a very well running bike as its only good for anything over 4500rpm and in running in period this could do more harm than good.

Will find a origanal filter but till then will try get it running better by adjusting the air screw.

Thanks

Martin
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RIDE : 1983 - Yamaha Rd125lc

[bikersupermot...]

martin, dont worry too much about high revs, 2 stroke engines are designed to rev high, they make their power at the top end of the rev range not in the bottom or mid revs.

(hence the saying - screaming 2 strokes!)

my bike has the red line at 10k rpm ... when i had a new piston and rings fitted i ran mine up to 8k in short bursts - your supposed to give it an even workload over a range of revs to spread then tension in the engine.

i wudnt ride at 8k for long tho, but its fine in short bursts. once you get used to riding your 2 stroke you ll only be at 4k as your revving up or down thru 4k - ull never ride at 4k!

my 4s bike is the opposite, a single 577cc, that makes all its grunt around 3k - 6k and much above that the torque/pulling power disappears.

so rev your bike harder than 4k - remember when they are tested at the factory, they are revved very hard for long times to test durability - your engine wont blow up even if you took it to the red line!.

and then you can enjoy the sweet 2s ride and come back with your clothes having that sweet aroma!
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[MARTIN RD125LC]

[Kickstart]

Hi

The air screw will have a tiny effect. It pretty much just allows a small amount of air through a fueling circuit at idle throttle openings. If you are using any reasonable amount of throttle then its effect is tiny. It is just really adjusting the mixture at idle and on throttle positions only just off idle. If you at doing 4.5k then I would expect you have the throttle reasonably far open, so the jet that is relevant for the mixture is the needle.

The RD is a non power valved performance 2 stroke, and that (especially combined with the Allspeed exhaust) is going to mean it has stuff all low down power. I would expect it to be very weak until the power band (at a guess, around 6k~7k) at which point it will take off like a scalded cat. Without using the revs (which you can't really use while running in) I would expect it to be left for dead by a CG125.

All the best

Keith
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[MARTIN RD125LC]

thanks for the advice fella's.. Ive adjusted it a bit as at traffic lights i always think its going to stall cause it drags alot of air in, its 10x better now and kickstart your right about the powervalve.. It goes like shit off a shuvel lol
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RIDE : 1983 - Yamaha Rd125lc

[Teflon-Mike]

[Shaft]

^^^^This.

But I would go out of my way to find a replacement airbox.

I worked on a few of these when they were new (I was a bit of a 2T nerd then) and they didn't like having their induction system messed with; all you ever achieved was to remove whatever of the miniscule low range power it had and make it a git to ride in traffic.

Going back to a standard airbox might give you half a chance of getting some clean throttle response under 3K.

IIRC derestricting 125LCs was a pretty involved task and changing the airbox was a no-no.
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[Teflon-Mike]

Think I have just remembered a 125LC 'quirk'... LC Nuts may elaborate, or correct me here. But from memory, the stock air-box had a tapered 'fin' in it, which a lot of elcie-Boys cut out with a stanley-knife, in the belief that it was part of the restriction pack, but in actual fact was a pretty 'key' flow corrector.
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[Shaft]