Rejetting advice needed

[Ziggy81scott]

Hi, I've recently replaced the standard airfilter with a K&N style filter, the bike runs ok however I've read online that it's recommended that you change the jet so that the mixture is slightly rich as this helps with engine cooling.
The question is, how much bigger do I go with the jet? And where can I buy jets from?

I've tried adjusting the mixture with little affect and fiddling with the mixture screw is really difficult as is right underneath the carb.
I bought a special tool for adjusting the mixture but on its first use it broke but managed to use a 1/4 ratchet with a screwdriver bit in a small socket. Even so it is incredibly hard to more the screw and pretty much impossible to count the turns, so any advice is welcome.

Cheers

[MCN.]

[MarJay]

Don't mess with it. Adjusting the mixture screw will just screw up the carburation. You won't know what jet to use unless you get the bike dynoed, or you really really know what you are doing.

Leave it alone, if it aint broke don't fix it.
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Rogerborg]

I insist that you don't tell us what bike it is. That would make it far too easy to offer any sort of useful advice.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Ziggy81scott]

My apologies Rogerborg.
Honda cd125 benly

[RhynoCZ]

K&N stuff is a waste of money.

Keep the motorcycle stock to have the best performance possible. Honda is a multi billion pound company, that invested quite a lot of money into developing your motorcycle. They are not stupid and they know what they are doing. All the GP racer stuff is not just about putting an expensive filter there.

I know I might get karma raped here, but the K&N filters are actually worse at filtering dirt/dust/etc. than the OEM filters. YES, people use K&N stuff in their race motorcycles, but then, those motorcycles could easily run fully opened intake as well, since they are not gonna do 50,000 miles on it, before they take it apart.

Paper filters are far the best at filtering particles, but there are motorcycles, with ram air intake mostly, that can't run a paper filter because when you run it in rain, the filter gets damp. Paper just disintegrates when it gets wet.

By the way, a ZX7R hiflo air filter is about £7.50, a K&N filter is abou £48.00, the difference in performance? Well, with the K&N stuff you need to re-jet the carbs (Dynojet kit - expensive or put it on DYNO and pay someone to fiddle with the carbs - not cheap) and the result is quite possibly the same, well worse MPG is expected. It's all funny because both filters are made out of quite the same material.

ALSO - running engine rich makes it run colder, but why would you want that + you do realize that rich mixture washes off the protective film of lubricant on the walls of the cylinder/s which ruins the rings and soon enough your engine's gonna start to run on oil rather than petrol?

Some engines are designed to run rich, but yours was not!
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Rogerborg]

[Kickstart]

Hi

The mixture screw only affects the fueling at tiny throttle openings; up to around 1/8th open, maybe 1/4 at most. While important to get a good idle and avoid it bogging down when you open the throttle it is probably the thing you touch last.

Main jet is the important bit. This controls the fueling around full throttle, and is the ultimate limit on how much fuel flows. With the main jet the idea is to do a plug chop. Basically put in a new plug and accelerate the bike hard up hill (fully throttle, revving it out). The hill just means you get more time on full throttle to make it easier. Then you pull the clutch and hit the kill switch (from full throttle), coast to a halt and remove the plug to check its colour. The try a larger or smaller jet and do it again.

Once that is done you can fiddle with the needle jet. The is a tapered needle, so you adjust the clip that locates it in the carb slide to alter its position, hence richening / leaning the mixture.

The above is a VERY brief description of it. It is possible to do it yourself, but it takes time.

For an overview of a slide carb (although aimed at the Dellorto ones, the basics are the same):-

https://www.bikechatforums.com/viewtopic.php?t=17124

All the best

K
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Rogerborg]

[Ziggy81scott]

Thanks Kickstart. Sound advice

[Ziggy81scott]

Ok. If you were me, considering I've put this airfilter on and already messed with the mixture, what would you do now?
Do I continue messing about with jets etc or put it back to stock setup?

The whole reason for tuning the carb was to try to squeeze an extra 5 mph out of the top speed or to create enough power so that I could put a 16 tooth front sprocket on with the aim of adding a bit more top speed.
The trouble with the bike is on big A roads the bike is running flat out for long periods, which doesn't do it much good. I like to tour on the bike and have been on 150 mile rides already. Having been a lorry driver I'm aware of getting in the way on these A roads so want to avoid thrashing the crap out of it but stay out of everyone's way.
I plan to do my test asap but due to lack of funds I can't yet so I'm stuck with a 125 for now. I avoid where possible bigger roads for these reasons but on long tours it's impossible to avoid them completely. It annoys me that I can't enjoy myself while going at my own pace without being in a vulnerable position due to cars and lorries being up my arse pushing me to get out their way. It's a bit of a pisser really what with having no money, hence my desperation to get more speed out of the darn thing. I appreciate your advice

[RhynoCZ]

You wanted what now?
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Ziggy81scott]

More speed. That's what I want

[RhynoCZ]

You sir, need a bigger motorcycle or a lot more than just a posh air filter.

Is this what we are talking about?:

https://icaruswept.com/wp-content/uploads/2015/01/Honda-CD125-T.jpg
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Ziggy81scott]

That's a newer version, but yes that's the bike and yes I agree I do need a bigger bike. Money is always the problem for me

[Rogerborg]

[RhynoCZ]

This might be a stupid question, but do you actually lay down on the bike, when you want to go fast, or you just sit there, being a huge air brake?

You'd be surprised how fast a motorcycle can go, when you put your chin on the tank.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Ziggy81scott]

ahh well I suppose I'll go back to trying to invent a time machine as it seems that may be easier than trying to make a 125 go faster lol

[Old Git Racing]

Put it back to standard, you can even keep the K&n because it will not make a bit of difference. You won't need to rejet it, the carburation will be so conservative to run in different markets you could probably run it with no filter on paraffin.
Sorry to say you won't get a power increase to give you the speed you require by messing with the engine. Try to lose as much weight off the bike as you can, Make sure the brakes aren't binding. Pump the tyres up to the maximum recommended for them. Go for a shite before every ride and save up to do the test and get something faster.

OGR.
____________________
2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Ziggy81scott]

I always laugh at young kids on 125s with their chin on the tank trying to go fast, so I'd feel a bit of a idiot doing it. I'm sure your right it would make a difference

[RhynoCZ]

[Kickstart]

Hi

Have you tried a larger sprocket on its own.

While more power might be possible, first stage is probably to get it as close to standard as possible, to try and get back as close to standard power as possible.

All the best

K
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Ziggy81scott]

I'll give it a try, should be good on the flat but the hills may slow me down. Suppose I can't have it all my way.
Cheers Kickstart

[RhynoCZ]

Au contraire, larger rear sprocket will make it easier for the engine to spin the wheel. The premise is, that the 5th gear (or what the top gear is) has very long gear ratio to keep the revs down at high speed = to make the MPG better. With stock final drive gear ratio, the engine will never get to the rev limiter at the top gear, not on a commuter.

Imagine a mountain bike.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Ziggy81scott]

So putting a larger front and rear sproket will give me more top end without affecting my sped up hills?