Carburettor Information

waketrip

Member
Red Ace, keep in mind that 30mm carburettors are what's being mounted Stock on 250cc engines, so it'll definitely be too big for a 125 engine.

An all around improvement (quicker pickup, smoothness, throttle response) could possibly be achieved by mounting a keihin or mikuni, good quality 26mm carburettor if they make them in this size.

A 28mm will probably give you better top end gains, but at low and mid range RPM's it might lose a little.

You have to realize that the size of the carburettor is not what increases performance and/or hp, but carefully choosing the correct size for the cc's and intended application, and the quality of the carb itself.

Generally speaking, a smaller diameter carb will provide more air flow speed, and will perform well at low and mid rpms, buy may not provide enough airflow at very high rpms, thus hurting top end power.

A bigger diameter carb, will enhance high rpm operation, giving you more hp at the top end, but the engine may run worse low and mid rpms.

And even more important than these two, is a properly jetted carb, with the adequate pilot, main and needle jets for the intended use.

Normally the stock (typical) carbs as specified by manufacturers for single cylinder engines are:

125cc --- 24/26mm
150cc --- 28mm
250cc --- 30/32 mm

but these vary wildly, as does jetting, because manufacturers may prefer to have an engine consume less fuel than to have it perform its best, or enhance low rpms, or high rpms, so that's why there is no ''ideal'' carb size for any given engine.
 

orrible-64

Member
On advice from other guys In the pits this week, 28 mm should work ok. I run a 26mm


Sent from my iPhone using Tapatalk
 

Racing Ruben

New Member
Hey guys,

Wasn't quite sure where to post this. It's about carburettors so I put it here.
I'm in the process of changing to a genuine Keihin PE28.
I'm running the 150cc big bore kit with the 125 head, open exhaust,... (you can look at my setup on the database spreadsheet)
It feels like I'm not getting the maximum out of my 150cc. Especially in fifth gear there's no real power. I can get to arround 115kph on a flat stretch of road but it takes a while to go from 100kph to 115kph. I tried different jets but that didn't make a real difference.

So I'm trying a bigger carb. I was hoping it would be an easy fit but that wasn't the case.
First problem: the holes on the rubber adaptor to connect the carb with the manifold didn't match. I tried to reposition the holes but it looked like it would never fit perfectly.
So I eventually found a manifold from a Yamaha XT 550 that would connect the carb directly to the cylinder. It has al the right holes in all the right places.
Second problem: the angle on that manifold is somewhat different so the carb gets really close to the upper engine mount and to a piece of the frame. So I can't use a velocity stack anymore because there's no room for it. I did find a foam air filter (UNI) that does fit.
Third problem: the carb fits nicely but it's tilted at an angle. So here's my question: What is the maximum angle you can tilt a carburettor? How would you notice it's tilted too much? Can this be compensated by adjusting the float height (no idea how that works)
I didn't get to starting the bike because I'm still waiting on the O-ring for the new manifold.
Does anybody else use this carburettor or a different 28mm carb?

Any thoughts on this?
 

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LOLitsPOOP

New Member
Yeah, you have the wrong manifold. You need an angled one.

You can get one from ebay: http://www.ebay.com.au/itm/Upgrade-30mm-Carburetor-Intake-Manifold-For-200cc-250cc-Dirt-Bike-ATV-Quad-Buggy-/262217103643?hash=item3d0d5b791b:g:qE8AAOSwJkJWhJTY

This is my setup. OKO 26mm Flatslide


 

2b

New Member
Hey Lolitspoop

Not sure if I have asked before, but I am going to change the Carb when I do my 150 upgrade over winter and like the idea of your set up.

Can you post links to the Carb and Trumpet that you have pictured above.

Cheers.
 

LOLitsPOOP

New Member
About the carb;
http://www.oko-australia.com.au/

Good site for more info including tuning;
http://www.mid-atlantictrials.com/OKO.html

I bought my gear from DHZmoto in Sydney.
 

Racing Ruben

New Member
Hey LOLitsPOOP,

Thanks again for the link to the correct manifold. It arrived this week and was pretty easy to bolt on. I even managed to fit a velocity stack. I've started rejetting. The Keihin PE28 came with 50-125 jets. That's just to big. Now I'm running 32-110 and it's running great. Great power gain in low range. Don't know about top speed yet. But it feels promising.

Ruben
 

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Sneikz

Member
The final mod to square away the input / output was to replace the OEM carbie with a Mikuni. Completed this job this arvo and the total package of CDI, header, exhaust and carbie has produced a bike with considerably better performance. This set of mods is a must if you wish to stay with the standard engine but enjoy the tinkering to produce something better than stock at a reasonable price that is easily completed by the novice.

Here's the Mikuni on the left and the OEM on the right. I can breath easy now!
 

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The Mariner

Adelaide, Australia
Hey Sneikz,

Care to share the mikuni model no., where you bought it & the size jets you have used etc..... pretty please ;D

Did you need a different inlet manifold ?
 

Sneikz

Member
I wish I could answer your question. The carbie was part of a kit provided by Michael Blumenfeld of Skyteam Canberra.

https://goo.gl/DGmxAC

Having said that I believe it to be a standard 30mm Mikuni with a 2.0 slide. Sorry I haven't dismantled to identify the jets and I forgot to check the needle. It did require a different spacer (bigger port) but was a straight swap in/out

Assassin Dirt bikes sell one.

https://goo.gl/5FqhqK

Not much help. Sorry.

But by golly a huge difference when matched with a new header and exhaust. Now looking for 17 tooth front sprocket.
 

Sneikz

Member
Gidday Phil,

Michael is good and true. Remember you're also getting his years of experience and set up of the kit.

He's got passion too.
 

The Mariner

Adelaide, Australia
I've been communicating with him for the last couple of days & will be calling him tomorrow to finalise my order from him. He seems to know his $hit. :)
 
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