non standard carbs? just thinking

[pepperami]

When I used to play motoball my/our bikes were usually a collection of bits from a number of bikes, but it worked.
However the bikes were made specially for motoball and did`nt need to be set up perfectly for the road.
I used all-sorts of carbs from Jikov`s to Bings, to Mikuni`s but it was always off road.

So I know you dont always need to have the standard carb.
What I want to know, is there an optimum carb/carb size for a road going 125 4-stroke? to gain performance over economy.

Yes I have a full license and a bigger bike so it`s not about me trying to get round any form of restriction.

Your input is appreciated mighty BCF
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I am the sum total of my own existence, what went before makes me who I am now!

[Paddy Blake]

To make much of a difference I think you would need to change more than just the carb.
You would need to change the cam and exhaust to allow more air through as you increase the amount of fuel it will use.
The bike will run best at a certain fuel to air mixture so if you up the fuel mixture you also need to up the air mixture or it will run worse rather than better.

I don't know which carb would be best but I ran a gn 125 on a gt 550 carb for 9 months.I messed with the main jet to get it right but it was not better or worse the the standard.
I just didn't want to buy a diaphragm for the gn carb.

[pepperami]

I understand what you are saying about possibly changing cams & valves.
But what I`m thinking is that the bike makers have to get thier bikes through all sorts of controls before they can sell them.
This surely means they set carbs and the like on the safe side of caution?

What I`m thinking is that there may be a little bit of easier breathing and just a tiny smidge of extra accelaration/top speed by just changing the carb to one that is more suited?

I doubt if the bike is worth £200 so a performance engine rebuild is not going to happen .
But going to the breakers and fishing out a better suited carb could be an option? possibly? maybe?.
Yes I could change the jets on the carb I have and I am thinking about that as well.
the reason I ask this question in terms of changing the carb is that as a 125 it will only suck in 125cc`s of air/fuel mix, so there`s no point in having a whopping great dell`orto on there but there must be an optimum size???? to allow optimum airflow whilst still having the venturi effect
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I am the sum total of my own existence, what went before makes me who I am now!

[pepperami]

With all the wise old sage`s? we have on the BCF this thread has failed and I am disapoint
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[Kickstart]

Hi

How big a carb do you want?

A larger carb will provide less of a restriction at high rpm. But as a carb works by creating a restriction , resulting in a low pressure area to draw fuel from the float bowl they tend not to work well if you use a large carb with at small rpm. They tended to become very non idiot proof.

There are ways round this. Most larger bikes use CV carbs. With these it doesn't matter if the bod riding it just cracks the throttle wide open at low revs, there won't be enough of an air pressure change to open the slide. Down side is that they do limit ultimate power and do slightly reduce throttle response compared to a slide carb (but also tend to give a FAR lighter throttle).

A lot of cars (and some larger bikes - often large Italian twins) used accelerator pumps on the carbs. With these opening the throttle squirted a load of petrol straight into the engine so when you opened the throttle hard at low revs the engine wasn't relying on the virtually non existent low pressure through the carb to draw fuel from the float bowl.

There are graphs around of carb size to the amount of power required, so you can go from that. Realistically a road going 4 stroke single isn't going to manage 20hp (most of them will be doing very well to produce 3/4 of that), so using a carb large enough to keep a 25hp engine happy is not going to gain you any top end but likely will lose you flexibility.

All the best

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

[pepperami]

99% of 4-strokes imported into this country will have a hard job producing 14 bhp.
But I`m guessing that for other markets around the world some of those same bikes will produce just that little bit more bhp due to some minor differences like carbs,carb settings, airbox sizes and nothing to drastic like engine differences.

what I`m trying to get to the bottom of is , is there a rule of thumb regarding carbs/carb sizes to get the best out of a 4-stroke?

The carb on this Sachs at the moment is a diaphram type Mikuni, 26mm I think.
Would I get a better response from this little engine with a ordinary slide carb of the same size? (26mm)

What I`m saying is that the engine seems like it`s not being allowed to give it`s all, or like it`s trying to breath through a thick wooly jumper.

Yes the engine is in good order ie valves, timing, & compression.
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I am the sum total of my own existence, what went before makes me who I am now!

[Kickstart]

Hi

Not that much difference in some ways. 125s have their main market in Europe due to learner laws while loads of other markets have different laws meaning different capacities (hence the 150cc bikes related to European 125s).

A 26mm CV is a fairly large carb for a 4 stroke 125. 26mm slide carb would be massive (enough for well over 20hp).

Probably more difference to be made from the length of the exhaust, cam timing and the length of the intake.

All the best

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

[Asimo]

If you really wanted to throw money at the sachs then consider the fxr150 conversion. Plenty of others have thought about it but getting parts shipped from new zealand was the main issue other than cost.

You'd then be looking at a 28 or 30mm carb I think.

Could always convert it to a two stroke, doubt it'd be hard since sachs made one to start with.

[stinkwheel]

My MZ660 had two carbs feeding the one cylinder. A small slide carb and a big CV carb with no slow running circuit. The linkage was arranged such that the slide carb opened for the first third of throttle opening, then the linkage picked up the butterfly for the CV.

Struck me as quite a good system for a single cylinder bike. Fine control of fuelling at lower revs, full-bore-roar at higher revs.
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I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]

[temeluchus]

[ClockworkJesu...]

Just wanted to add my two cents. Can't be 100% beneficial but, y'know; I have my excuses.

Anyway, I had a lil' carbed 50cc 4-stroke scooter. The previous owner had stuck an 80 jet in it and I put a K&N-style air-filter on the carb to allow it to breath better.

In all honestly, all I really did was decrease MPG and make it sound like a brute of a 50. Thanks to the lack of restriction, down-hill I could achieve 55 or so; but on a flat I felt no increase in total power.

Lesson I learnt was that bigger jets and more-open airflow is useless if you're not changing other parts of the engine; such as the total bore-size to warrant more fuel and increase down-drawn vacuum, and/or sports exhausts to open the engine up to allow for increased intake.

Once you start modifying engines, they stop working within the parameters they were designed for. Sometimes this means more power, but sometimes it means you just lose reliability.

Kay, I'm done. Good luck!

[pepperami]

Thank you people, I think you`ve answered my question

I really dont want to start mucking about with the internals of the engine and it appears that this carb is best left alone
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I am the sum total of my own existence, what went before makes me who I am now!