carb size vs inlet size

[lingeringstin...]

Just curious really.

The heads I use on my engines are either 19mm or 22mm inlets depending on whether they're a 125 head or a 250 head. The bolt pattern is the same so either head will bolt right on. They both use exactly the same valves and cam.

The 250 heads are generally modified for higher compression and the 125 heads are generally pretty bog standard with maybe only minimal skimming, so both types are pretty much the same thing with slightly different inlets.

This is a twin engine, 360 crank, with single carb.

I have never noticed any difference between the 19mm and the 22mm inlet sizes. My question is, this seems a bit tiddly and I am currently using a modified 27mm carb. Would there be any use in a larger carb with these inlets?

I'm thinking a 27mm carb might be about as big as these inlets can take and a larger carb might mean quicker initial response but no top end gain because the inlets/valves would restrict how much can get into the engine.

Since my engine is basically two 125 singles side by side I'm wondering what the usual inlet size is on a normal 125 four stroke single, and if they ever run bigger carbs.

I usually run a pair of YBR125 pistons with a single inlet and exhaust valve per cylinder. I wonder what the inlet size is on a YBR125 and what carbs they use.

[stinkwheel]

I'd have said it's only as good as the narrowest point anyway. There would be a good argument for ensuring it was the same diameter right through from a laminar flow perspective. I've even seen people fit a little alloy spacer tube in plate manifolds to keep the inlet tract diameter smooth and even.

I actually run a smaller carb than the inlet size on my trials bullet. Standard size carb on a ported head. I find the smaller carb gives better idle and more low rev control along with less risk of bogging down if you snap the throttle open.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[A100man]

My A100 is fitted with a 20mm carb, most likely for commuter economy. I'm toying with the idea of trying to fit a Mikuni VM22, like the higher performing GP100 of the same kind of era. In my mind this will involve dismantling the engine, clutch cover/ carburetor spigot mount and drilling that out from 20mm to 22mm to hopefully benefit from the improve flow.

As Stinkers says port matching appears to be the key here
so I'm not sure what benefits a 27mm carb would offer bolted on to a 19mm or 22mm inlet port.
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[MCN]

This question relates to Volumetric Efficiency.

How much gas you can push through an engine.

And some Thermodynamics thrown in the mix.

Trial and error is how the bodger manages this issue.

Calculus is how an engine designer manages it. With some trial and error thrown in.

Basically you can only get energy out of fuel if you can complete oxidisation in the machine.

Gas flow is limited to port size (and shape and routing.)

Blowers and Turbos interfere with the rules a bit bit there are calculated formula for those bastirts too.

Sukitansee is a proven method.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[A100man]

[MCN]

Bernoulli the Bastirt too. He has a finger in several holes.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Easy-X]

Taking the narrowest point and correct jetting into consideration I was told larger carbs might get you more overall power whereas smaller carbs are more responsive. For 2 strokes anyway.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[lingeringstin...]

What I've noticed with this particular engine type given it's inlet and valves (it's basically just a Honda 125 twin with bigger pistons) is that a 24mm carb works up to a point and then there's no top end.

It makes some sense that the diameter of the inlet is important, but then you have to remember that when the valve opens the piston is really drawing in whatever it can get past the valve, so a slightly bigger carb than inlet size (on a four strok) will work all the way to the upper rev limit whereas a smaller carb seems to work but only up to a point. If you don't need high revs it's not an issue, but I need everything I can get out of these feeble engines.

The air filter is also a deciding factor since they are basically strangling the potential air flow.

I don't have any an air filter as such. I take one of those cheap cone shaped pod filters and burn the paper out of it leaving just the screen so there is minimal restriction there but it still keeps children and small dogs from getting sucked into the engine.

However, I found that this then made the "proper" 27mm carb for this engine run like crap because the carb was jetted and needled with an air filter in mind. Just raising the needle and using a bigger jet didn't really sort it. In the end I used a bigger jet and the needle out of a 24mm carb because it's shorter and pointier, This seems to work very well now.

A standard jet for a standard 27mm carb on my engine seems to be in the range of about 110-ish but I am using a 125 now, so whatever that means. Anyway it seems to be the best combination so far.

[stinkwheel]

I presume we're talking CV carbs here, in which case is it actually opening the carb fully? Is the vacuum sufficient to fully lift the throttle valve? Maybe needs a weaker spring?

So many variables. Maybe you need an SU with the main jet metering valve attached to an advance retard lever with a bit of strimmer wire?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

[lingeringstin...]

After a lot of fannying about here's what I've discovered.

On this engine a carb the same size as the inlet is arse because it's just not big enough. The inlets are either 19 or 22mm depending on which head I'm using but to be honest I find no performance difference whatsoever between the inlet sizes. A carb that's way bigger than the inlets will work but not great.

The ideal seems to be that on this engine a single 27mm carb into two cylinders works best overall. A 30mm carb will SEEM like it's better but that's just because you get more initial response but no top end gain. Basically, it stops working after about three quarters of the way open, so there's no real difference between a 27mm and a 30mm carb. It's just that the 30mm opens more quicker so it SEEMS like it's more better but it's not.

The trick with mine was to get the right needle profile, which in my case was the needle out of a 24mm carb because it's shorter and pointier (which suits my setup as I don't use a filter). There doesn't seem to be any advantage in a bigger carb because there's only so much flow that can take place through the intake. Anything less than a 27mm carb is feeble because it struggles at the top range, and anything more than that is pointless due to intake restriction.

This is all completely different to my two stroke findings where the general rule is "If it looks stupidly too big it probably works".

[that_impulse_guy]

worth reading up about how they tried to slow down the (predecessor) of motogp by limiting carb sizes.

and when that rule kicked in, the bikes were quicker.

because...velocity stacks were "discovered"
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[Pete.]

I would imagine that the smaller inlets on the 125 engine are to bring up the inlet velocities to improve torque by increasing the intake charge momentum giving a little more cylinder fill in the low to mid-range. It would be at the expense of some top-end flow probably. The 250, being a twin, will have torque pulses at double the frequency of the 125 and so would benefit from the larger port without suffering too much in the mid-range.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good