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Old 03-29-2015, 03:58 PM   #1 (permalink)
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Bigger jet or restricting air flow?

I'm limited to 50cc and 28mph by law, so I ride cheap Chinese scooter for now.

I made exhaust lot quieter and in process I got rid of exhaust restrictions. This lead to air/fuel mixture to lean out.

After going up to 88 size main jet (original was 76) it is still running lean, but at least engine does not die instantly with more than half throttle.

I could limit area of air intake or I could install even bigger main jet, but which would be better for fuel economy?

I know that there is enough power for my usage with limited air flow, I'm already running needle at second lowest position so there is not much room to adjust part throttle mixture which is where most of the riding will happen and for best economy I would like to keep that as lean as possible.

Of course it will not be really lean, but probably close to neutral mixture.

I'm worried that with bigger jet there would be more fuel consumption, it would of course help with uphill speeds too, but maybe restricted air flow and smaller jet would be better despite spending more time when climbing hill?

I did install freewheel gear to final transmission for coasting downhills so I gain some mileage there.

Scooter has constant variable transmission, so as standard there would be no coasting possible.

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Old 03-30-2015, 08:57 AM   #2 (permalink)
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With bigger jet, there would be more pumping losses because of less opened throttle.

With restricted air intake there might be poorer atomization of fuel if air flow slows down.

Basically it is perhaps question which of the two causes less mileage drop.

Of course economically restricting air flow costs nothing while jetting costs same as 2-4 liters of fuel, depending if I would get lucky and get it right at once or at second try.

Difference probably is so small with two methods that even if jetting would be better it would take forever to actually it to pay costs of jets back.
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Old 04-13-2015, 03:52 PM   #3 (permalink)
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Improving engine breathing is the way to go in my opinion.
If you have done a freewheeling setup, a more efficient "Pulse" will reduce their duration, could help with mileage.

If you can not P&G, improving engine breathing can only return gains if you can alter gear ratios (maybe with a set of heavier than standard weights in your CVT ?)
Taller gear ratios (through closer front pulley)s means for a given level of acceleration, your throttle body is opened more meaning less pumping losses.

Obviously, using more air means more fuel so it seems counter intuitive.

But it has been the recipe for eco tuning vehicles for long : tall gear ratios.

Maybe you can play with inlet temperatures to further save fuel (and maybe offset the need for tuning it richer)

If you are after money efficient modification, then you can never beat a free mod ...
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Old 04-13-2015, 04:17 PM   #4 (permalink)
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If you're getting a lean stumble around half throttle I think I'd bring the needle back up a step, and then re evaluate. Might then be able to drop the main down a size or 2. Did you move the needle before the exhaust mod? If so, maybe now returning to stock jetting with the exhaust mod, would leave you running as lean as dropping the needle before the exhaust work
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Old 05-22-2015, 11:45 AM   #5 (permalink)
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Quote:
Originally Posted by renault_megane_dci View Post
Improving engine breathing is the way to go in my opinion.
If you have done a freewheeling setup, a more efficient "Pulse" will reduce their duration, could help with mileage.

If you can not P&G, improving engine breathing can only return gains if you can alter gear ratios (maybe with a set of heavier than standard weights in your CVT ?)
Taller gear ratios (through closer front pulley)s means for a given level of acceleration, your throttle body is opened more meaning less pumping losses.

Obviously, using more air means more fuel so it seems counter intuitive.

But it has been the recipe for eco tuning vehicles for long : tall gear ratios.

Maybe you can play with inlet temperatures to further save fuel (and maybe offset the need for tuning it richer)

If you are after money efficient modification, then you can never beat a free mod ...

I have been doing lot of adjusting recently, roller weights, needle height and so on.

What I have managed to do is to get scooter run lot stronger, I needed to enrich mixture quite a bit when I removed intake restrictions.

Roller weights provide interesting results, running engine around 7000rpm seem to provide surprisingly better fuel economy than when running it 5500rpm, that is perhaps because there are long 4-7% climbs I need to climb and at 5500rpm engine just don't have the grunt and speed is nearly half in climbs than with 7000rpm, time spend on climb starts to affect fuel economy.

6500rpm might be sweet spot for climbing, so I try to get 7000rpm rollers weighted and order bit heavier ones.

I have gotten two tanks with 110 and 115mpg now with 7000rpm setup, unrestricted air intake and modified exhaust, also mixture adjusted to be bit rich.

Problem with freewheel now is that when I let go from throttle, engine cuts off because mixture is too rich to burn, with leaner needle position I had less of that problem.

Lean stumble is pretty much away now, occasionally there is tiny bit of it, but mostly mixture is quite rich, I think that tip of needle could be bit different shape for it to work better for low to 1/2 throttle as that tends to be rich, even idle mixture is set to compensate that low throttle a little. I could put one size larger main jet I guess, but current is quite perfect.

Brakes like to bind slightly, that is causing some fuel usage, for longer trips I usually push calipers a little that brakes unbind, I did try to make pistons move more freely, but it worked only for week, might need new calipers.


Not sure if I keep freewheel setup, it is very handy in city and at parking lots, also on some hills, but because of issue with throttle close and engine shutting off and my typical riding being on rural roads, it might not give much benefit, transmission oil change is soon, so I might try without it for a while.
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Old 05-22-2015, 05:42 PM   #6 (permalink)
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Since we are dealing with engines with high specific power, we can assume they are a little cammy with quite some overlap which means they get more efficient at higher RPM.

usually an engine is consuming the least fuel a tad under peak torque with WOT.

I bet your peak torque is 8K and peak power 9K.

What I get from your testing is that there is an optimal compromise of gearing for every size of engine and that longer gearing is not always the best compromise when dealing with smaller engines.
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Old 05-23-2015, 12:45 AM   #7 (permalink)
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Quote:
Originally Posted by renault_megane_dci View Post
Since we are dealing with engines with high specific power, we can assume they are a little cammy with quite some overlap which means they get more efficient at higher RPM.

usually an engine is consuming the least fuel a tad under peak torque with WOT.

I bet your peak torque is 8K and peak power 9K.

What I get from your testing is that there is an optimal compromise of gearing for every size of engine and that longer gearing is not always the best compromise when dealing with smaller engines.
It is 50cc engine with around 3hp at 8000rpm and has peak torque at 6000rpm, but I don't know how torque curve looks, torque might not drop right after 6000rpm, it has not much pull at 5500rpm though, so maybe advertised specs are not what I'm actually getting.

I could get hotter cam that has 40 degree more, but same lift, big bore kit comes to mind also, hills are quite long and there is lot of weight to be drag around.

With CVT system rpm is quite constant most of the time, especially when power is needed so narrow power band would not be big issue, but should I try to optimize thing to higher rpm? As it can't really well climb hills at low rpm, I would guess I have to.

Noise is just something I really don't like and above 6000rpm it starts to make quite bit of noise, with BBK it might be possible to have bit more civilized noise and perhaps economy would improve too as climbing hills will not put so much strain and hill climbing would be shorter time.

It would cost around 70 to swap cam and move from 50cc to 80cc, which is of course bit on illegal side, but perhaps law was made for bit different users in mind.
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Old 05-23-2015, 02:58 AM   #8 (permalink)
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I'd go for a BBK but not wilder cam.
Doing it this way will make your current cam softer.
You will not unleash all the power out of the BBK but it will get its torque and power earlier in the RPM range.

The hills have a bigger impact on FE when your CVT is downshifting.
I have no idea if the time of the hills vs more capacity all the time will net you economy gains but more capacity will make the vehicle faster and it's hard to resist going faster and losing fuel in the process.

But honestly I would leave it as is because you can never recoup the cost of improvement through economy gain.
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Old 05-23-2015, 12:25 PM   #9 (permalink)
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Quote:
Originally Posted by renault_megane_dci View Post
I'd go for a BBK but not wilder cam.
Doing it this way will make your current cam softer.
You will not unleash all the power out of the BBK but it will get its torque and power earlier in the RPM range.

The hills have a bigger impact on FE when your CVT is downshifting.
I have no idea if the time of the hills vs more capacity all the time will net you economy gains but more capacity will make the vehicle faster and it's hard to resist going faster and losing fuel in the process.

But honestly I would leave it as is because you can never recoup the cost of improvement through economy gain.
But do we really calculate costs of our hobbies?

Downshifting of CVT is happening quite a lot, especially when there is headwind, so BBK might help there. For me wind starts to be very uncomfortable around 45km/h, so I usually ride only 40km/h or sometimes 35km/h because of headwind, often using only half throttle or less, except hills where I need to take everything out from little motor, sometimes for more than 10km.

With Chinese 4-stroke BBK is not making very huge difference though, quality brand 2-stroke will be faster at tuned 50cc stage, but fuel economy is bit better with 4-stroke.

BBK kit and choosing rollers that give bit lower rpm than 7000 might indeed be best way, less noise that way, also there is speed limiter at 45km/h ignition is starting to cut, there is sensor at CVT for that, of course that could be bypassed, but 45km/h is enough speed for me.

Problem comes when coasting with freewheel and speed exceeds 45km/h at downhill, engine dies, so I have had it disabled for now, but I need to re-enable it as I think I take freewheel out for little testing.

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