Topic: air of fuel ?

[aminemed]

Guys

how do you know, without doing a plug chop, if a bike lacks air or fuel. If a bike lacks power, not much answer from the throttle, how can i know if it is an air issue, or a fuel issue.

In my own case, i need to close the throttle and open it again to get some power.

thank you

[Yamahawk]

Put the choke on, and see if it improves performance... if it does, it is lean.. if it doesn't and gets worse, it is rich.
Charlie

[aminemed]

putting the choke would stop all air coming, right ?

[John Eberly]

The choke does not stop all air, but it restricts it enough that the manifold vacuum draws more fuel in.

Lean conditions cause surging and missing under acceleration, often backfires too.

With the kind of problems you are describing, I'd pull the plugs and see if they are fouled.

[carnivorous chicken]

With the kind of problems you are describing, I'd pull the plugs and see if they are fouled.

Yeah, you should be able to get a rough idea by simply looking at the plugs. Most manuals have a page that shows you plugs with various conditions, including lean and rich.

But... what kind of air filter and exhaust are you running? Have you checked your float level? Have you synced your carbs? Have you done a 3K-mile tune up? Checked compression? What makes you think it's air and fuel and not something else?

[Yamahawk]

Guys

how do you know, without doing a plug chop, if a bike lacks air or fuel. If a bike lacks power, not much answer from the throttle, how can i know if it is an air issue, or a fuel issue.

In my own case, i need to close the throttle and open it again to get some power.

thank you

Well, you don't want to do a plug chop, so first, what bike and year do you have? If it improves when you close and open the throttle, and your carbs have an accelerator pump in the carburetors, (later model) then that would mean your pump is adding fuel, and you are lean... that's why I said to add choke to see if it is lean or rich. It's a 5 second test and if it improves, the carburetors need cleaning. I would suggest pulling your plugs to look at them too. It can't hurt, and if you give the forum more information as to what year and model bike you have, it makes it MUCH easier for someone to give more than general advice...
Charlie

[aminemed]

I've already done a plug chop, see attached picture and it shows a decent color in my opinion, maybe a bit lean.

My carbs are 657B with main jet @ 105 and slow jet @ 40, aor screws set to 3/4

It is a 750 k2

I'm trying to understand the reason why it lacks power without the plug chop because as said earlier the plug color seems quite decent to me but can't hardly reach 60 mph as i keep pushing the throttle to have some responsivness, my hand is hurting at each ride.

 I do have pod filters instead of the oem airbox, there is then much more air getting into carbs than the original airbox. Logically, if i reduce the amount of air (choke lever on), i should imrove fuel / air ratio. But when choke is on, it does not run better, but runs poorly. I'm lost !

I ordered 115 main jet, but i know that this would make a difference only when i push y throttle more than 1/2.

[carnivorous chicken]

I've already done a plug chop, see attached picture and it shows a decent color in my opinion, maybe a bit lean.

My carbs are 657B with main jet @ 105 and slow jet @ 40, aor screws set to 3/4

It is a 750 k2

I'm trying to understand the reason why it lacks power without the plug chop because as said earlier the plug color seems quite decent to me but can't hardly reach 60 mph as i keep pushing the throttle to have some responsivness, my hand is hurting at each ride

Have you done everything else for a tune up? You should be doing close to 60 in 2nd gear if you want to on a 750. All cylinders firing? Timing? Valves? Points? Compression check?

[aminemed]

yes everything is good ! did all the tune up. It all points to a carb issue: fuel / air

Those pod filters are a real nightmare !!!

[aminemed]

Did a plug chop after taking the bike for a ride and its fairly lean! Plug is white ! I should rejet and move the needle

[Stev-o]

Plug is white !

I dont see that

[Gene]

Are you ABSOLUTELY sure all cylinders are firing? Check your pods - are they clogged up? You may not have enough air rather than too much. Your description sounds like you're flooding . . . .

[aminemed]

Here is another picture of the plug

[TwoTired]

I do have pod filters instead of the oem airbox, there is then much more air getting into carbs than the original airbox.

That's not true to science.   That's human expectation, which does not follow physical principles. You did not improve the engine cylinder volumetric efficiency.  It draws the same air volume as before with original air box.

You did not change the pressure at the valve inlet.  You only brought the atmospheric equalization pressure closer to it by shortening the intake duct length. 

Ever notice the carbs don't deliver fuel until the engine pistons move?  The falling piston provides the force that drives the fuel through the fuel jets by creating a pressure differential.  The atmosphere supplies about 14.7 PSI, and this pushes on the fuel in float bowl chambers.  The engine intake creates less than 14.7 psi and that differential is applied across the carbs fuel jets (carb throat to fuel bowl surface), driving fuel through them.  Shorten the duct and less differential pressure is applied because now the atmospheric source is brought closer to the carb throat where lower pressure normally exists when the engine moves its pistons.  This is the reason why all the carb jets need to be increased in size when PODs are installed.  A given jet orifice delivers a set amount when presented with a differential pressure across it.  Lower the pressure diferential, less flow.  Increase the orifice size the make the same volume transfer with less pressure differential.

Logically, if i reduce the amount of air (choke lever on), i should imrove fuel / air ratio. But when choke is on, it does not run better, but runs poorly. I'm lost !

No that is NOT logical.  The choke actually does restrict the air inrush, by blocking the carb inlet.  Less oxygen is then available to oxidize fuel = less power.

Cheers,

[aminemed]

Im going from 105 main jet to 115, just ordered the jets.

@TwoTired in my specific case and given my description of lack of power and plug reading, do you think it's an air issue or a fuel issue ? how can i check my pod filters to see if they are clogged up ? They are still new (less than 1 000 miles) Thank you

[TwoTired]

In my opinion, your spark plugs show an Air/Fuel ratio that is too lean.
The question is, under what conditions were those deposits created? (Throttle position and engine loading.)

Cheers,

[aminemed]

Full throttle, but not a heavy load..

[PeWe]

Needles might be lean too. Std carbs with pods might need 4:th notch from top.