I'd bought some Gates style thickwalled radiator hose a few weeks ago in Cebu and finally got around to swapping it in place of the slightly S shaped factory air box to carb boot in an attempt to straighten out that pathway out and pick up some CFM.

It was a bear to get in place and in the end I had to make second one that was angle-cut at each end for a better fit.

Going for a ride, the bike seemed different/down on power just a little. Felt like it was running rich. Since playing with main jetting with that tight fitting hose in place would have been impossible-I opted to just put the Kaw rubber piece back in place.

Test riding that; the bike was back to normal running crisp and pulling hard in the upper rpm range.

So what was happening with the modded section in place will have to be determined at a later date. Either when I use the Gates hose again-but shortened enough to not have it actually going over either end, instead I'll use short sections of rubber hose at each end going over both the carb/air box and Gates hose. That way i remove and replace it easily if I want to play with jetting. Or wait on the 2.25OD , .049 wall Al tubing on its way here.

The interesting part is it did make a change in the way the bike ran! So results were obtained. I don't see how straightening out the airflow to the carb would be a negative change, other than needing some rejetting that seemed to be necessary because of the mod. Maybe more flow was pulling harder on jetting that was previously working at less than 100% of max flow, and the mod now had the jets flowing closer to 100%?

Anyway since it did have a noticeable impact on performance, though initially down on power. It'll be interesting to see if a gain can be realized in the end by getting the jetting idealized once again.

It'll be easier to tell what's happening/right away after the a/f meter and exhaust bungs arrive and are installed. Then just change the jet to compensate

 

wouldnt it be easier to ditch the airbox completely and just mount a pod filter directly on the carb ? thats what i'm planning once i have a pumper.

 

It would, but even though my bike's mostly used on the street, I see any kind of clamp on filter as sucking up a lot of heat from the exhaust system; maybe the cooling system's hot backwash as well?


I'd thought about adding a clamp on K&N inside the air box to facilitate relocating a battery in there, but now I'd found a very small hi tech battery that I can attach to the bottom of the seat easily and will be in the upper air box opening and I can keep my current K&N Kaw air filter at the same time.

The battery pack is only 2" thick-I could also mount it inside the air box against the back wall..


I'm just fooling around with little mods to see what happens while waiting another 2mos for the 331 kit to arrive

 

This is very interesting. Another example of improving airflow that results in less than optimal results. Port matching the header was a performance failure until I saw that the A/F went way lean(From the dyno). After correcting that with the DJ132 jet, the engines' top end came alive with gains of right at 1hp.
Improving airflow can only cause the A/F to go lean - not rich. The only way more fuel will be pulled from the main jet at WOT is by a restriction increase in the intake tract upstream from the main jet.

 

I don't know about that. Guess it depends if you feel engines generate vacuum or if it's atmospheric pressure rushing to fill the void that acts on jetting. I know a vacuum gauge seems to answer half that statement but I've heard the argument that what's really happening is atmospheric pressure.

Remember that engine vacuum is just air pressure lower than atmospheric pressure. The starting point to evaluate engine vacuum is the intake manifold. When you connect a gauge to a tap on the intake, you're measuring manifold vacuum. (from MOTOR Magazine)

So let's see..if intake mass flow or velocity was increased because the Gates straight hose was less of a restriction in the intake tract, then the venturi effect in the middle of the carb/over the jets would be increased. Or so it seems. If it was, otherwise perfect jetting could now be too rich if a jet can be said to flow within a range.



I just enjoy talking out loud about it, and am open to any idea being a bad one, but I don't see why straightening out part of the intake tract would be a negative after jetting to compensate if necessary. That would almost have to be a first wouldn't it?

 

Main jet will operate at maximum flow at WOT. Increase the air flow and the jet can no longer provide enough fuel - the AFR goes lean - and you put in a bigger jet.

The increase in the venturi effect may cause some increase in fuel flow through the jet but it is not adequate compensation.

There is little to no vacuum at WOT in the intake tract.

 

Then there is the whole issue of inducing turbulence in the intake tract upstream of the carb, and all the crazy crap that happens when that occurs - the carb will be operating outside of its' design parameters and its' systems will fail to function properly.

I don't think your experiment would have induced turbulence.. Do you?

 

Guess we'll just have to agree to differ until I get some instrumentation on the bike and can know where I am at with jetting better.

I'd have to believe that each jet has high and low flow numbers-close as they may be, all dependent on the amount of air going over it.


Example:

Mikuni carb slides used to have an extra hole and a slot going from one hole to the other in the base of their slide to facilitate connecting the throttle cable to the slide. The cheaper round slide models may still have slides like this..

Decades ago the thought occurred to me cut a round gasket out of inner tube, poke one hole in it for the throttle cable, install it all in the slide and let the return spring hold the gasket in place over the bottom of the slide. Effectively sealing off the inner slide chamber from the intake tract. (think how a CV carb;s slide works)

Anyway, just as I thought it might, this increased throttle response; but I also had to reduce the pilot jet one size for any bike I did this on for it to start and/or run properly. Why then?

Because the same size pilot jet was now flowing more fuel than before in response to the increased airflow passing over it.


Thunder Products markets an add on 'space filler' for Mikuni round slide carbs that installs under the base of the round slide, smoothing out flow under the slide. Part of their installation tips include reducing pilot jet size by 50%!

Same principle? A jet that was previously sized perfectly is now too rich because of the increased flow over it.

 

As colleagues I am happy to agree to disagree. I hold out hope that whatever the A/F meter shows, the correction will lead to improvements. I am convinced this engines' systems and components were not designed for power production - but for compliance.
I wouldn't be surprised if the stock boot is restrictive.

 

The problem is not more air passing over it, rather velocity of the vacuum.

For instance if you put a fan in a room and let is blow air, increasing the size of the room doesn't cause it to blow more air. Putting an cone in front of the fan will increase air velocity.

The same is the the jets in a CV carb, air from the air box flows over the jets and pulls fuel. Opening up the airbox increases air flow which leans out the jet, more air doesn't mean more velocity. I'm not sure if I'm explaining it right, I'm pretty tired...

Air volume doesn't increase fuel from a jet, air velocity does. If anything it decreases velocity which leans out the motor. That's why with the 351 kit you can have the same jetting from a dyno jet kit on the 250 jug. The higher vacuum from the larger displacement pulls more fuel from the same size jet. We just jet up to take advantage of the larger displacement.