Topless Airbox
 

I'm attempting to make this age old mod work, but I'll need help from you carb tuning gurus!

particulars:
STG1 motor
billet long velocity stacks
K&N filter
no snorkel or top on airbox
no plastic heat guard around the carbs
195 front main
200 rear main
55 Slow Jets
3rd position from top for needle clip
2.25 turns out on pilots

It cranks and idles nicely but there are issues with mixture up top...

I've put just over 100 miles the fresh STG1 motor and it makes good power down low, but after 5K RPMs it seems to ramp up more slowly and stumbles at 7K RPMs. This seems to be more pronounced when motor is warmed up.

I'm guessing the mixture is just too rich after 5K RPMs and suspect it may be caused by the weak springs on the slides or mains that are just too big or both...

Thanks in advance for your help!

have you dynoed?
what do plugs look like?

please dont guess,you have a lot to lose

I haven't pulled the plugs yet and I was hesitant to dyno it as is, but maybe I should just to get the air/fuel mixture graph?

Oh, I get it. You're joking, right?

If I don't try it then I'll never know for sure it can't be done;)

Why not ring up Roger D as he is the one who suggested it? One would imagine he can help guide you in this.....

He gave me this starting point, so I don't think I'll bother him till I get between a rock and a hard place. I'm workin on a dyno run apptmt now cuz I want to see what the mixture is doin up top.

Dyno date is set!

I'll change the oil again and do the dyno run as soon as the rain stops from TD#5 out in the Gulf of Mexico...

we will be waiting!

could you have a low pressure area under the tank at speed?

Anything is possible...I know I have a low pressure area between my ears:(

damn man you owe me a keyboard!

spilled a beer in mine and still it works...just smells funny:cool:

i was just wondering if at speed could a low pressure area form?
if so that could throw off the dyno reading,since it would be sitting still.

The two big fans pointed at each side of the bike should help, but we'll have to see...

At this point (though you know it already, and I admire your commitment to proving for SURE it won't work well) you are in shady territory. If *I* were in your shoes, and dealing with a hand-built performance motor that adds a ton of variables into the equation, I would be wanting to know FOR SURE what my a/f was doing at all times. You are going to hit the nail on the head a lot quicker if you know where you are aiming. To that end, I would install a bung into the exhaust (if it doesn't have one already) and run a wideband O2 sensor, preferably one with data-logging capability. Dyno time is great and all, but really isn't going to simulate real world riding; you're not going to be doing WFO runs in upper gears all the time. You need to know how it is responding in regular riding conditions. With a wideband and the appropriate gauge mounted in the cockpit where you can glance at it, or again, data-log, you will know pretty quickly with less jetting changes whether you can make the modded airbox work, and not endanger your motor in the process. It's worth it weight in gold when tuning a forced induction system, but pretty valuable in general.
They aren't stupid expensive anymore either.
-R

Thanks Ross that's good info cuz I haven't even considered data logging yet...

This is the system I have experience with:
http://www.innovatemotorsports.com/x...cat=262&page=1
-R

Speak for yourself...... lol :)

Glad I'm not the only one......... ;)

Weren't you, just yesterday, espousing the virtues of a usable first gear? ;)

Why yes I was, not sure what your point is. Just because you open it up in the higher gears doesn't mean you don't use the lower ones in the real tight stuff. ;)

Yeah, but not WFO in 1st, unless you like sitting on your head. ;)
Again, since you seemed to miss MY point, most people don't ride around WFO. So, actually TUNING a bike rather than tuning for WFO on a dyno might be helpful, especially for somebody trying to run a modified airbox, which so far in SH history, hasn't been proven to work so well. Just trying to be helpful. :)

-R

Well... I'd like mine running right all the way, but that's just me...;)

'sides anything with big jugs that are topless is a good idea if you ask me...

Well it was a bit of humor is response to this quote:

You didn't say anything about 1st gear until later..... I know you're still a bit miffed about the carb heat discussion (or you would be giving mikstr a hard time also, as i just agreed to his comment) but come on, you are really trying to split hairs here.

Yeah, dyno time is useful too, I was just advocating for knowing what it was doing all the time ;)

It was actually initially directed at your response to his response, hence to both of you, but I forgot this forum doesn't include a quote inside of a quote ;)

I'm over the carb heat thing, but you're right, I've got a pretty hairy head, so there's always plenty of hairs to split. :)

No problem, I know what you meant, and I agree to a point... But you can tune for a lot more than WFO on a dyno if you know what you are doing... Altought the typical dyno operator is used to tuning IL4's at WFO, so you get what you pay for (at least sometimes)...

Besides a wideband is usefull... I have one... But I also have a dyno... ;)

you have ALL the cool toys :fawkdance::fawkdance::fawkdance:

Update
 

While waiting to get a dyno run I leaned up the mains:

running airbox lid & snorkel
190 front main
195 rear main
55 Slow Jets
3rd position from top for needle clip
2.25 turns out on pilots

Results are much better power w/o hesitation all the way to redline, but if WOT then I get a stutter around 7-8K RPM, so next mains going in will be 185&190 before the dyno run so she will make max hp&torque for the dyno run:cool:

so would you want to sell those 200/195 mains? :) or if you have another 190...:rolleyes: