jetting, again

[elbandtioCA]

thought I'd sorted things out pretty well--defiantly have noticed the bike running better since I put in the new thermostat.  But now that I've been riding it more I'm noticing that above 8k rpms, when I've got the throttle open pretty far--accelerating on the freeway or up a hill, the engine seems to make what almost sounds like a nocking noise.  It is different than the rattling at 6k that turned out to be my cam tensioner.  It also doesn't make the knocking at lower rpms. 

Could this be related to the mainjet size?  I've got a hunch that I still don't have the right size in there.  I'm running 105s with a factory pro kit and a yosh, no holes in the airbox.  Other boardmembers say my setup should have an 97.5 mainjet, but when I tried that it was too lean.  Even with the 105mjs, when I ran it at full throttle, cut the engine, and read the plugs, they looked pretty near white.  But would the wrong mjet cause a sound like this? 

[gsxr400 racer]

try 110's or 115's

[elbandtioCA]

so you think that the noise could be a lean symptom on the main jet?  are there other obvious causes, because i don't want to take the carbs apart again if there's an easier explanation!

[andrewsw]

You must have 33mm carbs?

I just took mine down from 105 to 97.5 and 105 was definitely rich...

If you have the 33mm carbs, then yes, I think Jay's right, you can go up a couple more sizes.

I think "knocking" could be caused by being extremely lean. The leaner you are the hotter it burns and the hotter the cylinder walls run thus making pre-ignition more likely. But that's just a guess on my part.

Either way though, if you get the mixture right and it's still knocking, then you know that's not the prblem any more 

also, don't run too long in the knocking state. If it's pre-ignition, it can tear engines apart.

A

[elbandtioCA]

nope, its the stock US 32mm carb.  everyone says that the 97 works for them, but the bike wouldn't even run with those in.  when i put a warmer thermostat in it seemed to run better with the 105s, which i think means that it was lean before, because i had to leave the choke in for longer before i put the 190 thermostat in.  so i'm confused about why my bike wouldn't take the same jet sizes as other peoples, but really i just want it to run well, whatever jet that takes!  is reading the plugs the best way to see if its lean?  i thought it might have an air filter leak but i've looked over the airbox pretty carefully.  the inside of the box has a funny mod to the intake tube, but nothing that changes the diameter of the opening, or any extra holes. 

its not quite "knocking" like on an old car that doesn't have the right fuel grade...but it is similar to that--that is the best way I could describe it.  i think pre-ignition would happen at lower rpms too? 

i'm not that far from factory pro...but i don't have the $$$ for them to mess with it.

[andrewsw]

What is your carb setup? needle position, air/fuel position, float position, etc.

I'm wondering if you've got a clogged pilot jet, or set the fuel level too low and you're making up for it with a really big mainjet. (see below for rationale)

just FTR, I'm running 97.5 main, stock pilot, 15mm float height and 3.25 turns on the air fuel at 1500 feet up. yosh 4-2-1, k&n filter in stock less snorkel airbox.

I don't know what the "best" way to check mixture is, but plug chops are definitely a good way without expensive equipment.

So here's my thinking, if you've got a too low fuel level, and/or clogged pilot jet, then you'd run lean at low revs. You could compensate for it just a little bit with a bigger main jet, I think, but not much. IOW, it would take a significant change in mainjet size I think to impact the low end like that, but I think it could. Meanwhile, if the fuel level is too low, when you start hitting the higher revs, it'll starve as it won't be able to suck enough fuel up and end up running lean. At some point, I read about mainjet size affecting the low end situation, but it was pretty minor. From that I extrapolate that if you're running really lean in the low end, you could make up for it with the bigger main jet. It's all a house of cards held together by a hunch. salt liberally.

You know, as I review this, you could have a clogged fuel filter or kinked fuel line... You could be getting enough fuel to run at low speeds, but no enough to run at higher speeds. I had the problem for a while, it was kind of a bugger to diagnose...

just a bunch of random thoughts, hope they help.

A

[gsxr400 racer]

I think these bikes have always had a fueling problem and the fuel valves are shit

[elbandtioCA]

andrewsw, thanks for the thoughts.  to answer your question, I'm running the factory pro Ti needles on the middle/3rd clip, 105 mjs, yosh 4-2-1 exhaust, 2.5 turns on the air screw (this significantly changed when I put the 190 degree thermostat in), float height is stock=14.6 (obsessively measured with calipers).  Originally I had the same thought that you did about the pilot jets being clogged so I took them out and made sure they were clear.  It doesn't feel like the low end is too lean--I mean it after just a few mins of warming up it idles steady and pulls away from the stop light pretty well.  I did notice a much crisper feeling in the low end after putting in the new thermostat and adjusting the air screws--before I think it was lean. 

But while I did all of this I also put an in-line fuel filter onto the fuel line.  At first the hose was too long and it got kinked, so I know what it feels like when the fuel supply is choked off, but I fixed that.  In fact, it pulls pretty strong at the high rpms now, it just makes this noise.  after I posted this morning I made my commute and listened to hear if I could come up with a better description than "knocking."  it sounds more like a "clanking" sound at the higher rpms, not nearly as intense or fast as the cam chain buzzing, but also not quite like an engine pre-igniting. 

If you're floats are at 15, then your fuel level is even a little lower than mine, so it doesn' seem like that should be the problem.  But you mention your airbox doesn't have a snorkel?  which part is that and what is the benefit of removing it.  I"m pretty sure mine is on there. 

[Chris H]

Hi guys, dont think it can be pilot jet related as this only affects fuelling up to 1/4 throttle, but your mains seem big for a road bike so somthing defo a miss, i always go back to stock and then test again,seems long winded but usually quickest in the long run.

[gsxr400 racer]

sound advice there

[andrewsw]

definitely sound advice... starting over, but if you're seeing a white plug in your plug chops, then you're lean on the main... but...

What kind of fuel filter? It may still be the problem! With no fuel pump to push/pull fuel through a filter, a filter will definitely slow down the flow. I recommend you test with it out just to make sure.

I had a nice inline filter and I had to take the filter element out of it because even though it was made to handle some pretty substantial flow, it still wasn't enough when flowing just under gravity. I took the element out of it, but left the canister in place and it does at least catch stuff because of the way fuel flows through it.

Clanking? that can't be good. How is it timed with the engine? Does it increase and decrease frequency at the same rate (or ratio anyway) as the engine? or does it remain relatively constant? 

It occurs to me that it's not engine related at all. I got the awful clanking noise at one point... I had lost the rubber bumper that the center stand pressed up against... every bump made this racket. Took me forever to find it.

The snorkel is this slightly tapered tube that extends into the airbox from the main opening. It has a little taper to it and the entrance point, where it meets up with the wall of the airbox is nicely rounded. Although it has a smaller internal diameter than the hole without the snorkel, I think its nicely smoothed shape probably helps airflow. Two thoughts on that:

1. the nicely rounded entrance and long straight run (about 6 inches I think) would provide nice smooth laminar airflow into the airbox instead of a turbulent mess around the opening.

2. That straight run could provide a "velocity stack" effect that would actually help pressurize the airbox a bit. As air flows down the snorkel tube, it has inertia. At the moment when no cylinder is pulling, that air continues down the snorkel due to that itertia. It could actually stack up and increase the air pressure a bit thereby making it all flow a little better. That's just a hunch though, and with all four cylinders pulling from the same box there may not really be a pulsing action in the airbox anyway.

I think the lack of snorkel contributes to what I consider my upside down jetting. I am running 3.25 turns and really need another .25 to .5 turns on the air fuel to kill the last of the lean surge. So that's pretty rich down low, yet I'm running a 97.5 main which seems kinda small for the amount of air-fuel I'm running. I think as the engine speed increases I get more turbulence around the airbox opening resulting in less air flow. But that's all just guessing. I'm no engineer.

A

[elbandtioCA]

thanks andrewsw for all of this--between this and that link in the jetting post I've learned a lot about the fluid mechanics of air boxes.  I'm wondering if restoring the snorkel to my airbox would have a positive effect on the bike--it sounds like it might be better to just leave it intact, even though a number of the people on the jetting thread have some sort of modification. 

I've been listening more carefully to my high RPM noise and I've noticed a few things, and I'm wondering if they are evidence that my problem has nothing to do with jetting.  First of all, the clinking noise that appears at 8000rpm and above does not occur if I just rev the engine in neutral.  It only happens when I'm riding in gear.  Second, it is much more pronounced when I let off the throttle and decelerate.  So it is very easy to hear if I stay in 2nd gear up to a high rpm and then start slowing down--I get a noticeable series of clanks there.  And they do seem related to the rpm.  Could this be another cam chain issue?  Or is this still consistent with an incorrect main-jet?

[Banditmax]

More consistent with a wobbly clutch basket i think.

[elbandtioCA]

that's interesting.  I haven't tightened the clutch basket because I thought I'd know it was loose by hearing a rattling when I was stopped at lights.  But I have had to adjust the clutch because it gets difficult to shift.  Would the clutch basket sound only show up at consistently at the high rpms?  It doesn't do it at all under 8k

[Banditmax]

You cant tighten a worn cush drive. I was thinking it was showing up due to deceleration not high rpms as such. But was only an idea may not be right.