Comprehensively jetting a cat street fighter (no ram air)

[Bmxr123]

I have been troubleshooting the carbs on a yzf600r streetfighter non-stop for the past week and have come to some conclusions.

As many of you already know, the 97+ Cat was set up rich right off the showroom floor. Furthermore, the yzf600r has a significant amount of cam overlap, making it even LESS of a performer in the low and midrange. I suspect this was to offer a bike that would get excellent EPA ratings(most testing is done at cruising speeds)but would come alive at higher rpms. This is the typical nature of the 4-cylinder engine but the Cat's case is borderline extreme.

Most people loved the idea of a rich mixture right off the bat, thinking it can be remedied by simply getting an aftermarket exhaust and k&n filter.(some Ivan's or dynojet needles and spacers are usually a must also if you want to get decent midrange as well)

However, it gets interesting when building a Cat street fighter, seeing as how you must remove the front fairing and ram air tubes in the process.This, of course, diminishes air flow, causing your mixture to go rich. This is especially noticable at lower speeds in which WOT is given and the main jets start to kick in..

But, not all is lost: The 94 95 96 yzf600r had the same engine with no ram air induction and no holes in the frame. Dynojet's 94 95 96 stage 1 kit had 102, 108, and 112 main jets(i am giving you round-about keihin jet-size equivalents as per the conversion chart i have included below ..The main jet sets in the kit were actually dj100, dj104, and dj108) and you were to intall one of those 3 sets of mains depending on whether or not you had the stock muffler, air filter, etc. I am STILL intrigued about this HUGE difference in main jet size between the two generations and am still wondering if i was sent a mixmatched kit or the chart i have been using is inaccurate. The kicker is that the needles in the kit gave the cat an insane boost in acceleration at midrange.

My personal experience playing with quite a few different jet setups has shown me that the ram air induction on our 97+ bikes is still mostly operational, and ultimately gives us considerably more horsepower than the 94 95 96 models even after eliminating the front fairing and ram air tubes. (correct me if im wrong, but im pretty sure the 94 95 96 didnt have ram air deltabox frame.)

I had an excellent conversation with a guy on customfighters.com that had his 01 street fighter Cat comprehensively dynoed and is running 144(keihin) mains on 2 and 3 and 142 mains on 1 and 4. This suggests that i am still running rich with the 155 mains but just not rich enough to foul out and misfire..There are visible signs of a rich mixture when the carbs are using the main jets..I had a buddy follow me and say that as soon as i hit about 5000rpm(right about when the main jet kicks in) a bit of black(unspent gas) smoke could be seen from my tailpipe but quickly dissapeared as i got higher in the rpms. This proves that for a brief period during ascention through the rpms I am only running rich in one small spot. But, there is NO noticable decrease in power during any of this..Keep in mind that i am running a completely stock 97 model w/ needles from a dynoet kit to richen up my midrange and air/fuel screws set from idle-drop. It goes without saying that the airbox mod is mandatory and will not be discussed further here.

Hope this sheds some light on the unusual situation we put ourselves in when eliminating the ram air induction. I just figured i would post my findings, hopefully saving some people the agony in making all of these discoveries the hard way.

The conversion chart(courtesy of jetsrus.com)is below:

KEIHIN DYNOJET MIKUNI
92 92 85
95 94 87.5
98 96 90
100 98 92.5
102 100
105 102 95
108 104 97.5
110 106 100
112 108
115 110 102.5
118 112 105
120 114 107.5
122 116
125 118 110
128 120 112.5
130 122 115
132 124
135 126 117.5
138 128 120
140 130 122.5
142 132
145 134 125
148 136 127.5
150 138 130
152 140
155 142 132.5
158 144 135
160 146 137.5
162 148
165 150 140
168 152 142.5
170 154 145
172 156
175 158 147.5
178 160 150
180 162 152.5
182 164
185 166 155
188 168 157.5
190 170 160
192 172
195 174 162.5
198 176 165
200 178 167.5
202 180
205 182 170
208 184 172.5
210 186 175
212 188
215 190 177.5
218 192 180
220 194 182.5
222 196
225 198 185
228 200 187.5

[coolaide]

The ram air problem is because with out the tubes the air passing the holes in the frame cause a vaccum the pulls the air out of the air box instead of pushing it in. Kind of the same thing that happens in a spray gun, the air passing the hole pull the air out of it. All the tube do is change the angle the air hits so air goes in not out.

[Bmxr123]

that make all the sense in the world! The name says it all too, right? .......those tubes redirect or "ram" air into the airbox..


Just another bit of info: I just jetted my buddy's stock 02 yzf only using the Dynojet needles and clips and its like a whole new bike! Screw the expensive Ivan's and dynojet kits!..The crappy part is that you HAVE to order the whole kit to get the needles!..I could imagine that by simply spacing the stock needles out a bit with an O-ring underneath the "needle seat" it would reproduce these performance-enhancing effects!

I will continue my efforts in sourcing out these needles sold separately without the obligation of buying the kit.

[Binksman]

I'm glad somebody has the patience to experiment with different jetting Fuel injection is the one thing I think would make the bike darn near perfect (mostly for ease of maintenance and modding). Thanks to the OP for the info!

I have noticed that on a rare occasion when riding my naked Cat with a strong crosswind, the tone of the engine changes slightly. It's never been enough to change the speed, but it does sound as if the engine is running better at that speed. So I am interested in fabbing something like the air tubes on aTriumph Speed Four to fit over the factory frame holes. It would really clean up the looks of the naked bike, which -at this point-irritates me more.

It's still a very good bike without perfectly reengineering to run naked

[trevtastick]

Good stuff to know as I'm going to be doing a radical (for the yzf) fighter. Actually just joined customfighters after months of following tons of builds for ideas. I like the 'cat and all, but I'm truly 'fighter kind of guy. Instead of selling her I'll be selling all the plastics and junk and put that money towards new tools to build what I want. :evil

[paolodipaolo77it]

Some good info above. I've been building/modifying bikes for the past 51 years. A few tips that might help.

1. What ever anyone might tell you otherwise: It is possible to get a street fighter running correctly without an Airbox. It takes good Ears and alot of patience.
2. Before you start changing needles, pilot and main jets first block the slider air vents to prevent carb slide flutter. Then start with the rest.
3. Decide what you want to achieve. Do you want low end speed or high end speed. The normal carbs are good for low end speed. 0-60 in about 1 second is possible with the standard carbs. If you want high end speed think about larger carbs. But if you start changing things other performance aspects with be effected.
4. The most efficent way to generate speed is to reduce weight! So think about cutting everything off the bike you don't need: Ignition block (Use a relay with remote), Saw the back of your bike off and move everything under your tank. Buy a lithium battery. They are about the size of your hand, weigh nothing, cost nothing and give a kick like a mule.
5. Instal (to start with!) an A/R sensor. This will then show you if your running rich or lean. Once you've got the bike to perfection add 4 lambada sensors in the down pipes and start rejetting again. Every clinder is different. 4 in a line will usually mean the two inside cylinders are running lean(er). It's takes patience. Do you need a dyno to test? Hell no. Instal an app that measures accelation by speed. And keep adjusting until you're happy!
6. Don't ever forget to check your carb fuel hight. Too high and you'll flood at low throttle. Too low and you'll run lean. The higher the float level measurement (ie 15mm) the lower the fuel level. The lower the float level measurement (ie 10mm) the higher the carb bowl fuel level.
7. Check your carb syncronisation!!! This is crucial! Make sure no springs are missing!
8. Once you get it right start thinking about your rear tyre traction. Your street fighter will be so fast that your rear shoe will slip. Don't forget that if you give full throttle and let the clutch out your bike will (believe me!) go vertical and you horazontal in an ambulance.
9. Once your tuned to about 0-60 in about a second start worring about cars hitting you that turn on to the road. They don't expect you to reach them so fast!
10. Don't get yourself killed!