Fuel Flow Diagram Keihin FCR's

I remember trying to dial in FCR's on my CBR900 back in the mid 90's. I got pretty close, but only with the help of this graph. I originally had it in a Sudco catalog which is long gone. Thank God Al Gore invented the internet!


My apologies if this is posted elsewhere, but I am sure it will help someone in need.

I am going through my bike now and trying to sort the jetting. This is the history to date:

1. As purchased (used & stock performance parts with Quickshot 2 AP cover)

Condition - ran like total crap. Idle was variable, but on the high side like it was running lean. Low end bog, but WOT was good.

Hard parts - 165 main, 48 pilot, and no spring over the hot start plunger. No idea on fuel screw.

2. Shipped to Zipty for carb mod 'kit' (includes fuel screw, drain plug, & hot start nut)

Condition - wouldn't start cold. had to feed in throttle while kicking to get the bike started...like a two-stroke. Figured out hot start plunger spring was missing. Ran much better overall, but low end still had a little bog. Everything else was clean and pulled nicely

Hard parts - same as #1, but fuel screw at 2 turns out & MRD full exhaust system.

3. Rode next time with spring over plunger, but plunger was damaged. Spring was installed, but cable not attached.

Condition - still a total bitch to start cold & still needed throttle w/ choke. Once warm, only once or twice it didn't start on the first kick. Ran the same as above

Hard parts - same as #2 but with plunger spring

4. Tonight (10Apr2008) installed 45 pilot, new hot start plunger (system now fully functional), and set the fuel screw.

Started at 2 turns out (zipty recommendation). Needed throttle to start. let it warm up and rode around my neighborhood. Set fuel screw and found out that I was best at just over 1 turn out. Ran very smooth around my neighborhood, but no WOT. This should be unchanged though. Throttle response seemed the best it ever has. Started a lot easier albeit warm. Will try again tomorrow night with cold start.

EDIT - 5. Tonight (11Apr08) Same as #4 but cold start. One kick, idle came up nicely with cold start, and ran very well once turned off. Progress!

Next step = install 42 pilot to give me some flexibility (want 2-3 turns out) for temp and altitude.

The chart, in another form but with the same information, is in the back of your manual. But before you get into confusing yourself with the minutia, always start by getting the pilot and main jets right before spending hours sorting the needle out.

Agreed. I really don't plan on messing with the needles. The midrange is fine for my weekend warrior needs at the moment.

How about the need to twist the throttle while kicking it over when its cold? Pilot?

A few quick twists to give a priming shot is common. Once it starts, it should stay running on the choke. If it repeatedly stalls, you will want to try a larger starter jet. I went from the stock #68 to a #72. Once my bike fires off, it stays running now.

Pilot/fuel screw should be set for when the bike is at normal operating temp and then left alone.


I have the carb apart on the bench and decided I would check the TPS.

At 0% open, the resistance is 760 ohms (spec is 0-2k ohms) - PASS.

At 100% open the resistance is 3300 ohms (spec is 4k-6k ohms) - FAIL.

I had disassembled the carb once before to clean it. It ran fine beforehand.

Would a bad TPS cause the need to hold the throttle open when cold starting?

Might affect it slightly but more likely affect while running. As long as it is adjusted per the manual, you should be fine regarding TPS.

Revisit the choke circuit.

My bike finally runs great, but I'm not happy because it doesn't really make sense.

The last time I rode was 30 miles of tight single track with the jetting at 45 pilot, 165 main and needle in the #2 position. The bike ran fine but needed the throttle held at 1/4 turn during the cold start and used the hot start every time when hot.

Afterwards, I removed the carb and cleaned it out completely. The only change I made was to move the needle to the #4 position. After reassembly, the bike would not start without 1/4 throttle being held and would not idle without the cold start pulled. If I pushed it in, it would stall after a couple of seconds and would backfire if I cracked the throttle no matter the fuel screw setting.

I upped it to a 48 pilot and it ran pretty well with the fuel screw at 4 turns out. Seemed to start easier when hot (no hot start needed) but cold was still a little difficult.

Next was a 50 pilot at 3 turns out on the fuel screw. The bike started on the first kick after a 24 hour soak, two nights in a row, with no throttle...nothing! Pull the cold start and kick once. It starts very easily when hot with no inputs of any sort needed. Hell, I even kicked it KTM style with my left foot and a flip-flop - no problem. Around my neighborhood, it didn't stumble and didn't bog. The response seems good, but I'm not going to chew up my rubber by banging gears on asphalt.

Its the first time ever that the bike has started cold without any input aside from a kick. Its also the first time that the bike has consistently started without any input when hot on the first kick.

So why am I unhappy? I'm actually thrilled that the bike is running well, but I'm left wondering what could cause my bike to need a 50 pilot when everyone else has a 45? Yeah it still pops a little bit on decel, but not too bad. I bought the bike used and was told the engine was stock. I checked the valves a couple weeks back and everything was within spec.

I've got an 06, I put a 48 pilot, stock main, 2 turns on the screw, 1 or 2 squirts on the throttle, and it starts 1 kick, nearly every time. I don't have to mess with the throttle at all. When hot, it starts the same whether I use the hot start or not. I'm at sea level, more or less.

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