Richer Jetting on the Inboard Carbs?

My Kawasaki J engine with FCRs was pretty well tuned on a dyno. But I've noticed the pipes bluing and the plugs a bit cleaner at #2 and #3.

I've read about the theory of jetting richer at the inboard cylinders to aid in cooling, since they get so much hotter. Also read that some Suzuki straight-fours went richer on the inboard mains from the factory.

Any incite on the theory?

Thanks.

Lots of people like to say things that aren't true. On a stock engine it shouldn't be necessary. Going one jet size like 60-65 is a big jump. The bleed screws should allow fine tuning for the pilot system, the needles for elevation changes and the mains for 1/2 to WOT.
Non stock applications might call for this but I doubt it.
Steve

I used to tune the center Amal carb, one step richer in my Triumph triple for this exact reason.
It did make a difference in that one.... but they were notorious for killing the middle piston in 5000-7500 miles.
Never had very much of a difference in plug readings on my 4 cyl Kaw engines if it was running right, and if I did see a difference, seems like a small pilot jet or idle screw setting would clear it up well enough. I don't think I would pull a needle up or down for this tweak and unless a track bike. Different main jets seems way over the top.
If you see that much difference on a street bike, I think you have other problems besides minor carb jetting.
Like cheap chrome, bad valve guide seals, valve lash, valve/seat wear,,, stuff like that.

I stopped listening to "Arm chair quarterbacks" long ago. :sick: If they get you in trouble, IT'S STILL YOUR FAULT. :whistle:
Steve

A size up on PFJ would definitely be too much. I had a nightmare with these FCRs eliminating a flat spot coming off idle. It turned out I needed to find the perfect correlation between PFJ and PAJ.

Think I'm gonna try 1/4 to 1/2 turn more out on the screws (viewed the plugs after idle), and maybe up one on the main. Seeing these beasts are at their hottest at high RPM, and it's bored making it even hotter.

I scanned the engine hot with an infrared thermometer, and the inboards are significantly hotter. Just common sense with two sides up against another combustion chamber, and way less cooling fin area.

Ultimate tuning on these things would be an O2 sensor in each pipe.

Funny thing is I also read that some bike's factory jetting (Suzuki again) was leaner on the inboard mains. Maybe due to air box design?

That depends on your carbs. I got rid of a flat spot by soldering holes some hot shot "Master tuner" drilled in the emulsion tubes. I went leaner on the pilot jets and mains. I'm now working on going leaner still on the pilots using my jet drills. I'll leave the air jets alone for now.
Steve

The Keihin FCRs have replaceable PFJs and screw-in adjustable PAJs. The carb set I bought were jetted for a semi-stock KZ1000 with pipes and pods. Mine is bored to 1075cc, 10.5:1, ported head w/ oversized valves, and high-end street cams. So it wouldn't even run with the out-of-the- box jetting.

Running it on a dynojet dyno never really picked up the lean spot just off idle. So I used the state inspection emissions analyzer at work, and rode the clutch while coming just off idle. Using the CO reading I finally achieved 13/1 air/fuel. The trick was maintaining the correct idle mixture at the same time. On the FCRs that means you should be able to cause a lean misfire with the screw all the way in, then find the best spot between 1 and 2 turns out. I lost count of how many times I pulled and disassemble the carbs, swapping PFJs and tweaking the the PAJs to get it right. But now it's runs great.

Not sure about inboard carbs on an inline four, but I know that some of the V twins like a Virago had richer jetting on the rear cylinder to keep it cooler because the front cylinder got most of the cooling airflow.

That was always a problem on Hogs with one carb.
Steve

I had a GSX1100 16 valve, 1980, I don't recall the fine details other than the inner carbs are jetted differently , also .IIRC there is another 2Hg? for the outer carbs on the balance.

In reality Suzuki made a legendary motor largely copied from the kawasaki z1 heritage, a lot of internal parts and the design is very similar. Kawi went with the uniform carb jetting, Suzuki with the different inner/outer jetting, plus a greater variance in carb balance .



It is a good question, just not sure of the answer. Maybe the jap big four were undecided on this issue too?

The Z1/KZ series was over engineered with a full roller bearing lower end, Babbitt cam bearings which made them superior to the other bikes of the day. If there was a oiling problem, the bearings in the head could be replaced. Not on the others. I have been able to do a complete overhaul of the engine three times in the 40 years I owned my bike. If I had a Honda CB or one of the others, it would have been in the scrap heap long, long ago. :whistle: I have never had issues with the two center cylinders overheating and I live in the desert. I may install a oil cooler though, maybe not.
Steve

swest wrote: The Z1/KZ series was over engineered with a full roller bearing lower end, Babbitt cam bearings which made them superior to the other bikes of the day. If there was a oiling problem, the bearings in the head could be replaced. Not on the others. I have been able to do a complete overhaul of the engine three times in the 40 years I owned my bike. If I had a Honda CB or one of the others, it would have been in the scrap heap long, long ago. :whistle: I have never had issues with the two center cylinders overheating and I live in the desert. I may install a oil cooler though, maybe not.
Steve

Hey Steve,

If you're thinking oil cooler, check out www.kipkawa.co.uk/parts.php?link=1981%20Z1000J

This is a real, quality, bolt-on oil cooler kit for a J engine. Not sure about earlier KZs.
I originally bought a kit from Earls, but nothing fit. But my hat is off to Earls, because they bought me the kit from kipkawa to make it work. You don't see that kinda stuff in the U.S. very often.