Ohh, Yes, he did...660 the easy(?) way

[reelrazor]

Im at 122.5 mains. FP needles half a clip higher than middle..three turns out....essentially sea level.

Right now it's pushing 90° and humid and the jetting is flat out spot on. Last week in 70° dry air I had a stumble at quarter throttle/low load high speed to whacked open transition..tells me I need either another quarter/half turn on my screws, one bigger pilot, and or needles raised that last half clip.


It's really really pretty silly. It's a 750 basically. Lunges like no 6 should..pulls revs like no liter will......

Roll-on wheelspin. Ima have to rethink the Avons

[JeCarter]

So how is it holding up? Are you still at the same carb settings? I want to do this mod but on a limited budget Id like to know what I need. Thanks for the write up

[Genesis]

The compression ratios of the FZR and YZF engines are the same, so the combustion chambers should have the same volume (= same +10% compression increase), but the FZR bore is 3mm smaller in diameter, making the chamber taller and narrower. Unless I'm talking out my backside, the extra 1.5mm radial distance of the YZF block should equate to a rather higher squish ratio. Unless the head gasket is some ungodly thickness, this should reduce the chance for detonation at the periphery, and also increase the turbulence in the main volume of the chamber itself. This should in turn improve combustion efficiency, allowing for both more spark advance and more fuel burned = more power.

From what I could see, Yamaha just increased that squish land on the 4tv pistons as you are alluding to, Don. The actual raised portion of the pistons (4tv and 3he) is identical. I'm an old two stroker, too...and know the value of squish.

It is my thinking that it is exactly that, coupled with the overall compactness of the chamber in general which allows me to use mid-grade fuel at 5° before base advance, and the increased compression.

That being said, and considering that chamber volume is the same between the two heads (4tv and 3he), and that aftermarket cams are sold as being usable for 3he and 4tv motors, I don't think yamaha re-engineered anything in the head except maybe the coolant passage casting...

It is my understanding that the 4tv bikes' intake manifolds will not fit the FZR carbs, but I am betting the 3he manifolds are a bolt-on proposition.

None of this thinking came into play in my choice of 3he or 4tv heads..the fact was, I had two 3he heads-one a ported 3he head with aftermarket valves, springs, and cams.

the first build used my stock 3he head..(because I was unsure of valve to piston clearance using the hot cams)....which fairly promptly spit a valve collet.

After talking to the spring people ( http://www.rdvalvespring.com/ ) and catching a very cool trick from them for checking clearance, the next install got the ported/sprung/cammed head.

So if I am understanding, the FZR head and carbs ends up working out better. so really the only YZF parts are the prisons and chambers.

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[Lastmile]

Are the fzr carbs better than the yzf's for this? I was under the impression it was the other way around.

[reelrazor]

Ehhh, it's a toss up.

With the YZF carbs you get a throttle position sensor, but the FZR wiring harness/ignition box aren't set up to use it. The YZF carbs are also set up to have a pressurized airbox. Tuning them back for the FZR airbox might be a chore. And, I am not sure if teh FZR throttle cables are a direct match up to the YZF cable wheel.

For me it was about easy and cheap. Had the FZR carbs and everything else.......

[DonTZ125]

Running TCat carbs with an FZR airbox is doable, but does require rejetting and probably setting the float levels. As RR said, the FZR TCI doesn't know what a TPS is or what to do with it, and the YZF TCI can't be used with the FZR flywheel - the trigger pattern on the OD of the flywheel is completely different. Also, the YZF and FZR crank ends use different tapers, so you can't simply swap the flywheels.

[Genesis]

Would you "have" to use the TPS?

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[DonTZ125]

With an FZR TCI, you CAN'T use the TPS - there's nothing to connect it to.

[DonTZ125]

Just to clarify - the TPS tells the TCat TCI to adjust the ignition timing according to engine load. This allows the 4TV ignition to apply far more advance at part-throttle, while still avoiding detonation at full throttle. The FZR TCI has no provision for a TPS, and only has a 2D (vs RPM ) ignition curve.

[Genesis]

Right it works similar in a car tbi, but thats computer controled injection. The YZF carbs are carbs not computer controled injection... right? So what if the TPS was eliminated? What would it hurt.... could you use the YZF carbs w/o? Just curios. Because the head is better.... right? Would the carbs have to be so detuned would it even make it worth it?

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[DonTZ125]

Correct - the YZF carbs are carbs, not EFI throttle bodies. They can be used with the TPS disconnected, or even dismounted; it won't affect the carburetion.

[Genesis]

Thanks Don, I might try that. But next winter.

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[reelrazor]

Right it works similar in a car tbi, but thats computer controled injection. The YZF carbs are carbs not computer controled injection... right? So what if the TPS was eliminated? What would it hurt.... could you use the YZF carbs w/o? Just curios. Because the head is better.... right? Would the carbs have to be so detuned would it even make it worth it?

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I don't think there is any real difference in the head, definately not in teh combustion chamber itself. But, the YZF intake ports are oval-ish, at least at the upstream end..which makes them larger, which in my opinion is detrimental to intake velocity.

from about when the first gen YZFs came out until about 2004, 600 intake ports got smaller and smaller(volume) and yet power went up. This tells me the manu's got seriously interested in charge velocity vs. raw flow....they haven't gone back to the large volume paradigm...tells me velocity is more important than huge static flow rates.

[Lastmile]

i don't think there is any real difference in the head, definately not in teh combustion chamber itself. But, the YZF intake ports are oval-ish, at least at the upstream end..which makes them larger, which in my opinion is detrimental to intake velocity.

from about when the first gen YZFs came out until about 2004, 600 intake ports got smaller and smaller(volume) and yet power went up. This tells me the manu's got seriously interested in charge velocity vs. raw flow....they haven't gone back to the large volume paradigm...tells me velocity is more important than huge static flow rates.

Very interesting, opposite of what I figured. I'm planning this build, and I'm trying to figure out the best parts. I'm selling my beloved dirt bike and will have a little extra money than normal.

[Genesis]

Right it works similar in a car tbi, but thats computer controled injection. The YZF carbs are carbs not computer controled injection... right? So what if the TPS was eliminated? What would it hurt.... could you use the YZF carbs w/o? Just curios. Because the head is better.... right? Would the carbs have to be so detuned would it even make it worth it?

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from about when the first gen YZFs came out until about 2004, 600 intake ports got smaller and smaller(volume) and yet power went up. This tells me the manu's got seriously interested in charge velocity vs. raw flow....they haven't gone back to the large volume paradigm...tells me velocity is more important than huge static flow rates.

so I wonder if 2004 carbs would fit my YZF heads.

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