FXR race cams

[gav]

[speedpro]

Basically you can do whatever you want with F4 four strokes, the basic limitations are.

Naturally aspirated, ie. no turbos or superchargers (Ram air is OK) . . . .

Obviously talking of the rules themselves here, not a particular capacity F4.

The reason that some of the CB125 twins go so well is that they've had 30 years to figure out what makes them go faster and stay together. The FXR's have a long way to go till they're in the same league.......

and NOW the FXRs are mentioned.

[richban]

We race on real tracks, not car parks
What would you suggest, currently running GN250 carb, ported head, big bore exhaust.
Have got 32mm Flat Slide carb coming.

Up the compression. That should make a big diff. FXR's don't have much at all. I have a Mikuni TM28mm flat slide on mine and it works a treat. Not a pumper but hard to find a small one pumper that is. I know my bike lacks power down low. I am sure a rise in compression will help heaps.

[Sideways Sam]

For top end power, which I take it you need from your comment about 'real tracks', I would be very tempted to get the Cam sprockets slotted so you can tweak the valve timing for top end power (you might lose your idle). It's an easy mod, you can do it all yourself or get a machinist to machine the sprockets for less than $100

Just need to buy a degree wheel (about $50-) and do a lot of study on the internet re valve timing. It's not particularly hard, just takes time (lots of time). Ideally you'd have access to a dyno but I know a few people who've done it here without one quite adequately.

Feel free to come to the Mt Welly 'car park', all the engine tweaking in the world wont do you any good if you can't maintain corner speed. TZ350 has more horsepower than anybody I know and once he gets the trainer wheels off he might win some races....... (sorry Rob, couldn't resist). As for me, I can't go round corners yet either, my son does about 4 seconds a lap quicker on exactly the same bike!

[Sideways Sam]

Yeah I figured that, wasw just winding you up for the hell of it......

[Sideways Sam]

Up the compression. That should make a big diff. FXR's don't have much at all. I have a Mikuni TM28mm flat slide on mine and it works a treat. Not a pumper but hard to find a small one pumper that is. I know my bike lacks power down low. I am sure a rise in compression will help heaps.

Up the CR is always good and it works right through the rev range. You need to be really careful though. Read my post on FXR conrods breaking....... I would suggest that you wouldn't want to up the CR until you've completely rebuilt (or at least blueprinted) the bottom end. That's the approach I'm taking on my next couple of engines.

[Sideways Sam]

We race on real tracks, not car parks
What would you suggest, currently running GN250 carb, ported head, big bore exhaust.
Have got 32mm Flat Slide carb coming.

Until you've done any serious mods inside the engine itself, I'd always be tempted to leave the airbox in place..... Most people will tell you to drill holes in it etc etc etc but that's all a croc unless they've done about 15 pages of calcs or just fluked it right. Even if you change the carb, try to leave the airbox in place.....

The Suzuki engineers spent millions in research on that airbox to overcome a midrange dip in the power curve, and it works.......... It will continue to work until such time as you change the internal dynamics of the engine eg CR, cam dynamics etc. If you aren't changing any of that stuff the airbox will give you the midrange boost at exactly the right point without sacrificing power anywhere else. Theres a reason most serious racebikes (including MX where weight is a really big deal) run airboxes.....

If you change the internal dynamics of an engine then the airbox needs to change, this is the point where most serious builders on a budget just ditch it. If you've got a degree in airodynamics (my daughter has, yeah baby.....) access to a dyno (still working on that) and unlimited time (that's the tough one) you can create a custom airbox designed specifically to rectify issues in the power curve of your seriously modified engine.....

What tends to happen is the less mechanically minded people just copy what others are doing. So one of the first things they notice on another bike is that the airbox has gone (cause they can see it) so they get rid of their's not realising that until they make some major internal changes to an engine it's a major performance booster

[Sideways Sam]

We race on real tracks, not car parks
What would you suggest, currently running GN250 carb, ported head, big bore exhaust.
Have got 32mm Flat Slide carb coming.

When you say 'ported' head. was it flow tested?

I had my last head flow tested.. (before it imploded)

The standard FXR head can flow the equiv of about 24hp (haven't seen anybody get anything like that out of one yet) by the time I'd finished with it, it was theoretically capable of around 28hp if every other component was delivering up to this level.....

I think the standard FXR head is pretty good and the major enhancements are to be had in other places....

[richban]

When you say 'ported' head. was it flow tested?

I had my last head flow tested.. (before it imploded)

The standard FXR head can flow the equiv of about 24hp (haven't seen anybody get anything like that out of one yet) by the time I'd finished with it, it was theoretically capable of around 28hp if every other component was delivering up to this level.....

I think the standard FXR head is pretty good and the major enhancements are to be had in other places....

Yep have had mine flowed and the same conclusion good for about 24hp.

[Buddha#81:Me]

Until you've done any serious mods inside the engine itself, I'd always be tempted to leave the airbox in place.....

The Suzuki engineers spent millions in research on that airbox to overcome a midrange dip in the power curve, and it works..........

What tends to happen is the less mechanically minded people just copy what others are doing. So one of the first things they notice on another bike is that the airbox has gone (cause they can see it) so they get rid of their's not realising that until they make some major internal changes to an engine it's a major performance booster

That good to here about the airbox's, My MotoFXR still has it fitted, no top and filter. I left mine there for wet weather riding, its always good to hear more sciencetific reasons. My motor is standard, carb and pipe are the only changes.

[Kickaha]

My motor is standard, carb and pie are the only changes.

Looking at you, I'd say plenty of pie to

[Sideways Sam]

That good to here about the airbox's, My MotoFXR still has it fitted, no top and filter. I left mine there for wet weather riding, its always good to hear more sciencetific reasons. My motor is standard, carb and pipe are the only changes.

You'll probably find that the filter and the top are all part of the equation. I know that when I removed the filter it lost power significantly so it's got the filter back in now.

Truth is, standard carb, standard airbox. Just some serious jet and needle adjustments and it goes as well as any other FXR at Mt Welly. I'm more suprised than anyone........

[Rick 52]

Are you sure it wasn't the the bike running to lean that buggered your engine ? The aire box was designed to run with the CV carb but if you get a new carb then its no good . Its all about getting a good amount of fuel air mix in and getting the the old air out . I tryed to get the CV carb to run good but its better with a slide carb you then get more control.

[TZ350]

Feel free to come to the Mt Welly 'car park', all the engine tweaking in the world wont do you any good if you can't maintain corner speed. TZ350 has more horsepower than anybody I know and once he gets the trainer wheels off he might win some races....... (sorry Rob, couldn't resist).

Bugger taking the trainer wheels of, trying to keep up with you two is scary enough as it is.....:

[Sideways Sam]

Are you sure it wasn't the the bike running to lean that buggered your engine ? The aire box was designed to run with the CV carb but if you get a new carb then its no good . Its all about getting a good amount of fuel air mix in and getting the the old air out . I tryed to get the CV carb to run good but its better with a slide carb you then get more control.

I've considered that, over and over. Truth is there was no indication of overheating which there should have been if she was running lean.... Who knows????

Right now I'm playing with a computer and injection, if I can make it work (which is looking good) I'll happily throw all the carbs away........ The hardest part is finding a suitable throttle body, rest is a pice of cake. Looking at using a TB off a quad bike. would like one from GSXR600 but don't really want to pull a bank of four to bits