This is probably the best info I have seen about tuning a carb.

http://www.mikuni.com/pdf/hs40_manual.pdf

David

 

Well, I got to test the FCR35 in anger in the dirt yesterday. Just over 50km's of riding in a very good mix of fire trails, tight single track, rocky loose terrain, hill climbs, and steep decents - pretty much the only riding conditions missing was mud, so it really could not have been a much better ride to thoroughly test the bike. Ambient temps were in the mid 20C's so it wasn't too hot either, but that didn't stop me getting my coolant temps up to just under 100C in the tight single track.

So, what do I think? Overall there is definitely an improvement in the bike and it's rideablility. It has NOT magically created more engine power, but it HAS changed the characteristics quite noticeably. How so? Well, I felt the power delivery is a lot more linear and predictable, and although I expected the AP to cause me some problems with breaking traction on loose surfaces when I snap the throttle, it did not do that once. What the AP did was get the power normally delivered after a second or two with the CV carb down when I wanted it. That change took me a while to get used to - for example, in the tight single track where I would normally hold the bike in 1st gear to keep the revs up I found I could actually ride a lot easier in 2nd gear without having to change down till there were gnarly corners and/or steeper uphills. Being able to stay the gear higher helped quite a lot with fatigue which for a fat bastard like me it a BIG bonus! We had to hop over several logs on the ride, and although I could not lift the front wheel by just snapping the throttle, doing that did lighten the wheel to make it easier to get over the logs - the predictable power delivery helped significantly compared with the CV when it came to log hopping. If I used humps to assist me, I could lift the front wheel pretty easily. This made negotiating braking/rolling bumps easier as I could just lift the wheel a bit and let the rear wheel follow the undulating ground and push me through - great for managing fatigue again

As far as jetting goes, yesterdays ride pretty much convinced me I don't need to do much more. I thought that with the richer pilot circuit I would suffer from low rev bogging and idling when my temps got up, but that wasn't really the case. It did stall once or twice when idling hot, but I think I was actually struggling more with the stock CV carb than I did with the FCR. In winter the jetting it probably going to be spot on, so if it runs fine now, why mess with it?

So... for those of you considering the FCR35 for your bikes, the burning question is "is it worth it?" This is not an easy question to answer IMO, but I probably went through a bit more pain with my carb than most would thanks to my spigot issues. Here is my opinion:
Is the FCR35 worth it just for the performance increase - NO
Is the FCR35 worth it just for making the engine more predictable and usable - NO
Is the FCR35 easy to fit - NO
Is the FCR35 worth it from the "learning experience" - YES
Is the FCR35 worth fitting for the overall improvement in the engine - YES

If I knew it was going to be as much of a challenge before I bought the carb, would I do it again - I dunno. If I did not have the spigot adapter issues, I would say YES. It ended up costing me more than just getting Bill Blue's Mikuni pumper which should just be plug and play - so I would advise going that route if you aren't up for a challenge and want to save some money. I don't know if the FCR is any better or worse than the Mikuni, so please don't ask. It looks like there is a lot more info and advice out there for the Mikuni, but the info available on the FCR is VERY LIMITED. Hopefully this thread will help people in the future.

At the end of the day I am happy. I have learned a ton, and am infinitely more confident working on my bike because of the experience. I have learned tons about jetting and tuning a carb which I think will serve me well in the future. Thank you very much to all those that helped with advice and encouraging me. I really appreciate it!

My final setup:
The FCR35 carb is a N424-25 series carb (the N424-21 series carbs are more common and jets are not interchangeable, so beware!). Pilot jet thread of 4.5mmx0.75mm and length of 15mm.
Pilot: #55
Main: #152
Fuel screw: 1 turn out at sea level
Needle: OCDVR with 3rd clip from the top
Accelerator Pump: Taffy-type mod to limit squirt duration using RC Car collar
Engine side spigot length: 27mm (important to make sure the carb fits without touching anything else)

 

Well, last week all of a sudden my KLX started running horribly. Would not idle at all, and when teh throttle was only a little bit open whe burped and spluttered more than a newborn being burped. I thought I may have gotten a bad tank of fuel so flushed it out and replaced it, but still teh same problem.

So this weekend I took it off and discovered the problem. My dodgy job of sealing the spigot adapter to the original spigot had failed, so I assume she was sucking in air there. I didn't have the time to repair it, so switched back to the stock carb.

Well, now that I am used to the FCR, I can appreciate how big a difference it makes. I think it's like the whole dishwasher analogy - you never realise how much you need it, you think you could easily live without it, until you don't have it... and then all of a sudden you can't live without it! When I took it for a quick test ride after re-fitting the CV carb, I turned a corner and openned the throttle expecting to accelerate away, only to be shocked at the lack of initial response, and then relieved when the revs slowly increased. It's going to take some getting used to while I sort the FCR out and refit it!

On the FCR, I think I am going to use some JB Weld to attach the spigot adapter to the spigot permanently so I never have to worry about it failing again. Will try do that one of the nights this week and try refitting it this coming weekend if I can find the time. Whilst it's out I am going to drop the pilot jet back to the #52 and see how I go with that as I am sure the #55 was a tad too rich