Gareth. J. Posted January 29, 2012 Share Posted January 29, 2012 (edited) Hey guys, I'm going to be replacing my Keihin CV CBR1100XX carbs with new Keihin FCR41 carbs. I have had to get two sets of four which I will modify to become six. Just wondering if anyone has any knowledge on a good place to start with jetting sizes? Has anyone done it, seems to be quite a few in japan but nowhere else. I have an L28 over bored 1.5mm, 10:1 comp ratio, P90 head heavily worked with high lift cam, Stainless headers, Msd6al-2 and I'll make a custom manifold with 40mm I.D. pipe. Thanks, Gareth. Edited January 29, 2012 by Gareth. J. Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted June 6, 2012 Author Share Posted June 6, 2012 (edited) I found a few other FCR41 setups with similar cubic capacity to my worked L29 running on average 185mains, 45 or 48 pilots, 200 main air, 100 pilot air, 2 turns on mix screw and the needle on the third clip. I have adjustable pilot air screws to help with tuning. This will probably be a good starting point that will run fairly well but won't have huge top end I suspect. Time will tell, stay tuned. Some pics of the setup so far... I had to buy them as two sets of four, then strip down to individual carb bodies and reassemble in the required spacing. Edited June 6, 2012 by Gareth. J. Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted June 26, 2012 Share Posted June 26, 2012 Most of the guys on here dont talk motorcycle but I have a few.I think you are a bit large on the main jets I would try 168-165.You really need to install a wide band o2 or you will be chasing problems for ever.I have a 41 fcr laying atound for my honda xr650r but i havent installed it yet. Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted June 26, 2012 Author Share Posted June 26, 2012 Yeah spot on Randy, I have a wide band air fuel meter installed already and I think it would've been a nightmare without it. I can't wait for the unique idle rattle and induction roar! The current carbs are 42mm Keihin cv's from a honda blackbird, initial jet sizes were around 160ish, it ran but with no real guts. After nine jet changes and drilled out slides, I finally settled on 220mains. The FCR's are a whole different ball game though and infinitely adjustable. Fingers crossed they are as easy to tune. I was actually thinking of removing the accelerator pump plunger rods for tuning, this should give me a more accurate view of the 'real' fuel mix, then reconnect them when it gets close. what do you think, have you heard of people doing this before? Cheers, Gareth. Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted June 26, 2012 Share Posted June 26, 2012 I agree on disconnecting the accel pumps and get your part throttle a/f worked out.there is a leak/bypass jet in the bottem of the float bowl that can be used to change the time length of the pump shot.Probably if you make the car drive smoothly rolling on the throttle and add just enough pump shot so you can stab the throttle with out a stumble it will be right the a/f will be correct. Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted June 29, 2012 Author Share Posted June 29, 2012 Thanks Randy, I'll post up results but it may be a while yet. Hehe. Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted August 9, 2012 Author Share Posted August 9, 2012 Here's the new manifold, with the carbs test fitted and plenty of clearance all round. I have a question for the velocity stack guru's.... I have come across a 'power now' stack (see pic below). The velocity stack is fitted with a horizontal plate across the centre, when the flatslide is open on low to half throttle the air is only drawn straight throught the lower section. This is meant to promote higher velocity,less turbulence and give better driveabilty. I guess it's acting like a choke at part throttle, at full throttle it has no effect. It claims to increase power/torque in the low to mid range rpm. My shift point is 7200 so I figure thats low-mid range. The theory behind these seems to make sense but I'm wondering what you guys think? Thought I'd keep it all on one thread. Also to correct I have adjustable main air screws and have just fitted adjustable idle mix screws for east tuning. Thanks, Gareth. 1 Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted August 18, 2012 Author Share Posted August 18, 2012 Got these babies on and running, engine runs quite nicely. My oxy sensor is dead so fine tuning will have to wait, plug color looks spot on! Check out the video's in the members video section. Quote Link to comment Share on other sites More sharing options...
Leon Posted August 20, 2012 Share Posted August 20, 2012 Very cool! Interesting induction sound. FWIW, I would not use those stacks. It's just marketing hype. You don't want high velocity (choked) flow, especially before the throttle plates. Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted August 21, 2012 Author Share Posted August 21, 2012 Thanks Leon. I said choke which was the wrong term, the actual wording was more linear flow with less turbulence. Basically I thought it would work like this... When the throttle is opened low-mid way on the FCR's the air coming in hits the slide, drops vertically to go under the slide and then continues on. With conventional butterflies the air would seem to have a straighter, smoother path at low-mid throttle. Thats my very simple logic, seems like I have been fooled by the snake oil I already bought two of those stacks cheaply, I might give them a try anyway to see if they get rid of the 'stand off' I can see at around 2-3,000rpm. Quote Link to comment Share on other sites More sharing options...
Leon Posted August 21, 2012 Share Posted August 21, 2012 Thanks Leon. I said choke which was the wrong term, the actual wording was more linear flow with less turbulence. Basically I thought it would work like this... When the throttle is opened low-mid way on the FCR's the air coming in hits the slide, drops vertically to go under the slide and then continues on. With conventional butterflies the air would seem to have a straighter, smoother path at low-mid throttle. Thats my very simple logic, seems like I have been fooled by the snake oil I already bought two of those stacks cheaply, I might give them a try anyway to see if they get rid of the 'stand off' I can see at around 2-3,000rpm. You'll have entrance losses whether it's at the slide or at the inlet of the air horn. You're just moving it further upstream. If you really want to see if they make a difference, dyno test the car and see if you get a part-throttle gain. Radiused air horns will improve performance for multiple reasons (decreased entrance losses, increased resonance effect), but that little slot down the middle won't buy you anything. If anything, I'd see that as a restriction at higher rpm. I'm getting into the details, but either way that's a cool and unique setup! Quote Link to comment Share on other sites More sharing options...
Gareth. J. Posted August 22, 2012 Author Share Posted August 22, 2012 It's all helpful information, thanks. Current stacks are the standard Keihin radiused type 50mm long, maybe ill just keep those. I have never dialed in the cam properly (74deg works grind with high lift) so that could also be a factor with the Stand off. I should look into that before I make any drastic changes, I'll be getting a new oxy sensor this week for the air/fuel meter, this should help to get it pretty close jetting wise. Quote Link to comment Share on other sites More sharing options...
Leon Posted August 22, 2012 Share Posted August 22, 2012 Cool, keep us updated! Quote Link to comment Share on other sites More sharing options...
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