Alternate carbs?

[fintip]

I found some info on this on the old fjmods site (I collected the relevant information on the wiki, see http://fjowners.wikidot.com/carbs#toc2)

One of the really interesting statements is swapping in Keihin FCR 39's supposedly is a huge, massive upgrade. Choke is no longer necessary, supposedly.

A) Anyone have any commentary on this (or other, or general) carb swaps? I have never personally known of a carb swapping example, except for a guy I work with at the shop who mentions a theoretical swap he would like to do.

B) There's a set with flatslides for $500 on ebay right now, if anyone is feeling adventurous... http://www.ebay.com/itm/Keihin-FCR-39mm-flatslide-carbs-gsxr-gsx-r-750-600-1000-CBR-ZX-YZF-fcr39-/161058081492?pt=Motorcycles_Parts_Accessories&hash=item257fcf4ad4&vxp=mtr

[aviationfred]

There are a couple of members that have done big bore kits, (over 1300cc's) that have installed Keihin FCR carbs. I think one of the members is Flynt. Also you may want to check with Randy, I believe that he built the engine and did the initial tuning.

Fred

[JMR]

 I run FCR's and have used RS 36, 38 and 40's. The FCR's in that ebay listing are downdrafts and won't work on the sidedraft FJ (unless you source H type float bowls). Even at that the side throttle pulley will not work. Grab some Mikuni TMR's....they are nice.

[fintip]

I had never seen mention of other carbs here, color me surprised.

Are the performance gains as significant as it says? What are some shortcomings of the stock carbs that aftermarket carbs outdo?

[red]

I had never seen mention of other carbs here, color me surprised.
Are the performance gains as significant as it says? What are some shortcomings of the stock carbs that aftermarket carbs outdo?

Fintip,

Any time that you are considering new carbs, there is probably enough wrong with the old ones to make a serious difference between the old and the new.  Parts availability may also be a factor, if you can not get new parts for old carbs.  At least one brand mentioned having an accelerator pump, so you give a shot of gas when you crank the throttle, which improves the bottom-end response.  Depending on the volumetric efficiency of everything in between the carbs and pistons, larger carbs may improve the top end power, but at the expense of bottom end response.  A new throttle-grip and cabling may be needed on new carbs also.  Get good advice on any alternatives, before you dive in.  My carbs are bone-stock, so I can't say more.

Cheers,
Red

[Flynt]

Keihin FCR 39's supposedly is a huge, massive upgrade. Choke is no longer necessary, supposedly.

The theory of operation with the FCRs is fundamentally different...  they are not a CV design like the stock Mikunis and will give a very different feel, but categorization as an "upgrade" will depend on what you do to complement their installation.  Regardless of what else you do to the engine, they come on much quicker and are very "touchy" compared to CV style carbs.  Accelerator pumps are essentially squirt guns that pour gas into the intake when you crack the throttle.  The other big difference is you control the venturi flow directly (the diaphram operated CV piston/needle is hooked directly to the throttle cable).  The throttle behavior is the biggest single difference you'll notice along with attendant gas mileage reduction...  assuming you just installed them without building the right kind of engine to attach them to.  


On the plus side, the FCR39's will flow much more than the stock Mikunis and have some additional tuning parameters (accelerator pumps for example) to optimize the fuel mixture across a wide operating range.  As stated in some of the responses, this is a trade-off to serve the high flow end of the RPM range...  I posted dyno results showing about 40% more power than enhanced (exhaust and dyno tuned) stock at 9,500 RPM, but my engine (1349cc, big valves, ported/polished head, Megacycle cams, Akrapovik exhaust, etc...  thanks Randy!) is built to take advantage of these carbs.  


The no choke is no joke...  they don't have 'em and you don't need 'em.  From cold, I twist the throttle once (to squirt some gas into the intake) then start and hold idle for 10-20 sec, then ride off.  The bike is rock solid stable throughout its operating range and could be mistaken for an FI system based on behavior.  I love it.


Here's the story:  http://www.fjowners.com/index.php?topic=5619.msg49134#msg49134

Frank

[Pat Conlon]

Kyle, I moved this topic over from the Introductions Section. I don't know why you posted it there...

In short, when you open the throttle, the CV carbs flows air/fuel it wants (based on velocity) vs. the flatslides, that flows the air/fuel you give it.

Also, the FCR's and Mikuni TMR's are superior to other flatslide carbs (e.g. Mikuni RS) in that the flatslides are on rollers which make the throttle action smoother.

The Flatslides are quicker to respond to throttle input, no doubt, but the CV carbs, by design, compensates for altitude changes.

A nice feature for a street bike.

Going over a mountain pass in Colorado and with the thin air at 10k or 12k feet, the air velocity is slower thru the CV carbs which causes the slide and jet needle to lower and thus, flows less fuel thru the needle jet. A good thing at high altitudes.

Something to consider.

[JMR]

I had to modify the throttle cable holder on the FCR rack to better accommodate the cables. I also had to change the cables ( which actually simplifies the whole thing as the the cable junction box is eliminated).

[JCainFJ]

I have a set of RS36's on my 89 and they do let the engine pull hard all the way to the redline. The stock BS36 Mikuni's stop making more power at about 8500 rpm. I did lose a few mpg, but I feel that the added power and controle make the carb swap worthwile. I did have to modify the stock cables, and the enrichment circut (choke) does come in handy in cold weather ( I ride until it is below 32 F.). My 89 is stock internally with a Suppertrap exhaust and a Dyna2000 ignition. I got these carbs from Carb Parts Wherehouse and had to change the float valves and set the fuel levels.

[JMR]

I have a set of RS36's on my 89 and they do let the engine pull hard all the way to the redline. The stock BS36 Mikuni's stop making more power at about 8500 rpm. I did lose a few mpg, but I feel that the added power and controle make the carb swap worthwile. I did have to modify the stock cables, and the enrichment circut (choke) does come in handy in cold weather ( I ride until it is below 32 F.). My 89 is stock internally with a Suppertrap exhaust and a Dyna2000 ignition. I got these carbs from Carb Parts Wherehouse and had to change the float valves and set the fuel levels.

That's interesting. Why did you do that?