Which Carbs? Stock of Flatslides?

[Mell00]

Ok, I'm sure this in the archives somewhere but I only have ultra slow web connection....

I am looking for opinions on carbs for a 1992 FJ all stock. I plan to put a K&N OEM style filter in it thats about it.

Carbs choices pros and cons? Not looking for major top end speed so larger carbs are not really needed. Best response and drivability is most important.

Any info/oipinions would be appreciated.

Thanks

Mel

[Pat Conlon]

For what you are planning on doing, the stock BS36 CV Mikuni carbs work just fine, why would want to change?

Barry Edwards has some carb discussion on his UK web site: http://www.fjmods.btinternet.co.uk/Carburation.htm
The Mikuni RS series are available for the FJ, cost new about $1100 (scroll down)
http://www.motorcyclecarbs.com/Yamaha_Street_Carburetor_Perfo_C595.cfm
The Keihin FCR38's are no longer available for the FJ (that I could find) The above carbs.com website shows the FCR's still listed for the FJ but in contacting them, they don't have them anymore. IOW the website is wrong.

Not discussed on Barry's site, but I think the cat's meeow for a big block FJ would be the Mikuni TMR carbs.
All the advantages of the Keihin FCR's, roller slides and accelerator pump, but unlike the FCR's these TMR Mikuni's have a choke.
They're only $2,400, chump change:  http://shop.yoshimura-jp.com/en/product/syousai.php?id=1156

[JMR]

 The TMR's are very, very nice. I've got a set for one of my other bikes.

[Mell00]

Alright then, I'll stick with the stock carbs.

How about minor upgrades like air filter and jet kit? Response and drivabilty most important. Not looking for major hp boost, I'll do those upgrades later.

[andyb]

Response and drivability?  Tune the stock bits and don't change much.

Power?  Throw it all out and start over.  Then wish there were new words to swear with, because the old ones don't always cut it.

You can get the stock carbs on a dyno and tuned if you really want to max out on how well it can run for what it is.  Just do your research first on the dyno place, and ensure that they understand you want usability foremost.

[JMR]

Do the XJR carbs have a different diaphragm/slide set up than the Mikuni's found on the FJ's? Those slides weigh something like 53 grams each if I remember correctly.
Something with a plastic flatslide would definitely by better

[andyb]

The XJR late models are FI  The early FJ and the later FJ have slightly different slides, believe it was an 84-only sort of thing (search, the answer is here someplace).

Why do you think there's a problem with the weight of the slide?  The spring adds a linear amount of resistance, making it functionally heavier as it rises.  I would wonder if the opposite is true, that heavy slides will dampen things from their inertia and cause less needle/emulsion tube wear.

You can change the rate of opening of the slides through a couple of means (drilling or epoxy`ing, springs), but it changes the fueling delivered by the needle.  If you're not changing to a fairly customized needle, I'm not certain that you'd see gains.  Sure, the motor could in theory get more air faster, but the whole reason for CV carbs is to cause a restriction.  Slam a set of flatsides open at low revs and you'll realize why that's a good thing.


Note to the original poster:  Don't do it later.  It means that you end up fighting things twice.  Figure out what you can afford, what you want, and do it all at once if at all possible.  The usual bolt-ons are a fairly standard set of mods (albeit they don't gain much power, they change the shape of the power moreso).  If you're going to go with pistons and headwork and etc in a few years, then yes, it may be worthwhile to do things in stages.  Fighting to nail the jetting isn't fun! 

[Mell00]

Well, then i'll go with the stock carbs then.

Any good recommendations on a quality rebuilder that is econmical? I just don't have the time or tools here to be able to do that properly since I have just relocated out of state for a new job.

m

[FJmonkey]

Well, then i'll go with the stock carbs then.

Any good recommendations on a quality rebuilder that is econmical? I just don't have the time or tools here to be able to do that properly since I have just relocated out of state for a new job.

m

Check out Randy, he is a club member and engine builder/racer for the FJ power plants. He services carbs if you send them to him.

http://www.fjowners.com/index.php?topic=69.0

[JMR]

The XJR late models are FI  The early FJ and the later FJ have slightly different slides, believe it was an 84-only sort of thing (search, the answer is here someplace).

Why do you think there's a problem with the weight of the slide?  The spring adds a linear amount of resistance, making it functionally heavier as it rises.  I would wonder if the opposite is true, that heavy slides will dampen things from their inertia and cause less needle/emulsion tube wear.

You can change the rate of opening of the slides through a couple of means (drilling or epoxy`ing, springs), but it changes the fueling delivered by the needle.  If you're not changing to a fairly customized needle, I'm not certain that you'd see gains.  Sure, the motor could in theory get more air faster, but the whole reason for CV carbs is to cause a restriction.  Slam a set of flatsides open at low revs and you'll realize why that's a good thing.


Note to the original poster:  Don't do it later.  It means that you end up fighting things twice.  Figure out what you can afford, what you want, and do it all at once if at all possible.  The usual bolt-ons are a fairly standard set of mods (albeit they don't gain much power, they change the shape of the power moreso).  If you're going to go with pistons and headwork and etc in a few years, then yes, it may be worthwhile to do things in stages.  Fighting to nail the jetting isn't fun! 

As a generalization lighter moving parts react to change in direction better. For instance, the lighter, later type slides found on 90's GSXR's snd CBRF2/3 work well. The use a much lighter spring, a better diaphragm (IMO) and are more sensitive to small changes in vacuum. The make for lighter carb assemblies and the carb slide bores are subject to less wear. Their design improves the signal strength similar to flatslde carbs like the RS or FCR. The flatslde creates such a high vacuum level the RS need that stiff return spring to compensate. The FCR uses a floating valve deign which works well and eliminates the need for the stiff spring. The TMR's have the rollers fixed to the carb bodies and the slides are sandwiched between them. They are very nice carbs...I have a set.
  Lighter moving parts (especially those that go up and down) are generally a good idea. Light valves, light shims, light buckets, light rods etc etc. Like i said...a generalization.
All that being said the stock carbs work OK....I put 15,000 on RS's with no problems. You can soften the return spring a bit from as delivered. If I remember correctly my 87 made 130HP with RS 38's, Yosh Stage 1 cams with 105LC and a Hindle pipe with 2.5 baffle. That was with stock pistons and quite a while ago.