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Got a set of FCR35s on my J engine with a Kerker 4-1. Been going crazy trying to get rid of a dead spot off idle. Tried almost every jetting combination possible. A race guy on another site told me he had the same issue with FCRs on a big straight-4 and a Kerker header years back. Same non-jettable low end dead spot which turned out to be a flow fault at the collector, and the only fix was a different exhaust (He thinks it was a Mack).

My Kerker is kind of a rare one that was discontinued. It has the turn at the collector to allow oil changes, and it's chrome not black. As far as I can tell the only J engine Kerker now available is black and straight (pull to change oil).

Anyone experience similar problems? Don't know the company history, but I'm guessing when Supertrapp took over they re-did the original Eddie lawson design to allow oil change and high stand. Then discontinued.

Gonna run it with the baffle out (2" comp) to see if there's an improvement. If so, I'll be pretty confident the exhaust is the root problem, and wondering if a straight Kerker may be a fix.

I think the problem I'm having applies to the FCR carbs only, and my next step will be installing OEM Mikunis and see what happens. Big drawback is the FCRs and chrome Kerker are works of art and hate to lose either.

I find it hard to believe that a header could cause a dead spot off Idle. the revs aren't high enough for the exhaust to affect anything. Possibly running pods could affect it, but not exhaust.

I had a dead spot off idle leaving the stop lights and it turned out to be my charging system had a shorted stator, affecting my ignition voltage until the revs came up.

old_kaw wrote: I find it hard to believe that a header could cause a dead spot off Idle. the revs aren't high enough for the exhaust to affect anything. Possibly running pods could affect it, but not exhaust.

I had a dead spot off idle leaving the stop lights and it turned out to be my charging system had a shorted stator, affecting my ignition voltage until the revs came up.

Tuning with Innovate LM2. Idles perfect. Lean surge at 1/8 throttle 2k rpm. I can correct it by decreasing needle diameter, but then cruise at 1/8 throttle at 4.5k rpm is way too rich. Trying to correct that with clip position, L1 or taper will lean it out too much above 1/4 throttle. It's a characteristic of FCRs.

Check your mechanical advance springs

baldy110 wrote: Check your mechanical advance springs

Dyna 2000

My first dragbike had a sidewinder built by George Kerker himself.

I think that just goes with putting a racing carb on a street bike. They were never intended to be used at 1/8th throttle.

VTEC wrote: Tuning with Innovate LM2. Idles perfect. Lean surge at 1/8 throttle 2k rpm. I can correct it by decreasing needle diameter, but then cruise at 1/8 throttle at 4.5k rpm is way too rich. Trying to correct that with clip position, L1 or taper will lean it out too much above 1/4 throttle. It's a characteristic of FCRs.

Wow, that's some high tech and expensive acronyms for some aftermarket go fast Keihn flat slide racing carbs along with a wideband oxygen sensor tuning tool. Neat stuff, hard to tune, nowhere near stock parts. It sounds like you are looking to blame it on the header, when as a general rule exhaust flow really doesn't come into play until you hit high revs.

I did google tuning the FCR35's for you and got 30,000 results on google. Here is the top hit's url. with the Keihn Flat Slide tuning manual. Hope this helps.

www.factorypro.com/tech/tech_tuning_proc...g_FCR_Burns,Pat.html

DoctoRot wrote: I think that just goes with putting a racing carb on a street bike. They were never intended to be used at 1/8th throttle.

No truer words have ever been spoken my friend.

It is getting a little old about all the comments on why such items as Keihin CR or FCR carbs are more trouble than they are worth. Out of the box, the CR or FCR carbs will run halfway decent when you bolt them to your bike. In most cases they are about 60% of being in the ball park. Now to get that 40 or so percent out of your carbs is going to take some massive amount of switching of internal carb parts, and a certain amount of talent to understand what your motor and plugs are telling you!!! The Keihin Cr's and FCR's are infinitely adjustable. Dialing your carbs in is more than changing main jets, and raising or lowering the clips on your needles.

To get my first set of Cr's dialed in to the point they had no hesitations or flat spots, I changed everything in those carbs except the slides, and the carb bodies themselves!!! By my old notes, those carbs were off and on that bike over 20 times before I got them dialed in. There was a time or two where I went the wrong way and had to go back, and try a different direction on the jetting. The FCR's have even more tuneable parts then the older round slide CR's. That FactoryPro article will definitely head you in the right direction from what I read of it.

If you plan on buying a nice set of aftermarket carbs, and expect them to run right on out of the box( they won't), or don't want to or not have the ability to work with the carbs to get them running at 100% or so. I would suggest that you stay with your bikes stock CV carbs!!!!!

Like I said, I changed about every jetting part in my CR's to get them right in the ball park!!

Dr. Gamma wrote: It is getting a little old about all the comments on why such items as Keihin CR or FCR carbs are more trouble than they are worth. Out of the box, the CR or FCR carbs will run halfway decent That FactoryPro article will definitely head you in the right direction from what I read of it.

If you plan on buying a nice set of aftermarket carbs, and expect them to run right on out of the box( they won't), or don't want to or not have the ability to work with the carbs to get them running at 100% or so. I would suggest that you stay with your bikes stock CV carbs!!!!!

Like I said, I changed about every jetting part in my CR's to get them right in the ball park!!

I don't recall reading any of these comments stating as you say, but perception is also part of your own mindset when reading others comments. Those are racing carbs and as such costing ~$1400.00 .. new rack, ~$400.00 for the wideband O2 tool, then the expense of the jet kit you have pictured (300 bucks?) and how much work to dial in?

Going fast is really a labor of love!!!!