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Help! Dreaded FCR Bog off idle.

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Hi,

Please help before I put back on my CVK carb. :banghead::busted: My buddy and I both put FCR 35mm carbs on our KLX300's at the same time. The jetting on both bikes is identical. Mine has a bad bog off the bottom and his is almost perfect. My mechanic went up one pilot size on mine and it helped a tiny bit, but not a lot. Eventually the mechanic told me the FCR I bought (used) has a bad low speed circuit in it. In an attempt to make it run right I put on a Boyesen Quickshot. It might run a bit better, but still bogs badly. (If I'm going slowly and snap open the throttle, the engine will die like hitting the kill switch.) Backing the fuel screw 3.5 turns out helped a bit, but the bike still dies then lunges hard! Very disconcerting.

Any ideas on getting this thing running right, or should I just get rid of it and buy a new carb? Running out on the trail is not to bad, but every once in a while - usually just when I REALLY need it - I'll hit the gas hard and almost go over the bars. I know that unless I can get this worked out, it will end up with me taking a big header over the bars.

Here's my jetting specs:

FCR35 on a KLX300

Pilot: 62 (up from 58)

Needle: OCDVR

Clip position from top: 4th

Main: 155

Fuel Screw: I've tried everything from 2 to 3.5 out

Main Air Jet: 200

Pilot Air Jet: 1.5 out

Any help is greatly appreciated. :banghead:🤣:busted:

Thanks,

Bill

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have you timed the accelerator pump squirt?

your running a huge pilot jet,that pilot air is out so far.

ideally i would like to see the pilot jet and pilot air brought down a bunch.

the DVR needle also has a very lean straight diameter and slow taper for the small carb.

try a EMP or a EKP.

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Burned, thanks for the suggestions. The current jetting specs are what Jerry at FSW has come up with over the years. He knows these bikes intimately (much better than I do) and has come up with them over the years. To make matters more confusing, they work perfectly on my buddies carb/bike.

I have not timed the pump. I know it's workinig because I can flood the engine. :banghead: The boyesen pump cover instructions showed a screw and the proper dimension, but my carb is built differently. There is no screw adjustment. It has what looks like a metal stop with a slot cut in it that you could put a screwdriver into the slot and widen it out. Is that the part that "times" the squirt? Sorry for the ignorant questions, but this is my first time dealing with an FCR. I don't want to mess it up any worse than it already is. Thanks for any more help you can give.

Bill

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the tab you see controls when the squirt starts.spread or squeze as needed.

it should squirt as soon as the slide moves and for around 1 second.

i bet yours either has way more squirt than your buddys or too little.

i still think you can find a better jetting combo as well.the huge pilot settings are needed to over come that needle.

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Burned - thanks again!!! Forgive me if this is a stupid question, but like I said, I'm completely new to the FCR. How can I check the squirt duration? Do I pull the carb off the engine, leave the gas and throttle hooked up and measure it that way? Can I see the squirt by pulling the bowl??

Now I feel like a complete newbie, but where is the pilot located on the FCR and can I get to it with the carb on?

The symptom's with the fuel screw 3.5 out were bog for a second then WHAM! Before going 3.5 out it was just bog........and dead.

I think I may try one size larger pilot before giving up completely on that circuit of the carb and moving on to the squirt duration. It seemed like the more I turned out the fuel screw, the less it bogged, so that makes me think that I might be headed in the right direction - until I read your post about the exceptionally large Pilot I have in already. I'm 3.5 out right now on the fuel screw and I think rather than going any further in the adjustment, a larger (:banghead::banghead:) pilot should be investigated.

The pump duration seems like it may have the best chance of success here. I'm busy for a week or so, but as soon as I have a chance to check into it, I'll report back. Thanks. Bill

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One last question, does this seem like a jetting / squirt duration problem to you, or could there actually be a faulty casting causing a slow speed problem with the carb? Thx!!!

bp

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remove the carb.with fuel in the float bowl operate the throttle (with the carb pointing away from you).observe when the squirt starts and how long the squirt is.

it seems like a squirt issue to me.

if you really want to richen the pilot circuit,just close the pilot air down a 1/4 turn.no need to keep buying pilot jets.

you have to remove the float bowl to get at the pilot jet.the intake bell needs to be removed to get at the pilot air screw.

i highly doubt you have a defective carb.

how does the bike idle?

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Burned, thanks again! The bike idles fine. And it runs like a scalded cat - except from closed throttle to open throttle. if the throttle is completely closed off and you hit it hard - the bog. Quarter throttle and you hit it hard and it's a 80% chance you'll take off just fine,

Starts like a dream when cold - three twists (no choke) and an easy kick. I just had a new piston, rings, etc. & new bronze valve seats put in and haven't had a chance to ride much due to work, but the one time I had a chance to ride just a bit it was a little harder to start when hot stalled, but not difficult.

I'll pull the carb and make sure the pilot screw is 2.5-3.5 out and mess a bit with the timing of the squirt and report back.

Cheers,

Bill

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