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FCR carb is beating me, help?

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First and foremost, I've been out of the sport for about 8 years.. back then, I was rebuilding a bike a week- if not more. I'm 7/10 familiar with the passage routing of the FCR carbs, the purpose in all of the jets, settings, etc.

That being said I recently bought a project. 09 kx250f, it was a "basket case" bike. Top end had been removed, the guy ran out of talent and decided to just sell it and move on. The rod bearing had failed, so I went ahead and rebuilt the engine completely.

Once reassembled, I was immediately fighting carb issues. Pulled it apart, set everything to factory spec and cleaned the bowl/passages as best I could (because the bike is in 100% stock format, and my elevation/weather accommodate those settings as well). Slight improvement, still issues idling. I pulled the VC, double checked the timing. The bike is dead nuts, 28 points between the dots, all marks (both flywheel and cams) match perfectly. At this point I ordered a replacement fuel adjustment screw (the one that was on it was a no-name brand, oring looked aged and the price of a replacement oring was basically identical to the price of the whole screw assembly.

At this point my big concern was just safely breaking the bike in, so I pulled the plug to make sure we weren't lean and got the bike broken in.

After I got the bike broken in, I went back to hunting my idle issues. Installed fresh orings in the mid body, soaked the carb while it was apart, blew out off al the passages with compressed air, reassembled the carb then blew it out again. Inspected both the jet needle and emulsion tube for wear.. set the jet needle clip to the 2nd slot (from the top, it was set to 4, 2 is factory for the 09). Verified once again that it does have factory jets (40 Pilot, 182 Main). I cleaned the slide, verified the slide plate wasn't upside down. My hotstart is NOT stuck, nor is my choke. I drained the fuel (that was new anyways), flushed the tank, and went to a different gas station to get fresh 93. Also verified float level, set to 8mm with a caliper.

The carb is front to back, cleaned and assembled to stock. I've also done the simple things like verifying the carb is clocked correctly (a rookie mistake from when I was a kid that still haunts me..  lol), and replaced the plug (that was done when the engine was finished.. just figured I'd mention it.. also tried to old plug just in case).

I've been through just about everything I could think of.

The bike at this point will idle.. sort of. Any blip of the throttle leaves it with a hanging high idle or kills it. When it stalls, the bike doesn't like to start without using the hotstart (Leads me to believe its a flooding issue.. which I suppose is kind of self explanatory).. and the bike doesn't really respond to fuel screw adjustments like a normal bike. Off idle, the bike runs solid. Snappy throttle response, no lean/decel pop, no bogging.

TPS has been a concern, however the general symptoms of a bad one don't match my issue.

Any input would be greatly appreciated, thanks

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On 5/24/2020 at 7:04 PM, mlatour said:

Perhaps this could be of help for ideas:

 

Thanks for the link, I read through all of it.. unfortunately everything seemed to be a dead end for me.

I unplugged the TPS today to put the bike into a "base map", and lowered the idle adjustment until it was no longer touching the throttle. I was able to manually "idle the bike" by holding the throttle, revved crisp and returned to idle fine (of course, once again with me holding the throttle). However, anytime I tried to match the idle with the adjustment screw/knob, I would get hanging idle. If I can sort out this hanging idle, we'll be good to go. I'm at this point diagnosing it like any other hanging idle issue (air leak, lean condition, sticking throttle) but nothing has stuck out to me yet.

Any input?

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UPDATE: Kept moving forward with diagnosing the bike today. It has about .60mm of play between the vac plat (no its not upside down) and the front of the carb body. Looked like way too much IMO. This was something I hadn't heard of being an issue- most likely because the last time I worked on bikes, the oldest FCR carb I put my hands on was about 5-6 years old- not 11. I shimmed it with some feeler gauges I had to see if that would help, if not eliminate my issue.. no luck.

I think at this point my best bet is to just order a new carb.. which is crazy to think about, I never had to go that far "back in the day" (once again, I should remind myself that those carbs were 5 years old and probably have 1/3 the hours..).

Whats the best bang-for-buck carb on the market? I would assume to avoid the FleEbay specials, however some honest input from experience would be nice. Thanks again

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I'm not sure if it's Tokyo Mods (?) but before considering buying an all new carburetor, (forget cheap Chinese copies)

you could have your's professionally verified and rebuilt/corrected if necessary.

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2 hours ago, mlatour said:

I'm not sure if it's Tokyo Mods (?) but before considering buying an all new carburetor, (forget cheap Chinese copies)

you could have your's professionally verified and rebuilt/corrected if necessary.

I used to work on bikes everyday and have a few buddies still in the business. I may run it by one of them just to get a second pair of eyes over it but its looking like the throttle slide play is my downfall. Definitely more than normal from what info I can find online, however there isn't any manufacture tolerance listed for it.

 

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13 minutes ago, 502Racing said:

How about the seal on the slide plate?

is it facing the right direction to seal against the slide?

ask me how long I chased that one....

Yes, but I'll double check tomorrow. May go pick up a new seal just to be safe before shelling out the big bucks on a new unit.

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Pretty common to wear out, need a new carb

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Update: S***ty news. Felt like you guys might had been as stumped as I was about the situation.. kept researching, testing, etc. Finally came to the conclusion that a new carb would just solve all my problems- and since I am so cheap in the bike.. it won't  hurt that bad to order it.

It showed up yesterday, along with a jet kit. Installed it with the jets provided.. exact. same. issue. Didn't get discouraged, put it up for the night and started on it today. tried various pilot jets, ending up at a 58 pilot.. yes.. a 58. The bike is responding to the fuel screw (I can hear it), and obviously is pig rich with those jets.. but the idle still hangs. I sprayed it down with penetrating oil in hopes of finding an air leak- no luck.. just made a mess.

Went back, tried setting the idle like I've done 2 million times before (before owning this bike.. I've had several carb 250fs), no luck.

The new carb includes a new TPS and hotstart plunger/spring (the cable is also free and not binding.. works properly).

Once again, to clarify. If I turn the idle screw all the way down, I can manually control idle with the throttle and it works/revs great. It is ONLY when I try to use the idle adjuster that it gives me trouble.

It wouldn't make sense to me, but are the any ignition related parts that would cause this issue?

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strange situation, if the idle adjustment screw is turned all the way out, there will be no space below the throttle plate and so the bike won't fire up, unless maybe the hotstart is open somehow (how else would the cylinder get air)….I learned this early on in my FCR carb rebuilds, now I set the idle after carb rebuild by measuring that the idle screw is 'screwed' in at 7.5 mm from the flat surface on the inside of carb...that gives a slight crack of daylight under the throttle valve plate ( the end of a cable tie will also just fit under that plate as another metric)...it would be interesting to know where the throttle wheel is at when you are able to idle the bike by the throttle alone, maybe your 'throttle' idle is actually way far off to compensate for the extremely rich conditioned caused by the 58 pilot jet or air leak at the hotstart...

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On 6/2/2020 at 8:50 PM, mattydavis said:

strange situation, if the idle adjustment screw is turned all the way out, there will be no space below the throttle plate and so the bike won't fire up, unless maybe the hotstart is open somehow (how else would the cylinder get air)….I learned this early on in my FCR carb rebuilds, now I set the idle after carb rebuild by measuring that the idle screw is 'screwed' in at 7.5 mm from the flat surface on the inside of carb...that gives a slight crack of daylight under the throttle valve plate ( the end of a cable tie will also just fit under that plate as another metric)...it would be interesting to know where the throttle wheel is at when you are able to idle the bike by the throttle alone, maybe your 'throttle' idle is actually way far off to compensate for the extremely rich conditioned caused by the 58 pilot jet or air leak at the hotstart...

Not ton a ton of variance from the 58 pilot from the 40 pilot regarding the hanging idle issue, the bike obviously feels overall pig rich with the 58 though, of course.. however it seems to not be in-line or directly effecting my issue. Bare in mind and well, the hotstart assembly is all new and the cable has been check and not hanging the assembly open.

I'll take your adjustment screw advice into consideration and give it a try.

Game plan is to start back at ground zero. Never had a bike give this kind of trouble- took a few days to reset my though process and plan on attacking it again Sunday or Monday. Gonna re verify timing, valve clearence, everything. The problem will eventually come to surface, just gotta keep fighting lol. 

Thanks again for all y'alls advice. I appreciate it.

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On 6/5/2020 at 8:36 PM, WannabeTech said:

Not ton a ton of variance from the 58 pilot from the 40 pilot regarding the hanging idle issue, the bike obviously feels overall pig rich with the 58 though, of course.. however it seems to not be in-line or directly effecting my issue. Bare in mind and well, the hotstart assembly is all new and the cable has been check and not hanging the assembly open.

I'll take your adjustment screw advice into consideration and give it a try.

Game plan is to start back at ground zero. Never had a bike give this kind of trouble- took a few days to reset my though process and plan on attacking it again Sunday or Monday. Gonna re verify timing, valve clearence, everything. The problem will eventually come to surface, just gotta keep fighting lol. 

Thanks again for all y'alls advice. I appreciate it.

Unplug the TPS and try it.

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11 minutes ago, Shawn_Mc said:

Unplug the TPS and try it.

Already did, check the 3rd response on this thread.

I did however finally figure out my issue, the neutral switch. I've never seen this be an issue on a bike, or at least with these symptoms. Unhooked it, reinstalled factory jets, factory settings, bike rides great.  Finally finished and cleaned up.. you wouldn't believe it was the same bike if you saw it a month ago lol

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