Picked up a 99 YZ400F - Questions (yes, I've been searching)

I doubt that will help.  The fact that it runs with the choke open and then acts up when you push it off tells me that it is both opening and closing as is.  If it wasn't,  there would not be the change in fuel delivery that is causing it to run poorly.

 

One unfortunate fact about the original Flat FCR carbs is that they can wear and internally gunk themselves up to the point where they simply can't be fixed in any practical way.  If you can find a carb body gasket (goes between the upper and lower halves of the main carb body that are separated by removing the 4 Torx screws; JD Jetting has them), then you might solve your problem by taking the body apart and scooping the goop out.  Just don't disassemble it if you don't actually have the gasket.

 

A badly worn or cracked/chipped vacuum release plate on the slide is another potential trouble area.  Also, try disconnecting the TPS.

you doubt, but you dont know for sure. I know that the choke port is the only port after the slide....if one drop of gas goes in as result of defective choke plunger seal....it can rock the whole warm up period, and idle performance.

you doubt, but you dont know for sure. I know that the choke port is the only port after the slide....if one drop of gas goes in as result of defective choke plunger seal....it can rock the whole warm up period, and idle performance.

:rolleyes: 

this site is like a facebook for bikers...i like it. All i know is i read 40-50 bloggs on peeps that spent 300-2000 bucks on rebuilding this and that....and i know that the carb kit does not come with a new plunger seal. Cause i did one. Yamaha's are notorious for failing plunger seals. They have a rubber plunger in there, and it shrinks like all the other carb parts, so i gather that, especially on a 400 cc thumper, it will not seal out gas when shut off, and when that big ass crank comes around on intake with the slider down, it will suck in gas....guaranteed. 

now...does any of you good ol' boys know where to get one ? lol

Can't buy the seal separate from Yamaha

You can try Sudco.com

You can probably source it from a Hydralics parts house, like did for the fuel inlet spigot flat rings

Yamaha's are notorious for failing plunger seals.

News to me.  And I think that if it happened on an FCR, it would be the first reported here in at least 13 years. 

 

now...does any of you good ol' boys know where to get one ? lol

 

Sudco, if anyone has them.

News to me.  And I think that if it happened on an FCR, it would be the first reported here in at least 13 years. 

 

 

Sudco, if anyone has them.

Krannie and Greyracer.... i just ordered one plunger, should be here today. got it for 38.00 plus shipping from ebay (a guy named KehinCRF, a bike parts guy in Michigan). After spending 80,00 on the carb kit, it is kinda dumb NOT to replace the plunger for a extra 40 bucks. I mean the bike starts up first kick with the choke pulled out, and the ignition checked out last week, i think like most 400F owners, the bike ended up scrapped after 4 years of use, and i got it for 500 bucks.. i mean the original factory tires are on it.....lol

I would suggest repacking the exhaust pipe.  Pack it super tight and your noise problem should be solved.  The FMF Q is a good choice if you buy a new pipe but that will need repacking after a while.

I would suggest repacking the exhaust pipe.  Pack it super tight and your noise problem should be solved.  The FMF Q is a good choice if you buy a new pipe but that will need repacking after a while.

Don't pack it super tight.  Too much packing will be just as bad as none at all and WILL affect your performance/jetting.  

 

I mistakenly packed my silencer too tightly and the bike wound up popping like crazy and shooting flames out the tail end (and no, it was not an exhaust leak).  I repacked with less material and everything went back to normal.  I figured that packed too tightly the exhaust gasses don't absorb in the packing, go through too quickly, with no back pressure like an open exhaust with no packing.  The gasses need to actually go into the material to provide the reduction in decibels, slow it down and provide the correct back pressure. I want to say 2 wraps of FMF 4 stroke packing is what I use with my FMF Q and is enough to get me to 94 dB.

Edited by Pooley

Yeah, overpacking can almost be like not having a can. Obviously it isn't that extreme, but there is such thing as overpacking. 

Yeah, overpacking can almost be like not having a can. Obviously it isn't that extreme, but there is such thing as overpacking. 

hey everyone, the New choke plunger came in, and sadly it wasnt the same length, but my trusty depth caliper said it should work (lol) so i installed it, and it seems to almost idle with the choke "in" but im giving it a couple of days. i think this carburator is a bad model, i have heard of this problem with these CRFlats from Kehinn. Is there a carberator specialists who could rebuild mine to factory settings ? cause it gets spark according to the gap tester (30 bucks from O-reilly), it jumps the 1/4 inch plus gap with a snap ! lol and it has crazy compression. I actually sent my cams to LA to be re-indexed, cause they slid on the shaft. Piston and cylinder was healthy. And ALSO i rebuilt this carb with a kit that contained the excellerator diaphram. It to has 200/100 air intake jets, but a 48 (not 45) in the bowl. Funny Kehinn is stamped on the 48, i thought factory jet was 45 ???  i reset the needle to middle position, i suppose when the guy installed the hi performance white bro exaust, he raised the needle all the way.....THEN THERES THE SLIDE ADJUSTMENT. Poop, im done. ugh. So far i got about 1000 bucks in this depressing monster......i wish i could have got a classic 1980 YZ250 2 stroke instead. 

  i think these bikes should run better than they do, a four stroke should fire up eazy, and tune eazy...this is bull $h!t .

Edited by Peter Thumpertail

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