hi .,.need help 2007 yamaha big bear 250
rebuilt carb intake manif boot all cleaned removed ,, all new jets ,,starts ,runs ,idles great ,,but ,,throttle is not allowing to rev ie: throttle response
now i finally discovered and check this out that my mechanics rag was covering the air vent by the /under the gas tank with the vent covered but only with this thin red mechanics rag and not doubled i get good throttle response and visually can see rag sucked ..but if you cover it too much it will stall
cant be riding with a rag covering the vent ...please any ideas ...
the shops here wont talk to you unless service charges to bring in and drop off and all dealers the motorcycle shops ,this is re build /repair /replace father son project
any help appreciated thanks Hunter56

 

any ideas on this ? checked all air hoses, air box is sound and lid is tight new gasket on lid starts good idles good but throttle response bad unless air vent by gas tank is slight covering . ,,,tried messing with it today cant figure out any ideas please ? thanks

 

I'd check over the diaphragm assy to make sure it's not damaged and all parts are assembled correctly.Sounds like it's not getting enough vacuum unless the air vent your taking about is partially blocked? Plus there is an air cutoff diaphragm on the side of the carb that a lot of people over look that can cause problems if damaged or leaking. Plus the usual questions on float level,jets the same size in the kit as in the carb. Plus as cheap as aftermarket carbs are now you might consider one if you can't figure out this old carb.Carburetor Yamaha Big Bear YFM 250 YFM250 Carb 2007 2009 | eBay

 

i had a thought about that ,,when we rebuilt we removed the diaphram assembly before soaking carb it all looked good we didnt want to dis assemble the diaphram and needle as to not to damage and we re installed ,,the diaphram on the side w /spring also looked good as we removed it and re installed ,,is it possible they didnt go in right ? thank you for reply ,,we had gone thru the diagram also ,, should we re move and reassemble those again? parts are expensive in that area of carb ,,the cheap aftermarkets honestly am afraid of and trying to keep original as the original ,mikuni is also ,,any more thoughts ? thanks

 

Just retrace your steps on the carb using the parts break down as a guide. If a diaphragm is bad,it can be replaced. Hopefully it's just something out of place.

 

so heres update ..i replaced and got new parts replaced the small air diaphram and spring on side ,.,replaced the throttle diaphram ,needle valve assembly and new spring for throttle diaphram .. cant figure out retraced steps, float is ok idles good starts good but still only good throttle response if air vent by tank is slightly covered and sucks air ..air chamber is sealed and all hoses ok .. got to be something simple ,,all air tubes are clear .. we used the correct Moose carb rebuild kit and all jets fit and replaced from kit ..puzzled .. thanks for help

 

You do have the air box and filter installed ? If not,that may be the problem. Plus do you have a good flow from fuel valve in the on position and also reserve? If not,just replace it as cheap as they are.http://www.ebay.com/itm/Yamaha-Big-B...5TkSez&vxp=mtr

 

Thanks for reply
air box is installed correct new gasket on lid , checked all and all hoses clear from obstructs has new intake boot to carb also new fuel petcock on its way ordered cpl days ago , otherwise old original seems to flow good checked all flow including reserve into gas can ... Geez

 

Maybe make sure you haven't accidentally doubled up on any o-rings for your set screws, etc. I rebuilt my carb a few months ago and did that on my idle set screw and I couldn't figure out why it wouldn't run right. There was a tiny o-ring buried in the threads that kept the screw from going in all the way.

 

all the ideas and help has been great ..update where i am at now ..besides frustration i replaced the fuel petcock ,,and got an oem replacement carb not mikuni original but a good oem exact replacement
put carb on and runs better but still have the problem of the air vent if covered will throttle better .heres the kicker also
we drove it and runs ,,but with choke all off the throttle response is non responsive but if i pull the choke out a little it gets response and shifts well but can get bogged down if the air vent is covered slightly with choke all off runs better
.any help is appreciated ,,,dont what else to do ..could valves need adjusted ? the intake ? it idles starts and runs shifts .runs better with choke out driving but can bog down,,see previous responses and help also here ,,,thanks all !