nofacemonster Posted January 16, 2013 Share Posted January 16, 2013 i have my SR 400 carburetor cleaned, slow jet was kind of too big, so i re-jet the carburetor with 40 Size pilot jet. I checked everything, the Needle was set to middle. Pilot screw is now set to 1 1/2 turns. Floater is properly adjusted so it won't overflow. Bike runs well, spark plug looks cleaner now. New air filter etc as well. I did all above because few reasons. 1.) Bad fuel economy, failed the green test (now passed) 2.) The IDL RPM goes up and down in a weird way and if i accelerate suddenly it stops, drop dead. Not a big issue while riding but i wanted to get rid of it. First problem i think is solved now because spark plug looks pretty good. Second problem still remains. I made a video for you so it will be really clear. The RPM goes up and down without doing anything with the throttle, and when i throttle suddenly it comes back down and stops. What causes this?. For your info * Pilot screw is 1 1/2 turns * New float needle ( So checked for any leaks, so far no ) * Pilot jet down from 45 - > 40 * Main jet is still what it was, 102.5 ( looks pretty small for a SR4 but i don't know the standard size * Carb is BST34 Mikuni * Exhaust wasn't there when i bought the bike, some stupid custom made pipe loud as hell, so i couldn't find the stock original exhaust. I added a honda GB500 Clubman exhaust. * air cleaner was shit, so i made one using air cleaner material, something similar to stock air filter. everything else seems pretty normal to me. please take a look at the video. http://youtu.be/xeysGZr5Z38 Link to comment Share on other sites More sharing options...
dt502001 Posted January 16, 2013 Share Posted January 16, 2013 You could try turning the mix screw out 1/4 turn,also are the valves adjusted proprely? Link to comment Share on other sites More sharing options...
blackhat250 Posted January 16, 2013 Share Posted January 16, 2013 Is it a diaphram carb?,, if so check the rubber for pinhole, if its a screw top, make shure the rubber gasket is seated, And it sounds like diesel locomotive Link to comment Share on other sites More sharing options...
Moderator Cynic Posted January 16, 2013 Moderator Share Posted January 16, 2013 Is it a diaphram carb?,, if so check the rubber for pinhole, if its a screw top, make shure the rubber gasket is seated, And it sounds like diesel locomotive Thats cos its one of those f.f.four strokes. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted January 16, 2013 Moderator Share Posted January 16, 2013 my throttle usually makes my revs go up and down seriously, its running a bit lean, the modern fuel now has ethanol (alcohol)in it which loves attracting water. tweaking the mix screw out to enrichen the air/fuel mix like dt50 said, on the xs400 its now 3 turns out. You may have to up the slow jet 1 step. Link to comment Share on other sites More sharing options...
shambles1980 Posted January 16, 2013 Share Posted January 16, 2013 sounds like it keeps suckinng in more air than is needed. (or is getting less fule than it wants) honestly havent got a clue.. its all that up and down valvey open close springy stuff.. Link to comment Share on other sites More sharing options...
nofacemonster Posted January 17, 2013 Author Share Posted January 17, 2013 Thanks a lot for all your help friends. i think i got it fixed. Today in the morning i removed the carburetor. Change the slow jet back to 45 which is the standard according to the Japanese website i found. The previous owner of the bike has done some serious crappy adjustments to the carb in thought of increasing the fuel economy ( which never will work in my opinion ). He has changed the main jet from 162.5 to 102.5 which is really bad. So i don't have 162.5 and spare parts are not easily accessible in where i live. Even though i never had a problem with the main jet i changed it to what i had. 140 size, which is still much larger than the 102.5. The float settings were screwed up too. So i found these information in the Japanese website saying it should be 12.5mm ~ 13.5mm so i set it according to the website. Phew! now it runs way better than what it was. Idle seems 90% OK as far as i see. I ran about 5KM ride, slow and fast and checked the spark plug and color seems to be pretty ok. But i'll have to check it again after a at least 25 to 30km ride. So i am not 100% sure on that. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted January 17, 2013 Moderator Share Posted January 17, 2013 good to hear Link to comment Share on other sites More sharing options...
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