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What causes RPM to go up and down when idling?


nofacemonster
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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.
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You could try turning the mix screw out 1/4 turn,also are the valves adjusted proprely?

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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 :babyha:

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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 :babyha:

Thats cos its one of those f.f.four strokes.

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my throttle usually makes my revs go up and down :eusa_whistle:

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.

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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..

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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.

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