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2008+ KLR650 Wrenching & Mod Questions For repair, maintaining or modifying discussions related to the newly updated 2008 and beyond, Generation 2 KLR650 Motorcycle.

Thread: Got Knocking Pinging Popping? Do The .22 Mod Reply to Thread
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  Topic Review (Newest First)
06-14-2016 10:45 PM
Damocles If this link is clicked on, and allowed to run, a couple (or maybe more) video clips discuss some interesting CV carburetor phenomena:


One additional detail might have been included; the BYPASS idle passage; on some CV carbs, an idle mixture bypass passage, in addition to the passage controlled by the idle mixture screw. allows fuel to be drawn into the venturi.

I think possibly (do not know for a fact) this bypass idle mixture passage permits engine operation in cases where the idle mixture screw may be closed off.

Idle bypass circuit discussion:
The typical set up is this: The pilot circuit get its fuel from the float bowl through the pilot jet. The circuit also has a pilot air jet, but the purpose here is not to provide all the needed air (as in the starting circuit), but to provide air to premix with and partially aerate the fuel before the mixture enters bore and completes the atomization process with air traveling through the bore. Such little air passages are sometimes called "air bleed" circuits.

This circuit typically has two outlets. One is called the "pilot outlet" and is located on the engine side of the butterfly valve. This outlet supplies the fuel mixture to support idling and deceleration (that is, off-throttle running). It has an adjustment screw which controls the amount of fuel mixture entering the bore under off-throttle conditions. This screw is sometimes mistaken for an "air screw" but it is not. Turning in (right) reduces the amount of fuel mixture, and turning out (left) increases it. In for lean, out for rich.

The other outlet is call the "bypass outlet" and is located right at the point in the bore where the bottom of the butterfly comes to rest when closed. Typically, the butterfly is set a tad open to permit just a slight amount of air to pass by at the bottom to support idle and decel, and most of the fuel for these functions is supplied through the pilot outlet. But as the butterfly is opened, more air flows past it, and the venturi effect starts to work on the bypass Additional fuel is now drawn out of the bypass to support low speed running and cruising at small throttle openings. (Note that if the butterfly valve is adjusted to 'fully closed' the engine will probably not start or idle. It needs to be open a tad. As mentioned above, these outlets continue "giving" throughout the rev range, but their contribution to the overall mixture diminishes as the slide rises.)

Written by George Burroughs (koolaid_kid)
Proofread and modified by Brian Wringer (bwringer)
From:Adjusting the Idle Mixture on CV Carbs
06-14-2016 09:14 PM
sonnyny That was my fear but no damage I can see. I checked the valves 1K ago and they are all in spec, no noticeable noise out of the top or lower end, and not burning oil (1/4 qt. in between changes). I attribute that to running the 91 octane which eliminated pre-detonation which would have damaged the valves. Crazy how this thing came out of the factory like that and ran perfect for 8K miles.
06-14-2016 08:43 PM
Fox Fader I wonder what damage has occured after running her that lean for 8k miles?
06-14-2016 05:29 PM
Got Knocking Pinging Popping? Do The .22 Mod

I have an 09' with 18K miles and since about 8K miles I started to experience pinging during acceleration or pre-detonation of fuel and popping/light backfiring when the throttle is closed at a roll in gear. I cleaned the air filter, tightened the exhaust bolts, checked timing, changed plug, decarbed the engine with seafoam, and ran seafoam through the carb but nothing worked.

As time went by it became worse so I started to just use 91 octane which cured the pinging but not the popping. I did lots of looking on web and there are lots of you out there with the same issues. The solution is the .22 Mod. Did mine yesterday and all problems are gone and the throttle response is incredible. If you don't know what it is do a search or look at the vid on youtube.

Strange thing is that when I drilled out the pilot mixture screw cap and tried to turn the screw in it was already bottomed out and would not turn in at all. I have been riding it this way since it left the factory but no symptoms of a lean mixture until 8K miles. Anyway hope this helps.

BTW, the #4 washer (mentioned in the vid) at Ace Hardware is too large to fit tight on the needle and inside the plastic spring seat legs. I had some tiny washers and one fit just right. The washer should be about the size of the needle head.

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