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Old 04-10-2010, 01:17 PM   #1
BarryT OP
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Airhead Mikuni question

My R75/7 has a tendency to want to die when braking up to a stop sign or braking to a stop in general. If you rev the engine and return the throttle to zero when it is idling in the garage is settles back down to idle, but when you are riding and have to stop (pull clutch in reduce throttle and brake) idle idles down to about zero and stalls. I was wondering what this could be.

My 1st guess is

Float Level

Second guess is Idle mixture


Besides that issue it idle/runs smooth.

-Barry
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Old 04-10-2010, 09:01 PM   #2
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Trend or event?
Just started happening or been getting progressively worse?

If it is an event, think about what changed and when.
Most likely it is a trend. Assuming all other factors are set up right, timing, valves, etc. Older carbs may have worn float needles, replace the needles before you start bending stuff. If needles or floats are wrong it will never be right.
Carbs can be mis-adjusted by a high speed setting masking an incorrect mixture setting. It is worth getting the mixture to smoothest/ fastest then bringing the speed down to the correct RPM then finely tuning the mixture again.
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Old 04-10-2010, 09:50 PM   #3
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Your thierd and fourth guesses might be intake leaks and a binding throttle cable. Those are both things that have caused similar issues with mine.
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Old 04-10-2010, 10:23 PM   #4
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I dont think its intake leaks or binding cables. Its only when I do a hard stop (gas sloshing in the bowls). If I do a light stop or its in neutral and not moving it returns fine. Couls this still be worn needle or would it being doing it all the time with worn needles. The idles been like this since I got the bike. I have put on new coils, electronic ignition new wires and plugs cleaned carbs new timing change and seals, removed the carbond from heads and piston tops and it still does it...


_Barry
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Old 04-11-2010, 01:22 AM   #5
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I assume that revving it the garage means it's not at its full operating temp. Thus, the mixture screw has to be adjusted when the engine is at operating temp (at least 10 miles of riding). The mixture tends to lean out when already hot so my guess if your have a rich mixture, reason why it can't hold its idle speed when already at operating temp.
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Old 04-11-2010, 06:56 AM   #6
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I will give it a try, but it does this warmed up or cold and ONLY when making a hard stop (the clutch is of course pulled in). I was just thinking that all the gas in the floats, if to low is sloshing forward and the engine is starving for gas, but I will give the mixture a try.

-Barry
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Old 04-12-2010, 01:50 AM   #7
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It would do that at operating temp if you're revving high then suddenly release the throttle with clutch pulled in. The idle just isn't stable enough to hold. Either too rich or lean (my guess is the former). Check the color of your plugs for a good indication.
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Old 04-12-2010, 07:10 AM   #8
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Disclaimer: I have not worked on Mikunis. BUT, most carburetors are sensitive to proper float levels, since the jets are buried in the fuel to a certain level, and depend on air pressure/vacuum to pull the fuel up into the venturi.... most main jets are indeed in the middle of the bowl for a reason: fuel sloshes forward on braking, and back on acceleration. You don't want major changes in fuel levels at either extreme, hence the "middle of the bowl"...!!

I would check your floats, and per the advice above, do it after you have done a plug chop. Run the bike like you do, pull the clutch and cut the ignition (off) without letting it idle, and pull over to have a look at your plugs. They tell a good story: light khaki tan is perfect, white is too lean, dark tan/brown is too rich. Try adjusting your float levels to correct what you are seeing.

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Old 04-19-2010, 10:02 PM   #9
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Mikuni's once set properly seldom change(float level, jet wear etc). If carbs were running good and then started exhibiting the problem and the carbs are not gummed up then, 1) adjust the valves, this is the first thing to do with an airhead when you have idle problems. 2) Check timing, your description makes me wonder if your timing is not correct at idle. 3) adjust cable slack, set idle balance and air screw. An out of sync condition would make the condition you describe.
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Old 05-07-2010, 08:40 PM   #10
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I will give that a try tomorrow before the snow starts again. The valves are perfect and have been quadruple checked and it isnt the timing. It has a brand new chain and it did this with the electronic ignition and the points. At idle my plugs are black as night. I have taken the airscrews out pretty far and it is still showing rich. I do have the correct jets so that isnt it. Maybe the float level is to high? I did make sure that my slides were resting on the idle needles at idle and my idle is about 950 rpm when warmed up. Im getting really frustrated. This has been a problem since I bought the bike.

Thanks
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Old 05-07-2010, 09:12 PM   #11
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Are you sure there air screws? Most mikuni pilot screws control fuel.
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Old 05-08-2010, 08:53 AM   #12
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If you are adjusting the mixture screws while the engine is running and they do NOT change the idle speed/ smoothness with a half turn in either direction then you have the wrong idle jets for your application.
Fastest/ bst idle should have your mixture screws between 1 and 3 turns from seated. if it is rich throughout then you need to try the next smaller idle jets.
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Old 05-08-2010, 08:55 AM   #13
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The instruction manual says air screw.
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Old 05-08-2010, 12:41 PM   #14
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Well i leaned out the air screws quite a bit and the plugs went from ridiculously black to a clean black so I am thinking that I have a super rich condition at idle and it may essentially be flooding when I go from half throttle to idle. I did increase the float level because I thought it was starving for fuel. So I am going to bust the carbs back apart and put the float level to spec and adjust the idle screws from there.
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Old 05-09-2010, 06:48 PM   #15
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Remember you are dealing with air screws so screwing them is richer and out is leaner. Check your pilot jet sizes - 30's should be about right for your bike.
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