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Old 01-12-2007, 09:07 AM   #1
honcho OP
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high idle issue

so i had left my 78 yamaha sr500 sitting for a few weeks (ok, a couple of months) and was in the middle of some other projects. The original issue with the yamaha was that it was running too rich. so i finally got back around to it and ordered a couple of smaller jets, along with a few other things i needed like a petcock rebuild kit, and a tach cable. Until the other night i had never taken a carburetor apart. this bike has an aftermarket mikuni carb... although i don't know exactly which one, i describd it to Mark at thumperstuff.com, and he was able to send me the correct jets. so i installed the new smaller jets, and the new tach cable, gave the bike an oil change ans slapped it all back together. the throttle cable was pulling smoothly. i rolled it out of the garage and it started on the third kick! but it immediately blew the end of the supertrapp out (it wasn't secured very well intially) and was loud as hell. i looked at the tach and it was running at 5000RPM. so i cut it off and couldn't do anything as we were headed out to dinner. well after dinner i was in no condition to work on it and this morning we left for the weekend, but i was hoping to gather some helpful info so i can try to fix it when i get home sunday night. any thoughts? thanks
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Old 01-12-2007, 10:48 AM   #2
Mista Vern
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Sounds like the slide is sticking or something is binding within the carb. I am surprised you got it started with that going on!

Pull the air cleaner and see if you can get your finger inside the carb to see where the slide is. Prolly hung up.
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Old 01-12-2007, 03:16 PM   #3
furiousfart
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can you back the idle screw down? also if you grab the throttle and give it gas does it return to 5k quick or does it take it's time to get there? You could be running really lean now.
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Old 01-12-2007, 05:42 PM   #4
Mista Vern
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He'd have to have the idle screw really whacked out if it's turning 5K, no?
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Old 01-13-2007, 10:20 AM   #5
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Slide in backwards.
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Old 01-13-2007, 11:31 AM   #6
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I would believe that the slide is in backwards... how do i know which way is correct? The slide is the large cylinder with the large spring inside?
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Old 01-13-2007, 11:37 AM   #7
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Quote:
Originally Posted by honcho
I would believe that the slide is in backwards... how do i know which way is correct? The slide is the large cylinder with the large spring inside?
Yup. You may be on to something!
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Old 01-13-2007, 11:41 AM   #8
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There is a cutout on the bottom of the slide, it faces down-stream(towards the eng.). There is usually a pin that locates the slide in one direction, but maybe it fell out or got pushed in.
You always want to make sure that there is some free-play at the throttle grip, but when the idle shoots up to 5k, there is something else going on.
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Old 01-13-2007, 03:44 PM   #9
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that makes sense... i thought i had lined it up correctly (little groove lined up with the pin) but i may have turned it backwards... we'll see on sunday... thanks for the help fellas
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Old 01-17-2007, 03:34 PM   #10
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so... the slide is and was in correctly... the throttle is smooth... it starts 1st or second kick, but then dies when i take the choke off or when it gets much below 3000 rpm. it's idling OK, but varies about 500rpm (between 4000 and 5000), and sounds like it's surging (?) a little bit. i played with the idle screw, which helped and let me get down to around 3, but if i try to shift it dies, and if i let the rpm get too low it dies... it blackened the shit out of my nice clean exhaust plates (supertrapp) real fast. maybe the new jets are too small and it's starved for fuel? still doesn;t explain the high idle. whats going on here?
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Old 01-21-2007, 04:05 PM   #11
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High idle, surging sounds like it is too lean. When it is running, if you give the throttle a quick snap does the engine speed come down fast or slow if slow it is definatly lean. And the blackening of the exhaust is from running lean.
did it run fine with the old jets before it sat? If so why not go back to those.
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Old 01-21-2007, 04:57 PM   #12
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I'm reaching here, but is the little plastic button that you can push to automatically set the idle to the right speed for starting purposes backed out?
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Old 01-22-2007, 03:45 PM   #13
honcho OP
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plastic button?

so i talked to a guy at my "local" shop (1/2 hr away) and he said it sounded like a classic case of a clogged primary jet? he explained to me that one jet is in use in the lower rpm range and as the rpm get higher (or when the choke is on) the main jet takes over. he also described the lean surging, which seems like the major culprit. so i took the carb apart again... cleaned every passage, and put it all back together. i also took a couple of plates out of the exhaust in hopes that it would be less lean. so it's all back together and time to test it out... starts first kick! seems to be running ok, got it down into the lower 3000, higher 2000 range, and it's feeling pretty good. i guess i didn't let it go on the choke long enough though, cause it died at about 2500 rpm, but it seems clear of the prior problem... now the new problem. i couldn't get it to start again. kicked it ove maybe 100 times before i got too tired. tried with the choke on, choke off, throttle partially open, all the way open... tried to roll start it, and in doing so came to realize that something is wrong with the tranny or the clutch. it feels like the transmition is dragging. in neutral the bike rolls around fine, but in gear with the clutch in, it's not so smooth. i guess this is the tranny dragging? i have zero experience with transmitions. so.... it's sitting on it's center stand in the barn while i turn my attention to the carbs on the /7 and research what's up with the dragging. blech.
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Old 01-22-2007, 04:29 PM   #14
Mista Vern
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If a bike has been sitting for a long time the clutch plates will drag. Not a problem!

I am not surprised you couldn't get the bike re-started. Lukewarm engines are a bear to start if you are not real aquainted with them.

Look on the upper left side of the carb above the choke lever, and you should see a small plastic button that you can push in to set the throttle opening for easier starting. That could be stuck and not letting your idle die down.

If you can't get this figured out, just sell the bike to me for $300.
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Old 01-23-2007, 05:32 AM   #15
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cool , i'll look at it this afternoon when i get home... thanks
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