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  #1  
Old 06-17-2011, 09:31 PM
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Cool opt, anyone on Mikuni VM carb throttle adjustment & richness - Big Boss

OPT,

I am able to start the big boss by: choke on, pull - almost starts. Hold thumb throttle open, choke off, pull. By second pull it fires and runs. After running and warming up, won't idle high enough.... Maybe adjust cable on top of carb, or at thumb area?

Also, seems low end response a bit sluggish, thinking too rich. Looking at aLCymer manual but all I see so far refers to jets and the idle mixture. Tried leaning the idle a tad, but it just wanted to quit. Interesting that it would idle along until it was good and hot... then idle would fall too low.

Also, fan never came on. Guess I need to check into that also.

You've been quite generous with advice.... so hoping I'm not wearing out my welcome. - Doug
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  #2  
Old 06-17-2011, 11:02 PM
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I'm no OPT but I will say this, it's been my experience that if your idle seems high when it's cold and as the motor warms up it slows to the point that it dies when it's warmed up then you would indeed be too rich or either your jetting or your mixture screw is severely too far out.
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  #3  
Old 06-17-2011, 11:46 PM
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Check what Jmuddin 284 said,plus on the old 89 adjust the throttle to where there is just a very little slack in the thumb lever,plus you can take some slack up at the carb first,then the final slack at the thumb throttle. As far as the fan coming on,they hardly ever do since the sensor is located on the rear upper engine bracket,usually they break any way from vibration and almost never get hot enough to kick the fan on since its not on the engine! PLUS did you ever check compression?? Could be loss of compression when it gets warm and won't idle correctly(piston rings not sealing against cylinder when warmed up),plus could be leaking crank seals,especially the one behind the drive clutch.Air/fuel screw out 2 turns from light seated position. OPT
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  #4  
Old 06-18-2011, 06:54 PM
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Angry Ohhhh boy.... Time for a chill pill

Gents,

Thanks a lot for your advice. Got it started this am with the technique of holding the throttle open. Once warmed up, I did my best to tune the carb starting with the idle/stop screw. Set the rpm's a little high and then tweaked the mixture... prob right at 1.5 to 2 turns. No idea about how a cold start will be tomorrow. While bleeding brakes and making a list for screws at the hardware store, I noticed that the overflow hose (the local mechanic put on the carb after supposedly re-building it) was a bit wet looking. I took it off thinking, "No way... that's exactly why I brought it in for a re-build. I had it apart except for taking out jets or needle4s. a little build up in the bowl, but got it clean... and it still dripped.

SO, I looked at the overflow and sure enough.... dripping. in fact, if I gave the bike the tiniest shake, it damn near poured out, just like before I took it in to be re-built.

What do you guys think the problem is.... and would you even trust the same guy again? Feel like asking for my $ 90 bucks back and ordering a $ 17 kit off ebay and trying it. Is there a plausible reason it would still drip if in fact it was truly re-built? Hate to assume I got shafted, but this is a bit hard to digest. Coming from South Florida originally, I was accustomed to looking hard for someone to trust. ANd OPT, I have not checked the compression... I don't own a gauge. Might look into buying one though since it is obviously a good tool to have. How much is one anyway? Local auto parts store for Chinese one, or ...?

Thanks..... hoping to have a running machine soon. Brakes are working GREAT!
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  #5  
Old 06-18-2011, 06:55 PM
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Default FOrgot to mention...

Despite th eover-flow issue, I have it running great.... but that might be a contradiction fo terms. Well, the warm idle is perfect and throttle response seems very good. Of course I suppose I'll soon be starting all over again!
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  #6  
Old 06-18-2011, 08:18 PM
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If the guy is close,JUST take the carb off(unscrew the cap and leave the slide out of the way) and EITHER take it back to him or you can pull the float bowl and check that the brass float twin arms are parallel with the carb body when held upside down,OR just a little up from parallel which will shut off the needle valve a little quicker and may not run out the overflow tube on the carb bottom(YOU can slightly bend the "tang" that contacts the float needle DOWN which may stop the over flowing) ALSO,you should be able to see if it DOES have a new needle valve assembly(will be BRIGHT brass versus the dull gray on the rest of the carb!! OR just flat take it back and say it leaks IF you halfway trust him to work on it again! 1 1/2 - 2 turns is normal setting on air/fuel screw. Compression gauges are cheap now if you buy at Harbor Freight,Northern Tool,etc. Will be Chinese,BUT what isn't now days?? Good enough for home use,but if it starts easy and idles ok now,you may hold off on the gauge set. BEST tools are on the tool trucks,BUT costly!! I have at least 15-18 thousand dollars in the tools I brought home(not counting my Matco box),BUT couldn't sell em for a 1/3 of what I paid for em if I tried,so I'll just pass em down to my son! OPT
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  #7  
Old 06-18-2011, 09:53 PM
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Default Thanks, AGAIN.

Yeah, I think I may as well pop it back off and take a look at the brass on the assy. as you mentioned... very good idea. The trust factor I am just not sure about. I have a long time best bud that is a top flight mechanic at a dealership... does all the vette work and the dfficult stuff.... He is always tidy and thorough. ANyone can have a bad day, but it seems this job weasn't properly check before returning it to the customer. Was surely leaking all along, just couldn't see it with the new overflow hose on the nipple.

I was trying to avoid any wasted time on a rebuild by having a pro do it... but perhaps I can swing it. Not sure if I need any specialty tools for pulling jets, etc., but I suppose I could wing it. Need to put a new throttle cable in when I install the oil-bypass kit. Hoping that project is straight forward. Guess I'll be into more cable adjustment. Have to admit I enjoy the satisfaction when I learn something new and actually accomplish it.

Happy Father's Day to you, OPT. I'll do another post to everyone else out there.
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  #8  
Old 06-19-2011, 04:14 PM
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Exclamation OMG!!! Unbelievable...!

I should post a photo of this mess... OPT that was a good bit of advice. I took it back off and opened it up. NO new bowl gasket, No new inlet valve assy, nothing has been replaced that I can tell.... all old and tarnished finishes. Think I'll go see the owner tomorrow and ask for my $ 90 bucks back... Wouldn't trust these weasels as far as I can throw them at this point. You can have a bad day, but to hand back a carburator under the label of being "re-built" and nothing has been replaced? If I was younger, I'd probably go get myself in a lot of trouble.

Hey, which model carb was on this machine? I really can't see any identifying marks... Lots of new Mikuni carbs on ebay, but none that are identical. I measured the interior that mates to the intake port on the engine, but I swear the measurement I get is 31...?? Is it a 32 maybe? I doubt anyone changed it from the stock one and the parts line up with the schematic in the CLymer's manual. Wonder if a current model of the same spec would fit my mahcine. One for Hinda has a gasket built in and looks like it would bolt directly rather than utilize a rubber gasket/boot with hose clamp.

Thanks as usual to you guys... Hope you're enjoying your day. I am, in spite of what I discovered.
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  #9  
Old 06-19-2011, 05:23 PM
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this is an old 32 mm Mikuni carb that has been around forever and used on some old Yamaha,Suzuki and Kawasaki engines for years. Honda has always used Keihin carbs. Carb off a newer 250 trail boss or Trail blazer will fit I believe. Not much has changed on these over the years. YES,sometimes can't trust any one anymore! These type of guys hurt all of the good mechanics out there! BRAND NEW carb like yous is 98 bucks at Dennis Kirk.com,just have to use your main and pilot jet from old carb if different OR new jets are cheap!! But just a new cheap after market carb kit for yours is good enough! OPT
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Whats right is whats left after you do everything else wrong (Robin Williams) RIP
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  #10  
Old 06-19-2011, 07:25 PM
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Question OK

I'll remember that... just in case I have any unrecoverable errors during re-build... but it seems pretty straight forward. Sounds like the adjustments are mostly the e-cliip and air mixture and idle... outside of throttle linkage and re-jetting....?

Hey, speaking of jets, I just read that I might have to re-jet if I go to premix... (32:1 I think was the magic number for my machine). DO you recommend re-jetting to bigger jets for the premix?
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Old 06-19-2011, 07:25 PM
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32, carb, chokewont, cold, high, idle, installed, mikuni, run, starting, starts, throttle, tuning, vm, wont



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