Mikuni Carburetors (Update)

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tank4all
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Mikuni Carburetors (Update)

Postby tank4all » Tue Mar 15, 2016 11:36 am

So I've made multiple post ranging from ignition coil to spark plug wires. I've finally narrowed my problem down to one cause (I think). My issue now is my bike losing power when I open the throttle (idle is fine). I was getting arcing when I open the throttle so I figured it was my coil or spark plug wires were bad. However I tried leaving the choke on my Mikuni Carburetors so idle is still fine and when I open the throttle with the choke on no lost of power. I think what was happening was I was putting in too much fuel that it was flooding my plugs and not firing. This was causing it to arc on the coil tabs when I open the throttle (my plugs were wet).

I'm assuming since the throttle opens fine when the choke is on this means there is a problem with my carbs in terms of how the fuel is getting into the engine. What parts of the carb should I pay attention to and what could be the issue? I'm really new to this. Thanks in advance for the help.

Here is a picture of my carbs. I'm not sure what model this is.

UPDATE:

I'm made a few post regarding the problem I've been having and I've finally resolved the issues. I ended up just buying new Mikuni Carbs from Amazon and now the bike is running like it did before. Thanks all for your help!
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Last edited by tank4all on Mon Apr 18, 2016 9:59 am, edited 1 time in total.
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rivers
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Re: Mikuni Carburetors

Postby rivers » Tue Mar 15, 2016 12:04 pm

Don't know what you got going on but if the carbs or throttle opening is causing the cylinders to flood and not fire. That is NOT what's causing your spark jump/arcing at the coil. I think you're heading down the wrong track messing with carbs to cure an electrical(ign) issue. Get the ign up to spec first, then mess with the carbs last. You may well have two separate issues ign and carb but straighten out the ign problem"first".
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Re: Mikuni Carburetors

Postby INSUBORDINATOR » Tue Mar 15, 2016 12:07 pm

Hi, 650s Came with Mikuni carbs. They are very good. Don't know the previous circumstances of your bike or how it ran previously. My 1994 650 didn't run at all due to clogged carb jets. As it is an older bike your jets could benefit from a thorough cleaning. Here is how I do it: Make a note of what jet goes where, Soak all jets in carb cleaner for days to soften deposits, in a small covered jar - so cleaner won't evaporate. Then put all jets in a small SONIC/VIBRO JEWELRY CLEANER with a detergent degreaser. They should be like new & gleaming clean with 24 hours of sonic cleaning.
Jets can be damaged by trying to poke a bit, wire or pin through them.
My latest Fixer-upper Ural came with badly adjusted carbs, & the PO must have retarded the ignition in attempting to make it run. It was loosing power when throttle was applied also. Will be worth double checking ignition timing. Knowing a bikes forensics can tell you a lot, as it was probably running ok before "X" changed or happened.
Good Luck & Best Wishes, C.
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Re: Mikuni Carburetors

Postby tank4all » Tue Mar 15, 2016 12:23 pm

So I forgot to mention that my bike was running just fine until I tried to clean my carbs. This is actually what started this whole mess.
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Re: Mikuni Carburetors

Postby rivers » Tue Mar 15, 2016 12:57 pm

Ok, so the coil arcing problem is no longer in the game? When you pulled the carbs apart to clean them you didn't bugger the float setting did ya?
Running stock jets or were they previous owner improved? Haven't messed with a mik carb in awhile but it's a pretty simple carb and there are online manuals avail.
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Re: Mikuni Carburetors

Postby tank4all » Tue Mar 15, 2016 1:08 pm

Honestly I just unscrewed everything I could and used carb clearer. I tried to be very careful and thought I had put everything back where it was. I'm trying to read up on how-to's to see what I did wrong. Besides the float is there anything else I should pay attention to and check?
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Re: Mikuni Carburetors

Postby TerryG » Tue Mar 15, 2016 1:24 pm

Sounds a lot like a clogged main jet. Could also be installed wrong if you took it out to clean it. Get some of the online diagrams/info available for the round slide Mikunis, and carefully re-do the cleaning. I find that spraying contact cleaner into the jets works fine if they are just a little clogged. Then spray into all of the orifices that they came out of, plus any of the other orifices. The jets should seat very positively without undue effort, and screw in easily. Going from a failing memory, there are some washers and gaskets that go under some of the jets. Make sure you didn't misplace one. Parts should be available at any old time dirt bike shop as many are sized for a whole range of carb sizes.

And as Craig said, be sure to check the float height too. All you need is a ruler, preferably in mm.
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Re: Mikuni Carburetors

Postby SNAFU42 » Tue Mar 22, 2016 2:43 pm

Had a similar problem (needing choke) with my 47el HD turned out to be an Intake leak.Spray a little WD40 or either around your Head/Carb boots (while its running) if it idles up tighten up the clamps.Good luck Markus
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Re: Mikuni Carburetors

Postby addseo1115 » Mon Mar 28, 2016 12:17 am

Thanks for the info
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Re: Mikuni Carburetors

Postby eastbloc » Mon Mar 28, 2016 12:10 pm

The Mikuni carbs come with an enricher circuit. This is useful to you for two reasons:

1. Make sure the enricher circuit is in the "off" position (lever up, plunger down, as it is shown in your first pic). On an otherwise properly tuned bike, running with the enricher circuit in the "on" position will cause a rich condition and poor running.
2. You can use the enricher circuit to diagnose a lean condition caused by an intake leak. If the bike runs _better_ with the enricher circuit in the "on" position, you have an intake leak. You can diagnose the side with the leak since each carb has an individual enricher circuit.

Check on those and get back to us. It's very likely that you've developed an intake leak when you've disturbed a crusty old set of compliance fittings.
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Re: Mikuni Carburetors

Postby tank4all » Thu Mar 31, 2016 10:31 am

eastbloc wrote:The Mikuni carbs come with an enricher circuit. This is useful to you for two reasons:

1. Make sure the enricher circuit is in the "off" position (lever up, plunger down, as it is shown in your first pic). On an otherwise properly tuned bike, running with the enricher circuit in the "on" position will cause a rich condition and poor running.
2. You can use the enricher circuit to diagnose a lean condition caused by an intake leak. If the bike runs _better_ with the enricher circuit in the "on" position, you have an intake leak. You can diagnose the side with the leak since each carb has an individual enricher circuit.

Check on those and get back to us. It's very likely that you've developed an intake leak when you've disturbed a crusty old set of compliance fittings.


So this is a dumb question but are you saying that I might have a leak here (red box in photo)? I haven't had a chance to work on my bike but will this weekend.
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Re: Mikuni Carburetors

Postby Steve7959 » Thu Mar 31, 2016 11:13 am

tank4all wrote:So this is a dumb question but are you saying that I might have a leak here (red box in photo)? I haven't had a chance to work on my bike but will this weekend.


Without wanting to steal eastbloc's thunder, yes that it where he means.

They're the fitting between the carbs and the cylinder head. If the fitting is damaged or the carb isn't seating properly, air gets sucked in and weakens that air fuel mixture.

By operating the enricher on the carb, you artificially improve the mixture to compensate for the leak
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