'77 mikuni carbs

brasco18 · Posted: April 28, 2008, 4:58 pm Post subject: '77 mikuni carbs

I have '77 mikuni carbs on my 650. I can't seem to get the mixture right. I have on the bike a pair of sportster mufflers. besides that the bike is stock. Here is what I need to know:

ain jet size
Pilot jet size
needle jet size
jet needle size
which notch on the needle to use
ixture screw - in or out to lean mixture
number of turns

I ask all this because I cannot seem to get it all right.

jayel · Posted: April 28, 2008, 8:30 pm Post subject: Re: '77 mikuni carbs

XS650D (US) Mikuni BS38 ID Mark 58400
ain jet size 122.5
Pilot jet size 25
needle jet size Z-8
jet needle size 4M1-3
which notch on the needle to use 3
ixture screw - in or out to lean mixture 1 1/4
number of turns
Float Height 25+- 2.5mm

the sportster mufflers should have minimal effect (adequate baffling) unless they have been gutted then you're running straight pipes hope this helps

Cooltouch · Posted: April 28, 2008, 8:31 pm Post subject: Re: '77 mikuni carbs

Before you start tearing into the carbs (the BS38s on the 77 are the best of the lot, from what I've been told), make sure you don't have any leaks around your carb boots. You can check by spraying WD-40 around the boots while the bike is running. If the idle changes, that's a good indication that they aren't sealing tight.

Also, can you be a bit more descriptive about what you mean when you say you can't get the mixture right?

Best,

ichael

brasco18 · Posted: April 29, 2008, 11:04 pm Post subject: Re: '77 mikuni carbs

My mileage is horrible, like 30mpg. It seems to always smell rich. I smell like gas after riding it. I did some tinkering with it the other day. I raised the needle clip a notch and got it good and hot then tweaked the air mixture screw to figure out which way does what (Out leans it). It seems to be running good. I don't notice any gas. I need to check the plugs and run a few tanks of gas to see where I am at.

xsjohn · Posted: April 30, 2008, 2:38 am Post subject: Re: '77 mikuni carbs

Out richens it.....if you havn't taken them apart and verified the jets inside it might be good to do that........the PO may have changed things in there.....had a 77 myself......cant quite remember where I set the mix screws but 2 turns out might be a good place to start.......check the floats....that can make it rich if set to high.......too much fuel will wash out the rings..........it should get 48 or more to the gallon if set right........verify that the needles are 4M1 in the third position (center notch)......and it might be a good idea to change out the needle jets if the needles look like there is any wear near the top of the slope......if they wear the mettering hole gets bigger and richer......

xsjohn

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

The mixture screws on all stock 650 CV carbs are true mixture screws that regulate fuel flow. They are not air screws. Turning them out adds fuel and richens your mixture, turning them in leans it.

Really bad mileage is many times a sign of worn needle jets. You run on the needles and needle jets for the majority of your riding so they will have a big effect on fuel consumption. Another cause could be way too big of a main jet. If the P.O. followed some of the recommendations on the internet and put 140 - 145 mains in, then that may be it. That's way too big a jump (7 to 9 sizes) for the '76-'77 carb set which only ran a 122.5 as stock.

brasco18 · Posted: April 30, 2008, 7:05 pm Post subject: Re: '77 mikuni carbs

I rebuilt the carbs about a year ago using a rebuild kit from Mikexs. It seems the sizes of jets included in that kit don't match specs. What gives?

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

That's the way many of these aftermarket rebuild kits are. Rather generic and "one size fits all", 'cept they don't Smile

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