Mikuni Carb issues - Motorcycle-Journal Forums

jmensch · 12-13-2006, 05:25 PM

Hey all I was given a 1980 Yamaha XS650G a couple of years ago. This is my first bike so I am learning, and finally decided to try to fix it up. It has been garage kept and there is no rust in the tank, so the fuel comes out clean. Right now I am at my wits end with it. After alot of research I found out what it needed and got it all put together. I totally cleaned out the BS34 Mikuni carbs took out all the jets and cleaned them throughly. I was able to get it running for a short while before cleaning all the jets but gas was coming out of the air box and now I can't get it running.

I am have spark, compression, and fuel is getting into the float bowls I am just not getting fire. The engine turns just fine. After cleaning the jets I adjusted the float level all the way up and I would get a little fire at first but then nothing. I even tried the kick starter and the first kick a little fire, then not after that. I then adjusted the float level down some and now it still won't start but gas is coming out of the air boxes. I am getting tired of taking the carbs on an off. Anyone have any ideas?

Thanks and God bless

FLYONWALL · 12-13-2006, 08:33 PM

Have you tried to use any starter spray? With the air box off
can you spray a touch down each carb throat?

DrBob · 12-14-2006, 12:49 AM

With the air cleaner off, get a small amount of fresh gas in a cup (fresh as in right from the gas station, and NOT out of the same tank that is on the bike). Using an eye dropper or straw with your finger over the end, put about what would fit in an eye dropper down each carb. Hit the starter. The bike should start, run for a second, then die. If it does not, the problem is not with the gas or the carbs, but more likely a stuck valve or a valve timing issue.
If it does start, then die, try it again using gas from the bike's gas tank. If the bike started with the fresh gas, but not with the gas from the tank, the problem is with the gas in the tank. Drain it, wash the tank out, dry it good with a hairdryer, and refill with fresh gas.

If it starts then dies with both sources of fuel, the problem is with the carbs, such as a stuck float, misadjusted choke, or bad needle/ jet.

snakeyes3602001 · 12-15-2006, 08:39 PM

i do agree mostly with dr bob, while i am new to being a mechanic, i have learned a lot about carbs because it seems to be the thing my boss the owner of the shop always gives me the carb jobs. i would check them chokes and make sure the little brass tube coming down in the float bowl is clean, and giving fuel to the choke. also, i do disagree about if it does not start with good fuel, he is right maybe, but the cheaper quicker thing to try is make sure you have GOOD spark. change the plugs, they are cheap, and sometimes you cant even tell they are no longer any good. even if they spark some, sometimes it is misleading and it is the whole problem. this happened to me once, we tried everything, then i just changed the plugs, started right up. and the old plugs were clean, they jsut were not doing as good as new ones.

jmensch · 12-18-2006, 03:32 PM

Well I don't know what I did exactly but I got it running today. I took off the carb and adjusted the floats to the right level. It runs now but it is real ruff. It will backfire as it comes down and if I don't keep the throttle on it will die after a few minutes. So that is my next step, getting it to run right. Thanks for you suggestions.

AZRyder · 12-18-2006, 04:07 PM

Quote:

Originally Posted by jmensch

Well I don't know what I did exactly but I got it running today. I took off the carb and adjusted the floats to the right level. It runs now but it is real ruff. It will backfire as it comes down and if I don't keep the throttle on it will die after a few minutes. So that is my next step, getting it to run right. Thanks for you suggestions.

That sounds like a timing/carb sync issue

83dr250 · 12-24-2006, 11:39 AM

your carb lines are not in the right place the carb is not venting I had the same problem

jmensch · 12-24-2006, 06:56 PM

How so? I have one line going from a nozzle on the back of the fuel tap to the carb T for the fuel line, and then one going from a nozzle on the side of the fuel tap to the carb manifold, which is the vacuum tube. Is there something I am missing?

83dr250 · 12-25-2006, 04:44 PM

don't remove the fuel line remove the other one and try starting it up and see what happens? Do you have a manual? I was kind of in the dark until I got my manual for my specific bike. Get manual and all problems will be solved

ChuckD · 01-11-2007, 09:01 AM

Your vacuum petcock is bad, thats why it's leaking through the air box. Go to MikesXS.com and order one of his after market replacement petcocks. They're a simple on-off-reserve petcock that bolts right on to the tank. Those vacuum petcocks aren't the best thing out there, but can be rebuilt! I went for the replacement myself. Check the diaphrams on your CV carbs as well, 26+ years will kill the rubber and make hard starting issues, a pin hole in them and they're a goner. I replaced mine with VM34 round slides.
As far as not starting, your coil could be bad, notoriously weak and cheap. Mikes has a much better replacement for that . If the battery isn't fully charged you may have problems starting it as well.
80%(??%) of percieved carb problems are ignition related
Hope that helps

jmensch · 01-23-2007, 06:25 PM

Well I replaced the vacuum petcock and I have inspected the diaphrams over and over and over again and they look fine, no leaks. As I said, before I was able to get it started but I just replaced the vacuum petcock with a non-vacuum type and now it won't start. It will turn over just fine but no fire. I am stumped!! I have spark and compression as well.

12-13-2006, 05:25 PM	#1 (permalink)
jmensch Newbie Joined: Dec 2006 Posts: 4	Mikuni Carb issues Hey all I was given a 1980 Yamaha XS650G a couple of years ago. This is my first bike so I am learning, and finally decided to try to fix it up. It has been garage kept and there is no rust in the tank, so the fuel comes out clean. Right now I am at my wits end with it. After alot of research I found out what it needed and got it all put together. I totally cleaned out the BS34 Mikuni carbs took out all the jets and cleaned them throughly. I was able to get it running for a short while before cleaning all the jets but gas was coming out of the air box and now I can't get it running. I am have spark, compression, and fuel is getting into the float bowls I am just not getting fire. The engine turns just fine. After cleaning the jets I adjusted the float level all the way up and I would get a little fire at first but then nothing. I even tried the kick starter and the first kick a little fire, then not after that. I then adjusted the float level down some and now it still won't start but gas is coming out of the air boxes. I am getting tired of taking the carbs on an off. Anyone have any ideas? Thanks and God bless

12-14-2006, 12:49 AM	#3 (permalink)
DrBob In The Zone Joined: May 2004 Bike: several Location: Decatur, GA Posts: 2,763	With the air cleaner off, get a small amount of fresh gas in a cup (fresh as in right from the gas station, and NOT out of the same tank that is on the bike). Using an eye dropper or straw with your finger over the end, put about what would fit in an eye dropper down each carb. Hit the starter. The bike should start, run for a second, then die. If it does not, the problem is not with the gas or the carbs, but more likely a stuck valve or a valve timing issue. If it does start, then die, try it again using gas from the bike's gas tank. If the bike started with the fresh gas, but not with the gas from the tank, the problem is with the gas in the tank. Drain it, wash the tank out, dry it good with a hairdryer, and refill with fresh gas. If it starts then dies with both sources of fuel, the problem is with the carbs, such as a stuck float, misadjusted choke, or bad needle/ jet. __________________ Dream like you'll live forever, live like you'll die tomorrow. You MUST obey the pug dog!

12-18-2006, 03:32 PM	#5 (permalink)
jmensch Newbie Joined: Dec 2006 Posts: 4	Thanks Well I don't know what I did exactly but I got it running today. I took off the carb and adjusted the floats to the right level. It runs now but it is real ruff. It will backfire as it comes down and if I don't keep the throttle on it will die after a few minutes. So that is my next step, getting it to run right. Thanks for you suggestions.

12-24-2006, 06:56 PM	#8 (permalink)
jmensch Newbie Joined: Dec 2006 Posts: 4	??? How so? I have one line going from a nozzle on the back of the fuel tap to the carb T for the fuel line, and then one going from a nozzle on the side of the fuel tap to the carb manifold, which is the vacuum tube. Is there something I am missing?

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12-13-2006, 08:33 PM	#2 (permalink)
FLYONWALL Seat Tester Joined: Nov 2006 Bike: 89 Honda VFR400r Location: DAPHNE, AL Posts: 73	Have you tried to use any starter spray? With the air box off can you spray a touch down each carb throat?

12-15-2006, 08:39 PM	#4 (permalink)
snakeyes3602001 Newbie Joined: Oct 2006 Posts: 15	i do agree mostly with dr bob, while i am new to being a mechanic, i have learned a lot about carbs because it seems to be the thing my boss the owner of the shop always gives me the carb jobs. i would check them chokes and make sure the little brass tube coming down in the float bowl is clean, and giving fuel to the choke. also, i do disagree about if it does not start with good fuel, he is right maybe, but the cheaper quicker thing to try is make sure you have GOOD spark. change the plugs, they are cheap, and sometimes you cant even tell they are no longer any good. even if they spark some, sometimes it is misleading and it is the whole problem. this happened to me once, we tried everything, then i just changed the plugs, started right up. and the old plugs were clean, they jsut were not doing as good as new ones.

12-24-2006, 11:39 AM	#7 (permalink)
83dr250 Newbie Joined: Dec 2006 Bike: 1983 dr 250 Location: dallas Posts: 12	your carb lines are not in the right place the carb is not venting I had the same problem

12-25-2006, 04:44 PM	#9 (permalink)
83dr250 Newbie Joined: Dec 2006 Bike: 1983 dr 250 Location: dallas Posts: 12	don't remove the fuel line remove the other one and try starting it up and see what happens? Do you have a manual? I was kind of in the dark until I got my manual for my specific bike. Get manual and all problems will be solved

01-11-2007, 09:01 AM	#10 (permalink)
ChuckD Newbie Joined: Jan 2007 Posts: 2	Your vacuum petcock is bad, thats why it's leaking through the air box. Go to MikesXS.com and order one of his after market replacement petcocks. They're a simple on-off-reserve petcock that bolts right on to the tank. Those vacuum petcocks aren't the best thing out there, but can be rebuilt! I went for the replacement myself. Check the diaphrams on your CV carbs as well, 26+ years will kill the rubber and make hard starting issues, a pin hole in them and they're a goner. I replaced mine with VM34 round slides. As far as not starting, your coil could be bad, notoriously weak and cheap. Mikes has a much better replacement for that . If the battery isn't fully charged you may have problems starting it as well. 80%(??%) of percieved carb problems are ignition related Hope that helps

01-23-2007, 06:25 PM	#11 (permalink)
jmensch Newbie Joined: Dec 2006 Posts: 4	Well I replaced the vacuum petcock and I have inspected the diaphrams over and over and over again and they look fine, no leaks. As I said, before I was able to get it started but I just replaced the vacuum petcock with a non-vacuum type and now it won't start. It will turn over just fine but no fire. I am stumped!! I have spark and compression as well.

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