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carb question

  • Thread starter Thread starter schmmt
  • Start date Start date
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schmmt

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I have a 1981 GS 750. It hasn't been driven for a few years. It was very hard to start and would not idle at all. Ran good above 3500 prms. plenty of acceleration. So I changed the oil, :confused: air filter, drained the tank, took the carbs off and cleaned them(rubber plugs and gaskets were in good shape), put it back together with new, clear blue, fuel line. Now it fires right up, but it has no power and it backfires when I down shift or after I rev it up a little. I noticed that the fuel in the fuel line is swirling around and bubbleing????? I really don't know much about this stuff, but I am pretty handy with a wrench. Any suggestions on what could possibly be wrong? They are Mikuni carbs if that help at all(maybe they all are) no fuel leaking from anywhere.
 
schmmt said:
I have a 1981 GS 750. It hasn't been driven for a few years. It was very hard to start and would not idle at all. Ran good above 3500 prms. plenty of acceleration. So I changed the oil, :confused: air filter, drained the tank, took the carbs off and cleaned them(rubber plugs and gaskets were in good shape), put it back together with new, clear blue, fuel line. Now it fires right up, but it has no power and it backfires when I down shift or after I rev it up a little. I noticed that the fuel in the fuel line is swirling around and bubbleing????? I really don't know much about this stuff, but I am pretty handy with a wrench. Any suggestions on what could possibly be wrong? They are Mikuni carbs if that help at all(maybe they all are) no fuel leaking from anywhere.
Click to expand...
The swirling isnt nothing to worry about but I would check the timing and your points and condenser.Get a clymers manual to learn howto do it properly.You need to sync your carbs too.I would probably start there.How did you set your jets? 1/14 turns out or more?Also check for air leaks around your intake manifolds and also your o-rings. is your airbox tightly sealed?An air leak could cause your problems.Start with the simple things first.The one's that dont cost any money! Let us know what you find out!
 
O.K. Here is where my inexperience will start to show! Which jet should be set 1 1/4 or more out? I removed what I think is the pilot jets(under the rubber plug in the float bowl) cleaned them and put them back in, all the way to the bottom of the threads. And the brass screw with the hole in it right next to the rubber plug(main jet???)... I unscrewed one but nothing came out when I flipped the carb over and tapped on it. I only took the bowl off and cleaned what I could with a few cans of cleaner. I didn't want to dig around in there not knowing what I was doing so I put it back in(all the way) and just sprayed cleaner threw the hole in the brass screw until it started coming out other parts of the carb, then repeated this a few times. Then there's the little orifice, I think it's the slow or idle or start jet... the one that draws from the bottom of the bowl, threw the side of the bowl. I cleaned those too. If you could point me in the direction of which jet is the one that should be adjusted, that would be great! The airbox is definitly sealed. I had problems with it before and made sure that wasn't the case again(it's deformed and hard to get sealed) I see I need a manual to learn more about this stuff, but I'm desperate to get this thing on the road quick and cheap!!! that's why I'm talking to you guys. As far as syncing the carbs, I had that done along with having them rebuilt 3 years ago, then some intermitent electrical problems took it out of commision less than 100 miles after. I just figured the electrical stuff out this spring. If they were synced and then hardly ran, do they still need it? I'll order the manual and make sure the other stuff you mentioned is up to par.
thanks a ton for the help! I didn't know if I would actually get any response... Thought maybe it was to stupid of a question to get an honest response!!
 
schmmt said:
O.K. Here is where my inexperience will start to show! Which jet should be set 1 1/4 or more out? I removed what I think is the pilot jets(under the rubber plug in the float bowl) cleaned them and put them back in, all the way to the bottom of the threads. And the brass screw with the hole in it right next to the rubber plug(main jet???)... I unscrewed one but nothing came out when I flipped the carb over and tapped on it. I only took the bowl off and cleaned what I could with a few cans of cleaner. I didn't want to dig around in there not knowing what I was doing so I put it back in(all the way) and just sprayed cleaner threw the hole in the brass screw until it started coming out other parts of the carb, then repeated this a few times. Then there's the little orifice, I think it's the slow or idle or start jet... the one that draws from the bottom of the bowl, threw the side of the bowl. I cleaned those too. If you could point me in the direction of which jet is the one that should be adjusted, that would be great! The airbox is definitly sealed. I had problems with it before and made sure that wasn't the case again(it's deformed and hard to get sealed) I see I need a manual to learn more about this stuff, but I'm desperate to get this thing on the road quick and cheap!!! that's why I'm talking to you guys. As far as syncing the carbs, I had that done along with having them rebuilt 3 years ago, then some intermitent electrical problems took it out of commision less than 100 miles after. I just figured the electrical stuff out this spring. If they were synced and then hardly ran, do they still need it? I'll order the manual and make sure the other stuff you mentioned is up to par.
thanks a ton for the help! I didn't know if I would actually get any response... Thought maybe it was to stupid of a question to get an honest response!!
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I dont know how many turns out on those without the book.My father-in-law has a 79 750 but they should be the same.Someone will come up with the answer on here soon.Were the jets all the way to the bottom of the threads to start with?It could just need to be set right.I will see what I can find out and get back to ya.....if nobody else does first. You wont have points either if its an 81.Sorry my fault.
 
Wow, I've learned alot since I have been searching around for info on this site! Unfortunately I don't know what the jets were set at. I didn't even know what they were when I pulled them out! But now that I am a little more educated in this area, I did try to back them all out 1.25 turns. It ran really good for a while!!!! Now I have a spark problem(no spark on #2,#3 cyl.) I am going to do a little research on it before I start a new thread asking for help. Chances are it has been covered somewhere on this site before. I'm still interested in the proper jet settings if you can find them though.
thanks
 
Any GS before 80' uses VM-carbs and yours uses CV(BS)-carbs. The "idle-screw" is really a fuel screw and it is located on the manifold side of your carbs. There is probably an aluminum cap covering the screw head and the cap must be drilled out to access it. If you do this be careful not to let the drill go past the cap or you will bugger up the screw. If you choose to access them, turning the screw out (counterCW) will actually richen the mixture.

However, you probably don't need to touch the fuel adjustment screws. If you are using the stock air-box and exhaust there is no need to access the fuel-idle-screws. I see that you have checked the seal on the air-box side. It simply cannot be stressed enough to insure that both sides of the carbs maintain a serious seal. These bikes are extremely sensitive to the fuel-air mixture and the behavior you explain (loss of power and popping/backfiring)would indicate, to me, an air leak. Make sure all boots, both air box and manifold side, are lock-tight. If indeed it is too lean you may be able to change the float level to richen the mixture. If you set the floats to ride higher in the bowls you'll richen the mixture. Wrong float level may be your only problem.
 
schmmt said:
Wow, I've learned alot since I have been searching around for info on this site! Unfortunately I don't know what the jets were set at. I didn't even know what they were when I pulled them out! But now that I am a little more educated in this area, I did try to back them all out 1.25 turns. It ran really good for a while!!!! Now I have a spark problem(no spark on #2,#3 cyl.) I am going to do a little research on it before I start a new thread asking for help. Chances are it has been covered somewhere on this site before. I'm still interested in the proper jet settings if you can find them though.
thanks
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On those cv carbs you should run them down all the way.I have not check mine though.It runs great.Try a carb sync that will tell you a lot.Chek you coil connection on 2and3 if your not firing.Clean the connections and check the wires and boots.Could be something simple(hopefully)!
 
81 750 Should have 34mm BS type carbs. The jet under the rubber plug in the float bowl should be hand tight + a little, very delicate. The air mix screw is on the top/intake side of the carb under the alum plug . If they have not been removed you need to remove the aluminum plug they should be set anywhere from 1 1/2 to 3 turns out for stock bike
 
spark problem fixed. a broken wire on the front side of the coil. I'll see if it runs good today. it was raining out yesterday so I didn't get a chance to test it out. I did turn the jets under the rubber plugs out 1.25 turns!?!? It seems to be running good. Is this something I can get away with? or is it necesarry to pull the carbs apart again and screw them back in? Maybe I can do this by only pulling the bowls off, not all of the carbs...for the 5th time. Seems I learn everything the hard way! oh well. as long as I get to ride this summer. If the plugs read o.k. can I leave everything the way it is?
 
The pilot jets are not made to allow any direct adjustment. As mentioned above, the pilots (the jets under the rubber plugs) must be tightened down. The fuel/idle adjustment screws are the ones that folks were referring to when they said to turn out 1.75 turns.

However, as I said before, if you are "stock" you don't need to touch the fuel/idle adjustment screws. Unless, when someone cleaned/manipulated the carbs in the past , they changed the setting. If the setting has been changed you should be able to visualize the screws from the top on the manifold side of the carb-rack.
 
O.K. I'll turn the pilots back in. The only fuel/idle screw that has been messed with is the #1 carb. All the others still have the aluminum cap on them. I found the slide(or whatever you call the cylinder that slides up and down) on #1 wasn't returning all the way down. I took it apart and cleaned it up. It works now. Someone probably compensated for this by adjusting this fuel screw. Does that make sense? P.S. this bike is stock w/13,000 miles on it.
 
You may have just successfully diagnosed the initial problem. Good Luck
 
schmmt said:
O.K. I'll turn the pilots back in. The only fuel/idle screw that has been messed with is the #1 carb. All the others still have the aluminum cap on them. I found the slide(or whatever you call the cylinder that slides up and down) on #1 wasn't returning all the way down. I took it apart and cleaned it up. It works now. Someone probably compensated for this by adjusting this fuel screw. Does that make sense? P.S. this bike is stock w/13,000 miles on it.
Click to expand...
Sounds like you found the problem,now go ridin!!!!
 
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