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Carb rebuild now won't start

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Guest

Guest
Hi all, 1979 GS850G. I took the VM carbs apart and rebuilt them with the help of this site. Bench synced them. Fuel mixture screws are at one turn out except for the #2 carb that had its hold widened, that mixture screw is 3/4 out. Air mixture screw are set basically factory. All new o-rings including manifold. Rebuilt the air box. Set the timing manually. Clean plugs. Put the gas tank on, set the petcock to prime for a few minutes. All the cables seem to be ok. The bike won't catch. Turns over but eventually sputters and back fires. I rechecked the ignition cables, all in the right order and good spark. Rechecked the timing manually, all good. I'm going to reset the air mixture screws to factory setting but other than that I'm at a loss to what to do next. Any suggestions??
 
well I'm only guessing so but I didn't check (not exactly sure how to check)
 
Pull a drain plug out, or just loosen it a little bit, gas should spill out.
Did you do the whole carb dip thing?
 
I would be inclined to also check fuel is getting to the bowls? but backfiring? I reckon your timing is off or leads in wrong order mate, check it again would be what i would do, lack od fuel or wrong mixture from carbs isn't going to make it backfire.

Hope that sheds a little more light on the situation. :cool:

Skid.
 
I checked the ignition wires and the timing, everything is spot on. I rechecked my fuel mixture and air mixture screw. They were off from OEM, not sure how that happened but I reset them all to; fuel mixture 5/8 out; air mixture 1 1/4. Again with the exception of the #2 carb. I set the fuel mixture in a little more. I need to get my impact screw driver for the drain plugs. They were the only thing I didn't take off (couldn't get off really) when I did the rebuild, and yes I did do a carb dip, and spray.
 
i too have a '79 850 and had kind of a similar problem. Although mine would only run on full choke and would not back fire but would pop a lot and eventually die out on me after 3 mins or so. Then would be very difficult to start. Before the cleaning, although i was able to install new o-ring drain plugs, it ran much better pre cleaning. I did dipping just to clean it b/c i had no idea when the PO last did this. According to all the threads i read here the Gs 850 is know to run lean. When i counted the turns for both air and fuel screws before removal i had air screws out about 1 turn and fuel screws out 1 to 1 1/2 turns out. I know this sounds screwy but thats what i counted numerous times. I too set the screws to the same settings as you and it still ran bad. Plugs were either very white - lean or with factory specs, as you mentioned it ran rich - carbon.

What i did was set the screws back to what i had counted pre cleaning however i discovered that it ran 1000% better when i left the PRI on continously. I kinda found this out by accident. So, either my petcock was not functioning properly, which was the case, or by setting the screws to precleaning setting fixed the problem. I think it was definatey the petcock but after leaving the screws set as they were precleaning the plugs are the tan color they should be. Maybe i could not run the bike long enough to determine the proper color of the plugs but the plugs are neither white or all carboned up like they were before. I am in the process of getting it syncronized now.
 
just a thought, did you disconnect the coils from the harness at all? Perhaps they are hooked up backwards there??
 
Interesting point Taz about where your setting were pre rebuild. Mine were the same, very off from the factory settings according to my Haynes manual. I am guessing once I try and synch the carbs maybe I will have to adjust farther out. I took the coils and their attached plug wires off the frame but I never dismantled them any more than that. The bike started fine before disassemble. It didn't idle that great once it was warm (it would stall at idle) hence the rebuild. I haven't had a chance to try starting it again since I reset the air and fuel screws to factory settings. Tomorrow, I am going to keep my fingers crossed. I have to laugh though, I'm so new to the bike seen that once the battery went dead the other day I cleaned up and figured that was it until the battery charged.......................KICK START!! newbie...
 
Arrrrr.......tried staring it again, no go. Just popping and back firing. It sounds just like a miss firing thing, as if I have the timing way off, or the plug cables are mixed up but I've checked numerous time. I did check the float bowls and they have gas in them. Any suggestions??
 
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She runs (had the wires for the coils crossed, opps). She runs a little rough though. I'm waiting for my Carbtune to arrive (2 weeks). My # 2 carb had the fuel jet hole expanded so I set the needle farther in than OEM. How will I be able to tell if it is too far or not far enough?
 
azr said:
She runs (had the wires for the coils crossed, opps). She runs a little rough though. I'm waiting for my Carbtune to arrive (2 weeks). My # 2 carb had the fuel jet hole expanded so I set the needle farther in than OEM. How will I be able to tell if it is too far or not far enough?
Click to expand...

Set your mix screws at the recomended turns out. ( not sure for your model check the manual )

Use the carb tune to adjust the mix till you see it dark blue. Yellow is rich light blue is lean. ( don't strip the threads attaching the extension lead to the carbtune plug )

Then check the plugs and adjust screws again without the carb tune trusting the color of the plugs during plug chops, should be tan or grey but not black or white.

I bought a carb tune and this is the method that worked best and has the engine running great through out the rpm range.

Hope this helps
 
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Carb syncher. I'm going to set the fuel mixture and the air mixture screws by the "best run" method (I think that's what it's called) tomorrow and see how much better it'll run. Right now it's running a bit rough.
 
I tried the best run method with not much sucess. I pulled the carbs and re bench synched them (felt I got it bang on this time). Tried setting the air and fuel mixture screw by best run but couldn't get a decent idle. The carbs are making a "popping" noise. I'm sure it has to do with the mixture screws and the fact I haven't syched the carbs yet (my sychronizer is still on its way) but I though I would have been able to get it pretty close by hand and then be able to set the screws. Any thoughts about what is making the popping noise? When it does it the idle drops, enough sometimes that the bike stalls.
 
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