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stalling when warm, black smoke on startup

  • Thread starter Thread starter Guest
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I'm waiting for 2 new mixture screws from Z1 and then I'll let you know. I can't really afford the carb synchronizer so I'm trying the old make your own. I know it will work fine for a 2 carb system..but I have 4...so I'm hoping it will do for now.
 
azr said:
I'm waiting for 2 new mixture screws from Z1 and then I'll let you know. I can't really afford the carb synchronizer so I'm trying the old make your own. I know it will work fine for a 2 carb system..but I have 4...so I'm hoping it will do for now.
Click to expand...
Save yourself some money and a LOT of aggravation. Read this thread to see what I mean:
Trying to synch GS650 carbs, too much vacuum?


Lodogg2221 said:
Mike,
BTW, isnt it the #3? Left to right? I thought the two on the left, #1 and #2 looked good when we pulled them, and that #3 was fouled and wet, and #4 was pretty dark, but firing...
Click to expand...
For the VM carbs on the '79 and older bikes, #3 is the vacuum source.
For the BS carbs (also known as CV carbs) on the '80 and newer bikes, #2 is the vacuum source.
AZR (the original poster) has a '79.
Mike81gs850 (hijacker) has an '81.

Yes, the cylinders are numbered left-to-right. Clutch hand to throttle hand.

.
 
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I took everyones advice and ordered my Carbtune, should be here in 2 weeks at most. I got the bike together but it won't start. I rechecked the timing, the plug wires, all good. I reset the air and fuel mixtures to factory settings and I'll try again tomorrow. It turns over but won't catch, just pops and then eventually backfires with black smoke...arrrrr. Not sure what I've done wrong.
 
I tried staring it today, no luck. Still just pops and backfires, and I am using the choke. It's as if I have the timing way off, or the plug wires are mixed but I've checked all of them and they are all correct according to my manual. The plug wires actually have cylinder makers on them for a no brainer, and I've reset the timing twice by the book. this isn't making any sense. And I did check to make sure there is fuel in the float bowl and there is.
 
double check that you do get a spark on all 4

the airbox+filter has to be on
 
Thanks PS, I checked my wiring and I had crossed the Coil wires (White...Black...what's the difference...BIG DIFFERENCE!). So she lives once again. She running pretty rough though. I don't think I did the bench sync right. My Carbtune is 2 wks away though. My #2 carb had the fuel mixture hole expanded so I set the needle in farther that OEM. Any ideas on how I will be able to tell if it is too far or not far enough?
 
once you have the carbs ballanced you can dial that screw in completely and then work your way out until you get the highest idle / smooth sound

or you can use "colortune"

or replace that one carb body

get the valve clearances sorted BEFORE syncing the carb
 
I wasn't going to set the valve clearances simple due to the fact that it was supposed to be done last year by a shop the PO had it sent to. And I'm running out of money for valve shims. I am just hoping this will do the trick for now and I can get a bit of ridding time on it. I was going to try setting the air mixture and fuel mixture screws by the best idle method tomorrow. Is this a bad thing to do before I have my carbs synched?
 
well, anything's better than nothing when you wanna ride...:)

but ideally you would have it in this order:

1. adjust valves
2. sync carbs
3. adjust mixture
4. check sync
 
More carb problems

More carb problems

ok....what the heck? I went to adjust the fuel and air mixture screws and then all of a sudden this weird knock/pop noise appears. I can feel it when I'm holding onto the carb or air box. There doesn't seem to be any rhythm to it and I can improve it or worsen it by adjusting the fuel mixture screw. Is this the cylinder in the carb bottoming out?? I've pulled the carbs off the bike (again.....). I had a couple of fuel leaks coming from the float bowl cover screws and it was odd but when I went out this morning there was fuel on several of the fuel mixture screws. This was ongoing throughout my adjustments, not much but a bit. I put new o-rings on all of them. My only guess is they are not sealing.
 
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Ok, no more fuel leaks, not that I can see anyways. But..the popping noise continues. I redid the bench sych and felt I got it bang on this time. I started her up and tried to adjust the air and fuel mixture with not much success. It's running rough at idle. It seems to hold at 1000, then all of a sudden it pops and drops RPM, sometimes enough to stall. I have not done a carb synch as my synchronized is not here yet. I'm just wondering if that is why I can't get the air and fuel mixture screw to work properly? On the #3 and #2 carbs there isn't really any difference on adjustments to either one. On the #4 and #1 carbs the fuel makes a small difference (it is almost completely set in) and the air mixture makes a large difference in RPM when doing it by the best idle method. It may not be running that great, but it is running and I'm learning lots. Any thoughts on all this??
 
if it does idle, that's probably as good as it gets without the valve adjustment and the vacuum sync
 
Before you pull anything down, put half a can of Seafoam in a full tank of gas and run it through the carburetors and see if it clears up your problem. It may not work but sometimes does. If it works you will save yourself a bunch of work. You can get it at NAPA and other auto parts stores. :-) Tom
 
finished

finished

So she's all back together. I did wind up doing a valve adjustment and 4 valves were too tight. It was a really easy job with the zap strap trick, they worked GREAT. I synched the carbs with my Carbtune, they are pretty good but when I had the carbtune hooked up the cylinders would bounce more than I think they are supposed to (I'm thinking I have a vacuum leak somewhere). I went for a long awaited ride and she is running great, except for when I am sitting at idle and go to give it gas she stalls unless I feather it a bit. I notice that when I had the carbtune attached. When I gave it a bit of gas the cylinders would go right to the bottom then quickly shoot up. I am thinking maybe I have a lean mixture? Thanks again for all the help so far. I couldn't have done all this work with out everyone's help.
 
Lesson to be learned from this is never take for granted what the PO tells you about the bike. He is just looking to get rid of it. Apologies go out to all of the honest ones, but for sure be skeptical of MOST of the sellers.

When I bought mine in August, the PO made it sound way too good to be true and you know what they say about that. Within a week of owning the bike I figured out he lied about just about everything on the bike so I took it upon myself to treat it as a bike in dire need of a tune up and found 5 of the 8 valves needed adjustment. One set of points looked like new ones while the other set was shot and gapped incorrectly. And on and on and on. It is finally coming around and as soon as the carbtune arrives later this week, I think she will be running good enough to scare the crap out of me when overtwisting the throttle.

Good to hear you finally got yours running so much better. Now you know better days are ahead.
 
Valve shims are no longer a pricey subject. In fact there is a member here that has a very nice exchange program for shims. He is Ghostgs1. Give him a try.
Also there is a salvage yard in Lubbock, Texas that sells them for $4 each. Phone # is (806) 745-4447. Another source is lloydrivers1964@yahoo.com phone # (952) 334-5838. Apple Valley, Mn. and he sells them for $2.50 ea.
Good luck
 
Clean you carbs, replace your plugs check your plug leads are tight..... replace your air filter....

then try again...

Don't adjust the idle until it is warm. :cool:
 
azr said:
So she's all back together. I did wind up doing a valve adjustment and 4 valves were too tight. It was a really easy job with the zap strap trick, they worked GREAT. I synched the carbs with my Carbtune, they are pretty good but when I had the carbtune hooked up the cylinders would bounce more than I think they are supposed to (I'm thinking I have a vacuum leak somewhere). I went for a long awaited ride and she is running great, except for when I am sitting at idle and go to give it gas she stalls unless I feather it a bit. I notice that when I had the carbtune attached. When I gave it a bit of gas the cylinders would go right to the bottom then quickly shoot up. I am thinking maybe I have a lean mixture? Thanks again for all the help so far. I couldn't have done all this work with out everyone's help.
Click to expand...

The carbsync will bounce some. That is normal. Also, the carb sync is measuring the vacuum in the intake manifold and when you open the throttle this vacuum does go down. That is normal as well.

The popping/thumping that you could feel from the carb body/intake is a sign of a lean mixture.

AZR, sounds like it is getting there. Did you set the mixture screws on the bottom of the carbs near the intake manifolds by best idle? The period right off idle is fed from these. I suggest turning each of these screws out another 1/8-1/4 turn.

Also, my '79(s) benefited greatly from raising the needles up one notch. However, this does involve pulling the carbs again and taking the throttle rod out and pulling all four needles and readjusting the vaccum sync. Maybe something to do in the spring.

Chris
 
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