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carb backfiring/popping

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G

Guest

Guest
1979 GS850. Rebuilt my VM carbs (thoroughly) Rplaced all seals on air box. New o-rings for the manifold boots. All boots seemed nice and soft. Manually set the timing (checked it three times). Starts up fine and once if warms up it starts backfiring through the carb (mostly, in the first 5 min I might get one or two back fires through the exhaust) I did a bench synch and set all the air and fuel screws to factory. I tried to synch the carbs but the vacuum is almost nil and when I increase the RPM's the vacuum goes up but then speratically drops, then increases making it very difficult to adjust. I sprayed carb cleaner around attempting to find air leaks but it is back firing so much that I can't tell if there is a leak or just a back fire. I haven't done a valve adjustment yet. The bike wasn't backfiring before the rebuild, it was running fairly well with the exception of stalling at idle when warm (I figured the two broken fuel screws I found during the carb rebuild was causing that). I'm at a bit of a loose where to go from here. Any suggestions?
 
When you say it starts up fine & warms up is that on choke or off choke?

If on choke you may be lean when shutting the choke off? Stock airbox? Check you don't have one of the rubber boots rucked up or trapped leaving a gaping hole... I did that one once (on a 550, the 850's airbox is cake to fit in comparison).

How's your air filter?

How's your ignition? I had some backfiring through carbs with a weak spark caused by a faulty ignitor....

Dan :)
 
The bike starts fine when cold, choke on. Once is warms up and choke is off the backfiring really starts. I also starts fine when warmed up, choke off. I'll check the lean tomorrow when I go back at it. The air box is stock, cleaned, new seals the works. I checked all the rubber boots and they seem to be where they are supposed to be, none folded up under themselves or anything like that. The points seemed to be fairly new when I checked them, very good condition anyways, no wear. I'll check the spark strength tomorrow as well.
 
Just because you set the mixture screws to the "factory specs" doen't mean it's what the bike wants. The mixture screws need to be set CORRECTLY with the engine running. With the bike idling, turn each screw in or out, one at a time to get the bike to idle the fastest on each screw. After you set the first one, move on to the next. If the idle goes up while adjusting, turn the idle back down & move on to the next. After adjusting all 4 this way you will be surprised at the difference in ease of starting. My bike doesn't even require the choke to start anymore unless it is REALLY cold, or it hasn't been started in over a week. Good luck, Ray.
 
rapidray said:
Just because you set the mixture screws to the "factory specs" doen't mean it's what the bike wants. The mixture screws need to be set CORRECTLY with the engine running. With the bike idling, turn each screw in or out, one at a time to get the bike to idle the fastest on each screw. After you set the first one, move on to the next. If the idle goes up while adjusting, turn the idle back down & move on to the next. After adjusting all 4 this way you will be surprised at the difference in ease of starting. My bike doesn't even require the choke to start anymore unless it is REALLY cold, or it hasn't been started in over a week. Good luck, Ray.
Click to expand...


I'm lost. sombody 'splain this to me again...( or tell me how to use the search for this) many thanks!
 
This method refers to "best idle" method of setting your fuel and air mixture screws. You simply start your bike, have it running at factory idle (for mine 950-1050RPM) and then slowly start to adjust your fuel mixture screw (you do this to one carb at a time, once you have one carb done you need to reduce your idle RPM's back to factory with your main idle adjuster), you keep turning (one way or the other) it until you get the fastest, best idle before the engine starts to crap out, once it does your turn your mixture screw back just slightly...bingo "best idle". Once you have your first carb done readjust your idle to factory and move on to the next. You do this with your fuel mixture screw first then adjust your air mixture screw. Hope that helps. You can also do plug pulls to obtains the best setting for your bike or you can purchase a special spark plug called a color tune.
 
fixed...sort of

fixed...sort of

The popping/backfiring was from my fuel/air screws being way off. Once I set them with the best idle method that virtually stopped the backfiring. I did wind up doing a valve adjustment. 4 of 8 were too tight. Once that was done the carb synch was way easier. The Carbtune worked just great but the cylinders still bounced a bit, just a little. And when I go to put the gas on I loose vacuum for a second and then she goes. The bikes back together and running pretty good except when I am at idle and go to give it gas it will stall if I don't feather it. I'll look into that. The idles pretty good though, she might have a faint vacuum leak somewhere as every now and then she is a bit rough but she's running much better than when I bought it.
 
fixed

fixed

So I go into the timing again and this time check the points with a multitester. Turns out I am getting power to both points at the same time. So there is a short somewhere. I trace it as far as the harness going towards the tach/speedo and figure this could get ugly so I just unplug the two wires white and black and yellow and hook them directly to the coils by splicing them with the two orange wire from the coils and WHAMO!!! no back firing, no bogging, she ran and pulled like I've never seen her before. If I ever get ambitious I will trace down the actual short..maybe.
 
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