Pop/Backfire on decel - lean,rich or other?

[dbarnes42]

I have a 79 GS850, all stock except for Dyna ignition and Dyna coils. This bike is giving me fits! I have replaced the intake boots, air box boots, intake o rings, exhaust gaskets, rebuilt the carbs (dipped for 48 hours), new o rings in carbs, sealed the air box and adjusted the valves about 500 miles ago.

The bike has VM carbs. The pilot air screws (top) are adjusted out 1 1/4 turns. The pilot fuel screws (bottom) are adjusted out 5/8 turn. Both of these settings are stock I think. The air box is stock and I am using a UNI filter. The needles are stock and are set on the second notch from the top. The carbs were synched this summer, with the assistance of Brian Wringer.

The bike has developed a pop/backifre on deceleration. It only pops one time, but it is every time I decelerate. It does not do it as I am accelerating and up shifting. And today I noticed in 5th gear, 60 mph, 4K rpm, it sounds like it is only running on 3 cyclinders.

The bike starts easily using full choke and after about 15 -20 seconds I can start to close the choke. After about a minute I can close it completly. It seems to run well on the highway, other than the pop.

Once it is warm, it seems to hunt for an idle, sometimes. Sometimes the idle hangs up and if I blip the throttle once or twice, it comes down. But it almost always seems to start to climb back up after that. Other times, it dips below set idle and then starts to climb.

 

[almarconi]

I would start by checking for an air leak. Use some spray carb cleaner on the carb boots and see if you have any leaks.

If no leaks are found, you may want to try 3/4 on the fuel screw and 1.5 on the air screw and see if this helps.

 

[Steve]

I believe the stock setting on the needle is the third notch. Using the second notch will lean it out a bit and might cause the symptoms you are observing.

I also agree about moving the mixture screws out a bit. You may have them at the stock settings, but that does not mean that it's the right setting.

.

 

[Kcwiro]

I believe the stock setting on the needle is the third notch. Using the second notch will lean it out a bit and might cause the symptoms you are observing.

I also agree about moving the mixture screws out a bit. You may have them at the stock settings, but that does not mean that it's the right setting.

.

+1 to steve on this one...reference is a good starting point, does sound like you are running on the lean side of things.

 

[dbarnes42]

On the needle, the 3rd notch is the stock setting. I moved it to the second position because the bike was wandering/surging on the highway. The bike doesn't do that anymore. On the highway, it runs great, doesn't surge and has great acceleration.

I really noticed this problem after the vavles were adjusted the last time. Is it possible that in some wacked out way I got the valves slightly misadjusted and this is causing the problem?

This problem is really starting to get to me, mostly because this is not the first time it has happened. I have had carb problems with this bike since I have had it (2 years) and I am really at the point I want to give up.

Do any of you have a set of VM's that you would be willing to part with?

 

[JTsGS650]

Hanging idle/popping on decel sounds like a lean condition, what do your plugs look like?

You also mentioned that it seems like it is running on three cylinders some times. You may have a bad connection at one of your spark plug caps. Try snipping off 1/4" or so of the plug wire and reconnecting the plug caps or buy a set of fresh caps. This could also be related to your popping.

 

[Steve]

On the needle, the 3rd notch is the stock setting. I moved it to the second position because the bike was wandering/surging on the highway. The bike doesn't do that anymore. On the highway, it runs great, doesn't surge and has great acceleration.

Surging on the highway is an indication of lean running. Moving the clip up, toward the blunt end of the needle allows the needle to sit farther down in the jet, leaning it out. It baffles me that lowering the needle cleared up your apparent lean running. :-k

Did you sync the carbs after the valve adjustment? If you had to change the shims very much, especially if you only had to do one or two cylinders, you will affect the balance between the cylinders. Synchronzing the carbs mainly helps at idle and just off-idle.

.

 

[dbarnes42]

This is what I found today in my research. Plugs looked good, although they may have been a little on the white side. I adjusted the fuel screw 3/4 turns out and the air screw 1.5 out.

I also found out that even though it is a 79, it has a 80 exhaust on it. Where the pipes meet in the premuffler, one of the gaskets in there is broken up pretty bad, allowing a exhaust leak. Could this be the source of the problem? Sucking in air possibly?

Steve, I was incorrect in my statment about the needle. I the clip was in the second position and after talking with you because of the surging, I moved it down to the third.

Any thoughts on the exhaust leak?

 

[49er]

This is what I found today in my research. Plugs looked good, although they may have been a little on the white side. I adjusted the fuel screw 3/4 turns out and the air screw 1.5 out.

I also found out that even though it is a 79, it has a 80 exhaust on it. Where the pipes meet in the premuffler, one of the gaskets in there is broken up pretty bad, allowing a exhaust leak. Could this be the source of the problem? Sucking in air possibly?

Steve, I was incorrect in my statment about the needle. I the clip was in the second position and after talking with you because of the surging, I moved it down to the third.

Any thoughts on the exhaust leak?

The popping on decel will be caused by the exhaust leak. The hanging up on idle when the throttle is closed is a sign of leanness as stated by others.

As a reference, I'm now running 17.5 pilots at 7/8 and the airs at 2 turns out. The stock settng for the VM needles was #2 clip position, which is a bit lean. I now run mine with a shim over the #3 clip, effectively making it a #2.40 position. When I ran it in #3, it was a bit fat in the mid range rpm. Still running the airbox like you.

 

[Guest]

I've been playing with my 79 850 with lots of the same problems. Check some of my older posts but they say alot of the same things. Fix that exhaust leak first. It's amazing what problems come from a leak, and then do a best idle adjustment to your air mixture screws. Your factory settings are 5/8 for fuel and 1 1/4 for air, but remember thats just a starting point not what is necessarily best for your bike. Keep at it.