Carb problem?

[Anonymous]

I just bought my first street bike, a 1981 GS650e. It runs good and I'am having a lot of fun with it. The problem is that after riding for about 15-20 min, the bike starts to backfire slightly when decelerating(little popping noises) and Idles at about 2500 rpm. When the bike is cold it idles at 1100 rpm. I'am new to motorcycles and would really appreciate some help.

[Anonymous]

Its probably fine.

Its probably fine. You could try adjusting the air screws if they are not still caped. Run them in all the way and back them out 1 3/4 turns. I have a small flat head screw driver that is about 1 1/2 inches tall that works for this job. I still get some poping on deceleration though. I don't think you can get rid of it all together.

[Anonymous]

I get the same popping on mine. Always when engine braking. As I'm a fairly new rider, it's one of those "I don't know what I don't know" situations. The bike seems to perform well otherwise.

I need to rent an expert for an hour, have her ride it around, and tell me what needs to be done.

Drew

[KEITH KRAUSE]

If a bike starts and idles at 1100 rpm and then reaches 2500 rpm after it's warmed up,then the bike is running lean because of an air leak. Too much air increases the rpm's.First try tightening the clamps on all the carb rubber boots. The o-rings in the boots between the engine and carb are also a common problem.They get old and crack.If you replace,put some grease on them.Some popping when you let off the gas is something most of us can live with,but that high of an idle is not.

[Jethro]

My bike has popped since I got it 10 years ago! Doesn't seem to affect anything (although it may run a little hotter than it should...hmmm).

[Anonymous]

I think I'll do the leak test.

I think I'll do the intake leak test. I have the poping. It isn't bad and doesn't bother me. I also have the Idle that rises after its warm.

I figured I just need to do the valve shim adjustment that I neglected to do. If it could also be an intake leak I had better check that as well. I have new O-rings in the boots, but I didn't know you had to lube them.

[KEITH KRAUSE]

Brand new o-rings don't depend on a lube or grease to make them seal initially.The grease will help them last and stay pliable a lot longer so they will seal longer.It's still a guess as to the best stuff to use.I tried white lithium grease first and it disappeared in a year.This time I tried regular bearing grease.I don't know how it's doing but I don't have a reason to take things apart,which is fine with me. I believe you have the 'CV' carbs.I'm not as familiar with them as I am with 'VM' round slides.If your 'CV' diaphragm fails,I think this will also cause a high idle?The rubber diaphragm as it warms up,becomes more pliable and if it has a small crack or tear the air will find its way in,increasing the rpm's.Also,though not as common,a throttle valve(slide)will start sticking and hanging up especially when the engine is warmed up,causing a 'warm only' high idle.

[AOD]

Keith, both of my late 70's GS550's do this...cold idle is lower...warm idle is around 2500. i no for a fact my 78 was running way rich...so i dont think its a lean mixture. i just think the bikes are a bit cold blooded.

~Adam

[KEITH KRAUSE]

Keith, both of my late 70's GS550's do this...cold idle is lower...warm idle is around 2500. i no for a fact my 78 was running way rich...so i dont think its a lean mixture. i just think the bikes are a bit cold blooded.

~Adam

No bike is supposed to idle at 2500 rpm.Being cold blooded has nothing to do with a warmed up high idle.You can have a motor that is set up too rich and still have an air leak.You can still have a high idle.The 2 seperate problems do not necessarily cancel each other out.I also said that a leaking diaphragm or sticking throttle valve can result in a high idle.Here's a common story:A guy rides around for awhile and notices he has a high idle.So he turns down the idle adjusting knob.Everything is fine. He parks the bike and lets it cool down or it sits overnight.The next morning the bike won't start(or is harder to start) unless he opens the throttle to allow some air in BECAUSE he lowered the slides/idle.This makes it even harder to start because when you open the throttle,you are by-passing the choke circuit.With an air leak you will find yourself turning up the idle or constantly giving more throttle just to warm it up,but once it is warmed up you have to turn the idle back down again.This happens because a cold engines fuel/air ratio requirements are different than a warm engine.As the engine warms,an air leak will become more noticable.This up and down stuff will not stop you from riding,but it is an intake problem or possibly a sticking throttle valve.Sometimes the rubber intakes/o-rings or diaphragms warm up and become pliable and then expose an air leak.I have read in several books how too much air will raise the rpm's.I don't always believe what I read,but I also learned this by working on a lot of bikes and cars in my life.I've seen some cars lower their idle because of a VACUUM leak,but not because of a lean intake mixture.

[nabrams]

I just wanted to add that air leaks at the boots that attach to the airbox (containing air filter) can be just as bad as air leaks at the boots that attach to the engine. I know this because I had the same problem (popping/back-firing). After sealing the leaks (with silicone), the bike ran smooth as silk. That airbox and air filter were designed for a very precise air flow rate suitable to your bike. Any air leak will throw the whole thing out-of-whack. The air filter element is critical as well. If it's rotted, you can bet you're bike won't run right.