What causes the extra burst of fuel in the carb when you give it throttle?

[Roemy24]

I feel as though my '80 850L is starving for fuel. It starts and ideals great but bogs with any kind of throttle movement. Before I move onto electrial (plugs pretty good, maybe a little rich) I want to make sure I have the carbs checked away. Also no air leaks around boots or air box.

The main questions is what causes the extra burst of fuel in the carbs when you give it throttle?

Thanks
Joel

 

[zuki1000]

Its sounds too rich to me. Does the stumble get better when the bike warms up? Whens the last time the carbys were cleaned? Whats the voltage at your coils?

 

[Roemy24]

To rich, back off the air mixture screw? Doesn't really stumble but does run slightly better after warmed up but I can't get it over 1500 RPM, however if I pull the choke I can rev it up to 4,5,6K if I want to. Carbs are clean, just done. How do I check the voltage at the coils?

 

[Billy Ricks]

It's a passive system, meaning you don't have accelerator pumps. It relies on air flow through ports in the carbs to supply fuel.

 

[Big T]

And that air flow is messed up because you have air leaks.

Not being able to rev up is the classic sign of air leaks, or really lean jetting

Stock airbox or pods?

Stock exhaust or header?

 

[duaneage]

Some motorcycles have accelerator pumps to supplement the mixture when the throttle opens suddenly, like on a car. But these carbs are different and the engines have different requirements as well.

First, the idle speed is much higher than most cars, the engines are smaller, and the carbs are close to the head. This all means less of a need for accelerator pumps.

If you have a stumble or bog it is most likely due to leaks between the head and carbs around the rubber boots. The leaks reduce the metering signal strength and the carbs can't accurately meter fuel.

 

[Roemy24]

I'm running stock airbox and stock exhaust. I had an air leak that I'm taking care of and will repost tomorrow if I got it fixed. Anyone have or know of where I could get boots without breaking the bank? I already replaced the o-rings and boots looked in good shape and flexible.

Duaneage, would a faulty or going bad R/R cause this?

 

[Big T]

The 850 is really sensitive to airflow

Is your airbox sealed up??

If not, get some weatherstripping from Home Depot (or equivalent) and seal up the sides.

New intake boots are expensive, no doubt about that. Try Flatout or Bike Bandit or even your local dealer.


Also, when you say you replaced the O rings, both in the carbs and between the head/intake boots?

 

[Roemy24]

New o-rings everywhere. The airbox is sealed the best I could. It does have the black rubber cover "gaskets" that I even used rtv gasket sealer around them so that should be good. Do you think I need to still seal it even better?

 

[bwringer]

You also have to seal the air filter cage.

 

[Pacman]

How did you go about cleaning the low speed circuit in the carbs? Did you blast thru using carb cleaner?

The reason I ask is that your symptoms are classic for a lean condition. You have said you replaced all the o-rings in the carbs as well as the o-rings between the intake and head. Also you sealed up the airbox. Therefore, your lean condition is likely caused by lack of fuel rather than too much air (either of which can cause the problem).

Lack of fuel is most likely caused by the low speed circuit as it clogs up much easier than the high speed circuit/needle jets and it is much more difficult to get completely clean. So, my bet is that the idle jets are clogged and/or the passage from the idle jet to the carb throat is clogged. Even a partial clog will cause a lean condition.

As to your question of where the extra fuel comes from there are two places in your CV carbs. If you look in the throat of the CV carb you will see one small hole. This is the passage that supplies your mixture at idle with the butterflies closed. Then if you open the butterflies you will see another little hole in line with the first that is blocked of by the butterflies when they are closed. This extra hole supplies mixture when you crack the butterflies. Then, if you crack them enough the vaccuum will draw on both of these while the vaccuum raises the needle which supplies even more. This also happens while the piston the needle is seated in blocks off extra air from entering too quickly before the carb needs it. The balance of all this results in allowing extra gas to enter when needed for acceleration
without allowing too much air too soon. All without needing an accelerator pump like other carbs.

See, clear as mud.

Chris

ETA: BTW, if you go to Bikecliff's website he has a PDF of the BS32 carb manual. In this manual they have a graphic explaination of how the CV carbs work.

 

[Killer2600]

How did you go about cleaning the low speed circuit in the carbs? Did you blast thru using carb cleaner?

The reason I ask is that your symptoms are classic for a lean condition. You have said you replaced all the o-rings in the carbs as well as the o-rings between the intake and head. Also you sealed up the airbox. Therefore, your lean condition is likely caused by lack of fuel rather than too much air (either of which can cause the problem).

Lack of fuel is most likely caused by the low speed circuit as it clogs up much easier than the high speed circuit/needle jets and it is much more difficult to get completely clean. So, my bet is that the idle jets are clogged and/or the passage from the idle jet to the carb throat is clogged. Even a partial clog will cause a lean condition.

As to your question of where the extra fuel comes from there are two places in your CV carbs. If you look in the throat of the CV carb you will see one small hole. This is the passage that supplies your mixture at idle with the butterflies closed. Then if you open the butterflies you will see another little hole in line with the first that is blocked of by the butterflies when they are closed. This extra hole supplies mixture when you crack the butterflies. Then, if you crack them enough the vaccuum will draw on both of these while the vaccuum raises the needle which supplies even more. This also happens while the piston the needle is seated in blocks off extra air from entering too quickly before the carb needs it. The balance of all this results in allowing extra gas to enter when needed for acceleration
without allowing too much air too soon. All without needing an accelerator pump like other carbs.

See, clear as mud.

Chris

ETA: BTW, if you go to Bikecliff's website he has a PDF of the BS32 carb manual. In this manual they have a graphic explaination of how the CV carbs work.

Just like mud isn't clear, your explanation just confuses me. In simpler terms the vacuum controlled slide eliminates the need for a accelerator pump.

 

[Roemy24]

Thanks Chris! I believe so but easily could have missed something passages cleaning, I am fairly new to this. Where is the BS32 carb manual? I only found the cleaning one on his website. What passages are you refering to? The carbs have all new jets and are extremely clean so it must be a passage(s).

 

[Pacman]

Thanks Chris! I believe so but easily could have missed something passages cleaning, I am fairly new to this. Where is the BS32 carb manual? I only found the cleaning one on his website. What passages are you refering to? The carbs have all new jets and are extremely clean so it must be a passage(s).

On the right side just above the cleaning link on Bikecliff's website he has the BS32 manual in PDF format. Here is the link:
http://www.mtsac.edu/%7Ecliff/storage/gs/BS32SS_Carbs.pdf

When cleaning the passages for the low speed jets you kinda need three hands. The air jet on the front of the carb leads to the pilot fuel jet which then leads to the orifices in the neck of the carb. You need to use carb cleaner to blast thru the air jet to make sure you have no obstruction for air getting to the pilot jet. Then you need to block the air jet with a finger or something while blasting thru the pilot jet to make sure you get plenty of flow from the orifices in the neck of the carb. While doing this make sure you are able to hold the red tube from the carb cleaner can tightly against whatever you are trying to blast thru to get the most flow and pressure possible.

I have found that the orifice that gets blocked off by the butterfly tends to be the dirtiest so it is a good idea to block of the open orifice in the neck of the carb with a finger to get the carb cleaner to go thru just the one orifice. It also lets you know how well that hole is flowing.

I know it sounds confusing at best but once you start blasting thru these passages with the carb cleaner and think about where it's going you will understand better about how these passages are set up.

The amount of carb cleaner blowing from the orifices in the neck of the carb should be almost the same as the can puts out without restriction. It should really blast out of those holes.

Chris

Here is a modified version of the carb diagram showing the low speed circuit.

 

[Pacman]

Just like mud isn't clear, your explanation just confuses me. In simpler terms the vacuum controlled slide eliminates the need for a accelerator pump.

Sort of.

The more time you spend on your carbs the more my explanation will make sense. Or maybe not.

Chris

 

[Roemy24]

I got it. Looks like I have another Sat. morning project. I'll keep you updated!

 

[Pacman]

BTW, have you done the valves yet? If not, I would do those while you are at it. They probably wouldn't cause your problem but they still are a good idea to do.

Chris

 

[Roemy24]

Valve clearance has been checked and taken care of. Thanks for the FYI.

 

[Roemy24]

Pacman and others, it runs...sort of. I cleaned all the passages and it runs and revs up to 4500 but then bogs. I can hold it at 3500 but that's it. I had some backfiring also when I got it to 4000. Running off a satellite tank gravity feed. When I shut it off for a few minutes then tried to start it I couldn't. Looking around I noticed gas was dripping out the airbox overflow tube. Haven't had anytime and won't until tomorrow but what are your thoughts? I had the floats in spec but know I was in a rush and mixed and matched so I'll recheck them. Otherwise float pins function properly. What to do next?

 

[Pacman]

Pacman and others, it runs...sort of. I cleaned all the passages and it runs and revs up to 4500 but then bogs. I can hold it at 3500 but that's it. I had some backfiring also when I got it to 4000. Running off a satellite tank gravity feed. When I shut it off for a few minutes then tried to start it I couldn't. Looking around I noticed gas was dripping out the airbox overflow tube. Haven't had anytime and won't until tomorrow but what are your thoughts? I had the floats in spec but know I was in a rush and mixed and matched so I'll recheck them. Otherwise float pins function properly. What to do next?

Fuel flowing out of the airbox tube means that your carb float bowl(s) are overflowing. You can probably see on the airbox side of the carb which one looks different and shows fuel coming out. The fuel is probably flowing out of one of the openings on the face of the carb. This is a vent hole.

Once you figure out which carb (hopefully they aren't all four doing this) you will probably need to check the float valve needle and seat to see why this valve is allowing fuel to keep flowing into the float bowl when it shouldn't. You may need to replace the needle and seat or just clean the passage leading to it extremely well.

BTW, I recently had an issue with my float bowls overflowing and it turned out to be crap in the fuel passages leading to the needle and seat area that kept coming loose and getting stuck under the needle thereby causing a leak. I chased this problem for most of the winter before I finally figured out that it was just dirt where dirt shouldn't be that was causing the problem.

Chris