runs better with carb cover off?

[Anonymous]

Thanks Keith. The crosswind/bog-o-thon happened again today on the way home. Hit an area where there is a strong crosswind for about 1/2 mile and the bike just bogged along then started pulling again (couldn't hear if it was pinging b/c I wear earplugs for the commute).

Earl mentioned something similar when I talked about blowing into the vents; said it should move gas into the carb throat, richening it. I guess I'll have to adjust the floats *again* and raise them a little more. It is kinda strange though. When it won't start in the AM, it will almost always catch, the RPMs will go to about 3K for about 1 second, then it dies. A quick puff in either tube and it starts right up. What surprises me is that even 1 second at 3000 RPMs isn't enough to keep the gas flowing. The upside is that I'm getting really good at removing and installing the carbs. The bad thing is the airbox is getting harder and harder to get sealed on the carbs.

[Anonymous]

Is this line the only vent to the outside of the bowl? If so, wouldn't blowing into it force fuel out through the jets? That would sure prime the pump!

Anyhow, I wanted to enter the info into this thread that removing the fuel vent lines solved the strange mystery "running out of fuel" with a full tank that I have been trying to figure out for over a year. I checked everything possible, and the bike ran great, but still died if I tried to cruise over 60. I pulled the vent lines like Keith suggested, and the problem just disappeared! Amazing. And this happened even when my bike was bone stock. Airbox... the works. So I suspect this may be a problem for a lot of folks with mysterious "carburetor" issues.

Thanks!

[Planecrazy]

I'll be checking this possiblity out soon on my '80 GS550L, as I've had this "mystery problem" for years, and have even missed out on several weekend trips over the years because I didn't trust the bike to run properly...

I'll let you know what I find...

Regards,
Steve 8)

[mrquadriga]

Thank you to all that helped with this issue. Kudos Keith Krause!-Pete '80 1100L

[KEITH KRAUSE]

Anyhow, I wanted to enter the info into this thread that removing the fuel vent lines solved the strange mystery "running out of fuel" with a full tank that I have been trying to figure out for over a year. I checked everything possible, and the bike ran great, but still died if I tried to cruise over 60. I pulled the vent lines like Keith suggested, and the problem just disappeared! Amazing. And this happened even when my bike was bone stock. Airbox... the works. So I suspect this may be a problem for a lot of folks with mysterious "carburetor" issues.

Thanks!

Glad to help!
I once made a topic here about removing the float bowl vent lines if you're running pods or a modified airbox, and most members seemed to think I didn't know what I was talking about. I guess they figured it couldn't be that simple. :roll:
A lot of people have gone nuts trying to richen the jetting to eliminate it. Can't be done! That's why I mention it now and then.

[KEITH KRAUSE]

Thank you to all that helped with this issue. Kudos Keith Krause!-Pete '80 1100L

You're welcome.

[Anonymous]

Keith, I am completely convinced, I just wish I understood! It's like, a religious experience, but on a bike, at least, I expect things to make sense in the physical world.

Work with me here. The fuel is pulled out of the bowl, through the jets. That fuel is replaced by fuel from the tank. So... the net pressure in the bowl should stay at its starting pressure. Conceptually, bowl vents aren't even necessary to keep bowl pressure at atmospheric. And as there is little net movement of air through the vent lines, how does a vortex form? Unless it is due to passing air, like a venturi effect? That is the only plausible explanation I have thought of. Do you suppose that's it?

[KEITH KRAUSE]

Keith, I am completely convinced, I just wish I understood! It's like, a religious experience, but on a bike, at least, I expect things to make sense in the physical world.

Work with me here. The fuel is pulled out of the bowl, through the jets. That fuel is replaced by fuel from the tank. So... the net pressure in the bowl should stay at its starting pressure. Conceptually, bowl vents aren't even necessary to keep bowl pressure at atmospheric. And as there is little net movement of air through the vent lines, how does a vortex form? Unless it is due to passing air, like a venturi effect? That is the only plausible explanation I have thought of. Do you suppose that's it?

Well, bowl vents are necessary to allow the jets to draw fuel easily.
If the venting is poor, the jets can't draw fuel easily so there must be a counter vacuum building up. I don't know. The "vortex" idea was told to me by a guy at Dynojet.
I don't worry about the science behind it. I just know that with the engine flowing more freely, the standard bowl venting isn't adequate. A vortex, turbulence or ? compromises the venting and the jets don't draw fuel easily.

[themess]

carb vents

First, I know almost nothing about carburetors.
Second, carbs should need a vent for one of the reasons that a gas tank needs a vent, to accomodate thermal expansion and contraction. If the bowl becomes pressurized from heating, a lot more gas should be forced through all the jets. I would think that this would not happen unless the engine is off. Same thing should apply to cooling.
The Venturi Principle is fundamental to the operation of carburetors. When air blows across an orifice, the pressure behind the orifice lowers. If that orifice is the carb vent, pressure in the carb would be lowered, exactly what isn't desired. A solution might be to add a tube to the vent orifice, and run the end of the tube to a region of still air, where there can't be any wind across the opening of the tube.
As a test, I put a 3/8" vinyl tube in a glass of water, and blew compressed air past the end, and water was pulled about 3/4" up into the tube.
As for me, I used to own an '82 GS650G. The gasket at the base of the cylinder failed at about 6K, but other than that, the only maintenance that it needed for 25K miles was fluid and filter changes, two sets of tires (the second one at 24K), plugs and two or three valve adjustments. I bought an '82 GS850G that runs well up to about 25 mph and then accelerates very slowly. Based on these forums, I'm going to start with the fuel petcock and hoses, replace the air filter and seal the air box (it needes it). Only then, if it needs it, I'll disassemble and clean the carbs and put in Robert's O-ring set.
This is a great forum!

[KEITH KRAUSE]

Re: carb vents

The Venturi Principle is fundamental to the operation of carburetors. When air blows across an orifice, the pressure behind the orifice lowers. If that orifice is the carb vent, pressure in the carb would be lowered, exactly what isn't desired. A solution might be to add a tube to the vent orifice, and run the end of the tube to a region of still air, where there can't be any wind across the opening of the tube.

The solution you mention here wouldn't work.
The tube is what's causing or contributing greatly to the problem. Many stock vent tubes are 16-18" long and run to still air under the seat. Even if you try cutting the stock vent tube much shorter, it doesn't work either. You still get fuel starvation.
If you have pod filters, you want to remove the tubes and leave the vent ports open.