Gushing carburetor - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

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post #1 of 25 Old 10-15-2015, 11:25 PM Thread Starter
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Exclamation Gushing carburetor

Hi all

As of yesterday, whenever i start up my 1987 KLR 650, the fuel gushes out of the float bowl overflow. Now i understand that this generally means the float needle is not sealing.

I spent all of last night removing the carburetor, cleaning it and re-assembling it. This morning, startup, and same problem

I must ad, that the fuel pump diaphgram has 2 tiny holes in it. Are they supposed to have these holes in the rubber or not? Could this be causing my gushing problem.

Please help.
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post #2 of 25 Old 10-16-2015, 05:40 AM
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Welcome to KLRing.

The KLR doesn't have a fuel pump so it doesn't have a fuel pump diaphragm. You might want to look at a parts diagram such as found on BikeBandit.com or other retailers to find out what that part is.

The carb has diaphragms for the air bypass and the slide. The petcock has a diaphragm that turns on gas only when the intake manifold has partial vacuum. I don't think holes in these would drain fuel through the float bowl drain.

I forget if the KLR float needle has a brass point or a rubber point. If it's rubber, check it for cracking and make sure it's still soft.

Have you checked the float level and checked that the float still floats? A bent tab or a float that fills with gas can both cause an overfull float bowl. Does the float pivot VERY easily on the pivot pin? Any snag there can also cause an overflow.
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post #3 of 25 Old 10-16-2015, 01:10 PM Thread Starter
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Hi, thanks for advice. One of the boys from work thought that maybe the smaller diaphragm on the side acts like a "fuel pump". I gather this must be the air bypass then. Anyhow, these smaller one seems to have 2 little holes in the rubber, goodness knows how they got there. I'll re-strip the carb and check the condition of the rubber tip of the float valve. Thanks again.
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post #4 of 25 Old 10-16-2015, 01:21 PM
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The needle has a soft tip.

Even if there were an after market diaphragm fuel pump and all the diaphragms in the carburetor and pump and petcock had holes, a properly functioning float needle and seat would not let fuel in the carburetor above its set point.

The word gush bothers me. A slightly damaged/dirty needle and seat would only drip a little not gush. For it to gush the float valve would have to be wide open as in stuck open, improperly installed or sunk floats.

Also, I wonder how it gushes out the vent line with the engine started up. It seems like fuel that high would kill the engine.

I doubt it, but have to ask if the fuel line may now be connected to the vent line elbow.

When you solve the problem, check the oil to see if it smells like gas.
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post #5 of 25 Old 10-18-2015, 04:22 AM Thread Starter
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Just had the carburetor off and rechecked the float valve, everything is clean, and the tip is still soft. Float level is damn near bang on 17.5mm as per the clymer manual. I have a funny feeling that when I reassemble I'm going to have the same problem.

Here is another thought. When the bike is running the carburetor diaphragm is creating a vacuum, which in turn is sucking fuel into the carburetor. The float bowl fills with fuel, causing the float to lift, and hereby the float valve needle shuts off the fuel intake. The fuel is still being "sucked" into the carburetor ........, with the float now shutoff, where does this fuel coming into the carburetor go?

Thanks
Justin
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post #6 of 25 Old 10-18-2015, 06:33 AM
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With the float valve closed there is no gas being sucked into the carb.

There is also no diaphragm sucking gas*. The vacuum in the carb is an air vacuum. It's caused by the piston and valves pumping lots of air in through the intake and out through the exhaust.

As air flows through the carb the airflow is reduced in diameter which increases the air's speed and counter-intuitively reduces the air pressure. (Venturi was the first to discover this so it's called the Venturi effect.) This drop in pressure pulls fuel up just a fraction of an inch from the fuel level in the bowl and starts mixing air bubbles into the fuel so the mixture is lighter and can easily be pulled even higher into the carb throat. The float bowl has an air vent to prevent a vacuum.

As long as the carb float is high fuel won't flow in and won't be sucked in. As long as the float is low, fuel will flow in and can flow out through various paths including the overflow, the air vent, and the carb throat (and therefore into either the airbox or the combustion chamber). When the float moves and is at the right level, only an air/fuel mixture flows and only when there's enough air flow to cause the venturi to make a pressure difference.

*The KLR carb does not have an accelerator pump. Carbs with accelerator pumps, called pumper carbs, DO have a small diaphragm that squirts a small bit of gas when the throttle is rotated open. This is to prevent a common problem of opening the throttle and having the mixture temporarily very lean. The gas for this still comes from the float bowl on the pumper carbs I have.
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Last edited by Grinnin; 10-18-2015 at 06:36 AM.
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post #7 of 25 Old 10-18-2015, 10:55 AM
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A shorter version of Grinnin's answer to this question. The fuel is sucked from the float bowl through the jets into the throat of the carburetor where it mixes with the air being sucked through the throat by the engine. While this is happening the float valve does not "SHUT OFF" the flow of fuel. I just meters the flow to keep the fuel level in the bowl at a constant level. When you open the throttle more the engine draws more air through the throat which sucks more gas from the bowl and the float drops just a little to let gas into the bowl faster to maintain the level in the bowl. When you close the throttle some the opposite happens.

When the engine is stopped and no more fuel is being sucked from the carburetor the float valve should close completely and keep the fuel from overflowing from the bowl.
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post #8 of 25 Old 10-19-2015, 10:04 AM
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A very common problem today is the build up of deposits on the needle valve and in the bore of the needle seat. This causes the needle to bind and not to move up with enough pressure to shut off fuel. I often have to clean needle seat bores with crocus cloth on a wire. Some have had success with a cotton Q tip.

I'm also troubled by the apparent volume of fuel, how much fuel by volume, is "gushing" out?

FWIW, the carb is a low fuel pressure design so having a fuel pump in the system can over power the needle valve and force it open. I don't recall how much pressure is required for these carbs but have seen that problem with others when someone has adapted an engine to a different application requiring a pump.
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post #9 of 25 Old 11-04-2015, 04:17 PM Thread Starter
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I am glad to report, that after cleaning the carb for a second time, and then CAREFULLY re-assembling....Walah! She's all gravy and i am hitting the tarmac again!
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post #10 of 25 Old 11-04-2015, 04:41 PM
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Congratulations!

I would ask if you replaced the little air cut-off diaphragm on the side of the carb, the one that had 2 tiny little holes in it?
If you have trailing throttle exhaust 'after-fire' / popping, you'll know where to go, to fix it.

pdwestman
Modify at "YOUR OWN RISK"!

Still riding my 1987 KL650-A1. 85,000+ miles & counting
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