Air or Gas - 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 12 Old 09-03-2014, 07:17 PM Thread Starter
2nd Gear
 
Join Date: Nov 2012
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Air or Gas

With the bike off and the choke off, if you start the bike and crack the throttle while doing so, are you

adding air or
adding fuel?


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post #2 of 12 Old 09-03-2014, 07:37 PM
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Both, if I understand your question correctly.

If you start the bike without cracking the throttle, the carburetor is set up to allow a bit of air to pass the butterfly valve (which is what the throttle cables operate). That bit of air passes through the venturi and creates a low pressure over the needle jet and gas is drawn through the main jet in the proper ratio for good combustion at idle.

If you start the bike with the throttle cracked, then even more air is let through, drawing even more gas. The higher RPM creates more air velocity (the piston draws in more air on the intake stroke) and thus a more vacuum, which raises the slide. That allows more opening in the needle jet/mainjet, which draws more fuel to create the proper ratio once again.

However, you may be asking very specifically "What does cracking the throttle do?". Looked at that way, the throttle opens the butterfly valve, which allows more air in. The carburetor takes care of adding gas to the whole mess. Compare and contrast that to a pumper carb (like the CVK40 found on the older Vulcans). In that case, the throttle operates the butterfly valve to add air and also operates a pump to shoot a bit of fuel into the venturi, so that's a carb that truly does both.

Tom

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Last edited by Tom Schmitz; 09-03-2014 at 07:41 PM.
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post #3 of 12 Old 09-03-2014, 07:41 PM Thread Starter
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Thanks. The reason I asked is my KLR won't start with out either the choke on if its cold (normal) but when its warmed up it won't start unless I crack the throttle a tiny bit.

Does that mean I have become to rich to start or too lean?


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post #4 of 12 Old 09-03-2014, 07:44 PM
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That may be a symptom of tight valves. As the valves tighten up (the clearance between the valve shim and the cam closes up) the engine can become hard to start.

It's kind of unusual for the carb to changes it's idle mixture. If it has, all you probably need to do is clean it, and at this point some SeaFoam would probably do the trick.

How long since the valves have been checked?

Tom

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“On the way downtown I stopped at a bar and had a couple of double Scotches. They didn't do me any good. All they did was make me think of Silver-Wig, and I never saw her again.” -Philip Marlowe

“'Why' and 'How' are words so important they cannot be too often used.” -Napoleon Bonaparte

Sting like a butterfly.
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post #5 of 12 Old 09-03-2014, 07:50 PM Thread Starter
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Start of the spring, LE and RE were at 00.8 inch.

Otherwise no change I see. Bike run strong and pulls the same as it ever has.


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post #6 of 12 Old 09-03-2014, 08:10 PM
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Should still be OK. I'd try some SeaFoam and see if things don't improve.

Tom

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“On the way downtown I stopped at a bar and had a couple of double Scotches. They didn't do me any good. All they did was make me think of Silver-Wig, and I never saw her again.” -Philip Marlowe

“'Why' and 'How' are words so important they cannot be too often used.” -Napoleon Bonaparte

Sting like a butterfly.
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post #7 of 12 Old 09-03-2014, 08:13 PM Thread Starter
2nd Gear
 
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Have that in the tank now. I will check and do the valves in the spring again, I just don't want to pull it all apart now in the fall and miss the riding time.


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post #8 of 12 Old 09-04-2014, 11:38 AM
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Good call, Tom. I wonder if the idle speed has remained the same or dropped a bit recently?

That might indicate that the mixture has changed or some other factor has changed engine efficiency. If the idle is on the low side, engines can be harder to start because of lower compression pressure.
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post #9 of 12 Old 09-04-2014, 08:10 PM
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First guess is the idle speed screw on the side of the carburetor is set too low and cracking the throttle a a little is the same as turning the idle speed screw up a little.

Much less likely would be the idle mixture screw on the bottom carburetor outlet is too rich (to far out).

This is related. http://www.klrforum.com/showthread.php?t=36457

Last edited by GoMotor; 09-04-2014 at 08:22 PM.
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post #10 of 12 Old 09-06-2014, 05:41 PM Thread Starter
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Well I fixed this one. The air filter was much dirtier than I thought and it was restricting the air flow. Cleaned it out all good and the idle issue is fixed. I am still adjusting the idle to find the right spot.


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