Chunky start in the morning...

[2wheelN00B]

Maybe five or six tries to get it started, and runs sluggish for a little while after...

Dad says maybe the carbs are getting flooded while it sits overnight. What could be the issue? I'm assuming whatever mechanism is in place for cutting fuel to the carbs is malfunctioning, and the petcock has no OFF position.

Model is 1981 GS750L

 

[KitKat]

the petcock doesn't have an off position because it is vaccuum operated. The vaccuum that the carb creates pulls the diaphram at the back of the petcock open, allowing fuel to flow in.

Ideally, if your petcock is working, there should be no fuel flow in the on position while the bike is not running because the carburators are not creating a vaccuum.

A quick way to test this is to pull the fuel line that goes to the carbs and tilt the tank ( and shake, to ensure gas is over the petcock) , make sure no gas is coming out and be prepared with a bowl or something to catch it if it does. (this is all with the bike off). if it doesn't, then it looks like a new petcock is in order... try www.partshark.com (great prices, slow shipping), www.z1enterprises.com , or www.bikebandit.com. You can try rebuilding the petcock, but that rarely succeeds.

if the petcock is working properly, then make sure the floats in the carbs are set to the correct height, if not, they won't shut off and whatever fuel is in the fuel line will build up in the float bowls, causing the float bowls to overfill and overflow somewhere

Hope this helps

 

[2wheelN00B]

Thanks, I'll give that a shot, hopefully soon!

 

[KitKat]

get back to us with results so we can help you further.

Usually if the petcock fails, it dumps fuel down the vacuum line while sitting overnight, the fuel builds up.

My bike had a bad petcock and i would get a nice little pool of gas out of the overflow tube that attaches to the air box and goes down under the bike (of course i put something there to catch the gas)

i bought a new OEM petcock and new air filter and checked float heights and cleaned my carbs while i was at it and vuala, no more leaking.

Hope this helps

 

[BassCliff]

Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. 2wheelNOOB,

You'll find lots of GS lovin' on my website. Please stop by and feed your mind.

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

 

[Steve]

Does it seem to start better after it's warm? :-k

If so, it could also be that your valves need to be adjusted.

.

 

[2wheelN00B]

BassCliff,

Thanks for the hearty welcome! I will check out those resources.

Steve,

Yes, it's just the morning (or long periods). After it gets warmed up and worked out everything seems to clear up. Perfect starts actually, no real need for the choke.

 

[Matchless]

Are you goosing the throttle when the choke is out when starting from cold?
You should not open the throttle when the choke is out, especially not while the starter is turning it, but you have most likely read the owners manual and know this!

 

[2wheelN00B]

Matchless,

Owners manual? Working on that.

GS300Lowner,

A quick way to test this is to pull the fuel line that goes to the carbs and tilt the tank ( and shake, to ensure gas is over the petcock) , make sure no gas is coming out and be prepared with a bowl or something to catch it if it does. (this is all with the bike off). if it doesn't, then it looks like a new petcock is in order...
​

Did you mean if fuel DOES come out, a new petcock is in order?

 

[BassCliff]

Hi,

There are a couple of 16-valve GS750 manuals on my website for download. Help yourself.

Yes! If the petcock is in the ON or RES position, there should be NO FUEL coming out of it unless the engine is running and applying a vacuum to the petcock diaphragm. The only position that should allow fuel to flow without vacuum is the PRIme position. The PRIme position does not need a vacuum to open the diaphragm that allows fuel to flow. There's a little more info on this page of my website:

Where Do These Hoses Go?

Thank you for your indulgence,

BassCliff

 

[KitKat]

yes. if fuel DOES comes out of the petcock, while the petcock is in the on position and the bike is not running, a new Petcock IS in order

Hope this helps

 

[2wheelN00B]

I'm new to motorcycles and all things mechanical. Thank you all for your willingness and patience.

 

[KitKat]

no problem!

 

[2wheelN00B]

Ok. Fuel and vac lines disconnected. Tank disconnected. Petcock on ON, shake vigorously, no leaking. Switched it over to RESERVE, maybe a thimble full of fuel spilled out, then nothing. Back to ON, shake, RESERVE, shake, 4 or 5 more times, but nothing else.

What's my next step?

STEVE:

IT does start better, much better after it's warmed up. Valves, as in, a carb rebuild? Why, for my own general knowledge, would that be letting the carbs flood? I guess I just don't know the function of the valves.

 

[BassCliff]

Hi,

For your petcock, the next step would be to see if the vacuum diaphragm is operating correctly. Have something handy to catch any fuel that might escape. Attach the vacuum line to the petcock. Turn the lever to ON. Suck on the vacuum line. Fuel should flow from the petcock. The flow should stop when you stop sucking on the vacuum line. Do the same for the RES setting. Then try the PRI (Prime) setting. Fuel should flow from the PRI setting without sucking on the vacuum line.

Intake valves let the fuel/air mixture from the carbs into the cylinder. After the mix is burned the exhaust valves let the hot fumes out of the cylinder to escape through the mufflers. These valves are not part of the carburetors but they do work together along with other parts to make the engine create power from gasoline. For some basic theory on the 4 stroke internal combustion engine, here's a link:

http://auto.howstuffworks.com/engine1.htm


Thank you for your indulgence,

BassCliff

 

[2wheelN00B]

Ok, but really. I'll give it a try, but isn't the petcock the ONLY reason it could be flooding? If fuel is getting to the carbs when it's off, the petcock is the only culprit, correct?

 

[BassCliff]

Ok, but really. I'll give it a try, but isn't the petcock the ONLY reason it could be flooding? If fuel is getting to the carbs when it's off, the petcock is the only culprit, correct?

Yes, that would be my diagnosis. But there may be more than one problem. The definitive words of wisdom on the subject have been collected on my website on the following page:

Carb Notes (sync, etc)

See the "Overflowing Carbs" article. My thanks to Mr. bwringer.


Thank you for your indulgence,

BassCliff

 

[2wheelN00B]

Ok

Ok

Finally turned the bike off to work on it, but not before...

I let it sit a week because it rained all week, now the only way it starts is with starter fluid, and I started it up this evening and revved it up some and around 4000 to 4500 RPM's it loses power & dies.

But, the petcock was replaced. I can't even test if it did what it was supposed to do now, due to this issue.

 

[Don-lo]

You probably fouled a plug. One of your needle valves is probably leaking very slowly, so that the fuel in the line (after the petcock) is draining off into one or more carbs. Overnight, it was enough to cause just a rough start, but after a week it was enough to foul a plug. Putting the bike on the centerstand can slow the drainoff.

 

[Steve]

STEVE:

IT does start better, much better after it's warmed up. Valves, as in, a carb rebuild? Why, for my own general knowledge, would that be letting the carbs flood? I guess I just don't know the function of the valves.

No, the valves are the devices on the top of the engine (under that hourglass-shaped cover) that allow the mixture from the carbs into the cylinders to be burned, then allows the burnt mixture out through the exhaust pipes. When these valves are mis-adjusted (usually too tight), they do not properly contain the mixture in the cylinders for combustion, making it hard to start the engine.

Look on BassCliff's site, he has factory service manuals for your bike, as well as personalized tutorials (with pictures) on how to adjust the valves. If you look at the bottom of my signature line (the section below every post I make), you will see that I offer an Excel spreadsheet that helps determine what shims you might need to adjust your valves. Read through the procedure several times. It's not hard, but does take attention to detail. Feel free to ask questions. We have all had to do this, many of us have done it MANY times.

.