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Carb question

  • Thread starter Thread starter apchasse
  • Start date Start date
A

apchasse

Guest
Ok, just finished building a GS1000 that I got from a friend... Got the book for the 1980 GS1000G... In doing the carb rebuild, the carb in the book looked nothing like the carb that is on my bike. I also ordered a whole set of stainless screws for the 80 and non of them fit. So I have a reason to believe these carbs are from another bike or model. So... to get to my question:

I think my floats need adjusting, because when I start my bike, it acts like the choke is on, but I dont even have the choke connected. It starts right up first push of the button, and then as it warms up, it revs higher and higher. Its sucking gas down really fast too. When I pull the throttle, it boggs down badly. (that does sound like the floats right?)

So.... I have the book that tells me what the measurement for the float height for the stock unit, but if this isnt stock, do I use the same measurement? Or do I need to find out what carbs these are and find that float height? The carbs are Mikuni and Ive attached a couple pics....
 
Hi Mr. apchasse,

What book are your looking at? That picture is the CV type carbs that come on the '80 and later GS bikes. What carb rebuild procedure did you follow? This one I hope.

The rising idle and bogging is a sign of running lean and/or leaks in the air intake system (intake carb boots and O-rings). It could also be that you have not re-jetted to compensate for the pod filters. Feel free to check my website for manuals and guides. Keep us informed.


Thank you for your indulgence,

BassCliff
 
Ok, so I guess I do have the right carbs... My books carbs look different than the ones you have in the carb cleaning site there.....

Ok, I will check the boots and o rings. Could it be that I dont have a hose attached to the vaccume hose that is next to the #2 carb that says its going to the petcock?

Is there an easy way to diagnose the air leak if there is one?

Thanks!!
 
apchasse said:
Could it be that I dont have a hose attached to the vaccume hose that is next to the #2 carb that says its going to the petcock?

Is there an easy way to diagnose the air leak if there is one?

Thanks!!
Click to expand...

Why are you asking us about the vacuum hose? Step away from the computer and head to the garage to check.;):p

Some people will tell you that you can spray WD-40 or similar on the carb boots to determine a vacuum leak but in my experience that technique will only pick up gross leaks. You are better off just changing the intake O-rings unless you know they have been changed before.
 
Nessism said:
Why are you asking us about the vacuum hose? Step away from the computer and head to the garage to check.;):p

Some people will tell you that you can spray WD-40 or similar on the carb boots to determine a vacuum leak but in my experience that technique will only pick up gross leaks. You are better off just changing the intake O-rings unless you know they have been changed before.
Click to expand...


Ha Ha, no Im asking if not having a hose connected to the vacuum port will cause problems. (im not asking if it IS connected) I ask because I dont have one connected. I just have fuel going from a funnel to the main fuel port in between #2 and #3 carbs.

Sorry for my ignorance.
 
Hi Mr. apchasse,

If you are using an external tank with gravity feed, you may want to plug the vacuum hose while you're working on the motor and/or adjusting the carbs. A golf tee works pretty well.


Thank you for your indulgence,

BassCliff
 
Ok, sorry to ask so many questions.... one more...

Im going to try plugging that up. But I dont have a stock GS tank... I used an old yamaha tank and petcock to build the bobber.... The yamaha petcock has two ports, one for normal "on" and another port for "prime". (Im pretty sure the stock GS tank petcock is the same, right?) Does a hose go from the "on" port to the "intake port" on the carb, and one from the "prime" port going to the "vacuum" port on the carb?

As I understand, the prime port is a gravity feed port, and the main port uses vacuum pressure to get gas through.... Am I way off? And just for my own knowledge, and this is the case, when I put the petcock on prime, will the bike run? Or is the prime only used to put gas in the system for easy start?

Thanks for all the help!
 
It is always gravity feed. Prime is an always open position on the valve. On and reserve positions on the valve require engine vacuum to open the valve. In either case gravity provides the force that moves the gas. If you have been running the engine, there is fuel in the float bowls. With fuel in the float bowls, the bike should fire if things work right. When the float bowls are empty, and the petcock is in the on or reserve position it takes a long time of cranking the engine with the starter to keep the petcock valve open long enough for gravity flow to fill the bowls. That is why there is a prime position. I wish there was an off, but there is not.
 
apchasse said:
I ask because I dont have one connected. I just have fuel going from a funnel to the main fuel port in between #2 and #3 carbs.
Click to expand...
plug the vacuum port between the carbs.
but most importantly, move it away from anything that can catch fire and if your are going to continue using a funnel to run it on get a fire extinguisher. that method if I may say so, is dumb.

apchasse said:
Ok, sorry to ask so many questions....
Click to expand...
no such thing as to many questions.
apchasse said:
I dont have a stock GS tank... I used an old yamaha tank and petcock to build the bobber.... The yamaha petcock has two ports, one for normal "on" and another port for "prime".
Click to expand...
It "should be" one port for vacuum to "open" it (the fuel flows when vacuum is applied, fuel flow stops when vacuum is stopped ) and the other one is the fuel outlet.
is there a lever on the Yamaha petcock?
a picture of it would help make sure it works like I described.
 
Ok, I checked the boots and seals... they are good... I checked all the float heights and they were all off, so set them all to 23mm (book says between 23 and 25).. Connected the vacuum port to the prime port on the petcock, and started the bike up, and it started fine, with no accel after warm up. I actually had to increase idle just a tad to keep it running.....

Problem now is it is running really rich... Jet black plugs, and bogs down when I pull the throttle. I have a cut exhaust and pods on there, shouldnt that make it run leaner??

What would you suggest I start with, adjust the floats to 25mm?

Oh, and I checked and the carbs are completely stock as far as jetting goes. (115, 40)

thanks guys..
 
Last edited:
Also the air mixture screw on the top is out 2.5 turns for all of them... should I play with that some more?
 
apchasse said:
Connected the vacuum port to the prime port on the petcock, and started the bike up, and it started fine, with no accel after warm up. I actually had to increase idle just a tad to keep it running.....
Click to expand...
What is this prime port you speak of?
you didn't hook up the vacuum port on the petcock to the fuel inlet port on the carbs did you and connect the fuel outlet on the petcock to the vacuum port on the carbs did you?

pictures??


apchasse said:
Problem now is it is running really rich... Jet black plugs, and bogs down when I pull the throttle. I have a cut exhaust and pods on there, shouldnt that make it run leaner??
Click to expand...
yes.
 
I can get pics tomorrow... but no, I connected the #1 (main) port on the petcock to the fuel inlet port (in between #2 and #3 carbs), and the other output on the tank to the vacuum port on the carb (engine side of the #2 carb). the way I have it now though works great....

So do you think I should start with 25mm on the floats to address the rich problem?
 
I have the same rich problem on my 80' 750. Book says mixture screws 2 turns out after soft seat, i have 3 screws set to 1 3/4 turns and one set at 1 1/2. I found my plugs a lot cleaner. I am still working on these setting to get it right. Don't want to go too lean! You might want to give this a try. I use the smaller flat blade screwdriver end from a 4-1 screwdriver. Fits in the mixture screw holes and clears the fuel tank. Makes easy work of it. Speaking of which go EASY when you do this, start with the screws all the way down softly hitting the bottom. Too hard, they will break. Good luck!
 
I think its the petcock... I pulled the petcock apart, and all the rubber was in pieces... So this was probably sucking gas in through the vacuum line, thus making it rich... My petcock rebuild kit should be in tomorrow, and we'll see if that helps... Ill post up my findings

Thanks!
 
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