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Please check my Carb tune logic..........

  • Thread starter Thread starter Tim
  • Start date Start date
T

Tim

Guest
I just did the complete carb tear down and dip, O'ring process off of Bike Cliff's website. When I fired the old girl up she would only run if I applied maximum choke and she spun about 3,000 RPM. That tells me that my mixture is too lean and needs to be a little bit more rich. Does that logic sound true ? By the way I set my fuel and air screws to one and a half turns out when re-esembled. My carbs are Mikuni VM26SS 1979 GS850GN.

Thank you for your Time
 
I'm having the same exact problem, and when i say exact, i mean same EXACT problem.

I was told to re-check my float levels which i have yet to do, but based on my recollection that my stock float needles were a little worn, my setting at 24mm was more like 25.5 mm with the 1.5mm play in my needles. (ordered new ones, if you haven't figured this out before I finish mine I will post back)

Thus, bike running too lean. Did you install new needles or leave the old ones? Curious to find out if we both made the same mistake, or if perhaps this is simply a matter of needing to re-adjust the pilot fuel / air screws.
 
Try setting the fuel screws 3/4-1 turn out and air screws 1.5 turns out. Did you bench sync them?

As the bike warms up, the rpms will rise, you should reduce the amount of choke. Once it is fully warm, the engine should idle at the whatever the baseline is for your bike.

With float height set, fuel and air screws set, and bench synced the bike should idle. Make sure you don't have any air leaks.
 
I was told to re-check my float levels which i have yet to do, but based on my recollection that my stock float needles were a little worn, my setting at 24mm was more like 25.5 mm with the 1.5mm play in my needles. (ordered new ones, if you haven't figured this out before I finish mine I will post back)
Click to expand...

If you really want to get the levels set right, remove the carbs, take them inside and set them in a vice. Level them side to side and front to back. Get a piece of clear tubing that will fit the drain tube on your carbs (or an adapter for your drain screw. Get a small container of gasoline (I use a radiator overflow container) and hook up your carbs. Let the gas fill the bowls. Open your drain screw and watch as gas fills up the clear tubing. Place the tubing against the side of the carb next to the gasket mating surface and measure the level of gas in the tube. If you look in your repair manual, there should be a spec for fuel height. Lets say is 5 mm +/- 1mm, you want to see the meniscus ( U shape) 5mm below the gasket surface. Do this same measurement for all the bowls. Once you know where this level is you can adjust the float height. Once you make an adjustment to the float tang, put it all back together and measure again. The whole process takes me about 3 -4 hours to get it set right depending on how out of whack the floats are.

It is a time consuming, repetitive, pain in the *ss series of measurements. When you are done you can be assured that your floats are set to the correct height. The process is way more accurate than measuring the float height with a calipers since it takes into account the condition of your needles, floats..etc.

Don't try to do this out in the yard, at night in the middle of a snow storm :p
 
My float needles are only about a year old, I did do a bench manual sync of the carbs. Almarconi what do you mean by 3/4-1 ? Does that mean 1 and 3/4 turns out for the fuel mixture ?
 
Make sure the air box and filter are installed on your bike and are well sealed.
 
You think my float levels need adjusted ?? That is way different from what I was thinking. I'll check..........that's why I'm asking.
 
You think my float levels need adjusted ?? That is way different from what I was thinking. I'll check..........that's why I'm asking.
Click to expand...

It's hard to tell because I wasn't able to watch you put the carbs back together. Did you measure the float height with a calipers when you reassembled them?

Its clear that something is screwed up because normally, when you put the carbs back on the bike it should fire up, come off the choke and settle into a nice smooth idle.

Did you also check to make sure your throttle cable was routed properly and is also adjusted? A throttle cable could cause your carbs to act weird if not adjusted.
 
I am leaning out and I have my pilot fuel screws set to 1 & ? turns. Seems like the only logical reason would be the float heights.

Did you measure from gasket bump, or gasket mating surface Tim, and how many MM did you set them to?
 
Tim said:
My float needles are only about a year old, I did do a bench manual sync of the carbs. Almarconi what do you mean by 3/4-1 ? Does that mean 1 and 3/4 turns out for the fuel mixture ?
Click to expand...
No, 1 and 3/4 turns would be written 1 3/4. What he meant is between 3/4 and 1 turn.

Stock setting on the fuel screw is usually between 1/2 and 5/8 turn. This was meant to run lean enough to meet the EPA regulations that were starting to appear. This was just barely enough for the bike to run, and it took forever to warm up enough to be able to turn the "choke" off. Depending on the state of tune and whether you are using stock airbox and exhaust, most have found that somewhere between 3/4 and 1 turn on the fuel screw is a good starting point. If you have pods and a pipe, you might have to add another 1/4 to 1/2 turn. Again, as a starting point, the air screw is set at about double of the fuel screw.

Note that these settings are for the VM carbs found on '79 and older bikes. The BS carbs on the newer bikes are different.

.
 
As mentioned already, set the fuel screw to about 3/4 and the air screw to 1-1/2. I hope you changed the intake pipe boot O-rings while the carbs were off, they are very prone to leak due to deterioration of the O-rings and will cause the mixture to be lean - which will cause a creaping idle as the bike warms up.
 
shirazdrum said:
Steve is getting cranky
Click to expand...
Whaddaya mean, "cranky"? :-k

He asked a question:
Tim said:
Almarconi what do you mean by 3/4-1 ? Does that mean 1 and 3/4 turns out for the fuel mixture ?
Click to expand...
I answered it.
Steve said:
No, 1 and 3/4 turns would be written 1 3/4. What he meant is between 3/4 and 1 turn.
Click to expand...

Apparently, when I opened up my computer this morning, the page had not been refreshed and several others had posted about the same thing.
What I saw on my screen was that I was the next poster. :o

.
 
Nessism said:
As mentioned already, set the fuel screw to about 3/4 and the air screw to 1-1/2. I hope you changed the intake pipe boot O-rings while the carbs were off, they are very prone to leak due to deterioration of the O-rings and will cause the mixture to be lean - which will cause a creaping idle as the bike warms up.
Click to expand...

That would be these little devils, pain in the arse to get to but well worth the effort..

700carbintakes.jpg
 
Steve:

Go to tools, internet options, settings, and then make sure you set to refresh the page every time you visit :p

About the intake boots, I think when I did round 1 of my carbs I had the same issue and the intake boots did fix it for me, if you get the boot rings from cycle orings get the new bolts too. You will fight to get the old ones off but wont be anything worth using again.
 
Skateguy50 said:
Steve:

Go to tools, internet options, settings, and then make sure you set to refresh the page every time you visit :p
Click to expand...
The "problem" is that I was still "visiting". :o

When I go to bed, I merely close the lid on the laptop and it goes into "sleep" mode.
In the morning, I open it up and it's ready to go.
This time, it did not refresh when I visited.

.
 
I've changed the intake boots and O'rings, changed all O'rings in carbs after a good dip cleaning, changed boots going into the air box, changed air filter and sprayed it, resealed air box. I set the air and fuel screws to 1-1/2. I'll try fuel 3/4 and air 1-1/2. As far as the floats go I just don't understand how that could be causing this. If the floats were not allowing enough gas in wouldn't the bike be cutting in and out ? With maximum choke it has a consistent humm.

Once again thanks for everybody's help
 
Um, perhaps I should stop distracting Steve from answering you as he likely knows much more than I do on this subject, but isnt the "choke" more of a fuel thing than an actual choke so the fuel level might be part of it.

One thing I did learn doing my carbs is it took me about 5 times of taking them out and putting them back before I got it right, everything matters and if you did not physically measure the float levels to be in spec you should even if that is not causing this issue.
 
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