[tboooe]

Preface: I havent messed with carburetors in over 25 years, and even then I was not very good at it so please be gentle. Being a total carb newbie I want to take a methodical, stepwise approach to diagnosing the issue and ultimately identifying the correct fix, otherwise I will likely get overwhelmed making too many adjustments and probably end up making things worse.

I finally got one of my Superlights (#669) running. It is 100% stock. I adjusted the idle so it sits at 1100-1200 rpm. The issue now is that I think it is running incredibly rich. When I rev the bike it starts to pop through the exhaust at around 3500-4000 rpm. Additionally, the ends of the exhaust is covered in dark black soot. Plus based on the exhaust smell I think its rich.

First question, does this sound like the bike is running rich?

 

[OddBird]

Accelerator pump diaphragm leaking? Stuck float valve? It sounds like something beyond an idle mixture being out of adjustment- when was the last time the carbs were apart? I am no expert on Mikuni carbs, so I'm going to shut up now.

 

[chris_f]

I claim no particular expertise but popping in the exhaust is usually lean, there not being enough fuel in the cylinder to fire properly so it dumps into the exhaust where it eventually does ignite. So I would say rich idle circuit, lean mixture off idle.

 

[duc96cr]

Clean and rebuild them first , new diaphragms etc. then you’ll know how much tuning you need to do. It doesn’t sound like they are working well.

 

[tboooe]

Thank you guys. Before we jump to potential fixes can we first determine if the bike is running rich or lean? Here is more information. At idle, the bike seems to run ok. There is no popping or surging. It idles nicely at 1100-1200 rpm. When I apply throttle, the bike will pop in the exhaust starting at 3500 and continuing through 4000 rpm. Does that help diagnose if the bike is running rich or lean? Is there any other tests I can do to confirm this?

 

[duc96cr]

If the slides are opening smoothly and the carbs are synched, try opening the air screws about 4 to 4 1/2 turns out and try it again.

 

[tboooe]

If the slides are opening smoothly and the carbs are synched, try opening the air screws about 4 to 4 1/2 turns out and try it again.

Thank you. Turning the air screws out will allow more air into the carbs right? That means you think based on my description that the bike is running rich? Sorry for the questions I am just trying to understand the problem first so I can then understand the solution. Also, the air screws for each carb is behind the cap right? You are suggesting turning them 4 - 4 1/2 turns out from full closed right?

 

[chris_f]

It can be both, rich at idle and lean off idle.

https://www.mikuni.com/fs-tuning_guide.html

Tuning Tips & Manual Downloads | Mikunioz

Technical information

Technical information on Arrow karts, MOTA software , Mikuni carburetors

www.iwt.com.au

 

[tboooe]

It can be both, rich at idle and lean off idle.

https://www.mikuni.com/fs-tuning_guide.html

Tuning Tips & Manual Downloads | Mikunioz

Technical information

Technical information on Arrow karts, MOTA software , Mikuni carburetors

www.iwt.com.au

Thank you. How can my bike be rich at idle? I had to turn in the idle adjustment screw because it was idling way too low. Also the suggestion by @duc96cr is to turn out the air mixture screw which means to increase the amount of oxygen right? If so that would suggest the bike is rich? Sorry again, total newb here just trying to learn.

 

[chris_f]

Do you know specifically what carb they are? Mikuni TD type? Looks like that is what was on these.

The pilot circuit has an air bleed and fuel. The floats being set to high could be an issue. Or the air bleed could be to far closed. Or the jet orifice could be ovalled out allowing to much fuel. Once the throttle is open far enough it could still only be getting fuel from the pilot circuit, which would cause a lean condition.

Mikuni carburetor operation and tuning

Mikuni motorcycle carburetor operation and tuning

www.iwt.com.au

 

[tboooe]

Do you know specifically what carb they are?

These are the stock 38mm Mikuni BDST carbs.

 

[chris_f]

Here is another good reference, at the very bottom there is a section on diagnosing issues.

Mikuni Tuning and Jetting Guide

What kind of Mikuni do you have? You may not know it, but not all Mikuni carbs are the same. That 24mm Mikuni that needs replacing on your 2005 Yamaha TTR125 isn't exactly the same as the VM24...

www.vintagebikebuilder.com

 

[Ricko]

I second duc96cr, rebuild first. Good cleaning, and see what you have, clip needle? Jets sizes? Float ok?
See where you are and go from there. Inspect rubber intake flanges and every hoses while carbs are out.

 

[Mal’s900SS]

By ear, and a long way away, I was led to believe mid range popping or fluffy is rich, high revs fluffy or popping is lean. On the over run- decel is air bleeding into the exhaust, unless you’re working for an exhaust shop then it’s a tuning issue!

you can’t go off exhaust soot or colours anymore as all unleaded runs black. Grab an exhaust co meter.


I understand you are doing an original resto so don’t get upset then I say:

grab a drum of ‘carbrasol’ or the molasses based carby cleaner, strip the carbs and place in the drum to soak and clean for 3 days. Take the lid off an pour the mixture out into the waste oil drum.

all the carby bits will be dissolved and gone from the planet, (read Carbrasol gets rid of unsightly carburettors )

buy some FCRs and smile on

 

[Stillrollinalong]

Preface: I havent messed with carburetors in over 25 years, and even then I was not very good at it so please be gentle. Being a total carb newbie I want to take a methodical, stepwise approach to diagnosing the issue and ultimately identifying the correct fix, otherwise I will likely get overwhelmed making too many adjustments and probably end up making things worse.

I finally got one of my Superlights (#669) running. It is 100% stock. I adjusted the idle so it sits at 1100-1200 rpm. The issue now is that I think it is running incredibly rich. When I rev the bike it starts to pop through the exhaust at around 3500-4000 rpm. Additionally, the ends of the exhaust is covered in dark black soot. Plus based on the exhaust smell I think its rich.

First question, does this sound like the bike is running rich?

The smell is a good indication of being rich. Since most fuel is now unleaded the old pipe colours are not such a good indicator any more.

As for tuning there are so many overlapping varaiables. As shown below each part you are adjusting has a different amount of influence depending on how much throttle is being used. If its been 25 years maybe best left to an expert.

 

[belter]

it's an idle mixture screw, not air. want to be at least 4 turns out.

they're cv carbs, the slide carb guides don't really apply.

are you riding it or revving it in neutral?

 

[JEFFRPM]

I'm a carb tuning God (If that didn't sound arrogant) I'd really like to help out here but tuning a carb on a keyboard is totally impossible in my opinion.

I, you, we have to actually hear what it's doing to come close to a diagnosis but I'd start with a rebuild complete with ultra sonic cleaning if it's sat around for more than 4 months with ethanol infested fuel in it, replace those Diaphrams, sync them, check intake boots, reinstall them with fresh fuel & start from there.

 

[ducvet]

1. Make sure the tuning guides you read are for the MikuniCV carb and not a mechanical slide carb .

2. As Belter mentioned you have fuel screws not air screws there is a difference in how the air/fuel changes.
Fuel screw : in = less fuel or leaner
Air screw : in = less air or richer

3.Do not remove or rebuild your carbs if you do not even know how to tell if they are rich or lean, diagnose BEFORE surgery not after.

4. engines can pop or backfire out the exhaust for both rich and lean conditions, you need other evidence to prove one or the other. Air box popping = lean does it do this also?

5. Have you checked your spark plugs? Fresh clean plugs held at the problem area and then kill the ignition, a overly rich condition can blacken things pretty fast depending on how far off.

6. Have you verified your enrichener plungers are both connected and shut off? if one has gotten the other side of the arm or held open due to being held open you will be rich as it warms up the engine.
7. Has anything changed with the ignition system....
new non-resistor plugs.
New non-resistor plug caps
new aftermarket coils
New aftermarket ignition boxes

 

[tboooe]

Thank you everyone for takin the time to provide your input. It is greatly appreciated!

Here is another good reference, at the very bottom there is a section on diagnosing issues.

Thank you for this. Definitely very informative. Just skimming it I did learn something that was causing me a lot of confusion: "The mixture screw (also referred to as the air-mixture screw, fuel-mixture screw, idle-mixture screw or pilot screw) works in concert with the pilot jet." I was getting confused because people use these terms interchangeably. I know now that because the adjustment screw is closest to the engine, this carb has a fuel mixture adjustment screw. Baby steps!

are you riding it or revving it in neutral?

Just revving in neutral for now. I assume by your question I need to ride the bike and put it under load?

3.Do not remove or rebuild your carbs if you do not even know how to tell if they are rich or lean, diagnose BEFORE surgery not after.

Yes! This is exactly what I plan to do. I want to first understand the situation before I make any adjustments or start taking thing apart.

4. engines can pop or backfire out the exhaust for both rich and lean conditions, you need other evidence to prove one or the other. Air box popping = lean does it do this also?

I only hear popping out the exhaust. If it was popping in the airbox would it be obvious or do I need to listening carefully?

5. Have you checked your spark plugs? Fresh clean plugs held at the problem area and then kill the ignition, a overly rich condition can blacken things pretty fast depending on how far off.

Good idea!

6. Have you verified your enrichener plungers are both connected and shut off? if one has gotten the other side of the arm or held open due to being held open you will be rich as it warms up the engine.

You mean the choke right? It seems to be functionally correctly. If I start the bike when cold the engine will rev up to about 3K rpm and if I then close the choke the RPM will drop and the engine will stop.

7. Has anything changed with the ignition system....
new non-resistor plugs.
New non-resistor plug caps
new aftermarket coils
New aftermarket ignition boxes

Completely stock. The bike has only 6K miles on it. Its in remarkably great shape, just not ridden much at all because it was more of a show bike. As I understand it, the previous owner did start it on occasion.

 

[ducvet]

" If it was popping in the airbox would it be obvious or do I need to listening carefully? "

You should easily be able to tell if it is a airbox pop.

"You mean the choke right? "
Like the air screw, fuel screw a "choke" is a different device than an enrichener, a choke blocks air where an enrichener opens a port and adds extra fuel. Since no one uses the term except mechanics people call any start up device a "choke" , you only need to understand what you have to diagnose a bad one. The stock carbs use a thin bar activated by a cable on the handlebars, the bar has 2 forks which pull open a plunger on each carb opening a port and adding a bunch of fuel. If one of the plungers does not return closed then after the bike is warm and you shut the enrichener off the carb will still get too much fuel. Simply observe that both plungers are closing and opening with the handlebar lever.

To test lean or rich one thing to try is a more air test, remove the airbox lid and see if the condition gets better or worse AT THE PROBLEM AREA. Do not concern yourself with other throttle settings or rpms you are only looking to see if the problem gets better or worse with more air.

While the airbox lid is off there is another test you can do.

1. step one make sure NOTHING can fall in your engine and blow it up, sounds simple right?

2. step 2 remove the airbox lid and air filter

3. run the engine and verify the slides are opening as they should,likewise observe each carb for abnormal amounts of fuel.