Tuning a carb?

[jeffyh]

Can someone make a sticky on how to properly tune a new build carb and what to expect while break in and after?

I've been gathering bits and pieces from all the thread I've been reading, but I'm still not sure what settings and symptoms are attributing to my issues.

I just finished my gasbike GT6 build, and been trying to get it running "right". I'm running an NT carb with 16:1 fuel ratio @ 51° ambient temp. I can't get it started unless the choke is open. The screw is about 3.5 turns from closed. I've tried so many different carb pin/idle screw settings and i can only get it started choke open. Also on throttle, anything past 40% and I feel a loss in power.

Can anyone help me understand what's going on or how to fix?

 

[Chaz]

here's a carb tuning basic thread with pictures

http://motoredbikes.com/threads/nt-carb-tuning-basics.24867/


Your mix is way too rich. Go directly to 32:1

The manufacturers suggest high oil mix to protect the warranty. The above mix is much better for you.

 

[2door]

Not sure you're saying it correctly. "Choke open or off" means the lever is down. "Choke closed, or on" means lever is up. You'll want to start a cold engine with the choke on, or about half on then lower the lever to the open position after the engine starts and runs for a few moments.

I'll also concur with your fuel/oil mix. 16:1 is too much oil. It doesn't matter what the kit instructions tell you. Listen to people here with experience with motorized bicycles. Drain your fuel tank and mix up a new batch of fuel and go no higher than a 24:1 mix but 32:1 will work even better.

Don't worry about "tuning" until you have 200 to 300 miles on the engine. The Chinese 2 stroke doesn't break in quickly and needs to be run some before you start worrying about performance. A new engine will not run as well as it will after you have accrued some milage. Then you can begin to experiment with needle positions and jet sizes.


That "screw" is the idle speed screw. It isn't a needle valve but simply a mechanical stop that determnes how far closed the throttle will go and sets your idle speed. Number of turns open or closed is unimportant. After the engine is warmed you can adjust that screw either in, 'clockwise, or out, counterclockwise to increase or decrease the idle RPMs. It has no other function as far as fuel to air mix or low to high speed running conditions.

Tom

 

[jeffyh]

Your mix is way too rich. Go directly to 32:1

Thanks for that link, it was helpful. When you say rich, you mean the amount of oil is rich right? Wouldn't a rich air:fuel using 16:1 gasil make air:fuel even more rich using 32:1 gasil? I'll switch to 32:1 tomorrow. How accurate do these mixtures have to be. I barely used 500mL to test the engine so most of the original 16:1 gallon is there. Can I just add like 0.85 gals of gas to it to make it around 32:1?

 

[jeffyh]

Not sure you're saying it correctly. "Choke open or off" means the lever is down. "Choke closed, or on" means lever is up. You'll want to start a cold engine with the choke on, or about half on then lower the lever to the open position after the engine starts and runs for a few moments

Yup, I could only start the bike when the level is down, with it up, it won't start/run at all.

Getting more gas tomorrow to run at 32:1 as recommended and hope it fixes some issues. Should my needle c-clamp be on the middle position still?

My goal is to understand what the symptoms of rich or lean are and how to remedy. How gasil ratios play into the mix on how the engine runs and how it affects rich and lean running conditions. What a choke does and why it does what it does. I understand maybe 60% so far. I'd really like to make a write up about all of this so I can contribute to the community.

 

[Chaz]

Usually you'll need to choke the carb to start a fresh engine but not always. So if yours is starting up without the choke that's fine. You only have to use the choke if needed.

Yes, you can just add more gas to your gas can to change the mix ratio. So the ratios work as 16 parts gas to one part oil is heavier mix than 32 parts gas to one part oil.

You can leave the needle in the middle for a while to see how that goes. If your plug keeps fouling black and wet then try the needle c clip at the second from the top of the needle ( the blunt end). The stock carb jet is also a bit on the rich side, again to keep people from seizing pistons.

Best to just get it running and get some miles on it before you worry about fine tuning since things will change as the engine breaks in. You just really need to concern yourself with getting it running and not running so lean that it will cause problems and not so rich that it's just chugging and fouling.

There's a thread or two about checking the plug colour to see what's happening in the cylinder.

For now just remember to check all the bolts and fasteners for tightness as the engine settles in. Keep an eye on the chain tension as the chain will stretch (or wear in) a fair bit at the beginning of your break in period.

 

[2door]

Jeff,
Let's see if we clarify a couple of points for you.

Rich verses lean: When you hear those terms generally it is in reference to the ratio of air to fuel. The carburetor is tasked with mixing the air and fuel to the proper ratio to achieve the most effecient 'burn' or to get the most power from the explosion that takes place in the combustion chamber.

Rich, would indicate that the mixture has more fuel in the mix than is required. The results will be a loss of power, smokey exhaust, sluggish acceleration and if checked, the spark plug will appear wet or oily and dark in color, possibly even black. There are other symptoms, but typically a 'rich' condition is not damaging to an engine but can prolong or even halt the 'breaking -in' process whereby the piston rings don't seal as designed resulting in low compression.

Lean, indicates that there is less fuel in the mix. Determining the ratio, as I said, is the job of the carburetor. The air side is for the most part 'fixed' with minor changes being accomplished by air filter design, intake manifold shape, length and modifications to the engine such as porting and piston modifications. With a somewhat predetermined amount of air being drawn in, it is the engine tuner's responsibility to help the carburetor find the most effecient amount of fuel to mix and deliver to the engine. This is generally an experimental process and is done by changing the 'C' clip position on the needle valve, getting the float level right, and sizing the main jet orifice to deliver the proper amount of fuel for a given condition. An overly lean condition is to be avoided. It can damage an engine in several ways and can be determined by high engine temperatures, a spark plug that is light grey to white in color and other symptoms. Lean = bad.

You might also hear the term, "oil rich". This simply means that the ratio of fuel (gasoline) to oil contains more oil than is often recommended. It has nothing to do with the fuel to air ratio explained above. However....

There is a correlation with how much oil used to the air fuel mix. Unlike a four stroke engine that has a seperate lubrication system, a 2 stroke relies on the oil that is mixed with the fuel for lube. Briefly what this means to the air/fuel mix is that the less oil used, the richer the air fuel mix will be because there is more fuel (gasoline) in a given quantity of the mix. The oil doesn't burn or play into how much power the combustion process produces. Therefor the less oil you use, the richer the air to fuel ratio is going to be. Conversely, the more oil you use in a mix, the leaner will be the air/fuel mix.

The choke: Usually the choke of a carburetor is a movable plate, or cover that when used, restricts the amount of air that can be drawn into the carburetor which as explained above, will change the air/fuel mix, making it overly rich which can assist starting a cold engine. It usually isn't needed if the engine or ambient temperature is high enough or other conditions allow the engine to start without a rich condition.

These are over simplified explanations but I think it addresses some of your questions and hopefully clears up some of those issues for you.

Tom

 

[Chaz]

Great post, Tom. I had completely overlooked the confusion of oil/gas and fuel/air mixes.

 

[jeffyh]

@2door - lol, I think that alone would be worth a read for new people like me. Very well written too. Thanks for that.

Just switched to ~32:1, and the ride difference is amazing. I did not think that 100 mL of oil would make that much of a difference. Even the engine temp lowered significantly. My idle is still jumping all over the place, and I'm having some 4 stroking, but I'll figure that all out once things are broken in.

I'll report back and hopefully get to write up something after I get some miles on it and start playing with things which will probably be a while cause I built this thing so I didn't have to walk 2 miles a day, hah.

Happy riding!

 

[Bikeguy Joe]

I hope you meant to say 'just switched to 32:1'

 

[jeffyh]

I hope you meant to say 'just switched to 32:1'

derp... that's what I meant, hah.

 

[sbest]

Hey Jeffyh,
I had the same experience as you. I ran 20:1 oil for 3 tanks and eventually fouled the plug. I switched to 40:1 (32:1 or 40:1 makes little difference) and the bike ran so much better with no oil dripping out of the pipe any more. Mine didn't need choke either, and the plug was black. I lowered the float level and dropped the needle so the plug runs tan and I now need choke to start when cold. Bike runs great now. Also had to do some work on the head because I had rattle on the hills.

Steve

 

[jeffyh]

I lowered the float level and dropped the needle so the plug runs tan and I now need choke to start when cold.

Hmm, didn't know you could adjust float. Came across a great guide that 2door referred to in another post. Looks like I'll be doing that this weekend...

I finally have to use choke to get things started. Temps just dropped into the low teens and I can't keep the bike in the basement anymore. It's amazing what cold will do to an engine.

 

[sbest]

If you don't need the choke your jetting is too rich. What is wrong with too rich if it runs and and you are happy with it? Nothing, if you are happy with it, but you are losing fuel and performance.

First of all, I have to confess I am new to Happy Time motors, but I have a 40 year history with 2 strokes. 2 strokes (especially tuned pipe 2 strokes) are very susceptible to jetting issues. Too lean they run hot and detonate, burn pistons, destroy bearings. Too rich, they run lazy, never "make pipe", foul plugs and ports, use gobs of fuel, and just don't perform like they should. The manufacturers usually push them out the door a bit on the rich side. My China Girl Grubee was no exception.

So what is involved with tuning a Carb? First you have to know how to read if the fuel mixture is burning rich or lean, then you have to know what to adjust and how.

Reading the mixture, rich or lean is done by reading the base of the insulator of a new plug. You warm up the engine, put a new plug in and run it full throttle, full load as long as you dare. A few seconds to a minute will work. Then you look at the base of the insulator. This is what you usually see:

Pretty hard to see the base of the insulator isn't it? You can just barely see a dark band.
So we often cut the threads off the plug to see the "smoke ring"
"CUT A NEW PLUG! ARE YOU CRAZY?" So waste a new plug or waste a motor, your choice.

This whole series run from very rich to over lean to give you an example.

Here is another graphic I like about reading a plug:

It is the lower smoke ring you read, not the top one. The top of the insulator is more about the plug heat range and reads much the same as mixture, but takes longer to develop. Black is too cold, white is too hot.

If the smoke ring doesn't push all the way down to the base of the insulator, it is a sign you are not developing much pressure in the cylinder. More power can be had.

Whew, not to adjust. You have 3 mixture adjustments on most carbs. Idle/low-speed/pilot which is a fixed (nonadjustable) jet on the NT carb. The needle for the mid-range, you can learn about that later, and the main jet for Wide Open Throttle (WOT). The main jet is most important as a starting place and where performance is made and where engines are made or blown up.

You need a selection of main jets. Any jet that fits will work as long as all the jets you have are from the same supplier and are marked for sizes. Oddball jets will give oddball results which can be more expensive than buying a selection of good quality jets. Old jets corrode and varnish up and go lean, wire cleaned or needled jets go rich with time. Bigger numbers are richer. So what if your present jet is not numbered and you don't have a selection like I find with my NT? Here is the backyard hack to jetting:

If your jet is very rich, you can put the smallest strand of copper wire through it. Ball it up a bit on both ends so it doesn't fall through or restrict flow. If this leans it out too much, you are left with removing it and lowering the float lever a milimeter or two. If the jet as supplied is too lean, you have to make the hole larger with either a special micro sized drill (can drill it by hand) or filing or wearing it out with a wire or welder's tip cleaner set.

That is it guys, back alley jetting.

Steve