Tuning Two-Stroke Engines

As promised last week, here is the instructable inspired by the violent failure of this instructable

Step 1 Why it Failed

The faliure of the 50 mph trike was most likely not caused by wear and tear on the engine, or even damaged parts, but by a simple mistake in the tuning process. Two stroke motors are VERY touchy in the tuning process, if they run too rich, they bog down and quit. if they are too lean, they will over-power themselves, and 90% of the time they explode violently. I'm not a certified mechanic in the least. but I do know what I am doing when it comes to re-building and tuning small engines, mainly two strokes. The reason the trike's motor blew up, was because it was running too lean, and it was a racing motor (potentially deadly combination).

Comments

[LiquidLightning]

This guide is great, but you should add that this process will only apply on walbro carburetors. Also, after you finish, pull the spark plug and check the color of the tip.
If it is mocha brown, it's perfect, black means too rich, and white means your motor may just explode (too lean). Also, when it bogs down, typically that is over lean, and when it sputters (known as four stroking), it is too rich.

[2 stroke]

I LOVE 2 STROKES

[ilpug]

I have a Husvarna weed whacker. recently it started to become impossible to start. i would pull the cord about 5 times before it would even turn over a couple times. i took it all apart, cleaned it, greased it, and reassembled. it has a new spark plug and a new fuel filter, although the air filter is a bit dirty, but i have run it before much dirtier. i have been looking for the tuning pins, but i cant find them! i found two little pin things that are right on the carburetor, but they have no markings or any slots for a screwdriver. any ideas?

[bleachworthy]

Firstly, the air filter being dirty blocks air coming in, and will make the engine run rich, and your spark plug will eventually foul out. Second, its hard to tell which needle on the Carb is which. Best way I've found is to tighten both all the way closed, and gradually open one a quarter turn at a time until it sputters. If you open one more than 3 whole turns with no results, close it back up and try the other. In your case this may be difficult, since you have no way to grip the heads of the needles well. If you can, take a photo of your Carb so we can get a good look at it. Its completely possible that yours is not adjustable (thanks to the EPA), in which case I advise an aftermarket (different) carbourator. Most have a simmilar bolt pattern, and will interchange nicely. just be sure your donor motor is about the same size. My mccoulloch has a homelite Carb, because replacements and rebuild kits don't exist anymore for my make. Hope this helps!

[bcampbell61]

i have a 2006 kawasaki bayou 250 it runns good if u can start it and once u get it running u must drive it but as soon as it gets hot it runns fine but if its cold an u go to start it ull have to pull start it like 300 ft b fore it even tries to start plz help irf i cannot get it fixed ill scrap it!

[bcampbell61]

yea my montgomery ward chain saw did the same thing except it was the gap on the magneto!!

[ilpug]

thanks for the quick reply. today i cleaned the air filter, and checked the sparkplug, which was fine. then i took the whole machine apart, cleaned what was dirty, and put it back together. same problem. i will look into aftermarket carbs, because it really looks like this one was made to not be adjustable. on closer inspection of the pins, I found that they are perfectly round, and impossible to turn even with needle nose pliers. The instruction manual does say that you must turn those pins to tune it, but i think it was for a different model.

[bcampbell61]

check the gap on the magneto!!! and the gap on the spark plug!

[bleachworthy]

Well, you can always try to file or grind a flat surface on the sides of the needle heads for grip. Your best bet is to use a dremel, or other small rotary tool and make a slot for a screwdriver on the heads. If you can get them to turn, take them out and clean their seats. Make sure when tightening that you do not overtighten, this can permanently damage them! These things are super-fragile when it comes to tightness

[sergeantday]

Got a Stihl BG 85 that will not run at high speed. New air filter, new sparkplug, even cleaned the spark arrester. Factory book limited in problem solving. Attempted to adjust carburator settings but this failed. Gas filter is clean. Anyone got an idea how to correct problem Thanks for your help and stay safe

[big trav]

Could help if you stated what the problem is!!!!!

[ilpug]

@seargentday: have you tested the distance that the throttle lever moves hte actual throttle?

@big trav: he did state it: it wont run at high speed.

[TSC]

When your done tunning it will it idle just right ?

[MrTrick]

I'm sorry, but 2-stroke motors are more efficient (speaking in power-weight ratio) than 4-stroke motors because they have one power stroke out of 2, whereas a 4-stroke motor only has one power stroke out of 4. The number of moving parts is a minor aspect of efficiency that is dwarfed by other things like compression ratio, and displacement is a measure of engine size. Also, if you are going to take a picture from the wikipedia article discussing 2-stroke engines, it would be a good idea to: a) quote the article directly when talking about theory b) attribute the source of the image.

[big trav]

In theory you are right but in practice you are WAAAAAAAAAYYYYYYYYYYYY wrong, four strokes are more efficient and will run for easy 4 times as long on the same amount of fuel. Yes per cc 2 strokes USED TO produce more horse power, however some of the modern 4 strokes have matched or exceeded what equivalent 2 strokes produce with much higher torque. 2 strokes are ineficient in the way they use fuel due to a number of reasons one is the fact that they blow half of their fuel charge up the exhaust to achieve nothing, another is the fact that the engines lubricants are being carried in the fuel instead of in a sump completely separate to the combustion cycle. I think you should look at how a 2 stroke really works and all of the data which shows just how inefficient and polluting 2 stroke engines really are.

THEN you will see why I love them...lol

[MrTrick]

I didn't say fuel-efficient.
I said weight-efficient. As in, for a given power output, 2-stroke motors can be lighter.
(Even if, as you've said, they use more fuel)

;-)

And yes, 2-strokes make a wonderfully awful noise. :-D

[bleachworthy]

ah, good point, and I did not get that image from wikipedia, I got it from http://www.southernskies.net/page_info/runningtwostrokeengine.html and the displacement comment is a "duh" thing. I mentioned that the displacement difference, because it really does matter in this world of bady needed efficiency.

[TheBlunderbuss]

What choke setting should the engine be at, at any time? That is, if the engine has a choke (both my weed-eaters do).

[jbeidle]

The choke is only for cool weather starting. It provides a richer fuel-air mix for the initial start. Most 2-cycles instruct you to choke, pull until the machine tries to start (usually 2-3 pulls for a well tuned engine). Then you move to the half-choke position and pull to start. Once you got the engine running, leave it at half-choke for a short time (anywhere from 30 - 60 seconds, depending on the manufacturer) until warm. Once the engine is warm, you won't need the choke.

For this tuning procedure, you need to be able to start and warm the engine. Tuning a cold motor causes more problems.

[big trav]

If the engine is cold and has not been started for a couple of hours or more then use full choke, after a couple of pulls the engine should fire, then you flick to half choke and it should fire and run within the first 1 to 3 pulls, then you should immediately flick on to the run position. you may find it a little "doughy" for a few seconds while it warms up, but then it is right to go. This said I have a ryobi brushcutter which is a right pig if the fuel is more than a week old, and find with stale fuel it will not run unless it is half choked, Piece of crap that it is!!!

BUY STIHL, they are the best

[BIGHAIRYDUDE]

hey i need to ask you a question i got a new motor and the primer bulb will suck in and out pretty fast and there are no visible cracks. but i can see any gas going through the lines, on the side with the big screw where gas is sucked in, when i unscrew it it releases pressure and gas just spews out like crazy from the lines. i cleaned the carb and i just can fix it i dont know what is wrong if you could help it would be well appreciated

[bleachworthy]

blah, I need to read things right the first time... I'm assuming you have a diaphragm style carb, it sounds like a valve is not opening when it should. It's probably going to be underneath the diaphragm, you may need to remove one or two SMALL screws to release a little lever that the diaphragm plate presses down. Under this lever should be a spring, and under the spring either a ball or a pin/needle like thing. Be sure everything is clean, and be as gentle as possible in every step. In testing, to prevent fuel spray, open your tank cap first. Let me know if this helps!

[BIGHAIRYDUDE]

i did exactly that before and it didn't make a difference! i have no idea what to do, and i have worked to small engines for a while like rebuilding golf cart engines and weedeaters of all sorts but this stumped me.

[BIGHAIRYDUDE]

yeah

[wow-amazing]

Heh, more like 15,000 rpm (that's what my goped does, but it's a little farther from my vital organs). Is there a way to tune the high-end without full throttle?

[bleachworthy]

sorry it took so long, the only good way to tune it is to use WOT, I'm sure you can fine tune it in any range, it's just very tedious, and can require specialty tools like airflow meters, and air/fuel ratio meters (Spendy).

[BIGHAIRYDUDE]

my weedeater only has one screw what do i do

[bleachworthy]

Look for an H or a L on the carb near the needle. I'll bet the needle is for high speed mixture.

[BIGHAIRYDUDE]

okay well i have messed with the needle a billion times and the mix just is'nt right it doesn't seem to be doing anything it will idle but if you give it gas it bogs down but doen't quit and it won't accelerated its like theres no torque or something i made a motor bike and it has lasted about 140 miles so far and i have cleaned the carb a couple of times, and the motor is really old to, its a 25cc craftsman from i dont know when because i found it in some guys trash and fixed it up.

[bleachworthy]

It's possible that your carb is plugged up a bit, mine does all the time, all you need to do is unscrew that screw all the way (making sure not to lose the spring wrapped around it), press your primer button several times until gas comes from the screw hole, or pull the spark plug and pull like crazy until gas comes out. Check the needle, make sure it isn't bent, or otherwise broken, then clean it up and put it back in, unscrew it about 2 turns for starters. if this doesn't work, you may need to pull the whole carb apart for cleaning, it's not difficult, you just need to be very gentle, and remember where things go. Most carbs I have taken apart have re-usable gaskets and seals, this makes the fix FREE!!! hope this helps, and good luck to you!

[BIGHAIRYDUDE]

o.k thanks

[dirtbiker809]

some of this information is potentially misleading. what i was told was a 2 stroke engine has the same horsepower as a 4 stroke engine about twice its size in displacement. however, i thought that 2 strokes consumed more gallons per hour than a 4 stroke of equal horsepower and displacement. now granted, 2 strokes have tons of nuts, i just thought they were always less fuel friendly than 4 strokes

[Rectifier]

Nope, you're right. 2 strokes are much more weight/size efficient, but in no way are more fuel efficient.

When you see that green fuel-air mix rebound back in the animated pic - not all of it makes it back into the cylinder... Especially when not running at the resonant frequency of the pipe.
With a CVT (modern moped belt transmission) the engine is maintained at the optimum RPM through a wide range of road speeds - with a geared transmission, much gas is wasted outside of the powerband as well as the bike acting like a pig at low RPM.

On the other hand the latest direct fuel injection 2-strokes have completely insane power and efficiency - they avoid the fuel loss problem (and burning all that oil, too) by injecting fuel after the ports close, like a diesel. I personally test rode a 50cc bike with DFI up to 90kph, TOTALLY STOCK.

Source: I own a 2-stroke moped that has tons of nuts for its 50cc displacement. It also eats gas compared to similar, gutless 4 strokes on the road (almost twice as much, in fact - 3L/100km)

[sawdan]

This is a subject i am interested in. can you quote figures that show what "you where told"?
Just wondering.

[www.microbike.ie]

 Hi on the first picture you have 49cc pocket bike engine, that engine have only one idle screw how would you tune up that engine?

[shuadog]

This is very helpful, thanks. I have a Stihl weed wacker that needs a tune, I think. The issue is that it starts and runs fine, but sputters when I give it the gas and try to open the throttle all the way.

[finalreminder]

Mine has a H, L and I screw...and a choke.

[jbeidle]

The I screw is idle adjust. It controls the low-end throttle position to keep your motor running at idle. It's the last thing you touch.

[finalreminder]

Many thanks!! :) Followed your tut and my hedge strimmer purrs...well purrs as well as a 2-stroke can

[jbeidle]

Good instructable! Thanks.