I have a backpack blower with a small walbro carb on it which has 3 screws on it. The larger one is the idle and there are 2 smaller screws below it labeled H and L. What are the H and L screws and can anyone tell me how to properly tune the carburetor. Lately it has a real bad hesitation when I hit the throttle. When i give it gas it will bog down for a few seconds then it will rev up the the throttle speed. Any opinions on what it could be other then just needing a carb tune? I have checked the air filter and I believe it has the right gas mixture. Don't have a manual for it because its my fathers blower. thanks for all the help.
Carburetor Tuning
- Thread starter SR77
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H and L, high and low speed. 1 to 1.5 turns after going in all the way in snug, then fine tune it when its fully warmed up, a nice slight smoke is good, should be easy to rev up with little hesitation. though of course if the fuel isn't getting to it fast enough....the fuel filter in the tank may be a little clogged, also there will be a little mesh screen that can get clogged in the carb. another thing to make sure of is that the carb is on tight, and the fuel lines are good.
Small two strokes have very small jets in the
carb. Wind the screws in to determine the approx
setting then remove them and spray Carb Cleaner
through the jets to clean out any gum. Replace
the needles in there proper jet to the approx
setting and start and warm up th engine.
Then you can fine tune it from there
carb. Wind the screws in to determine the approx
setting then remove them and spray Carb Cleaner
through the jets to clean out any gum. Replace
the needles in there proper jet to the approx
setting and start and warm up th engine.
Then you can fine tune it from there
thanks for the replys. So what does the H and L screws adjust? I thought they were air and fuel screws and thats why i wanted to know how to properly adjust them, but you referenced them as speed screws. So basically i can remove the screws, spray carb cleaner in there and replace the screws and adjust by screwing them all the way in then 1 to 1.5 out and fine tune. Hope I got this right, first time messing with a carb. i will look at the fuel filter/screen and fuel line also. Like i said early, it really bogs down when i give it throttle, then will gradually increase in RPM until it reaches throttle speed. I am hopeing its just the carb or clogged filter. thanks
SR77,
I will relate to you what I know about chainsaw carbs with three screws - it should pertain to your blower more or less.
First, be VERY careful when you turn those jet screws in to seat them. The needles/seats are easy to damage if you turn them in too tight.
The way I was taught, the H (high speed jet) controls the fuel mixture at full throttle.
The L (low speed) jet controls the mixture at idle and initial acceleration. The hesitation you had when opening the throttle indicates the L screw needs to be turned out a bit until it accelerates instantly (it was starved for fuel). It's that simple - assuming you're at the 1 to 1.5 turns to start.
At full throttle, open the H screw until the engine JUST starts to run a bit rough (we call it "4 cycling") then turn it back in just a tad until it runs smooth. On a chainsaw, you leave it "4 cycle" just a tad, then it will smooth out under load).
You also have to adjust the idle speed - I'm not sure what that is on a blower. Whenever you adjust the idle speed, you need to check the L screw also, and vice versa.
Run your engine too lean and you will burn it up. Too rich and you will carbon up the rings and muffler. Always remember, two cycle engines are designed to run best at full throttle to reduce carbon build up. Idling/partial throttle is not good for them.
L = low speed/acceleration mixture adjustment.
H = high speed/ full throttle mixture adjustment.
I usually get my two jets a hair on the rich side them tweek them down to the sweet spot to avoid running too lean.
Good luck.
[ October 03, 2005, 08:34 PM: Message edited by: doitmyself ]
I will relate to you what I know about chainsaw carbs with three screws - it should pertain to your blower more or less.
First, be VERY careful when you turn those jet screws in to seat them. The needles/seats are easy to damage if you turn them in too tight.
The way I was taught, the H (high speed jet) controls the fuel mixture at full throttle.
The L (low speed) jet controls the mixture at idle and initial acceleration. The hesitation you had when opening the throttle indicates the L screw needs to be turned out a bit until it accelerates instantly (it was starved for fuel). It's that simple - assuming you're at the 1 to 1.5 turns to start.
At full throttle, open the H screw until the engine JUST starts to run a bit rough (we call it "4 cycling") then turn it back in just a tad until it runs smooth. On a chainsaw, you leave it "4 cycle" just a tad, then it will smooth out under load).
You also have to adjust the idle speed - I'm not sure what that is on a blower. Whenever you adjust the idle speed, you need to check the L screw also, and vice versa.
Run your engine too lean and you will burn it up. Too rich and you will carbon up the rings and muffler. Always remember, two cycle engines are designed to run best at full throttle to reduce carbon build up. Idling/partial throttle is not good for them.
L = low speed/acceleration mixture adjustment.
H = high speed/ full throttle mixture adjustment.
I usually get my two jets a hair on the rich side them tweek them down to the sweet spot to avoid running too lean.
Good luck.
[ October 03, 2005, 08:34 PM: Message edited by: doitmyself ]
Great, thaks alot for the replys. That helps alot. How does a screw actually adjust the jets for more or less fuel? maybe i will take the screws out to do alittle investigating to see I can figure out how it works. Wish i had a manual for this thing. So if i got this right. I screw the H & L screws all the way in and seat them, then unscrew counterclockwise (which is how you richen it)out 1 to 1.5 turns, then start it up and fine tune. I will try this out and hope that the L screw fixes the bogging. thanks again for all the help
Those screws taper to a point at the business end and seat precisely into a funnel shaped "jet". Adjusting the screw in or out changes the sizes of the opening and the amount of fuel passing thru.
Makes sense. I will give it a shot and hopefully get this thing running right. Thanks again for all the help.
Every small 2-cycle engine with less than 60cc that I have dealt with have started fine with 1 1/2 on both jets, and usually I lean the high jet out until I get the best performance and then richen the high side a little bit to make sure that it isn't running to lean which can overheat the engine and also starve the engine of oil.
Your engine acceleration issue sounds like you need to richen the low speed jet, try 1/8 of a turn counter-clockwise(screwing the jet out)to richen it and see what happens. It's not a bad idea to richen before you lean as you can seize a 2-cycle if it runs too lean for too long.
Your engine acceleration issue sounds like you need to richen the low speed jet, try 1/8 of a turn counter-clockwise(screwing the jet out)to richen it and see what happens. It's not a bad idea to richen before you lean as you can seize a 2-cycle if it runs too lean for too long.
doitmyself and MN Driver are right on the money. Try their settings and you'll be in business. You might also be surprised how much better it will run with synthetic oil. Try some Mobil MX2T.
Thanks again for the replys. I'll let you guys know how things turn out
I've been running M1 MX2T synth oil in my cheapo Homelite chainsaw. No smoke, and the engine runs great. The above info is good, do not let the 2 stroke run too lean, it will overheat and sieze.
The problem I had with mine is it's a California emissions engine, so the H and L screws had little adjustment limiters on them so you couldn't turn them very far. I had to go in and cut the limiters off so I could get it adjusted to run right. Now that it's not running so **** lean, it does just fine.
The problem I had with mine is it's a California emissions engine, so the H and L screws had little adjustment limiters on them so you couldn't turn them very far. I had to go in and cut the limiters off so I could get it adjusted to run right. Now that it's not running so **** lean, it does just fine.
On adjusting a chainsaw, do you guys adjust with the chain on or off? On my baby husky (136) it was set at the factory to run just a little too rich (too much 4-cycling). I want to lean out the H screw just a little to fix her. My only problem is that the stupid California carb has these round posts with little ridges on the outside for adjustment screws. Need to find a tool that will work!
Chain on, you do want it to 4 cycle a bit when not under load. That way when you are cutting it will get enough fuel.
Well, I finally found some time to mess around with the carb this weekend and I set both screws at 1.5 and started her up. There seems to be alot of white smoke coming out the exhaust, more so when I give it gas then at idle. If I am right, white smoke = lean? I know 2 strokes smoke alittle, but this was more smoke then usual. I had the idle running pretty good and not much smoke at all, but when i give it gas it starts blowing out. How many turns richer do you go with the H screw? Is it usual to have the L at 1.5 and the H at 3+ turns out? I just couldn't seem to adjust it right to get rid of the white smoke. Could something else be causeing the white smoke out the exhuast? thanks again for the help.
A blower will get to its best speed with the high speed jet. I would keep turning it in a little at a time till it tops out, giving the best speed to push the air. I have noticed that engines tuned for air do hesitate a bit when they are leaned out right, but they clean out and really get whipping. More RPM's more wind.
Some screws have a key lock on them so you can't adjust them, for chainsaws that are under full and heavy load, they need to run rich, for blowers lean them out and let them rev all the way up there, the hesitation comes from the limited design aplication. Or to say, it is not a honda civic or a sewing machine.
It is not very sofisticated. All good though !
Some screws have a key lock on them so you can't adjust them, for chainsaws that are under full and heavy load, they need to run rich, for blowers lean them out and let them rev all the way up there, the hesitation comes from the limited design aplication. Or to say, it is not a honda civic or a sewing machine.
It is not very sofisticated. All good though !
Hello guys, it is my first time posting here, just found out about this website. One other thing you want to check for is the stupid spark arrestor screen usually at the end of the muffler somtimes behind the plate with a few screws covering it. I always take them out and pitch them when new. They get clogged up easily with two cycle mixture.
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