Gas Two-Stroke Engine Question

[Monkey Wrench]

Gentlemen, I am seeking your collective expertise on an issue I am having with my McCollough gas two-stoke leaf/grass blower.

Not sure if you tune a traditional gas two-stoke the same as a nitro two-stroke, but this thing only seems to run best if I leave the choke lever at 1/2. If I open the choke (i.e the engine is not choked at all) it sputters and never winds up to full RPM's no matter how long I pull on the throttle.

It's a 32cc engine and my thinking is, because it runs better at 1/2 choke, that I'm running too lean overall, but I'm not sure and I don't want to risk burning the engine up trying to figure it out.

So, am I running too lean? My fuel mixture is 40:1, if that helps any.

Thanks!

 

[Racer 1966]

Here's the way I've always tuned my small engines. Get it running, hold it wide open and tune the HSN. The rpms will go up then it will go flat,back it up until it smoothes out again, the back 1/8 -1/4 turn. Then tune the LSN until you get a smooth transition to high speed. Adjust the idle as necessary.

40:1 is a leaner mix than I use, I go with 32:1 Never burnt one up yet.

 

[4u2nv]

Theresa jet screw in there that sounds like it might be getting plugged. Best bet would be to take the bottom cup off the carb and give it a good cleaning. When cleaning that jet screw, make sure you clean it with something like a wood match stick or something like that. Dont want to bore that hole out at all. (as much as I know you like boring holes..lol)

 

[olds97_lss]

How old is it? Have you ever had work done on it? Did fuel get left in the tank for winter storage?

Most 2 cycle machines (leaf blower, lawn mower, weed eater, chain saw...) can be held WOT for an extended period of time without issue. Typically they are mechanically limited due to load, be it a fan for a blower, a weed wacker head, a clutch/chain of a chainsaw...

Try holding it WOT and richen up the HS jet until it bogs and gurgles like RC's do when they are running rich. Open the choke completely and see if it clears up and starts running smoothly again or at least better. Then you know if your in the ballpark. If it requires you to have the choke half closed, then either the settings are off or it's gummed up like 4u2nv said and a cleaning of the carb will be required.

I used to work on small engines as a kid/teen. My dad never took anything to a mechanic... we didn't need to. He would buy old lawn mowers at garage sales just to have a broken one to fix. I fixed my fair share that way as well with him. Then I did it for 2 years while going through college at a small engine shop that serviced pretty much everything a typical person could have... and in some cases, non-typical. Like an ice fishing auger or a go-kart. Mostly 4 cycle mower engines, but I grew up with both 4 and 2 cycle. I worked on quite a few 2 cycles in the shop though as well. I can pretty much build any 20hp on down to a 1hp gas engine in my sleep... well, I could 10 years ago when I was doing it every day.

 

[Racer 1966]

Theresa jet screw in there that sounds like it might be getting plugged. Best bet would be to take the bottom cup off the carb and give it a good cleaning. When cleaning that jet screw, make sure you clean it with something like a wood match stick or something like that. Dont want to bore that hole out at all. (as much as I know you like boring holes..lol)

Jet screw????? I wouldn't take it apart until a last resort. The gasket and diaphragm don't like to be messed with in these small carbs. For cleaning, use a carb spray(I recommend GumOut). Spray through the fuel supply line and in the intake venturi. Also,check the spark plug for a hot spark, take it out connect the wire, hold the plug against the motor and pull the start rope. Should be a bright blue spark.

I bet adjusting the screws will be your fix.

 

[olds97_lss]

Be careful with the carb spray though. The plastic fuel lines on all of those things has a tendency to get brittle and crack if you leave the carb spray inside them for very long. It would be better if you unhooked the fuel line from the carb and sprayed it directly into the carb.

 

[4u2nv]

Jet screw????? I wouldn't take it apart until a last resort. The gasket and diaphragm don't like to be messed with in these small carbs. For cleaning, use a carb spray(I recommend GumOut). Spray through the fuel supply line and in the intake venturi. Also,check the spark plug for a hot spark, take it out connect the wire, hold the plug against the motor and pull the start rope. Should be a bright blue spark.

I bet adjusting the screws will be your fix.

There will either be this in there, or a regular jet

Gas is so poor these days that if its let sit in anything over a period of time, it goes bad and gums stuff up. Id try some Sea-foam in the gas as well. That might clean it out before you have to open the carb. Its good maitnance to clean a carb internally every season anyway.

 

[Racer 1966]

Be careful with the carb spray though. The plastic fuel lines on all of those things has a tendency to get brittle and crack if you leave the carb spray inside them for very long. It would be better if you unhooked the fuel line from the carb and sprayed it directly into the carb.

Never heard of that, besides after cleaning you usually start it up to see if it's fixed and the cleaner is gone. However, if you put the brass jets, float valves etc in cleaner to soak for, well.... a year??? it will eat them up!!! LOL

 

[hb1111]

maybe your pressure line to the tank is cracked or maybe even or the primer bulb this would cause it to run lean

 

[olds97_lss]

Never heard of that, besides after cleaning you usually start it up to see if it's fixed and the cleaner is gone. However, if you put the brass jets, float valves etc in cleaner to soak for, well.... a year??? it will eat them up!!! LOL

It will ruin lines in a matter of a few hours if you let it soak depending on what the line is made of. I've had it happen on more than one occasion. In weed eaters and chainsaws (small 2 cycle applications), they typically use a clear plastic line, I'm not sure what it's made of. Carb cleaner makes those brittle.

 

[Monkey Wrench]

How old is it? Maybe two years old, at most.Have you ever had work done on it? No, has always ran like a top before getting it out this Spring. Did fuel get left in the tank for winter storage? Yes, but I did put stabilizer in it and ran it through. Have done that yearly with all my two-strokes and never had any issues. Boith the weed whip and the edger run fine this season.

I know the 40:1 is a bit leaner, but it's an average of what all my engines use. I'll try the tips presented and let you know if I still am having issues.

Thanks to all of you for the assist, I appreciate it!

 

[4u2nv]

clean the carb.. The higher the gas prices seem to go, the poorer the quality of fuel.. Good bet its gummed up.

 

[Racer 1966]

Let us know if you need more help/info. I'm sure Olds, 4u2nv and I can argue about what your next step will need to be! LOL

 

[Monkey Wrench]

Well, I pulled the carb off and gave it a thorough cleaning. The jet cannot be removed because it looks pressed in (but that may just be me). The two adjustment needles can only turn 1/4 and if I turn them clockwise all the way, it runs worse. Doesn't matter what combination I try, turn either needle or both and it runs worse.

I pulled the primer bulb off and checked for cracks and found none. Both fuel lines are fine as well, no leaks or cracks.

The manual actually calls for 40:1 mix and I may have my mix wrong, so this weekend I'm going to mix a specific 40:1 and run it through. I may think I'm running 40:1 but maybe did it richer. That's about the only thing I can think of to try.

Again, my thanks for the assist, fellas. I appreciate it!

 

[redneckracer1979]

Did you try a new spark plug yet? Sometimes that is the problem also.

 

[Monkey Wrench]

Did you try a new spark plug yet? Sometimes that is the problem also.

No, not yet. I plan on picking up a fresh one as well.

 

[olds97_lss]

I'm guessing since you run nitro that you already checked/cleaned the air filter...

 

[Monkey Wrench]

I'm guessing since you run nitro that you already checked/cleaned the air filter...

Yup, sure did, Olds. I I cleaned it, let it dry real good, blew it out with the compressor and put a very light coat of filter oil on it, just to be sure.

 

[redneckracer1979]

That could be the problem right there then. Weedwacker engine air filters aren't really designed to have oil on them. Try running it without the filter and see if it runs any different.

 

[Racer 1966]

Well, I pulled the carb off and gave it a thorough cleaning. The jet cannot be removed because it looks pressed in (but that may just be me). The two adjustment needles can only turn 1/4 and if I turn them clockwise all the way, it runs worse. Doesn't matter what combination I try, turn either needle or both and it runs worse.

I pulled the primer bulb off and checked for cracks and found none. Both fuel lines are fine as well, no leaks or cracks.

The manual actually calls for 40:1 mix and I may have my mix wrong, so this weekend I'm going to mix a specific 40:1 and run it through. I may think I'm running 40:1 but maybe did it richer. That's about the only thing I can think of to try.

Again, my thanks for the assist, fellas. I appreciate it!

Only a 1/4 turn on the needles??? Should be able to completely unscrew them and clean out the passages. May have an anti-tampering device on it, but they are usually easily defeated. If you do get the needles out, reset by screwing them all the way in and gently seating them, then back them out 1 1/2 - 2 full turns. Then adjust with it running.
When you had the carb apart, you are right, the jet is usually a press fit, the one 4u2nv pictured looks like from a mower carb. Did you carefully inspect the fuel pump diaphragm? It only takes a slight crack to mess it up.
A rebuild kit is usually only around $10, if you can find one, Mac went belly up for a while and some parts are hard to find but I believe they are back in business now. Let us know how goes, we'll get you through it somehow.