Homelite String trimmer- delay in engine response

[sjt78]

I recently acquired a Homelite ST-155 string trimmer. I bought it at an estate sale so I don't know the history on this unit. It needed a new primer bulb and head assembly which I just installed. The engine runs/idles well and will start in 3 pulls of the cord/ 1 pull if warm. I've noticed that the throttle response when I pull the trigger is delayed. I press the trigger and then slowly the engine responds and will get up to full speed, but it is not instantaneous. Is this just the nature of the beast, or is there an adjustment I can make? Gas is new and freshly mixed with oil (around 50:1) and the spark plug could be the original champion plug. The air filter was recently cleaned and then lightly oiled. Let me know if there is something else I can check (carb, fuel lines, etc.). The unit runs fine otherwise. Thanks.
-Steve

 

[DaleW]

I think mine is a model st155. bought it in about 1986-87.
Still runs great, but I know the diaphram in the carb is shot. I have had to take the carb apart once or twice to clean it out. For me, time for a carb rebuild with primer bulb, etc.

I remember this was the first gas string trimmer for under $100 bucks way back when, so I was happy to buy it.

For the most part, still runs good.

Yes, mine has the delay too. BUT, I bet the a carb overhaul would rememdy that for mine.

Have fun!

 

[sjt78]

I got to use the trimmer again yesterday and the delay was not as bad. Not sure if it just because I ran it two days in a row or what. This is my first gas trimmer (had multiple cheesy electric models) and I think it is perfect for the little trimming I need. With the replacement parts, I have about $50 in it so I'm happy. I know if I bought a new one I'd want a Stihl or Echo which run at least $140 around here. Maybe next spring I'll pick up a carb kit and see if replacing the gaskets make a difference in throttle response. The gaskets in it now are probably original, but seem to be holding up.

 

[yeehaw1960]

If it's old enough, it will have two air screw adjustments. You need to tweak the one marked "H" out a little and see how it does. If that doesn't make it better or makes it worse, go back to the original position and tweak it the other way. So little time, so much to tweak! Otherwise, turn the one screw available out a little and/or in a little to adjust. A dirty air filter can slow your throttle response too.

 

[sjt78]

The air filter didn't look too bad before I went ahead and cleaned it. I'll have to check for the air screws. Those are in or behind the air filter box, right?

 

[yeehaw1960]

You "should" be able to see them without disassembling anything. There's usually an access for them. They're right next to each other on the side of the carb. If you find them and they don't have screw slots, a dremmel tool with a cut-off wheel gets around that. Careful though, the screw heads are made of soft material. Crank lightly after slotting them. If there is only one screw and one blank, don't slot the blank...its not a screw, just a plug. Have fun!

 

[sjt78]

I mowed the lawn today and pulled out the trimmer to cut around the trees, fence, etc. I noticed the delay again so I looked a the carb and on the side facing the trimmer head are two slotted screws. I guess this model came out before there were any barriers to adjusting the carb. Anyway, I found which one was H and adjusted that one as well as the L screw. It seems to idle and respond to the throttle much better. It actually seems like it allows the engine to rev more as it seems more powerful. Now when I pull the throttle the unit twists against my grip, before it never seemed as powerful. Great tips, thanks.

 

[doitmyself]

yeehaw's advice was slightly wrong.

There are 3 adjustment screws on the carb and all three have to be somewhat "in balance" for perfection.

The idle speed control does just what it says.

The high speed screw (H) controls the fuel/air mixture at full throttle. Start at 1 to 1.5 turns open. Adjust open until engine starts to "4cycle" or run rough/rich at full throttle - you want to be just on the edge of too rich. Then turn it in just a bit until it just smooths out. On chainsaws, you actually leave it slightly "4 cycling" when not under load.

The low speed screw controls air/fuel mix at idle and acceleration. Adjust this screw to 1 -1.5 turns initially. After warmed up, accelerate quickly to full throttle. IF it hesitates, adjust it slightly (usually open more) until the unit accelerates quick and smooth from idle to full.

The idle screw should be adjusted as high as possible WITHOUT the head turning. Too low, and it may not accelerate smooth up to full throttle.

All three screw have to be adjusted together.

 

[sjt78]

I have to double check as I only noticed two screws on the side of the carb. These are the H and L screws. I'll have to look for an idle screw. Thanks for the help.

 

[yeehaw1960]

Does it run better SJT78? I sometimes treat newbies like experts and experts like newbies. Feels good to fix your own stuff huh?

 

[sjt78]

Yes, it runs better with adjustment. I just need to take a look at the carb again to search for that idle screw. I also lubed the part of the carb that the throttle cable is attached to, much smoother control now. I have a new spark plug on order that is coming in today, so I'll see how that goes. I'll probably only get to use the trimmer once more before the snow starts flying, so next year I plan on putting on some new fuel lines and a new fuel filter.