Outdoor Power Equipment and Small Engines - string trimmers

got 3 trimmers , same make and model , what kind don't matter here , 2 of these trimmers have same toubles , start right up , but when i try to give them the gas to get them up to speed , they bog down , now i have cleaned the filter in the tank , swaped carb from one that is running good , cleaned exaust ports and mufferler , got good spark , but need to know what the gap is for the coil to flywhell should be , cause i removed one coil and replaced it with another ,when i did it started alright , but same problem , sprayed to check for any air leaks to gaskets ,put thumb on spark plug hole and rolled it over , got plenty of compression , acts to me like the spark is a hit and miss thing going on ,but when i roll it over with the plug out and grounded , it has good spark , ready to ram head into wall here , could you help , thanks in advance

Sounds like you need to back out the high speed screw 1/4 turn or so. It does matter what kind you have if you want the air-gap specs. If it sparks, it should be ok though.

Have you tryed the clean carbon from the spark arrestor screen and out of the mufflers.
Check and clean the screen in the carb. It is located on the other side from the diaphram.

no adjustment screws on carb , just low and high rpm , thanks cheese , just did'nt have the info last nite when i posted , could you give me the gab for the coil to fly wheel , thanks inadvance ,any idea's as to what the problem is

A gap of .012 to .015 should be good and set with a brass feeler guage. 50 lbs of compression will blow your thumb off the hole and that is not enough to run the engine. The spark should jump a 1/4" gap if they are electronic ignition.

If you think it is an ignition problem, did you switch the flywheels too? They have been known to de-magnetize.

I still go along with the rest of the pack here though and seeing you swiped the carb off the one that runs and still have the same problem I lean towards a carbon problem in the muffler. Take the muffler apart and clean it or try firing up without the muffler to test it

oil doc , did take the mfferler off and it starts right up , but still won't get up to speed , will check the compression to see what that is , this has me scratching my bald head , got 3 of these darn things and 2 have the same thing wrong with them , the other one is running great ,seems like the magnets on the flywheel have lots of strength ,tryed a new plug in it today and same thing , these darn things start right up but just won't go , they want to bog down , i'v tryed everything i think ,

What do refer to as high and low rpm? 2 screws?

sorry for not explaning that better , it is just the rpm adjustment , this machine don't have any high and low speed adjustment

Let's eliminate a few things here.

The carb and ignition are from a machine that runs fine. No clogged ports or muffler. ASSUMING fresh gas/oil mix of the proper ratio (even though I run at 100-1 with synthetics).

We still have not ruled out compression without the use of a guage. Have a great story from the Homelite Tech School.. another time though :))

Try one other thing... the fuel cap.. if the fuel cap will not allow air into the tank, it will starve the engine for fuel. Allthough a bad fuel cap usually allows the engine to run for a while and then dies from starvation and will not restart until the vacuum equalizes.

Only other thing I have seen, once on an ECHO and once on a Stihl were crankshafts that had twisted and was binding the rod on the crank journal. Chances of you having two of these are slim to none.

the other slight possibility is that the fuel lines have an ever so slight air leak in the fuel line...or are collapsing under load.....how about fuel filters?? Are they clean? But once again...we need proper settings on the High/low jets...if this is an epa regulated carb...possibly the settings can't be met. Almost...when you say high and low rpm....are you referring to the two screws that say H and L and is there also an idle adjustment screw? Sorry...I am not completely sure if we are on the same page regarding these...

I agree on the fuel delivery problem. I believe you either have bad fuel lines, filters, or a restriction in the carb.

thinking you guys mite have steered me in the right direction , collaped gas line mite be the touble , will try to find one tomarrow and give it a try , thanks for the help and will let cha no if this is the fix , almost