Help! Troy Built trimmer won't run continously

zeroedn(NC)March 19, 2010

Trying to fix a Troy Built trimmer TB20CS for a family member.


The trimmer will crank easily and run but soon cuts off like it's out of gas. I can feather the choke back and forth and keep it running indefinitely either idling or high speed, but engine shuts down when I stop jiggling the choke.

What I've done so far:

Drained the tank and refilled with new gas(gas wasn't bad), seems to have good fuel flow, tried running with and without the muffler (no change), cleaned and totally rebuilt the carb (Zama)with new kit, even cleaned behind welsh plug, but it runs exactly the same as before.

Sure could use some words of wisdom cause I'm at my wits end!

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Did you install a new fuel filter, down in the tank? Do you know how to do that? I ask, because there is a trick to replacing the filter inside the tank. They do clog up. Also, check the breather hole in the gas cap, for a little wasp egg stuffed into it. Does happen!

    Bookmark   March 19, 2010 at 7:21PM
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Sounds now like it's more of an air leak somewhere.
There's a bolt on either side of cylinder, and 4 screws that hold the plastic piece to the case, the plastic piece has the primer bulb in it and carb affixs to it.
make sure they're all good and snug.
hope this helps,

    Bookmark   March 19, 2010 at 9:41PM
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Thanks, but I forgot to mention I eliminated the possibility of gas cap problem by loosening it. I disconnected fuel line from carb and fuel flows very freely, also no there are no bubbles in line that would indicate an air leak when running.
I've put too much time in it already, but hate to admit defeat. The unit looks relatively new.

    Bookmark   March 20, 2010 at 9:08AM
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The air leak I'm talking about wouldn't show up as bubbles in fuel line.
If cylinder has air leak, it will alter the fuel/air mix in the cylinder, making it run lean.
jiggling the choke would compensate for this.

    Bookmark   March 20, 2010 at 1:04PM
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Glenum, the screws were tight, but I took the plastic mount completely off to inspect for cracks or damage but found no problem. Reinstalled the piece and tightened the screws firmly, but still the same problem.
Thanks for the suggestion anyway, I see how it could have caused the same problem.

    Bookmark   March 22, 2010 at 8:14AM
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masiman(z7 VA)

The best way to test for an air leak is to a Pressure/Vac test. You can do a cheap test by putting soapy water or oil on your crank seals, but this requires the engine to be running to see well. To do a press/vac test you'll need something to create pressure and vacuum (I have a Mity-Vac). It's easy to find something to create pressure with a gauge, it is the vac that you should make sure the tool can do if you go this route. You'll also need to cover the intake and exhaust ports, I use a piece of plumbers rubber wrap. You can put the seal between the muffler and exhaust port and between the carb and intake port. You want to make sure that you do not pressure the carburetor as you can ruin it. I believe you pressure and vac to 3.5 psi but double check before doing it. Too much and you can ruin the seals, where the crank comes out of the engine case. Your engine should hold the press/vac for about 1 minute IIRC. If it fails the test then you will need to find out what is allowing the flow of air in and out of your engine. If you can't find the culprit, the best way to do it is to pressure the engine and dunk it in water. You'll see air bubbles where the leak is. Of course, take off your ignition module before doing this.

I've done this sort of test twice and I think I can do it in under 2 hours. At shops I think they could probably do it in under 30 minutes. You'll just have to decide if this engine is worth that amount of work.

    Bookmark   March 22, 2010 at 1:28PM
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Thanks for the info Masiman. I've already got more time in this trimmer than I care to admit and have gotten nowhere. I'm not real fond of these big box store 2 strokes as it is.
I think I may just give my dad an early fathers day present, perhaps a Stihl or an Echo since I've never been disappointed by either.

    Bookmark   March 24, 2010 at 2:18AM
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masiman(z7 VA)

That is my thought on also on this engine. I am guessing this is a I personally have a Stihl trimmer, which I really like. Although, I know I would probably any of the other manufacturers trimmers in the $250+ range. Decide if you need to use the trimmer as a brushcutter or not. If you need to be able to do brushcutting, you'll spend a little more for a bigger engine and a solid shaft. If you don't need it, you can save a little money and have a lighter machine.

    Bookmark   March 24, 2010 at 10:31AM
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Probably worn out or cylinder wall scrtched or scored causing low compression? I'd see if it has any compression? If the three or four screws that hole the head on are tight probably worn out from too little oil in the gas if it's a two stroke? too lazy to look up the model number. Hundred of them ususally on ebay, get one for 5 or 10 bucks and swap out the piston and jug. Done that quite a few times with homelite brands which by the way is one of the easiest weedwacker to work on.

    Bookmark   March 24, 2010 at 9:46PM
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masiman(z7 VA)

I forgot to mention the compression test that rcmoser suggested.

The compression test will check the integrity of the combustion chamber. Can it hold the charge without letting your mixture leak out of the combustion chamber by the piston and rings and into the crankcase. On two strokes you should be getting @150-200 psi. At least that is what chainsaws should be. I'll assume that other small two strokes are the same but someone correct me if I am wrong. A much easier test to do than pressure/vac.

The pressure/vac test ensures the integrity of the cylinder as a whole.

    Bookmark   March 25, 2010 at 11:30AM
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Thanks to all for the advice and suggestions. I'll have the compression checked and if it fails I'll pull the carb that I just rebuilt and chunk the rest. Dang I'm a packrat, I just can't bring myself to throw away a perfectly good rebuilt carb!

    Bookmark   March 26, 2010 at 7:26PM
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Zero: Don't feel Bad , We all do the Same . I have an Old Weed Eater 1700 that just won't die , I have to set the carb up every yr. But it still runs great for camp usage. I have two cast off Weed Eaters also just for parts should I need then of about the same vintage lol .

    Bookmark   March 27, 2010 at 11:28AM
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Success at last!!! Sometimes itÂs just plain awkward admitting to a careless error, but in hopes that it helps someone elseÂÂ.here goes. I had so much time in this project I just couldnÂt just abandon it without answers or without at least learning something. Just to satisfy my own curiosity I purchased a compression tester, (which I should have bought earlier.) Surprisingly the compression pumped up quickly to 138, so now the carb is once again suspect. Keep in mind, this is the new style Zama carb with those dang tamper proof adjustments, thus the one thing I never did when I rebuilt the carb was to remove the H & L adjusting screws. I got busy with the dremel tool and made a tool to fit them, removed the screws, sprayed cleaner in the holes and replaced the adjusting screws. I set both to 1 ½ turns, cranked it up, touched up the H & L adjustments and it ran like new again. As the famous radio commentator Paul Harvey once so eloquently said, "and now you knowÂÂÂthe rest of the story."

    Bookmark   April 27, 2010 at 10:04PM
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