# Rebuilt Trimmer worked great for 5 minutes, then no more



## Engineeringtech (Aug 9, 2008)

Hoping to get some help with my Homelite UT20046 straight shaft string trimmer. Sorry about the lengthy history, but I want everyone to know what I have done....

A year after we bought it (new), it quit idling when warmed up. It also would not accelerate without boggng down. To use it, I've been starting with full choke, very slowly throttling up, and holding the throttle full open for all my cutting. I have been putting up with this for two years, and finally decided to deal with it.

After reviewing a bunch of videos on You Tube, and reading some app notes on the Zama website, I bought the proper rebuild kit, new priming bulb, new hoses, new fuel filter. I bought a tool to adjust the mixture screws. I disassembled the carb and cleaned VERY thoroughly, even removing the Welch plug and screen. I made certain all the small ports were clean. I assembled it very meticulously. I returned to the unit, and just to be safe, cleaned the air filter, and muffler. Air cleaner was very clean. Muffler was just a little dirty. I cleaned the tank out, changed the fuel lines (cracked), and filter, just to be safe. I changed the spark plug. (It is gapped correctly.) I re-installed the carb. Everything was put back exactly where I found it, and tight. I bought fresh gas and a new bottle of 2 - stroke oil and prepared some 50:1 mix. Put it in the tank. 

I followed Zama's exact procedure to start the unit and adjust. After only a couple minutes tweaking, he unit was idling and running fine. It accelerated smoothly from idle to full speed. The prime bulb stayed at about half level while it ran. I cut a few weeds and the string broke. I noticed the gearbox was very warm. I left the unit on the front porch while I went looking for more line, and some grease for the gearbox. (Incidentally, there is no clutch on the unit._

About 20 minutes later, I put a little bit of grease into the gearbox, and replaced the line. Everything seemed normal. The head moved freely. I have fair to good compression. I then tried to start it. Very hard to start, and the moment I take it off full choke it dies. Even if I leave the choke on, it dies after a few seconds. If I try to accelerate it, it dies. I thought I had the jets adjusted to perfection. I opened the low jet a quarter turn to see if I had made it too lean. But still can't keep it running. 

What on earth happened to the unit? Any ideas? I am wondering now if I have some sort of cracked crank case or weird thermal problem. I spent $50 on the parts and supplies. Hate to give up.


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## 30yearTech (Sep 14, 2006)

Does sound like an air leak, make sure the crankcase cover is tight, insulator block between carburetor and cylinder head is tight and the cylinder itself where it attaches to the crankcase is good and tight.


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## Engineeringtech (Aug 9, 2008)

30yearTech said:


> Does sound like an air leak, make sure the crankcase cover is tight, insulator block between carburetor and cylinder head is tight and the cylinder itself where it attaches to the crankcase is good and tight.


Thanks for the suggestions. I will check those things. Insulator block? Not sure I saw one. Black fiber gasket there.


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## Sir Thomas (Dec 7, 2013)

Engineeringtech said:


> Thanks for the suggestions. I will check those things. Insulator block? Not sure I saw one. Black fiber gasket there.


I've always call the black plastic part between the carb and the block a Heat Dam because it adsorbs the heat from the block.
When you reassemble any 2 cycle equipment, it's imperative that you make sure EVERYTHING is tight. When you can get new gaskets.


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## jerrymc39 (Nov 12, 2014)

Might not be a bad idea to check the coil gap and as long as you're in there also check the flywheel key. From what I have read it does seem to be an air leak somewhere.


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## Engineeringtech (Aug 9, 2008)

Thanks. I have been so busy with work, that I have been unable to check. Will do so this weekend.


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## Engineeringtech (Aug 9, 2008)

Have looked the unit over and found no air leaks or cracks. Everything nice and tight. Opened mixture screws back up (2 turns out), and easily started. Warmed the unit up and tweeked screws to optimum speed per manufacturers instructions. Good idle, good throttle response. Trimmed the yard, then shut it down for 10 minutes. Now I can't restart. Using the choke, I can get about 5 seconds of operation, before it dies. Bogs down and dies when I try to go from idle to full speed. Have I just got this thing too lean? Something wrong with the choke?


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## jerrymc39 (Nov 12, 2014)

Go back to -2 turns out on your high and low needles and see if it will run. If not , close your high needle and try to start again. If it starts then readjust your high needle to a satisfactory settting. If you're doing a clod start use full choke and hold the trigger while trying to start---if it fires once stop and put your choke on half and try again with the trigger depressed. If it starts and runs, readjust both the H & L needles. Good luck !!


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## Engineeringtech (Aug 9, 2008)

Thanks for your reply. I can always open the needles two turns from the closed position and start the engine. (I have to use full choke and pull the trigger if the engine is cold.) But the two turns out position is not the optimum position for running, once the engine has warmed. Nor does it give smooth throttle response. I don't have any problem finding the optimum position for the needles. However, if I shut the engine down for more than a few minutes, and restart it won't keep running, regardless of the choke position. I haven't ever been able to run the thing in the half choke position, even when the unit was new.


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## jerrymc39 (Nov 12, 2014)

From what you mentioned I would suggest you inspect your fuel filter or look for a kinked line or a crack in one of the lines. It still seems to indicate that you have an air leak somewhere.


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## Engineeringtech (Aug 9, 2008)

Brand new fuel lines and filter. No kinks or cracks. Fuel filter is sitting in just the right position, and good fuel flow. Primer bulb is always full of fuel. I agree with you though. Seems like an air leak somewhere. How do I find it?


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## jerrymc39 (Nov 12, 2014)

Start the engine and as it runs spray carb cleaner in the areas around the insulator block and the crankcase itself and see if the engine speed changes. If so, you have a leak in the area where it changes speed. Have you checked the muffler and exhaust port for excessive carbon build-up?


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## Engineeringtech (Aug 9, 2008)

jerrymc39 said:


> Start the engine and as it runs spray carb cleaner in the areas around the insulator block and the crankcase itself and see if the engine speed changes. If so, you have a leak in the area where it changes speed. Have you checked the muffler and exhaust port for excessive carbon build-up?


Sorry... Got busy with other things, and forgot I started this thread. Yes, I already cleaned out the muffler and exhaust port. Next time I try to use the trimmer I will try the leak detect you suggest. Good idea...


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