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Good afternoon,

I have a Troy Bilt TB675EC 4 cycle string trimmer here. The original carburetor was leaking so I replace it along with the recoil pull start mechanism. I am now able to start this machine relatively easily, however, it only seems to run well enough at idle.

Anything above idle and the machine will bog down an die almost instantly.

The spark plug is clean. I have not removed the muffler to check it out for clogs nor have I inspected the fuel filter in the tank.

However, I am thinking that there is a fuel delivery problem. I am not getting any leaks from the fuel lines, however, it doesn't seem like much fuel wants to stay in the primer bulb, which is detached from the carburetor on this model. I have removed the primer bulb from the plastic housing so I can run the machine with the cover off. I do know that the two fuel lines running to the primer are correct, as reversing them would give me NO fuel flow what so ever.

What I see here is that one of the lines from the primer runs in to the fuel tank, but seems to terminate near the top of the fuel tank, generally above the fuel. This line does not run down in to the tank. The other end of the primer runs to the carburetor.

There is a second port on the carburetor with another line that runs to the fuel tank. This fuel line actually descends all of the way to the bottom of the fuel tank and does have the fuel line attached.

Is this setup with fuel lines correct or is something missing here? Are both of the fuel lines in the tank supposed to run in to the fuel? I'm a bit confused here.

I have also not done a compression test nor have I checked or adjusted the valves at this point.

By the way, when running at idle, I sprayed some starting fluid between the engine and the intake. No change in anything so I will assume that there aren't any air leaks between the intake and the carburetor.

Thanks for any help or suggestions in advance!

Where did the carb replacement come from. Some of these that I've run into have smaller main jets and you'll need to swap with your old jet. Are there any adjustments on the new carb, might have a cover over them too.

It is a genuine MTD Walbro carburetor, not an aftermarket unit. Identical to the one that came off of the engine, and the correct part number for my string trimmer. Unfortunately, there are no high or low adjustments, only idle speed, just a screw that adjusts the stop for the throttle.

In all honesty, I don't feel as if it is running well enough at idle, either. Sure, it will continue to run for the most part but I don't think it's running right. Occasionally, it will die on its own, and sometimes adjusting the idle speed doesn't affect much at all. So, when I said it runs at idle, yes, it runs, but not 100% correctly.

I'm not exactly sure what the next step is. I'll probably pull off the muffler and check out the filter in the tank. I am also confused about how this fuel delivery system works in regards to the return and supply lines, and the primer itself. I don't think it is getting enough fuel, personally. Like I said, I don't see a whole lot moving through the primer. When the engine is running the primer is entirely empty. You can see fuel in the fuel lines, though, it is flowing. Perhaps not flowing enough, though.

That set up for the fuel lies is correct.
Fuel is drawn up from the filter ( did you change it ) through the carb then through the primer then back to the tank.

Does the engine die out rich or lean ?

I have not changed the filter yet. I thought it looks like fuel is being drawn from the tank through the primer as well. That's what it looks like when I see the flow. The fuel line between carb and tank is black, so I can't see through it, but the fuel lines between carb, primer, and tank are transparent, and I can see the flow.

In regards to if it is dying when lean or rich, I can't tell. How do I know?

PS, if I pull the fuel line off of the carb that leads to the filter in the tank, just after the engine is running, it still seems like there is a vacuum on that fuel line because fuel continues to dribble out of the end that I just disconnected. Seems odd, there shouldn't be a vacuum any longer.

Lastly, this model has a manual choke. To be honest, there is a slight difference with choke on, partial, or off, but not a big difference. But then again, at idle, there wouldn't be a big difference normally.

Dieing rich sound like our mower if you leave the choke on for too long, usually blows a little black or at least dark grey smoke and has deep soft sort of exhaust note.

Dieing lean sounds like when you turn the fuel off to stop, it generally has a bit of white smke associated with it the exhaust note is higher and sort of sharp.

I'm not getting any smoke out of this engine to be honest. And also, it's a small engine, it all sounds the same to me. But I guess you would say that the tone of the engine gets "deeper" when I give it even a tiny bit of throttle. Before it dies.

EPA strikes again.
So you are going to pour out fuel everywhere to evaporate into the air because they mandate the carb can not have adjusting screws to prevent you running it rich and allowing 1/1000 th the amount of fuel being tipped out escaping unburned into the air,, real smart ain't it.

Pull the carb off the engine but leave the fuel lines connected.
turn it right angles and blow air through the carb with the throttle held wide open.
If you get a strong flow of fuel mist coming out of the carb it is good & the filter is good.
High volume / low pressure air is best like from one of those cheap spray paint guns or the discharge from a vacuum cleaner that has a blow function.
If not you can use compressed air, just turn the pressure down.

If the flow is broken or really weak, replace the fuel filter & try again.

Pull the muffler off and blow through it.
There should be no resistance whatsoever.
While the muffler is off, check the cylinder & rings.
If there is groves in there, the unit is toast.
You can get replacement cylinder & piston kits, but it will end up costing about 75% of a replacement.

Will do, I will check it out more tomorrow.

During testing the other day when I wrote this post, I burned my hand pretty badly on the muffler. It seemed like the muffler was REALLY hot. This might be a stupid question, but considering that it was operating at low idle, not under load, and for a relatively short period of time, it almost struck me that the muffler was much hotter than what I would find usual. Maybe I am wrong, but this thought crossed my mind.

PS, my father in law had a newer 4 stroke Troy Bilt, I believe it was a TB685 or something like that. I believe it used the same carburetor, because I replaced it for him. While it ran better than this and would idle up, I never thought it ran good enough, even with the new carb. And with no adjustment, there wasn't anything to do about it. I did sell it for him though - he got a Ryobi 40 volt and will never look back on a gas trimmer again. Although primarily, these Chinese string trimmers are junk, regardless if they are 2 or 4 stroke. Doesn't matter, because crap is crap no matter if you have to mix the fuel or not.

Blaine B. said:

Will do, I will check it out more tomorrow.

During testing the other day when I wrote this post, I burned my hand pretty badly on the muffler. It seemed like the muffler was REALLY hot. This might be a stupid question, but considering that it was operating at low idle, not under load, and for a relatively short period of time, it almost struck me that the muffler was much hotter than what I would find usual. Maybe I am wrong, but this thought crossed my mind.

Click to expand...

They get very hot very quick sorry about the hand,,,, you are not alone

It turns out that there was nothing wrong with the fuel filter nor the muffler.

The main fuel line between the carburetor and the fuel filter was deteriorating pretty badly. I went ahead and replaced that fuel line. The other two fuel lines between the fuel tank, primer, and carburetor were of a different transparent type and were not deteriorating.

This string trimmer seems to run a little nicer, but not by much. I was able to achieve "full throttle" under half-choke but that's it. With no choke it will bog down and die just the same. Either way, even under half-choke it still isn't running right, and there is no difference running it with or without the muffler attached to the engine.

I didn't take the head off to examine the cylinder for scoring. I think that's it for this one, I may as well try to sell it for parts to recoup some of the cost of the replacement parts I put in to it.

I also noticed to be getting quite a bit of oil blow by coming out of that little port behind the air filter. There is a tube running between the air filter housing and the engine. I assume this is the crankcase ventilation?

Granted, the oil level was filled too high before, but I rectified that today. Still getting oil coming out of the vent.

It must be FUBAR. And I believe this machine to be less than 4 years old. Not surprising for China made crap, I guess.