Weed wacker with Zama carb bogs on acceleration

[FergusonTO35]

Hey guys. I have a 20 year old Homelite ST-155 weed eater with a Zama C1U-H41 carb. I know this is a cheapo machine but it was free, has been used very little, and other than the carb problem works great. It starts very easily and will idle all day long. The problem is that it bogs on acceleration. By adjusting the screws the best I can do is reduce the bog to about one second before it accelerates. This carb is very sensitive, if the screws are not perfect it will just bog all day long.

The air filter is clean. The spark plug, fuel filter, and fuel lines are new. I'm using 93 octane gas mixed at 36:1 with fuel stabilizer. Any ideas here? Maybe I need to replace the diaphragms? Check the metering lever? Could I have a vacuum leak somewhere?

 

[dozerdan]

I actually like the cheap older Homelite trimmers. I would clean the carb good and install a diaphragm kit. That should get your going again.
Look around for a Walbro carb for that trimmer, they made plenty with the good Walbro on them.

Later
Dan

 

[Matt B]

x2
Same problem with a Ryobi. Rebuilt the Zama, sorted fuel, air and spark issues but it still had an unstable tune and some hesitation on acceleration. Put on an old, rebuilt Walbro and went well.

 

[FergusonTO35]

Thanks guys. I like the older homeowner trimmers too, seems like they were alot sturdier back in the day. I'll go ahead and get a kit and gaskets for it. Another question: the carb on this machine has a primer bulb and the usual two hoses coming out of it. What's weird is that on this machine the hose which normally would return fuel to the tank comes out of the carb and points downward, not going into anything. Fuel doesn't come out of this hose. Maybe it's an air purge or something? I've had alot of weedeaters over the years and never one with this setup.

 

[FergusonTO35]

Yesterday I used the weedeater again. This time I was able to get most of the bog out by adjusting the carb in a tiny increment, then cut for awhile, then adjust again, then cut some more. Still I would like to put a Walbro on it. I found a new WT458 on Fleabay for a good price. It does look alot different than the carbs from when this weedeater was made. Not to say it necessarily won't work, it just looks different. Would this necessarily be a problem?

 

[dozerdan]

Yesterday I used the weedeater again. This time I was able to get most of the bog out by adjusting the carb in a tiny increment, then cut for awhile, then adjust again, then cut some more. Still I would like to put a Walbro on it. I found a new WT458 on Fleabay for a good price. It does look alot different than the carbs from when this weedeater was made. Not to say it necessarily won't work, it just looks different. Would this necessarily be a problem?

I think you need a Walbro WT-71, 71A, 95 WT-220

Later
Dan

 

[FergusonTO35]

Thanks, I'll look for one.

 

[FergusonTO35]

Update: looks like the machine is finally sorted out. Running the return line into the fuel tank was the solution. The strangest thing is that nothing comes out of it that I can see, whereas fuel comes out of it back into the tank on other carbs. And it seems that it was made that way at the factory. When I got it the return line came out of the carb and didn't connect to anything, it was secured to the back of the engine with a little bracket. There was only one hole in the top of the tank which the fuel pickup line went in to. The machine never would run right plumbed in this way. With the return line plumbed into the tank it runs great.

So, maybe this carb was an odd design that was quickly abandoned? The machine probably dates to the time period when primers were becoming commonplace on small 2 cycle engines.

 

[Matt B]

Have had a number of small 2 strokes come back to life after clearing the return line and its tank attachment. It's now one of the first things I look at with carb issues.

 

[FergusonTO35]

I'm thinking the return line on this one spits out air but not fuel. You can see fuel get drawn up into the carb but doesn't come out the return line.

 

[bikesandcars]

Update: looks like the machine is finally sorted out. Running the return line into the fuel tank was the solution. The strangest thing is that nothing comes out of it that I can see, whereas fuel comes out of it back into the tank on other carbs. And it seems that it was made that way at the factory. When I got it the return line came out of the carb and didn't connect to anything, it was secured to the back of the engine with a little bracket. There was only one hole in the top of the tank which the fuel pickup line went in to. The machine never would run right plumbed in this way. With the return line plumbed into the tank it runs great.

So, maybe this carb was an odd design that was quickly abandoned? The machine probably dates to the time period when primers were becoming commonplace on small 2 cycle engines.

I was wondering where that line was supposed to go myself. I may try drilling a hole in the top of the tank and terminating it there, if it gives it 2 more HP I'll report back.

I hear mine spitting air out, but never fuel. I have a rebuild kit on the way however because I tried cleaning without new gaskets and its spitting fuel out the side. I think that line has something to do with how the fuel primer works. If there is an issue with the gasket or "check valve" in the carburetor, plumbing it to the tank may help balance out that problem (it will have same pressure as fuel feed). Since they didn't do this from the factory I must assume however that there was a reason why, and the fix of plumbing it to the tank is a workaround.

That said, it might be worth a try as a speed mod.

 

[bikesandcars]

Follow up to discussion... while rebuilding the carb I looked at the "overflow" line.

I tried to squirt some carb cleaner through it, it was plugged... I blew the obstruction out with compressed air. After getting everything together I drilled a hole in the fuel tank and plugged the hose from the overflow into the fuel tank.

^ all of that was a very bad idea, it ran horribly.

I now realize that I blew out the outlet check valve. Returning the fuel to the tank has an simple advantage of not spilling fuel, but I don't see any other after looking at the diagram. I discovered that if the check valve doesn't work the carburetor malfunctions and both the primer bulb won't work right and the carburetor will not operate correctly. Duh and now aggravating because finding another check valve or primer assembly is a pain.

Further update: finding part A-01194 (aluminum primer valve body) on ebay for $10.00 shipped wasn't that hard, finding the part number was a little tougher...

 

[bikesandcars]

Last and final update. Got the NOS check valve assembly and installed it along with cleaning the carburetor again. I plumbed the check valve hose into the tank so it doesn't waste fuel on the ground every time I use the priming bulb (it now returns fuel to the tank). Starts, idles, and runs great.

 

[FergusonTO35]

Glad to hear it. After running good for awhile mine is back to not running right. I got a Walbro for cheap from a friend so I'm going to put a kit in it and give it a try.

 

[bikesandcars]

Glad to hear it. After running good for awhile mine is back to not running right. I got a Walbro for cheap from a friend so I'm going to put a kit in it and give it a try.

Sorry to hear, please let us know what walbro if you get it running right.

My zama is now having a new problem, idles great but dies when accelerating. I don't understand it because it was running great and now it's not. I ordered (another) gasket kit and will try another rebuild.

 

[FergusonTO35]

I got a Walbro WT-71 and put a kit in it. Tried it out and its running great with minimal adjustment. I ran it for more than an hour trimming the church property last week and it did just fine. Hopefully this machine will behave from now on.