Ryobi 720r fuel problem


Old 05-28-09, 10:14 AM
Thread Starter
Join Date: May 2009
Location: MN
Posts: 4
Ryobi 720r fuel problem

I started this in another thread but I want to give a full list of what I've tried with this thing.

I'm cheap, so rather than pay $250 for a decent quality string trimmer I bought a Ryobi 720r in reasonably good condition from a local thrift store. By "reasonably good" I mean it wasn't too obviously worn, missing parts, corroded, or dirty. It wasn't siezed up either.

Taking it home, the thing I found wrong was that the fuel lines were rotted. Simple enough replacement, after two or three tries I found the correct way to connect the lines to the tank to get them to seal.

I'm still having problems though. I connected the lines to the carb so that the primer bulb sucks fuel from the top of the carb and pushes it into the fuel tank, and the fuel feed line goes to the bottom of the carb.

I bought brand new 3/32 tubing for this, which works pretty well as far as I can see. There might be a bit of an air leak at the carb top connection.

FYI, it's a walbro carb with a simple choke plate (not butterfly) and a primer bulb. The bulb is in working shape.

What happens at the moment is that I can attempt to prime it, pushing 5-10 times until I see that the bulb is full and the fuel feed is full, and I hear fuel going back into the tank. I do see bubbles coming out of the carb with the fuel no matter how long I pump, which says to me something is wrong in the carb.

Then I try to start using Ryobi's directions, first on choke then half choke. It starts on half choke and runs for about 5-6 seconds (every time) then dies. While it runs I notice lots of bubbles in the fuel feed that weren't there before. After I run it like this it won't start for a little while, then it will run 5-6 seconds again (after 40 pulls to start). I can prime it again and get it to do the same thing in 7-10 pulls.

I tried opening the carb to clean it, and found that it was mostly clean. The diaphragm had been installed with a gasket under it, which I think was wrong, so I moved the diaphragm to be next to the carb body. I carefully removed the mix screws, the pin, spring, and arm assembly that regulates flow, and cleaned with gumout, then blew out with canned air. I checked the rubber tip and seat for wear.

I also opened the other side and cleaned the screen and holes, and made sure the intake fitting was clear.

Then I put it all back together.

Now it does the start thing then kills, and if I put it down with gas in the tank, it will drain through the carb into the cylinder, so the next time I pick it up I have to remove the plug and drain out a half cylinder of gas.

Suggestions welcome. I'm leaning toward getting a carb repair kit even though I can't see any problems with the current carb except maybe a worn diaphragm?

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Old 05-28-09, 02:32 PM
hopkinsr2's Avatar
Join Date: Feb 2005
Location: canada
Posts: 1,563
Get the complete carb repair kit.. I have found that if the fuel lines are rotton,, The diaghragms in the carb are just as bad.... Roger
Old 05-29-09, 11:49 AM
Thread Starter
Join Date: May 2009
Location: MN
Posts: 4

The diaphragm looked fine when I checked it, but I got the full carb kit because it was cheap. I'll try to install it this weekend.


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