Ryobi 790R string trimmer

suzi51(7)August 31, 2008

Hard to start and when it does, I can't get it to stay running. New primer plug and fuel lines installed. Filter clean and spark plug fairly new. I hate to chuck it, but has anyone got suggestions.

I know I can buy a 4 cycle for under $100, but I'd like to see if I can fix this one.

It's at least 10 yrs. old, but really wasn't used or abused during that time.


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After 10 years it's probably time for a carb overhaul as the rubber parts will get stiff with age. You could try cleaning the interior of the carb and reassembling it with an investment of $3 for a can of spay cleaner and some time. The way I see it you have to take the thing half way apart to get the model # of the carb to order a rebuild kit so you might as well give it the cheap try and see approach while you're there. Once you price a carb kit you can rethink the situation.

    Bookmark   August 31, 2008 at 9:20PM
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I am suprised it lasted that long without rebuilding the carb. The rubber parts weed cutter is talking about are the pump diaphram, and the meteting diaphram. Wabro or Zama carb. kits are usually less than $10 bucks with the $3 can of carb. cleaner (which you should have already if you own a LT or Lawn mower) takes you total investment to about 15 bucks plus tax plus the labor and learning experience of trying to figure out how carb. work (that alone is worth $15 bucks if you can fix it). shops charge from 35 to 60 bucks for a carb rebuild. That's why most elect to trash them so I can fix them and get another 10 years of use out of them. Gas blowers are the only thing I can't get any more life out of. they are actually worn out between the piston and cylinder walls. Takes alot of power which wears parts to move air!!!!!!!!!!....atleast that's my theory????

i posted a great zama carb. breakdown, anyway if you try to rebuild it you will have to get the numbers stamps on the side of the carb. Wabro will be WTXXX, Zama will be CUXXX. the XXXs are the model numbers that will follow the letters on the carb. The zama download actually tells you where to find the model numbers. http://www.zamacarb.com/tips.html

    Bookmark   September 1, 2008 at 9:08PM
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Thanks for the responses. Not sure I can rebuild a carb, but I sure
can handle the carb cleaner routine, to see what happens.
It wasn't used much in those 10yrs., but not all things are built to last
as they once were.

    Bookmark   September 5, 2008 at 2:38PM
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New to this forum,but could sure use some help.
Have a Ryobi gas trimmer Model 780r. Fresh fuel,new plug.
Will only run if I keep pumping the primer bulb.
Thanks in advance!!

    Bookmark   September 30, 2008 at 9:42PM
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welcome steve, I would first check the fuel filter connected to the hose in the bottom of the fuel tank. I would also inspect the fuel inlet hose (Bottom one) and the fuel return hose (top one) for any cracks or colasping providing you don't have any loose screws that mount the carb. diaphram end caps, or crank cover.

Fuel hose and filter will cost you around 6 bucks if they are bad. see if that helps, Fresh gas usually don clean all the crud in the bottom of the tank I would also remove and clean out the gas tank.

If that don't work the next problem usually the diaphrams get stiff and quite pulsing thus the fuel won't flow. First I would try just to clean the carb. by taking the end plate off and spraying carb. cleaner down all the little passages.

If that don't work you will most likely have to spring aobut 8 bucks for carb. kit, if you go that route make sure you get the numbers off the carb.

I guess you probably have a zama carb. Try this web site for alittle help on putting the carb. kit in or just wanting to know how the carb works there is also a troubleshooting logic in the download also. http://www.zamacarb.com/tips.html

    Bookmark   September 30, 2008 at 10:00PM
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I`m kinda leaning towards the diaphram as I can keep it running by constantly pumping the primer bulb. I`ll get inside the carb and take a look. I`ll let you know how it goes. Thanks again,

    Bookmark   October 1, 2008 at 9:52AM
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own a 790R with all the attachment.
Trimmer, bush wacker, leaf blower and chain saw.
It blew a piston ring so I I thought I would rebuild it.
New piston, rings, cylinder head and rebuilt crab.
Parts $130.00 Labor 3/4 day
I bought it from a friend for $65.00.
For some reason AFTER i was done with the rebuild I checked to see how much a new one would cost.
Guess what!!!!
$99.00 from HOme Depot

    Bookmark   February 27, 2009 at 3:53PM
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My Ryobi 790r ran well for 12 years. Last time out it was running rougn and then quit. I can't even pull the cord to restart so I figure it is dead. Any thoughts?

Also, I used the proper gas/oil mixture but this time the gas had fuel stabilizer in it. Could that have contributed to the problem?

    Bookmark   June 11, 2009 at 10:31AM
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Can anybody tell me the correct ratio of gas to oil? Should I discard mixture left over from last year? Unlike some of your contributors, about the only trouble I have had is with the bump advance and getting into the trimmer attachment to restring. All the other attachments have been great. Thanks for your help.

    Bookmark   May 28, 2011 at 7:13PM
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If you are using a quality (premium) grade 2-stroke oil then 40-1 ratio is more than sufficient , i use 50-1 routinely with Amsoil Sabre . Never store fuel for more than 60 days and expect good things. I believe the forum members recommend that you just put the left over fuel in your auto . Your auto could probably use a little xtra top end oil anyways . If your interested Walmart has a great product Star*tron which is great for enabling you to store fuel for extended periods . Stabil is the least I would use for storing fuel for perhaps 6mths , however the Startron Product is advertised for a yr lol not that I would push the envelope that far ! Check it out on the web .

    Bookmark   May 29, 2011 at 10:08AM
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