What's wrong with my Ryobi trimmers???

[kebo]

I have two Ryobi 2 cycle string trimmers, one is an older model 790r, the other is a newer one but not sure what model. They both crank easily enough, and idle ok but as soon as you give them any throttle at all, they want to die. I mean just like you hit the kill switch. What's up with them?? It sounds like a air/fuel mixture issue to me.

One the older 790r, the foam air filter was soaked with oil/gas mixture so I tried cranking it with the filter off and it runs a bit better, but still wants to die when you barely crack the throttle.

Have not tried adjusting any of the mixture screws. Any ideas?? Thanks!

 

[Patches]

Sounds to me as though the spark arresting screen in the mufflers are plugged with carbon. Remove the muffler from the engine and you'll find a metalic screen, sometimes attached with screws, in the port of the muffler. Remove the screen and clean the carbon from said screen, making sure you can easily see through the screen. This should cure your problem

 

[kebo]

Thanks, will pull the muffler off of one of them take a look at it to see if it's all clogged up.

Keith

 

[Coyote machine]

If you're using gas that is older than 90 days get it out of the machine and go with fresh mixture. Ethanol based gas is absolutely NO good after 90 days of sitting. And check the spark arresters too as already mentioned. Don't screw with the mixture screws until you have done both of the above on any machine that takes gas!
Change your plugs too if they've been exposed to the ethanol gas that sat beyond its shelf date.

 

[kebo]

I pulled the muffler off of the 790r trimmer and there was no spark arrestor screen that I could see. If it has one, it is inside the muffler. Will try the new plug and fresh fuel next. Thanks!!

 

[2manyrocks]

Got the same issue with an Echo SRM 2200 trimmer. It was running fine until I ran it out of gas, and then put in some premix that had been sitting from last year. It starts, but then dies as soon as I give it throttle. I was thinking the fuel pump diaphragm in the carb was bad, but do you think it's the old gas?

It died like 2 strokes used to die when the points and condensors were shot, but this unit has electronic ignition.

 

[Patches]

I pulled the muffler off of the 790r trimmer and there was no spark arrestor screen that I could see. If it has one, it is inside the muffler. Will try the new plug and fresh fuel next. Thanks!!

Really! I haven't seen a 2 stroke lawn/garden implement, chain saw, etc. without a spark arresting feature in the last 30 or more years, unless it has been removed; I believe they're federally mandated. Some are located inside the muffler, if the muffler is a two piece unit(pull apart) and some are externally mounted(at the exhaust aperture), retained by a plate and a couple screws. I have seen them so thoroughly plugged as to look like a piece of metal. You should look more closely as I honestly believe it has some sort of arrester :thumbsup:

 

[Patches]

Googled Ryobi and clicked on SERVICE PARTS, which brought up two windows: click on "Buy MTD Online" and enter model # 790r. Part #'s
753-04312 arrester screen
" 04313 arrester plate
" 04314 arrester hood
" 04315 arrester screw

So your Ryobi #790r string trimmer does have a spark arresting screen: all you have to do is find it

 

[kebo]

Ok, I worked on the 790r some this evening. Dumped the old fuel and put FRESH fuel in it, with 2cycle Lucas Oil mixed to about 42:1. I will catch some flack for this I know, but the 790r is 32:1 and the other Ryobi trimmer I have is a 50:1 mix so I decided to split the difference. I do not want to have to keep up with two different 2cycle fuel cans. That would just be too much hasssle and I'm willing to deal with the consequences, if there are any. I also added a little bit of SeaFoam in the gas can. That stuff is costly!! About $10 for a 16 oz container.

On to the second part.... After putting in the fresh fuel, I changed out the old plug and put in a brand new one. Now, it cranks up really easy, but won't even idle now. It used to be that after cranking it several times to get it to warm up, it would idle good. Now, after it's warmed up some, it will idle for about 5-10 seconds and then just die on it's own. Giving it any throttle only quickens the shutdown.

As stated before, there is no spark arrestor screen on it that you can see, unless someone has removed it previously. This is possible since I bought it used. Or, could it be integral with the muffler on this particular one?? Don't know, but I do know that when I pulled the muffler off, the exahust port was clear, and the mating part of the muffler was clear.

I guess next I need to check the fuel filter to make sure it's not stopped up and check the fuel lines.
Thanks again for your help.

 

[windy acers]

I have run my ryobi half chocked for about 2 years now.I use prem gas with stp gas treatment. hold it by the string end an slam it against a tree.SAVE THE SPOOL. DON'T ASK HOW I KNOW :confused2::laughing:

 

[Coyote machine]

YES! To whomever has been running last year's fuel it is guaranteed to be useless and very bad for your engine. Ethanol gas at 10% is hygroscopic- which means as it breaks down and phase separates it does NOT recombine and its octane is lost and it absorbs atmospheric water like a thirsty sponge!
It is without a doubt the worst thing to happen to any internal combustion engine, and it adversely effects boat and mower, trimmer, chainsaw motors because they are mostly used seasonally and unlike their car/truck counterparts they use an open to atmosphere fuel system.

If the Ryobi starts easier now you could still have exhaust screen issues, but more likely you need to have someone who knows how to adjust your carb settings do so to let it run properly with clean gas/mix. A 42% mixture, assuming you could even hit it on the money is a waste of time. Virtually every manufacturer today specks out at 50% to get more oil in to keep the pistons from being fried by lean mixture gas. Us it in all you machines that require a mix. you can't go wrong with 50/50.

 

[ol'oilburner]

check the vacuum line, I was having a simliar problem with my chainsaw and it turned out the vacuum hose had a split in it and it wasn't providing enough vacuum for the fuel pump to work properly.

 

[arrabil]

A 42% mixture, assuming you could even hit it on the money is a waste of time. Virtually every manufacturer today specks out at 50% to get more oil in to keep the pistons from being fried by lean mixture gas. Us it in all you machines that require a mix. you can't go wrong with 50/50.

You mean 2% (50:1) and 2.38% (42:1) right? Not 50% (1:1)?

 

[Ryobi310]

Hi everyone,

New member here, thanks for a great forum, I found this older post but wanted to add to it, hopefully the OP will read this.

I have many of these Ryobi's similar to his 790r (31cc half crank with reed valve), and these do have certain quirks that you need to know about for the relationship to work out.

The spark arrestor is located under the muffler between the cylinder flange and the muffler inlet. I always remove it and leave the darn thing out. Never has a string trimmer started a fire in a yard from carbon particles that i know of, plus the clogging of the screen is pretty often and is a headache to service on this design. My 1996 model did not come with one from the factory. Only the west coast states on federal lands require them I beleive.

Air leaks are common between the cylinder and the flange of the crankcase. Use a new gasket but spay both sides with KW copper coat first, let the KW become tacky then reassemble.

Another thing to check is the reed valve under the intake manifold the carb sits on. Some of these 31 cc motors did not have proper clearence between the con rod big end and the tip of the reed stop, causing eventual breakage of the reed stop and internal damage from bits of metal from the reed stop. To correct, install new reed valve and reed stop, then, buy some small aluminum spacers (4) about 3/16" long and instal between the crankcase and the intake manifold. You won't use a normal gasket. Take some permatex ultra grey and fill the gap created by the spacers. I use a tool to smooth it into the gap, like you would if caulking a joint in a bathroom fixture. Be sure to seal perfectly. Let dry overnight. You will need to slighly richen the carb due to the change in crankcase volume.

Use a ester oil to keep piston rings clean - I found Motul 800 or Bel Ray H1R at 25:1 ratio is very clean. Another good option to avoid ethanol is 40FUEL available at Walmart and Sears. This is good stuff for those who only burn a couple of gallons a year. Hope this helps those that like to keep old stuff going.

 

[arrabil]

Wow, thats WAY more work than that stupid trimmer is worth. Welcome to TBN.

 

[doall]

Yeah that's great info. Buy a new one. How in the world did you ever figure that combination of fixes out? I had a Ryobi in 96 too, but not in 98.

 

[Ryobi310]

Hi doall:

I've by circumstance mostly ended up with many of the 31cc Ryobi's and they've become one of my hobbies to fix them up and see how good I can get them to run. No it's not practical, but MTD has all the parts and prices are cheap so it's mostly self-taught. I found the reed valve clearance problem when rebuilding a leaf blower for my girlfriend. I have soup ups I've done using larger carbs and tuned pipes but the crank bearings have their limits. Don't ask how I know that

The 31cc has a very, very long production, and is still used in some MTD brands today. The oldest I've seen is from a lawn-boy model 1460 trimmer from around 1982 or 83. I think these go back to the 1970's but haven't found proof of that yet.

 

[arrabil]

Well, its still a stupid trimmer to me, but I do respect your hobby work on it. :thumbsup:

 

[doall]

I respect all hobbies. keep it up. we all wish we could figure out things like that.

 

[Ryobi310]

Hi everyone-

thanks for your kind words, I thank the members of great forums like this that I've learned so much from over the years, still need to learn the picture attaching procedures to add photos to my posts.