Which engine/string trimmer should I get?
 

The local pawn shop has two string trimmer candidates on the shelf. One is a Ryobi 31cc for $45, the other is a Poulan Pro 31cc for $40. 15% off sale at the moment. The Poulan Pro is purple, top to bottom, the Ryobi is an older one I think as it has a squarish power head.

I'm not sure if they both run the same engine or not; or if one is better than the other. If they are both equal, should I go with whichever seems to have the best compression? Does one have better parts support than the other?

Thanks,

Matt

Though they are both pretty good engines, I would definitely go with the Ryobi personally. Is this for airplane or weedeater use? Is the Ryobi rear exhaust/rear intake, or side exhaust/side intake?

In the smaller sizes, the "Weedeater" made by Poulan is a very cheaply made engine. In the 30'ish size and up I think they are better made.

AV8TOR

Matt, sent you a PM.

The conversion will be for an airboat, though it may get used as a weed eater until fall. We haven't got one at my fiance's house.

I am not sure of the intake and exhaust port configuration as it was hanging on a wall above my head. Is one config better or easier to use?

I think I am going to get the Ryobi, and after another couple paychecks I may go get the Poulan. I will be definitely watching come spring clean up for trimmers. I like to tinker and I think I may need a stack of them to experiment and learn with. I'd like to learn how to do porting and all that.

I often buy large lots of used/discarded yard equipment and fix the good ones to sell, and steal engines from a lot of them to convert for airplanes. The Ryobis, while not a high dollar, top brand like Echo or Stihl, seem to be the best of the lower priced weedeaters and consistently run pretty well if maintained properly. If it is rear intake/rear exhaust, it is actually a Ryobi engine. If it is a side exhaust/side intake, it is probably a Homelite engine which is actually slightly better.

AV8TOR

I would go for the Ryobi too.
I used to go around the neighborhood on garbage day and look for gas trimmers etc being put out as trash. Most all just have problems with the powerhead, and the engine works OK.
Thus I got them for free that way. But you need to go around early before the metal scavengers come around to get them too.

Some of the small engine repair shops have piles of the things out back too. They'll usually give you one or two or maybe for a small pittance in cost.

Well, I bought the Ryobi off the pawn shop. Just a bit over $40. Just yesterday I was hauling some trash to our town dump and found a 32cc Craftsman that seems to have good compression yet. I haven't had time to play with either so I don't really know anything about how they do or don't run. But I do for sure have one for learning to port and such on now.

If I remember right that craftsman is rear exhaust/carb and didn't have a primer that I could find. I thought that to be odd, as well as an actual on/off switch. I have something to tinker on anyway.

I have another question, unrelated to my engine quest. I have a carb from my Baja, the stock carb from the 23cc fuelie. I believe it is a Walbro 618. I know it is a touch biggernthan the 11mm recommended, but I already have it so no cost involved. Will it fit/work?

The carb should fit/work. You might need to port the plastic insulator where the carb mounts to the same size as the carb throat outlet for best results, and if the pulse holes for the fuel pump don't line up, you'll have to deal with that. You can make a groove to send the pulse where you need it, or just convert to an external fuel pump pulse line. Easy to do...

AV8TOR

If it's a WT 618 it is smaller than an 11 mm venturi carb. It is 7.14 mm

Sorry, its a Walbro 668. I can't keep of track of all the numbers used for naming things.

If its the same bolt spacing I can fit it. My soon to be father in law has a small milling machine and an engine lathe, so I should be able to come up with a new spacer. Maybe make it 1/2 or 3/4 thick to get a little extra velocity out of the air/fuel charge.

Gotta tinker on the engines first though...

Ok,That ones a 12.7 venturi. I've used that size on moded 25 to 30cc engines with good luck.Be sure to do what av8tor1977 said to do to the carb spacer and open it up.

If you make a new carb mounting adapter, make sure to make it of something that will not transfer heat to the carb, such as phenolic material. The carb mounting screws and the carb must be isolated from direct contact with the engine and it's heat.

AV8TOR

If you could find an aluminum isolator with teflon gaskets that would work,like the ones used on Zenoah,RCMK and Sikk boat engines.These seem to stop heat transfer.I had them on my Zenoah and Sikk powered r/c boat engines.Does any company make them in aluminum for airplane use that has the twister type like the one made by Wacker Engines ? http://www.walbro.com/parts-service.aspx

If you could find an aluminum isolator that has teflon gaskets like the ones used on r/c marine engines like the ones used on the Zenoahs,Sikk and RCMK's that would take care of heat transfer and warping.Even better would be an aluminum twister one like the plastic ones made by Wacker Engines.Does any company or person make these for conversion engines ?

I can get, or used to be able to get, teflon gaskets from Dave's Discount Motors. They were about a quarter inch thick if I recall correctly. I still may have some for my Baja 5B. As far as the mount screws, I don't recall there being a second set to mount the carb to the intake.

Does it really make a difference? Its easy enough to make two sets, but I just can't see that much heat coming through the screws. I'm not 100% certain, but I believe on the Baja series cars the carb and intake bolt on with the same screws. They seem to run fine that way.

Mind you, I'm not trying to say its wrong or whatever, just what I recall of my Baja goes against that advice.

I had troubles with a Stihl 62cc with the carb mounting screws going right into an aluminum adapter. I had a 1/2" thick plastic spacer, but the mounting screws went into an aluminum adapter. It transmitted enough heat that I had some vapor lock and tuning problems, so I made a different spacer that had the screws isolated. That solved my carburetion problems. It gets very hot where I am.... maybe that was the difference. I work on a ton of two stroke tools; the only one I ever saw that had the carb mounted with the bolts going into aluminum was an old chainsaw that had sleeves and fiber washers on the carb mounting bolts to help isolate it from heat from the screws.

AV8TOR

Enough heat coming through the screws to vapor lock your engine is quite interesting. I took a peak at my Baja last night and, indeed, the bolts go through the spacer to the motor. I don't believe there is any other option for mounting with them. The stock carb spacer is some sort of plastic; it reminds me of bakelite. Maybe these gaskets may help you with your troubles http://www.davesmotors.com/s.nl/it.A/id.871/.f http://www.davesmotors.com/s.nl/it.A/id.872/.f An interesting property of aluminum is its ability to conduct heat, which it does about as well as it conducts electricity. I haven't been following hpibajaforum.com in some time now, but when I was, I never heard of anyone complaining of a vapor locked carburetor. It could be something unique to rear-carbureted engines.