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  1. #1
    SaCCaL72's Avatar
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    McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    I want to purchase a McCulloch 31cc weedwacker at Home Depot for conversion ($39.00). It has a rear carb and exhaust, is that a positive thing or a negative one? Would it make it a better engine for use on an R/C Plane with them on the rear of the engine? Thanks in advance for your help.
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Sounds more like a Ryobi 31cc engine. I have one and that is just what it looks like. If the Mac 31cc weedeater is a Ryobi look alike you can get parts from Peter Faith at ww.wackerengines.com. Peter is a great guy to work with he treats all of his customers riight. The Ryobi weighs in at 4lbs and is a long narrow engine. It will turn a 18x8 prop at 7200 RPM and the is enough to do some nice comfortable sport flying on a 14 to 16 lbs bird. The conversion is very easy and almost anyone can convert one of these engines with Peters parts. I will cost about $60 convert this engine. That is very cheap and it will be a very reliable engine.

    Thanks
    Jim Stanton
    Millington, TN 38053

  3. #3
    jstanton's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Here is a couple of picks of a Balsa USA String Ray 120 that I used for my Ryobi. I have a friend that is flying a Ryobi 31cc in a Four Star 120 and it flys great also.

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    Jim Stanton
    Millington, TN 38053

  4. #4
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Thanks for your help Jim, I just eMailed Peter with what I had and asked if he has the parts to convert this engine. I hope he can help me out. I am thinking of either a Four Star 120 or a GP Giant Big Stik, not sure. Are there any other planes this size engine will work in? Thanks again for your help.
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  5. #5
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Jim,
    Here are a few pix of the engine, does it look something like yours did? Peter also thinks it may be a Ryobi from my description, I just sent him these pix also, I am waiting for his reply to see if it is a Ryobi engine.
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Its what we call the new style Ryobi. It differs on the outside by having a cast on fan shroud rather than a bolt on one.

    Enjoy,
    Jim
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    I would say that is a Ryobi and Peter will have all of the parts you need. Remember to remove as much as possible to get get the weight down. The engine should weigh 4 lbs when converted. I would suggest to bend the reed value up about 1/8 of a inch the take out some tension. This will allow more full into the engine. You can us a MAS 18x8 prop and you should get 7200 RPM with that prop.

    Good Luck
    Jim Stanton
    Millington, TN 38053

  8. #8
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Thanks guys, Peter did get back to me and he confirmed that it is the newer style Ryobi engine. He does have the parts and it isn't all that expensive. I am excited to finally get to perform my first conversion. Do I have to turn down the fan rotor part at all? Also, do you suggest breaking in the engine before the conversion? How do you break this type of engine in as they do not mention anything about breaking the engine in in the manual.
    I would suggest to bend the reed value up about 1/8 of a inch the take out some tension.
    Sorry if this is a dumb question, but where/what is the reed? I am new to this so I am not sure I know all the part terminologies. I am sure once you tell me I will know what it is, just not by name. Thanks...
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    The Reed value is under the carb. Remove the plastic manifold the carb is mounted to and you will see the reed value. The value opens towards the engine so don't bend it any more the 1/8" or an inch. You only want to remove some tension from the reed value so it will open up a little more when the vacuum is created. This will along more full into the cylinder providing more power. I would get the cut down fly wheel from Peter along with the other parts. Peter makes sure they are balanced once they have been cut down. I would also spend the extra $25.00 and get the muffler Peter sells for this engine. It will give you about 300 more RPM over the stock muffler.

    Jim Stanton
    Millington, TN 38053

  10. #10
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Any Plane that has a 1000 sq in or greater wing area and weighs in around 8 to 12 lbs with out the engine. My Balsa USA Sting Ray weighed 10 lbs with out the engine and has a wing aera of 1276 sq in. That gave me a wing loading of 23 oz per sq ft. The plane flys just like a bigger sport trainer. Here is a list of plane I know the engine will work on.

    Balsa USA Sting Ray 120
    Sig Four Star 120
    Dynaflite Decathlon
    Lanier Stinger 120
    Any larger Cub
    Balsa USA 1/4 scale WW1

    Good Luck
    Jim Stanton
    Millington, TN 38053

  11. #11
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Jim,
    Thanks for all your help, you have helped me a great deal. Do I need a velocity stack for the carb or because it is located in the rear, it isn't needed? Thanks again...
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  12. #12
    jstanton's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    A velocity stack is not need for this type of engine. I would order the Ryobi conversion kit from Peter with the muffler. This give you everything you need to get the most out of your engine.

    Also just for kits my local small engine shop sells a brand of oil he says will never give you any trouble and it works on all weed-wacker and chain saw engines. It is called Opti Oil and you just mix one package with one gallon of gas. Here is the web page address http://www.opti2-4.com/ . Take a look at it and read about the Opti-2 oil. Since your engine is new I would brake in the motor with the oil I was going to be using. My local small engine shop guaranty's this oil and will repair or replace the engine if it fails while running this oil.

    Good Luck
    Jim Stanton
    Millington, TN 38053

  13. #13
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Here is a video of a Lanier Stinger 120 with a 31cc Ryobi engine. You can see the torque these engines are able to generate.

    http://macs-rc.com/ryobi.html

    Enjoy
    Jim Stanton
    Millington, TN 38053

  14. #14
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Jim,
    Thanks for the video link. I had eMailed the company who makes Opti-2 to find out where I can purchase it locally and the guy actually called me. He was very nice and explained his product to me. He found a place like 2 blocks away from me who sells it. It was only $1.39 for a pack that mixed 1 gallon of gas and $1.99 for a pack that mixed 2 gallons. Not too bad. I am going to try it out as soon as it stops raining here. It sounds like it is a great product.
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  15. #15
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    jstanton,
    I just about finished the conversion the other day and prepared it for a test run this weekend. It ran, to my surprise, and really smooth too. No visible vibrations that I can tell. It turned the MAS 18x8 at about 7700 rpm give or take, I was so excited. I was able to turn down the flywheel with a lathe they have here at my work's metal shop. It came out pretty good being the last time I worked with a lathe was in college 15 years ago. It was fun! I will post pix when I get a chance of my converted engine. It isn't perfect but it came out pretty good for the limited tools I had to do it with and being my first time converting. Anyway, I have a few questions maybe you or someone can help me out with:

    --When I started the engine, it ran fine but only as long as you had your finger covering half of the carb air intake or if you had the throttle fully closed for an idle. In idle position it ran at a higher than idle rpm most of the time but would slow up over time. Then at that point if you throttled it up, without your finger it would speed up for a few seconds but then die out suddenly. With your finger covering half the intake, it would throttle up and down pretty nicely and stay running. I am assuming that the air/fuel mixture may be off and it is getting too much air into the carb causing it to cut out. It ran fine as a weedwacker, but I am suspecting that since the air cleaner and all has been taken off of it, the air is rushing in too fast so it needs to be adjusted. I see a screw that is set into the carb some and can be reached with a screw driver, is this the high speed mixture adjustment? Does this sound right or does anyone have any other suggestions?

    --Another question is about the engine needing to be choked to start, how is this done if the engine is installed into a plane, say with a cowling, without having to take the cowling off? Is it still necessary to choke it to start it or is the primer bulb enough to do that?

    --That brings me to my next question, do you still need the primer bulb and if so, where do you mount this thing and how would it be hooked up to an R/C type fuel tank?

    I know I have a lot of questions, but I appreciate your help in advance...
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  16. #16
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    SaCCal72
    The RPM readings you are getting are amazing for a 31cc weedeater motor. The best I could get with mine was 7200 RPM with a MAS 18x8 prop.

    Answer to your question about the airflow is I think you are on the right track, but I don't know about the screw on the carb you are talking about. Mine carb has 2 screws. 1 for the idle and 1 for the high end. If your carb only has 1 screw I have heard that kind does not make a good carb for RC flight. You can get a carb with 2 screws from Scott Ellingson at www.brillelli.com for about $20.00 that will work good on your 31cc. You will not need the primer bulb so don't worry about installing it. Just choke the engine and give it a couple of flips than hit it with your starter and she should fire right up. Oh about the choke did'nt you have one when is was installed in the weedeater? If you through it away go to your local small engine shop and see if they have a choke for a ryobi 31cc. I bet they will and it should only cost a couple of bucks. You can install it on the back of the carb and make an extension if needed so you can choke it if there is a cowl you are installing.

    I hope this helps. Good luck!
    Jim Stanton
    Millington, TN 38053

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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Well converts here's my few cents on the subject, as posted in the "WHAT A DEAL" by Captinjhon I've already did the McCulloch 31CC conversion and as you mentioned it is a Ryobi Hybrid, it looks like a Ryobi but the insides are totally different meaning it has two transfer pots on each side, the compresion is higher and it does have 2 rings, the other thing that's different is the exhaust timing which is very low if I remember it was like 125 degrees maybe because it was a CALIFORNIA legal model. I was getting 7200 rpm's now after openning the exhaust timing to 145 degrees I am getting 8400 rpm with an APC 18X6W prop. The other thing is that mine came with a single needle carb and I replaced it with a 2 needle one. Also I converted mine to "Gasoline/Glow fuel and glow plug" (see the Gas fuel glow plug no ignition thread) which eliminates the flywheel and coil for less weight at the nose, better Idle and transition and smoother running I use my engine on a plane that I dessigned specially for that engine total flying weight including engine is 10 LBS and the McChulloch realy makes that plane "DANCE" I love it and my buddies here at my club want me to build them some because they also went and bought the McCulloch trimmer, we wish we could get some more trimmers in because they were all sold out and havent bring any more.

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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Aero nut: do you have any photos to post of the 31cc Mac after you converted it. Sound like a good engine to get. Thanks Capt,n
    I never met a engine I did not like !

  19. #19
    SaCCaL72's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    jstanton,
    I still do have the original choke. I didn't install it because the lever was hitting the motor mount but I think I can modify it to fit. I wasn't sure if I should keep it on the carb or not because when I looked at some of the sample pix on www.wackerengines.com I didn't see it attached. I think I will invest in the 2 needle carb though. My Walbro carb has a screw needle where the throttle lever is that limits the throttle arms movement and the other screw I was talking about is as I said, set into the carb some, it isn't visible unless you look in a hole in the carb. I wasn't sure if that was the high end needle or not. I can take pix when I get home but I am sure as you said, it may be a single needle carb and I should just get myself a 2 needle carb. I am glad I can get rid of the primer bulb, it is kind of an unsightly fixture. Thanks for all your help, I will keep you all posted soon with some pix and stuff. Thanks again...


    Edit Text Below:
    ps. Can these engines be painted with high temp paint without any ill effects to it's performance? I see yours is painted red, did you do that yourself? It looks so much nicer...
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  20. #20
    jstanton's Avatar
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    SaCCaL72

    yes you can paint these engine with paint that is designed for engines. Go to your local auto parts store and get the color you like. Mine is Chrysler Red

    Thanks
    Jim Stanton
    Millington, TN 38053

  21. #21
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    hey aero nut you said you increased your exhaust timing to 145 degrees from 125 . are you talking duration? did you cut the top of the port so it opens sooner ?where should it be from tdc?it really gave you 1200 rpm?! thats awsome!
    i too am converting Ryobi hybrid twin ring four port on gas electronic ignition (got two now) only got mine at harbor freight, one looks just like SaCCal 72's, same carb etc. one runs on old stock two needle carb and 18x6 zinger prop 7500 rpm's . do you agree that the corect larger carb is the walboro wa167? what is the drawback of the single needle carb for rc use?

  22. #22

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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    I did the duration timing by grinding the top of the exhaust port what this does is like you said it lets the exhaust come out earlier and stays open longer which lets more exhaust gases come out leaving a cleaner intake mix to be ignited, As for the position from TDC I didn't measure it all I did was to grind off about 1/16" from the top of the port and then measure the timing with a timing wheel until I had the desired measurement of 145 degrees. and the result is the one I mentioned 1200 rpm's I did this because my old ryobi showed that it's exhaust duration timing was 145 degrees and it also working great so I used it as a reference point to start, maybe I coud get more RPM's if I opened the exhaust port some more but then I don't know if I would have to deal with the transfer ports too and I don't want to complicate my life further.
    Now it is very important to keep in mind that the numbers I'm posting are using Gasoline/Glow fuel mix and that means 2.5 to 3% nitro and about 44 percent methanol which also boosts the fuel power, this mix works with magneto or EI very well giving you a boost of about 300-500 RPM's(other buddies use my fuel on their Ryobis), they run smoother and dont miss at all.
    As for the single needle carb question, the single needle doesn't allow the engine to get the max RPM's and the transition response from IDLE to HIGH doesn't work very well in a few words it doesn't allow you to get the MAXIMUM performance out of your engine that is needed on R/C usage

  23. #23

    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Don't get hung up on numbers for carbs. Any carb with an 11.11 - 13.5mm bore will work on the 31. It also need both the H and L needle. I have used many different number carbs both Walbro and Zama. The bore and 2 needles is all that matters.
    There is no replacement for displacement....you can allways throttle back!

  24. #24

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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Hey POCO why don't you try one of your toro engines with my or AV8TOR's fuel mix with your igniton and also try it with the glow plug adapter and let us know your results, I'm using right now a weadeater 25 with the glow plug set up an I'm getting 8300 RPM's on an APC 16X8 prop, it hand start very easy and the total weight of the engine with prop, spinner,engine mount, vibration isolators and muffler is 2Lbs 2oz, it would be interesting to see what your final weight without the flywheel and coil woul end being and also the kind of RPM's you would be getting from these two setups

  25. #25
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    RE: McCulloch 31cc w/ Rear Exaust and Carb, is it better?

    Scott

    I agree! It may cost a little extra to run with a glow/gas mix, but it would help those who cannot afford the the upgrade to EI when they purchase.

    Thanks
    Jim Stanton
    Millington, TN 38053


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