mac49

My Feedback: (5)

I purchased this one off of ebay. I haven't check with my local Lawn-Boy dealer but I think it can be purchased as a shortblock. That was the way it was advertised on the auction. The diagram I downloaded from Lawn-Boy also showed it as an assembly.

rockcrawl

I though some of you guys might like to hear about my experience with the Lawn Boy engine. I'm glad I'm not the only one who though it was worth trying. I'm running a giant scale buggy, so weight is not as much of an issue. My engine is circa 1991, the same type as pictured directly above. 121cc, 4.75 hp factory rated. It has roller bearings supporting the crank. The tapered shaft is hardened on the end where the blade attaches, but I was able to turn it on a lathe so I could bolt the clutch on. The stock Walbro carb is junk, it's a float type carb that's designed to run in a narrow RPM band. It's also too small. I'm running a Tillotson HL series diaphram carb with about .8" bore and a tuned pipe for a Yamaha 100cc kart. The carb limits it to 5500 rpm with no load. It will easily take one of the larger 1" tillotsons that are popular for kart racing. I have the engine mounted with the spark plug pointing up, and I've made an intake manifold to bring the carb out the front. I left the stock flywheel and ignition on it. The flywheel and cowling acts as a cooling system drawing air across the engine, it's aways packed with grass and leaves after I run it. The engine runs great and has plenty of power, I have no regrets for using it.



Why do you airplane guys get rid of the magneto ignition and convert to CDI? Is it for weight savings?

BillyGoat

Ok I have a Lawnboy that I took from somebody’s trash. The only thing wrong with it was a busted fuel tank and it needed a new plug. I’d start tearing into this thing today but I got rid of my old mower because this one ran much better and it’s self-propelled. If somebody gets this in an airplane that mower may become history real soon.

Rockcrawl, I’m not into cars but that thing is too cool!!! [sm=thumbup.gif]Is that your own design?
I could see myself building one of those for grins and giggles. Thanks for posting those pics.

Sport_Pilot

I don't do engine conversions, mostly because I don't have any machining equipment and don't have the cash to do so. But I would think the newer four stroke lawn mower engines would be fairly potent. The older flat head engines have evolved from simple 3 1/2 HP with contact ignition to overhead valve 6 1/2 HP with electronic ignition. I think I have even seen them advertised with fuel injection. Briggs also has a series of engines modified for extra power used for the Jr. Dragster series. The old flat head engines used to be governed at only 3,000 RPM, so if the modern engines are turning at that speed they should turn a very big prop at 6 1/2 HP.

BillyGoat

I would assume a four-stroke lawnmower engine would not be good for R/C aircraft due to it having a wet sump oil system. Take it inverted and within a few seconds it’s game over.

Sport_Pilot

Why wouldn't you simply add the oil to the fuel and vent the overflow as we do with glow? Or add an oil tank and oil pump.

Volfy

My Feedback: (23)

RC glow 4stroke's crankcase get its lubrication from blow by oil. Glow fuel has some 15-20% oil content and typically one single ring. Gas 4strokes used for lawnmowers won get much blow-by oil even if you mix in the typical 25:1 ratio for 2-stroke gasoline engine.

You can design a dry sump system, of course, but that is a little too much beyond the typical scope of conversion.

Sport_Pilot

Gas lawnmower's have enough blowby to blow the oil out the crankcase if you leave the oil filler plug off. So I don't see any reason for it not to work. You could use a 10:1 oil ratio. Four stroke oil will mix with gas, but smokes like crazy.

twostroker

My Feedback: (37)

It might seem like "blow-by", but what you don't see is the rod dipstick churning the oil up every revolution.

Maybe a new piston with fewer rings would do the trick?????

Sport_Pilot

True the con rod churns it up, but its blow by that pushes it up a 10" tube. Lots of blowby. You could leave the oil scraper ring off, that is the only thing I see as a problem. But IMO a two stroke Lawnboy is a better subject. Just wanted to say that 6 1/2 HP at only 3000 RPM is more power than it would seem. Even more power if you put a less restrictive carb on it.

twostroker

My Feedback: (37)

agreed.

bdtsr

My Feedback: (5)

You're not seeing blowby in a 4 stroke mower engine, you're seeing crankcase pressure build due to the piston on the down stroke compressing the air in the crankcase. By removing the dipstick, you provide an easier exit for the air which has the oil being thrown all about by the crankshaft. The 4 stroke mower engines are still too heavy and are not designed to handle rpm's over about 4000. Yes I've seen B&S race engines turn 7000 with no problem, but at major cost (cam, carb, intake, piston, stroker crank). Maybe in a Cub like plane, one of these engines might work, particularly a flat twin! Since you really wouldn't be flying inverted, the oil problem would be there. You'd need about a 16" pitch prop to get the same airspeed from 3600 rpm versus 7000 rpm on a 2 stroke.

Sport_Pilot

I recall a flat twin used on a cub like plane of almost 20 foot span. No mods to the crankcase as they didn't do any aerobatics. I would think one of those hopped up B&S turning 7000 RPM would be fairly potent. The lawnmowers are fairly heavy, but if you grind away at some of the excess and remove the heavy flywheel they would be a lot lighter. BTW they are not as heavy as they look. Much of the weight is in the crankshaft to withstand hitting rocks with the blade and the babbit con rod. All of this can be changed, not an easy mod perhaps.

I can't believe you people don't think a lawnmower doesn't have any blow by! If not why do they have a crank case vent routed to the carb? Lots of smoke coming out of it if you forget to attach the hose also.

Volfy

My Feedback: (23)

I don't think smoke lubricates very well.

There are many more reasons why a lawnmower engine does not a RC aircraft engine make. For one, the vibration will be horrendous. Mounted on the heavy lawnmower frame, mass damping makes the vibration much less apparent. Free it up, and the big thumper will do what it knows best - shake!

Secondly, running 10:1 in any gas engine, 2- or 4-stroke is asking for trouble. If you have seen how the NORMAL 25:1 cokes up a 2stroke engine internals after NORMAL use, you would not entertain the thought of using any more oil, particularly in a 4stroke with the valve train components. Besides, I have a philisophical abhorence to running 2stroke oil mix in a 4stroke engine. To me, it defeats the purpose of the clean 4stroke. If you gonna do that, why not convert a 2stroke industrial big-block engine instead? It will be lighter and more powerful.

Thirdly, you can only shave off so much metal. I admire DougT's work on shaving off every last gram of weight off his little Echo 21.2cc. I myself have ground off more aluminum off weedies and chainees and caused more aluminum dust to fly than I care to inhale. It was fun in a masochistic way. These days, however, I much prefer to start with a good light short block like the Poulan and Homie chainsaws that need practically no cutting & grinding. It is one thing to cut off extraneous appendages and brackets; it is quite another to reduce a diesel engine down to a sewing machine motor.

Can it be done? Well... any thing can be done if you pump enough money, time and elbow grease into it. But is it a worthwhile exercise? Those of us who have played with conversion engines for a while eventually realize that however tempting it may seem to sculpt a precision machine out of a hulk of pig iron, sometime a boat anchor is best left as a boat anchor.

Sport_Pilot

You obviosly haven't owned a Honda! LOL, but it would need to be mounted on a rubber damped mount. Pretty sure if you put some castor oil in the crankcase and some in the fuel it would work fine.

When I first posted on this subject I was thinking like you, it wouldn't be worth the time. But I see much of the work has been done. The engine seems heavy for 6 1/2 HP, but did you know that they have modified these things to run on alcohol and get 35 HP on them?

Check it out.

http://www.briggsmotorsports.com/raptorengine.html
http://www.briggsmotorsports.com/blockzillaengine.html
http://www.drag.race-cars.com/carsal...67982667ss.htm

Volfy

My Feedback: (23)

Those links are for go cart racing and such. We are talking about RC model Airplane use. If you are thinking of using such an engine on a 1/4 scale RC car, that's fine. Otherwise, I don't think it applies to this forum.

Sport_Pilot

Hey thats not go karting! This is engine conversion, I do think it would be possible to convert a modified lawnmower engine. In fact the engines modified for Jr. Dragracing would work with little modifying. LOL, but not me, I don't want to fool around with a plane that needs 30+ HP.

Thinking of what you mentioned about vibration. I worked one summer repairing lawnmowers in the mid 70's, lots of bent crankshaft replacement. The crankshafts back then had little to no counterbalancing, and a fairly heavy babbit rod. My Honda HR 21 (which I bought in the early 80's and is still running today) runs a lot smoother than the Toro it replaced. The modern OHV B&S seem smooth also.

Volfy

My Feedback: (23)

Great, Sport! Feel free to convert one for RC airplane and post some pictures here. I am just dying to be impressed.

Otherwise, even though the name of this forum is "Engine Conversions", this is still a RC site. Occasional anecdotes are fine, but let's stick to RC-related stuff.

Sport_Pilot

Why be a jerk? Converting a lawn mower to R/C use is not allowed?

Volfy

My Feedback: (23)

I think I will let w8ye take it from here. Have a nice day.

rockcrawl

I'll be the first to admit I don't know squat about airplane engines. I'm not going to argue the possibility of using a lawnmower engine in an airplane, but this forum is about more than airplanes. If there's any chance that anyone can get a shred of useful information from a post, I think it should be welcomed. Many people read this forum every day without ever posting their intentions, and many of those people may be interested in cars, not airplanes. This thread has many more veiws than most others in this category, so there is obviously very much interest. Even if Karting parts have no place in RC aircraft (and I'll bet some of them do) they are still of interest to large scale RC car builders. Afterall, the name of the forum is not "Airplane Engine Conversions", if it were I never would have been here because Airplanes don't interest me. I happen to have more Karting and Jr Dragster parts on my RC car than I do RC parts. It wasn't until I realized that there are a goldmine of parts available because of the Karting industry that my engine really came to life. If a link to a Karting site can better one person's RC, then I feel it was worth posting. It's obvious that you (Volfy) have no interest in using a lawnmower engine. You've made your point, but there's no need to take it any further than that. As far as I'm concerned, none of the posts in this topic have gone too far off subject except for the last one or two comments that you made.

Jon

geardaddy

My Feedback: (27)

Have an update on the red Briggs engine image posted over a year ago. Went back to the big factory flywheel, cut a prop hub and added a Tillotson carb. Is a breeze to hand prop with a 30-8 P-K wood blade and runs fantastic. Can try to post an image of the new configuraton if anyone is interested.

Willdo

Yes, I'm interested, - I just happen to have two of these engines and I like them with their huge torque, and was considering converting one.
Trouble is do I want to build that big an aircraft?

I have been told that they are actually designed (and built?) by Tohatsu.

4 stroken ron

My Feedback: (29)

Hey geardaddy can you post the weight of that engine with flywheel, hub, prop. carb, motor mount, and mounting hardware. I would be real interested in knowing how much it weighs. Any RPM or thrust figures? How about pictures?
Thanks
Ron

geardaddy

My Feedback: (27)

Have never had any luck posting images with a Mac, now fumbling with a Dell. Engine is a 16 pounder, perfect for a 1/2 scale pietenpol or some other big bird not needing to hover and still use some nose weight.