Topic: lawn boy 2 stroke?

[mullet_boy1969]

I have a lawn boy push mower, and a small light montgomery ward mower.  I'm wondering if anyone here thinks the little lawn boy engine has what it takes to run with stock briggs flatheads.  It's 120cc I'm pretty sure, and around 4-5 horses.  The rules around here for the "stock" class is the mower has to be a mower (imagine that?), and the engine has to be off of a mower of some type.  (the mod class is crazy, 900 kawasaki bike motors and such).  I have a book on perfomance tuning 2 strokes, so I can make a good tuned pipe. Anyone think it'd be bad enough to do the job?? or for that matter even worth getting running?

[Tom the Canuck]

you'd have to heavily mod that little engine to make it worth while, i would definitely modify the intake to accomadate reeds,run a small mikuni VM style carb, polish port,and build your pipe. 2 strokes are finicky, and the better you balance that thing the better it will run. racing sled engines push 14 000 RPM,but i doubt you should go that high (for the sake of pistons) my sled makes the most power at 8200 rpm, for you since its fan i'd shoot for around 6500 - 7000. Another thing your going to have to consider is parts availability. it might be possible to find a piston for another application that will work,but if you wreck that cylinder is there going to be anything left? can you get another one? theres lots of questions you gotta ask before you take this on.

[mullet_boy1969]

The great thing about this little engine is its already has reed induction.  I know ill have to run a much larger carb, because as it is its governed at around 4000 I imagine, and the stock carb has trouble keeping it there even sometimes.  As far as the cylinder going on me, I'm sure i have another one laying about the yard somewere, so parts wouldnt be much of a problem.  I'm imagining the pipe and carb alone will wake this engine up a LOT.  I think i remember reading somewere that yamaha k100's (something like that..i know it was a yamaha 100 though) gain like 6 horse from just a pipe over a 4 hole can exhaust.  So im assuming i can get similar gains from a pipe on this thing, as they seam to be very similar situations.

[Tom the Canuck]

oh you'll see huge gains with a pipe and carb,but i would seriously look into balancing it. as far as making your own pipe remember,that pipe has to be excellent to work properly,if you can time it just right it will make huge horsepower in a certain RPM range,if you screw it up they'll be no power,or the power band will be too low. but i'm sure anything you do will be better than the stock.

[mullet_boy1969]

I bought a book "Two Stroke Performance Tuning" by A. Grahm Bell.  Amazing book.  Read it cover to cover and prolly need to do so again.  But anyways to the point, that book has a ton of formula to figure the cone angles, tuned length, belly length, everything u need to build an exhaust to perform at a target rpm.  I'm wondering what a good redline rpm would be for this engine?  I know a lot of older engines are governed at 4000 (or atleast my pulling tractor is), so I'm guessing this engine is around there right now too.  With the stock carb i dont think i can get past that rpm by much anyways, but i figured id ask for when i find a suitable mikuni or tilotson.

[Tom the Canuck]

if its nice and balanced,and you can keep it cool 6500 - 7000 would be nice.

[mullet_boy1969]

would metahnol be worth it?  I think the head/jug is one peice, so i cant run high enough compression to take advantage of it other than the cooler run temps.  Oh and by the way i have my choice between this and a 10.5 briggs flatty.  I just think the idea of a 2 stroke racing mower is kinda cool, and it weighs 8 times less.

[Tom the Canuck]

DONT RUN ALCOHOL IN A 2 STROKE, alcohol and oil dont mix. I wont even run 10% ethanol in my sled. fucus on a nice carb set up, your pipe,and making those reeds work. If you can get a set of V Force reeds for it thatll boost the performance a lot. Focus on the inside more so for now, if the head/jug is one piece look into another piston or rod that you can get to make the piston go closer to the head, compression is good. to a certain point. If your serious about running a race 2 stroke fuel go to your local motocross track,and ask what there running,and buy some up.

[mullet_boy1969]

i know alky can be ran in 2 strokes, you just have to run it with certain oils. (clotz and some shell oil...cant remember it right now but that book i have gave a good list and ratios to run them at) Thats what the micro sprints at the local dirt track run.  I could run more ignition advance on alky too.  As far as the rod/piston goes, im goin to have to start on my home work there.  im sure no one makes any high compression peices for these engines.  Maybe a yamaha 100 rod/piston?  Not sure yet ill have to get some dimmensions.  In your other post you mentioned balance, do you mean like crank and mechanical assembly, or from a tuning stand point, like setting carb, pipe, and port timing to work to hit a target rpm?

[mowdak1]

Well... I could be wrong, but don't you think that common sense would dictate that if a little bitty 4 hp 2 stroke could be built to a point of competing against whatever the rest guys there locally are running, in an economical manner, that some of them would probably already have those engines on the track competing with them. In which case, you wouldn't be here asking these questions, you'd be at the local track looking at all the others that were being run. 

And, most organizations rules specify 4 stroke engines only.

[matt25001149]

good point rocky but but whats wrong with trying something diffrent and if they're allowed bike motor in their mod class im sure that 2 strokes are allowed aswell

[mowdak1]

My point being...

in an economical manner

I'm not saying it can't run with the others, I'm not saying it isn't legal under their rules, (although it would be in USLMRA and I believe ARMA), I'm saying it probably isn't feasible to build it to do so or others would be doing it already.

We are all aware of what kind of horsepower can be generated from small 2 stroke engines, the kart world has proven that point, the cost incurred in building it to generate that kind of power is prohibitive in most "mower" racer's budgets however. Even taking that into consideration, those small 2 strokes are designed to get up to speed and run at high RPMs on a paved track, they lack the torque necessary to run on a short dirt track where your machine weighs 3 - 4 times as much, where your center of gravity is 2 feet higher meaning slower cornering, and thus where you're going to bog down coming out of the corners of a dirt track.

It all goes back to the old adage, "There is no replacement for displacement!" Given displacement you can generate more horsepower for less money. It's a known fact that the higher your ratio of horespower to displacement, the more expensive your build becomes.

So the question is... Do you have $5,000 - $10,000 to stick in this 4 hp motor to make it run competitively with 12 hp motors that have been built for $1,000 or less? Because that's what it boils down to guys. 

[mullet_boy1969]

Normally i would completely agree with everything youve said about not being economical and all that.  BUT the race around here is once a year, so no one thinks outside the box much, just your normal briggs flat heads.  the extent of the mods in this class arent much over discontecting the govenor and maybe a do it yourself port job.  my resoning for using this motor (or rather considering it) is that at 4 horses it weighs only around 15lbs.  I just looked at the shipping weight for a 10 horse from a small engine site, and it was 130 lbs.  now im sure it weighs less than that, but still even if it weighed as little as 80 lbs the power to weight ratio is much worse than the 2 stroke.  The track they run on around here is the outside of baseball field so not to many sharp turns and such to worry about having a ton of excess torque to use.  Oh and i almost forgot about the speed limit, your not "supposed" to exceed 12mph.  So i could gear to have torque from this little motor.

[Brad Butler]

Personally i dont think youll be able to do much with 120ccs, but then again, yz125 dirtbikes can make like 30 hp, BUT they also have powervalves to make some of that hp too.

2 strokes are better mod for mod, you can probably double your hp by just porting it and getting the carb right. Also making a good pipe would be nice, maybe a yz125 pipe since its used around that amount of CCs?

Heres an example. I have a blaster, stock it puts out 16 hp, i got it ported, put a FMF pipe on it, jetted the carb, and put a K&N filter on, im now pushing 25-27 hp on it i guess    Now if i actually got a bigger carb, and got my head rechambered, i could probably open up another 5-10 hp in it. I could be over 50hp with it if i went with a stroker crank and a drag pipe, and etc.

My guess is the rating is 5hp since u said its governed to 4000 rpm, a 2 stroke makes most power between 7 and 10k usually, so that little motor is probably making more hp than most would think. I bet you could make 20hp + with that thing, with the correct mods.

Your best bet is to get a drag port and polish (makes the most power IMO) from a reputable quad engine builder (that works on banshees, 250rs, blasters, etc.) A friend of mine is badass at porting, you should go with him, heck get that 2 stroke cylinder right  he ported mine and the gain was amazing. Call Neil at 459-9320 and he can tell you what would be good for that motor
heh, i just noticed i started out saying "not gonna make that much power" then at the end i kinda contradicted myself. I got to thinking about how much power 2 strokes can make. Sorry if i confused you 

[mowdak1]

Uhmmm... No, you cannot "gear to have torque" out of a motor that is designed to produce none. That motor is designed to spin a 22 inch blade; at most spin that blade and pull 40 lbs of motor and mower around the yard with someone behind it to push when it encounters anything too difficult. It is not designed to spin 2 or more blades, and pull 200+ lbs of mower, plus a driver around, that requires torque. If the 4 hp were capable of that through gearing alone, and it can be produced at a third of the cost, don't you think the manufacturer would be building riding mowers with 4 hp motors and 8 speed transmissions on them?

And, your horsepower to weight ratio should be factored into the complete machine, not just the motor, how does your 4 hp stack up against 400 lbs of mower and rider, versus 10 hp with 400 lbs of mower and rider? Your weight to horsepower theory is sound IF you're racing it on the bench, when you add the weight of mower and rider to that equation, the 4 hp engine loses dude.