mangocrazy
Italian V-twin nutjob
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- Thread starter Memmeddu
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dyno-tron
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Yeah, the newer 2 stroke engines usually have phenolic stuffers in the balance holes. Most of the classes of racing now don't allow any modifications whatsoever and are very eager to find evidence of tampering!9EHad a 210 Upton villiers in my kart , crankcase stuffing was a big part of the tuning process. I don’t suppose it applies when tuning modern two strokes , doubt if there is any wasted space.
Bob
Moose McAlpine
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This might help:
Memmeddu
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I'm following this channel , he made me understand how two strokes really works
Always thought that the fuel was injected in the chamber via depression as in 4 strokes but it's actually the opposite .
Positive pressure in crankcase because of the piston coming down .
A very nice channel .
Screwdriver
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Dude without a basic understanding of how a two stroke works, you are unlikely to improve on a tried and tested design developed over the years by teams of trained engineers. There are still a couple of options open to all of us for home grown improvements.
1. Blueprint it, i.e. make sure that the item matches exactly the design specification
2. Copy a known, tried and tested modification that someone else has shown to work
The first option is invariably the best. Engines wear, gaskets shrink or grow (with pre owned items, people often stick a cornflake packet gasket in there), seals leak, exhausts get bunged up and the manufacturing process itself has tolerances. Just make sure it is physically tip top.
The second one is more tricky. When I was racing strokers back in the 80's it was Stan Stevens (expensive) and I think Steve (?) Colsta
who were the big names. If you could get your hands on a modified engine, you could copy it. But for a grass trimmer, I doubt if there's a "known" tuner so you'd be largely guessing.The elephant in the room is the expansion chamber exhaust. The science behind that is baffling (
) with high pressure shock waves and reverse cone engineered gas flows. It is a minefield and even the very best designers will iterate a design using dynamometers or rolling road data to home in on an optimum setup.It's really easy to modify anything but the difficult bit is making it better and not just noisier and peakier. Quite how you establish whether or not your lawnmower runs better than a standard factory spec engine is probably a moot point. in racing, you look to see who's winning and try to emulate that. I guess you could go ask the guy with the best lawn...
Memmeddu
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I have to start somewhereDude without a basic understanding of how a two stroke works, you are unlikely to improve on a tried and tested design developed over the years by teams of trained engineers. There are still a couple of options open to all of us for home grown improvements.
1. Blueprint it, i.e. make sure that the item matches exactly the design specification
2. Copy a known, tried and tested modification that someone else has shown to work
The first option is invariably the best. Engines wear, gaskets shrink or grow (with pre owned items, people often stick a cornflake packet gasket in there), seals leak, exhausts get bunged up and the manufacturing process itself has tolerances. Just make sure it is physically tip top.
The second one is more tricky. When I was racing strokers back in the 80's it was Stan Stevens (expensive) and I think Steve (?) Colsta
The elephant in the room is the expansion chamber exhaust. The science behind that is baffling (
It's really easy to modify anything but the difficult bit is making it better and not just noisier and peakier. Quite how you establish whether or not your lawnmower runs better than a standard factory spec engine is probably a moot point. in racing, you look to see who's winning and try to emulate that. I guess you could go ask the guy with the best lawn...
This weed whacker/string trimmer/brush cutter is going to be thrown in the trash anyway ,so well worth trying .
Nobody does port work here so nobody that can give advice .
So essentially what I'm going to do is
Drop cylinder hight
Clean the ports an transfers imperfections .
Open the exhaust little bit
It should run the same but better
Memmeddu
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0.9mm squish with base gasket
0.75mm gasket
30% shrinkage so the gasket is 0.225mm thinner ,we have a 0.525 gasket right now .
0.9mm squish is what is recommended on 90cc
Strimmer is 24cc ,it should have 0.25mm squish
Removing the gasket it should go down to roughly 0.35mm squish so plenty of room considering the carbon build-up on the piston
Final squish after cleaning would be more than 0.4mm
I think that it will run way better without base gasket
0.75mm gasket
30% shrinkage so the gasket is 0.225mm thinner ,we have a 0.525 gasket right now .
0.9mm squish is what is recommended on 90cc
Strimmer is 24cc ,it should have 0.25mm squish
Removing the gasket it should go down to roughly 0.35mm squish so plenty of room considering the carbon build-up on the piston
Final squish after cleaning would be more than 0.4mm
I think that it will run way better without base gasket
dyno-tron
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The key thing with 2 strokes is the relationship between inlet/transfer/exhaust ports. The inlet & exhaust generally need to go in opposite directions, eg earlier opening inlet = lower but earlier opening exhaust = higher.
Dropping the barrel will = earlier inlet + tighter squish so try that. Then you can take it apart and file the top of the exhaust port so that it is open a bit earlier, and see what happens.
If it wants to rev a bit higher you can turn the carb high jet in a bit - tiny amounts - while it's running on full throttle. Be aware that if you go too far the piston will overheat and sieze, I have shelf with dead pistons & barrels as a reminder of my experimenting days
Popcorn
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As above dropping the barrels will lower the output of the engine, instead of removing the base gasket use 2 and remove the head gasket. Lap the head and barrels together with grinding paste (obviously cleaning off after) this works provide it's air cooled and not water cooled. Also doing this means you can try it without actually modifying anything just to see your going in the right direction with the modifications.
Screwdriver
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If you remove the base gasket, you'll be lowering the exhaust port. This drops the powerband lower in RPM giving you more torque but less power.
My memory is a bit rusty after so many years but I seem to recall a racers trick (I was competing in production racing) was to add an extra base gasket to raise the exhaust port. Thing it, everything you did had to work with everything else because you weren't trying to make "better" power than say Yamaha could but you just wanted a load more of it all in in spot. Everything had to be "tuned" to be optimised for a very narrow RPM; expansion chamber, port timing, port volumes, ignition timing, compression etc. etc. If you changed one thing to affect the power band, you had to change everything to match.
The most difficult part for me was carburation, I had a devil of a job getting that spot on. Endless plug chops and even if you got it bang on, your tuned engine was now so peaky and fussy, it would probably run like a dog first thing in the morning, sing like a highly strung violin lunchtime before going flat in the evening.
I doubt if your grasscutter even has a tuned expansion chamber so if I was you and you're determined to have a play, I'd start there.
PS The name Steve Colville is ringing a bell, from Colsta Tuning
PPS In the bike world, you weren't really trying to be better than the OEM designers but you could remove the detuning manufacturers had to include to pass emissions and noise regs.
PPPS The advantage of fitting a tuned expansion chamber is that your engine will probably sound a whole lot better, even if it isn't.
My memory is a bit rusty after so many years but I seem to recall a racers trick (I was competing in production racing) was to add an extra base gasket to raise the exhaust port. Thing it, everything you did had to work with everything else because you weren't trying to make "better" power than say Yamaha could but you just wanted a load more of it all in in spot. Everything had to be "tuned" to be optimised for a very narrow RPM; expansion chamber, port timing, port volumes, ignition timing, compression etc. etc. If you changed one thing to affect the power band, you had to change everything to match.
The most difficult part for me was carburation, I had a devil of a job getting that spot on. Endless plug chops and even if you got it bang on, your tuned engine was now so peaky and fussy, it would probably run like a dog first thing in the morning, sing like a highly strung violin lunchtime before going flat in the evening.
I doubt if your grasscutter even has a tuned expansion chamber so if I was you and you're determined to have a play, I'd start there.
PS The name Steve Colville is ringing a bell, from Colsta Tuning
PPS In the bike world, you weren't really trying to be better than the OEM designers but you could remove the detuning manufacturers had to include to pass emissions and noise regs.
PPPS The advantage of fitting a tuned expansion chamber is that your engine will probably sound a whole lot better, even if it isn't.
dyno-tron
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As @Screwdriver said, the exhaust can is the odd thing here. Go karts & bikes have a tuned exhaust, the cone at the end reflects the pressure wave back towards the exhaust port and stuffs the escaping air/fuel back into the cylinder just before the piston closes off the port. The timing of the pulse can move the power band. Short exhaust = quicker reflection = more effective at high revs. Long exhaust = slower reflection = more effective at lower revs. Huge amount of maths & trial and error goes into exhaust design, all done taking into account the port timings.
This is ok for 2 strokes with a separate cylinder head, but with the smaller engines the barrel & head are one piece (much simpler and cheaper) so no head adjustment is possible.
Memmeddu
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So let's start telling.
Engine runs so I didn't messed up everything
So I've cleaned of the carbon build up from the piston and cylinder head .
Removed the base gasket because the squish was horrific high for such small engine.
What I've done later .
Lifted little bit the top edge of the exhaust port as suggested .
Once done ,I didn't be able to stop
So removed material from the bottom edge of the inlet port .
Made both them wider , nothing special maybe 4mm wider (2 on each side )
Once done that I give a look at the cylinder base , noticed that the transfers were restricted 5 plus mm of waste .
So ground them in diagonal to not open a hole .
Essentially those restrictions were a like a wall in the transfers .
Once done that I've gave a look at the crankcase also here a small restriction in the transfers (no piston ported) ground other 1mm on each side .
Smoothed out everything I've ground using 400 grit sandpaper .
During reassembly I've snaped the lower piston ring (my fault)
Lubricated everything , and applied red liquid gasket .
Prior to mount the carburettor I've sprayed some starting fluid and it started rev up and died.
Carb is connected to the cylinder via a plastic thing
So well you already know
Exlarged it to match it with the inlet port without any restriction .
Didn't touch the carb side since it's 11mm as the carb output .
Opened up the muffler by removing the spark arrestor plug .
Put fuel mix .
Started it up and felt different .
No vibrations at idle but higher RPMs at idle
Had to turn counterclockwise the High jet little bit to make it rev up .
It sounded rich but not bad .
I can't describe it .
Tomorrow I'll buy new pistons rings and gasoline so I'll be able to proceed .
But it didn't blown apart so
I won't go further with the carb adjustments because I need to replace the pistons rings .
Remember it's not because the ports but because I miss positioned it like an idiot
I have something to read thanks to @dyno-tron
Engine runs so I didn't messed up everything
So I've cleaned of the carbon build up from the piston and cylinder head .
Removed the base gasket because the squish was horrific high for such small engine.
What I've done later .
Lifted little bit the top edge of the exhaust port as suggested .
Once done ,I didn't be able to stop
So removed material from the bottom edge of the inlet port .
Made both them wider , nothing special maybe 4mm wider (2 on each side )
Once done that I give a look at the cylinder base , noticed that the transfers were restricted 5 plus mm of waste .
So ground them in diagonal to not open a hole .
Essentially those restrictions were a like a wall in the transfers .
Once done that I've gave a look at the crankcase also here a small restriction in the transfers (no piston ported) ground other 1mm on each side .
Smoothed out everything I've ground using 400 grit sandpaper .
During reassembly I've snaped the lower piston ring (my fault)
Lubricated everything , and applied red liquid gasket .
Prior to mount the carburettor I've sprayed some starting fluid and it started rev up and died.
Carb is connected to the cylinder via a plastic thing
So well you already know
Exlarged it to match it with the inlet port without any restriction .
Didn't touch the carb side since it's 11mm as the carb output .
Opened up the muffler by removing the spark arrestor plug .
Put fuel mix .
Started it up and felt different .
No vibrations at idle but higher RPMs at idle
Had to turn counterclockwise the High jet little bit to make it rev up .
It sounded rich but not bad .
I can't describe it .
Tomorrow I'll buy new pistons rings and gasoline so I'll be able to proceed .
But it didn't blown apart so
I won't go further with the carb adjustments because I need to replace the pistons rings .
Remember it's not because the ports but because I miss positioned it like an idiot
I have something to read thanks to @dyno-tron
I have a gutless Honda 125 scooter, which is crying out for a tuning and an expansion chamber. Allas, not enough hours in a day...
Quick question for the knowledgeable on here though - why does it smoke like hell on startup when its been left for a few days?
Quick question for the knowledgeable on here though - why does it smoke like hell on startup when its been left for a few days?
Memmeddu
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I'd like to replace the rings I need torque not High speed .
I've opened up the inlet manifold just because it had restrictions.
I'll never touch Venturi tube on a carb .
I could ask to a friend if he can 3D print an horn for the carb
About smoothing out the inlet it doesn't work on 4 strokes but on 2 strokes you have to avoid any turbulence from what I've understood.
About your saw did you try both an hotter and colder spark plug?
As a learning step this work is usefull, but if you want to take this further you need a way of measuring your gains/losses. Google engine dynomometer, design and make your own bench top version. That will really take your learning foward.
