Is a mild gx390 faster than a wild 197cc clone
Here you go,last year damn near stock.
http://www.oldminibikes.com/forum/off-topic-discussion/57664-needs-more-work.html
http://www.oldminibikes.com/forum/off-topic-discussion/57664-needs-more-work.html
Motor:
18cc port and polished head milled about .080(Curtsy of Tim(Txpowdercoater)
Stainless valves(stock size) with good keepers and caps, on 26 pounders
1.3 rockers
cm pushrods
black mamba cam
flat top piston(stock size)
billet rod and fw
hogged out oil return hole and debaffeled valve cover
22 mil mukini
header(2-stage)
Timing at 28(some run higher but I don't want to take away form my low end)
Drive line:
max torque clutch(3000 rpm stall)12 tooth
promods jack shaft set-up(15 tooth primary,11 tooth secondary)
75tooth on back wheel
8.3-1 gearing by agk's jack-shaft calculator
Now keep in-mind I Race to 100 yards, i'm not looking for 50-60mph. Its about pulling to about 40ish, any fasters is not safe on a mini. But heck, having to lean over the handle bar when racing to keep the front wheel down is not safe ether. Helmets when I race.
Although I have not yet played with a big block engine in a Mini Bike yet. And in that respect this is off topic But I thought for engine comparison reasons I would share my latest experience with a modified Lifan 420 dirt oval kart I built. I recently had the opportunity to compare the potential of the big block against some other pretty savvy two stroke race engines . We were able to run it with some of the open UAS guys during the last two races on a high banked 1/4 mile clay oval track.
This track is known for being a HP track and motor eater. The UAS guys that showed up were top competitors in our area. There were a variety of engines being used.. The.17K+ rpm Sudams in Wedge karts have pretty much dominated at this track in the past. And for the last race a friend of mine even showed up with a Honda CR250 powered Wedge Kart he put together. I am highly familiar with this engine after 13 years of building them for my Son who was one of the top competitors on the west coast in a 250 Micro sprint. The CR250 is one of my all time favorite engine designs and makes 42-45hp stock on gas.
65+ tweeked on Alky
So for the first race.I was really excited to see how the 420 Lifanzilla Kart engine I built would hang with these UAS guys on this big track..
The first race night started with everyone spreading out on the track for a hot lap session.
I had my son hang way in the back as we had not ran this kart and engine package before,
I wanted him to feel it out good. Some of guys had gathered around our Kart before the race and claimed there karts were in the 40hp range, Maybe Slightly intimidating our Chinese pressure washer engine powered kart.
So Hot laps start.... and I watch my kid pass all but one kart in three hot laps.
I hear a couple of other pit crew guys say "Man I can't believe that four stroke is hanging with us"
I giggle and head back to our pits.. We later walked away with heat race and main event wins.
The last race of the season we got to start side by side outside pole position with the CR250 Kart in the main..The drag race down the front shoot on the start put My son Shaun about a kart length in front of the two stroke. Unfortunately someone didn't lift enough into the first turn and plowed him up in the marbles. The Cr250 kart took the lead and a Sudam powered Kart got under as well. But after getting pointed straight and hammering down Lifanzilla he passed back the Sudam halfway down the straight and was then shortly on the CR250's rear bumper. One more lap was spent all over it, then finally a pass on the inside.and a walk away main win by about a 3/4 lap .
I honestly can say that these engines have a lot of potential. And are a blast to play with.
I have experimented with them for the last two years or so and as far as the clones go I like the Lifan 420. And I know the Honda spec rods and cams fit it. Forged 90mm pistons in different configurations are readily available. The stock head can be modified to make good power to around 7-8k rpm. (our peak RPM on the 1/4 mile track was 7700.)
You can yank the balance shaft and bearings, The crank is forged and the rod journal is heat treated. Great engine platform in my book. High 30's hp and a whoop azz mid 30's torque output is pretty easy to come by with the right combo of parts. A lot more is involved than building the smaller engines though.. And for you guys running these in a Drag Mini Bike, Hats off to you. That would be one mean mini bike ride. Especially at the extreme level that these engines can be built.
This track is known for being a HP track and motor eater. The UAS guys that showed up were top competitors in our area. There were a variety of engines being used.. The.17K+ rpm Sudams in Wedge karts have pretty much dominated at this track in the past. And for the last race a friend of mine even showed up with a Honda CR250 powered Wedge Kart he put together. I am highly familiar with this engine after 13 years of building them for my Son who was one of the top competitors on the west coast in a 250 Micro sprint. The CR250 is one of my all time favorite engine designs and makes 42-45hp stock on gas.
65+ tweeked on Alky
So for the first race.I was really excited to see how the 420 Lifanzilla Kart engine I built would hang with these UAS guys on this big track..
The first race night started with everyone spreading out on the track for a hot lap session.
I had my son hang way in the back as we had not ran this kart and engine package before,
I wanted him to feel it out good. Some of guys had gathered around our Kart before the race and claimed there karts were in the 40hp range, Maybe Slightly intimidating our Chinese pressure washer engine powered kart.
So Hot laps start.... and I watch my kid pass all but one kart in three hot laps.
I hear a couple of other pit crew guys say "Man I can't believe that four stroke is hanging with us"
I giggle and head back to our pits.. We later walked away with heat race and main event wins.
The last race of the season we got to start side by side outside pole position with the CR250 Kart in the main..The drag race down the front shoot on the start put My son Shaun about a kart length in front of the two stroke. Unfortunately someone didn't lift enough into the first turn and plowed him up in the marbles. The Cr250 kart took the lead and a Sudam powered Kart got under as well. But after getting pointed straight and hammering down Lifanzilla he passed back the Sudam halfway down the straight and was then shortly on the CR250's rear bumper. One more lap was spent all over it, then finally a pass on the inside.and a walk away main win by about a 3/4 lap .
I honestly can say that these engines have a lot of potential. And are a blast to play with.
I have experimented with them for the last two years or so and as far as the clones go I like the Lifan 420. And I know the Honda spec rods and cams fit it. Forged 90mm pistons in different configurations are readily available. The stock head can be modified to make good power to around 7-8k rpm. (our peak RPM on the 1/4 mile track was 7700.)
You can yank the balance shaft and bearings, The crank is forged and the rod journal is heat treated. Great engine platform in my book. High 30's hp and a whoop azz mid 30's torque output is pretty easy to come by with the right combo of parts. A lot more is involved than building the smaller engines though.. And for you guys running these in a Drag Mini Bike, Hats off to you. That would be one mean mini bike ride. Especially at the extreme level that these engines can be built.
where do you get 110 octane gas 
?? Do you mix half with regular gas and did you have to change your jets for that? Man i'd love to put some 110 in my mini and blast off :scared::laugh:
?? Do you mix half with regular gas and did you have to change your jets for that? Man i'd love to put some 110 in my mini and blast off :scared::laugh:
I buy it from a place that sells race gas
Catmac 83
The first problem that I ran into when building the 420 was with the valve train geometry.
The 1.3 ratio Rev rockers + the longer valves I used were not a bolt on and go combo.
Had to space the rocker stands up a lot and custom cut my own push rod length to get things right. I could see where a lot of mistakes could be made in this area causing a short lived engine. The next problem was with the monster torque and the side cover coming loose. Even with it studded and solid dowels. So I made an extra side cover support to tie into the burris mount and a sturdy steel top plate/block girdle which also ties into the side cover. The top plate ties into the block/side cover at 8 points. The next thing I ran into was excessive cam and lifter wear. After some dyno pulls, examining what was going on with the lifters and cam lobes, plus the tendency of the big cam to rev into valve float like a light switch. I had Clements custom grind a cam for me and went with some different valve springs and billet lifters.
Piston is a forged pop up,Gapless ring set, Rod ARC, Small Rev Flywheel/timing adjustable.
Crank cut down and shoulder cut back to get the clutch in as close to the side cover as possible. 34mm flat slide jetted for alky. Head Mods... and I now only run Amsoil 4T oil in my air cooled engines after a great controlled test I was able to do involving 15 World formula JR Sprint engines I built last year. Can tell more about that later if interested.
Anyway , After each race and Shaun running the doo doo out of it I did a complete tear down and inspection. No more valve train problems, lifters and cam lobes looked like new.
With the new cam grind the valve clearance had changed a few thou after the first run. But was still perfect after the second race. Rod bearings looked like new, Pretty happy overall with how it looked. To sum it up I think the key to making these live at high RPM would be to have a good valve train set up, rod,piston flywheel,and the block and side cover girdled. n run good oil.
The first problem that I ran into when building the 420 was with the valve train geometry.
The 1.3 ratio Rev rockers + the longer valves I used were not a bolt on and go combo.
Had to space the rocker stands up a lot and custom cut my own push rod length to get things right. I could see where a lot of mistakes could be made in this area causing a short lived engine. The next problem was with the monster torque and the side cover coming loose. Even with it studded and solid dowels. So I made an extra side cover support to tie into the burris mount and a sturdy steel top plate/block girdle which also ties into the side cover. The top plate ties into the block/side cover at 8 points. The next thing I ran into was excessive cam and lifter wear. After some dyno pulls, examining what was going on with the lifters and cam lobes, plus the tendency of the big cam to rev into valve float like a light switch. I had Clements custom grind a cam for me and went with some different valve springs and billet lifters.
Piston is a forged pop up,Gapless ring set, Rod ARC, Small Rev Flywheel/timing adjustable.
Crank cut down and shoulder cut back to get the clutch in as close to the side cover as possible. 34mm flat slide jetted for alky. Head Mods... and I now only run Amsoil 4T oil in my air cooled engines after a great controlled test I was able to do involving 15 World formula JR Sprint engines I built last year. Can tell more about that later if interested.
Anyway , After each race and Shaun running the doo doo out of it I did a complete tear down and inspection. No more valve train problems, lifters and cam lobes looked like new.
With the new cam grind the valve clearance had changed a few thou after the first run. But was still perfect after the second race. Rod bearings looked like new, Pretty happy overall with how it looked. To sum it up I think the key to making these live at high RPM would be to have a good valve train set up, rod,piston flywheel,and the block and side cover girdled. n run good oil.
Holy cow promod, that's no small build, that's far from a mild big block.Thanks for the post! Very cool!Looks like ALOT of work went into that motor.Heck I only have one tool box worth of tools at my hands. So everthing I mod on my 390 will have to be bolt on. I think they sell plenty of stuff to get the 390 to sing enuff for a mini bike.