Hi all, this is KartFab from the youtube interwebs.
I am working on my next engine build and need some good input on my budget honda gx390 stroker build (budget+stroker=oxymoron right?)
So far i have a used honda gx390 block that I am taking to a shop to have bored to 90mm. I also purchased the -0.050" piston that arc racing sells. According to my research, if i put in the +.200" clone crank, that will put the top of the piston .010" in the hole at TDC with a stock length rod.
The purposes I am using this engine for will be to put on a mini bike -baja heat to be exact (dumb yes I know) and have to ride around on the trails as well as zip around doing a little drag race.
Now, I dont want to go ALL OUT to be the top dog with the biggest baddest, highest revving engine out there. The goal is to keep this within a reasonable budget, and have a target max rpm of around 5K, and keep the stock flywheel.
I am hoping I wont need to get chromemoly rods because the lift isnt very high, also hoping I can use stock valves and lifters as well. Just do a lap job and set lash after it is pieced together.
With that said, here is what I plan on purchasing (mostly from NR racing):
1)4.410" billet rod (stock length)
2) high torque cam - requires single valve springs, says it wont beat up valve train (http://www.nrracing.com/product-p/390clonecamshaft460.htm?CartID=1)
3) chinese 69mm crank (+.2 stroke) http://www.nrracing.com/product-p/69crank.htm?CartID=2
4) UT1 vs UT2: gotta check into this, but if mine is UT2, ill have to purchase this coil? http://www.nrracing.com/product-p/30500-z5k-801.htm?CartID=4
5) cheapo chinese tachometer to monitor my rpms.
6) will get a high flow intake adapter and filter, but will get htat after i determine if i can use the stock gx390 carb with good results.
From what I have gathered, ill check the valve clearance with modeling clay (i dont think there will be an issue as the cam is pretty mild). I will also need to check cam lobe clearance, and i will likely need to clearance the counterbalance from the crank as well as the rod from the bore. I feel pretty confident that I can do this. I know there are balanced cranks you can buy, but im trying to avoid spending 4-$550 on a balanced crank, and hoping that the counterbalance in conjunction with the ~5000 rpm target limit will keep things from flying apart.
So I am now trying to figure out:
A) when valve float occurs on the stock gx390 springs. Again, i only want to go to about 5000 rpms as my max target rpm.
B) what single valve springs are suitable for this application (e.g. not too heavy)
C) carb setup. Can i get away with the stock carb, and rejet? If so, what is a good starting point? Emulsion tube replacement, or is it good?
D)I plan on welding a new exhaust (can mig/tig) and am wondering what i need to stage it to? E.g. x length of x ID+ xlength of xID
E) what would be a good muffler for this thing? Are those open RLV mufflers even worth it? I dont want it to be ungodly loud, but i still want the flow to be good.
f) i have seen some modified engines run a fuel pump to the carb. Is this necessary for my application? What sup with the crank case cover being tapped and drilled in some?
g) can i use the electric start on this setup? (e.g. another reason I want to keep the stock flywheel yes i can always get the billet electric start one, but im trying to avoid this).
h)timing?
i) valve lash?
Anyway, Just thought I would ask these few questions in hopes that somebody with some foresight will reply with good information.
I am working on my next engine build and need some good input on my budget honda gx390 stroker build (budget+stroker=oxymoron right?)
So far i have a used honda gx390 block that I am taking to a shop to have bored to 90mm. I also purchased the -0.050" piston that arc racing sells. According to my research, if i put in the +.200" clone crank, that will put the top of the piston .010" in the hole at TDC with a stock length rod.
The purposes I am using this engine for will be to put on a mini bike -baja heat to be exact (dumb yes I know) and have to ride around on the trails as well as zip around doing a little drag race.
Now, I dont want to go ALL OUT to be the top dog with the biggest baddest, highest revving engine out there. The goal is to keep this within a reasonable budget, and have a target max rpm of around 5K, and keep the stock flywheel.
I am hoping I wont need to get chromemoly rods because the lift isnt very high, also hoping I can use stock valves and lifters as well. Just do a lap job and set lash after it is pieced together.
With that said, here is what I plan on purchasing (mostly from NR racing):
1)4.410" billet rod (stock length)
2) high torque cam - requires single valve springs, says it wont beat up valve train (http://www.nrracing.com/product-p/390clonecamshaft460.htm?CartID=1)
3) chinese 69mm crank (+.2 stroke) http://www.nrracing.com/product-p/69crank.htm?CartID=2
4) UT1 vs UT2: gotta check into this, but if mine is UT2, ill have to purchase this coil? http://www.nrracing.com/product-p/30500-z5k-801.htm?CartID=4
5) cheapo chinese tachometer to monitor my rpms.
6) will get a high flow intake adapter and filter, but will get htat after i determine if i can use the stock gx390 carb with good results.
From what I have gathered, ill check the valve clearance with modeling clay (i dont think there will be an issue as the cam is pretty mild). I will also need to check cam lobe clearance, and i will likely need to clearance the counterbalance from the crank as well as the rod from the bore. I feel pretty confident that I can do this. I know there are balanced cranks you can buy, but im trying to avoid spending 4-$550 on a balanced crank, and hoping that the counterbalance in conjunction with the ~5000 rpm target limit will keep things from flying apart.
So I am now trying to figure out:
A) when valve float occurs on the stock gx390 springs. Again, i only want to go to about 5000 rpms as my max target rpm.
B) what single valve springs are suitable for this application (e.g. not too heavy)
C) carb setup. Can i get away with the stock carb, and rejet? If so, what is a good starting point? Emulsion tube replacement, or is it good?
D)I plan on welding a new exhaust (can mig/tig) and am wondering what i need to stage it to? E.g. x length of x ID+ xlength of xID
E) what would be a good muffler for this thing? Are those open RLV mufflers even worth it? I dont want it to be ungodly loud, but i still want the flow to be good.
f) i have seen some modified engines run a fuel pump to the carb. Is this necessary for my application? What sup with the crank case cover being tapped and drilled in some?
g) can i use the electric start on this setup? (e.g. another reason I want to keep the stock flywheel yes i can always get the billet electric start one, but im trying to avoid this).
h)timing?
i) valve lash?
Anyway, Just thought I would ask these few questions in hopes that somebody with some foresight will reply with good information.