Honda GX390 Stroker Build

[KartFab]

Making the electric or starter only flywheel work with pull start so i can race it this weekend. HA ambitious goals, we shall see, we shall see.

 

[KartFab]

porting the cylider head

so this is the third head i have ported now. Not perfect, not sure if there will be any improvement, but I did try to not go all out on it. Worked the runner to bowl edges down on both intake and exhaust. Took the hump from the intake and exhaust and blended it to the valve thingee, took the valleys in the intake down just a tiny bit, and blended them more into the bowl. I tried to blend to about the halfway mark on the intake and exhaust valve thingee (gimme a break i got 3 hours sleep last night) time to go into zombie mode at work.

Also, not pictured, but the stainless valves i got were just over a quarter inch longer than the stock valves. This created some real issues with spacing, so i got a valve cover spacer, and a thicker guide-plate for the pushrods. I ended up doing the following:

valves ~7.22 mm longer,
took rocker rollers, spaced up with a thicker pushrod guide plate
spaced up roller rockers even more (ended up being 2-3 washers IIRC that are M.8 washers.
So the rocker studs now have about (you guessed it) 1/4" less available thread than before. So i went out and bought some m.8 x 1.25 pitch x 45mm allen head studs. These are the same ones used on the rockers, but it looks like the rocker studs actually were put on a lathe or ground on the edges of the allen head to decrease the head diameter to make them fit. I will have to grind mine tonight and double check with the calipers. Not sure how im goign to do this, was thinking file/drill, but now im just thinking ill use my bench grinder with an unused grinding wheel and hands.

While i was figuring out this spacing, i was putting the dry erase marker on the valve tips and checking the spread/sweep of the roller tip using an adjustable POS pushrod (the one i got wasnt long enough). Once I got it spaced correctly. i cut a chrome-moly rod and made sure the halfway point of the ball end fit exactly at the first thread of the valve lash adjustment at zero lash (as it says in the instructions).

Also, i dont know if i mentioned this, but clearance was also a struggle. I had to grind a small portion of the edge of the connecting rod down so the pto side of the cam lobe woudl clear it. Also had to flatten out the spot between lobes just about 10-20 thousandths as the rod was hitting it there too. After all that, I am thinking there is hope. There is light at the end of the tunnel, or im just hallucinating from lack of sleep.

 

[KartFab]

picture time

TIMING: Got the flywheel on, found top dead center with dial gauge, then confirmed with piston stop. Was dead on with both methods. Set timing to 28 degrees before top dead center as ill be using 93 octane pump gas for this one.

HEAD: lapped the valves to the seats. I had to cut out the center of a washer to space up the intake side to get the correct spring install height. I then had my wife put in the retainer clip thingees as i sacrificed my thumbs against the force of the 50 lb HD springs for the greater good of this build. They hurt for a good bit after doing this. I installed the 1.2 ratio rockers, which was a nightmare. The valves were too tall, so i had to get longer allen head bolts for the rockers, which i chucked up to my drill and ground with bench grinder to match the 12mm OD that the original allen head bolts had been cut on a lathe to. Dialed in the length of the rods. According to my calculations i needed 179mm ball end to ball end, but after doing the dry erase marker method to check roller tip contact area, i needed them a tiny bit longer. I settled at jut around 180mm length. Both rods were within .001" of that measurement. Set valve lash to .003" on the intake and exhaust. got the valve cover polished up. The hard part of the build seems like it is well behind me, but i am still very nervous about the carb. Ideally i just want it to work when i bolt it on haha. I wish i had more time to dink around with it.

 

[KartFab]

okay, so after days of working into the night and most of friday, i got the engine together. Two major problems

1) carb troubles - its a mikuni flat slide so i was able to get it hooked up with the correct throttle cable and twist grip. I broke the engine in idling about 2000 rpm for 20 mins with zddp breakin additive added to conventional 30 weight oil. changed oil filled up, then i went to go ride it, and 1/4 throttle the bike wheelies and the forks bottom out i ride it for about 10 seconds then the bike just shuts down. Gas is coming out of the very bottom tube on the carb (which i am thinkin g is the bowl vent) so maybe the wheelie jarred a float or something? i dunno. I am really in crunch time now because drag race tomorrow.


2) clunk sound - probably a clearance issue, cam to crank, or cam to connecting rod i carefully checked this and clearanced windage tray for rod and rod bolts, exhaust lobe to rod, and camshaft between lobes to rod, if i lean the bike towards the PTO side of the engine it makes a tiny clunk clunk clunk sound with a slight hesitation you can feel on the bike. UGH.

 

[KartFab]

tested for fuel, sprayed carb cleaner in intake, spoon full of fuel in spark plug hole, feels like it wants to start, but just wont.

checked for spark, spark ok, coil gap to .040" per ARC racing instructions for their gx390 flywheel.

feels like it has compression, but Im hoping its not a bent valve. running out of ideas here

 

[Whitetrashrocker]

Your clunk might be the end play in the crank is more than you clearance for. Just a guess.

---------- Post added at 11:38 AM ---------- Previous post was at 11:35 AM ----------

Perhaps your hard landing made a push rod jump out of place?

 

[gman3850]

it is most likely your cam and rod tapping each other. The vegascarts cams are clearanced pretty heavily for this reason.

As for the carb i could never get my 32 to run right, always idled spuraticaly and would bog out. i went to a 30 oko carb with a power jet and it seems to run mhhhhh/ok. I will be going a 33mm pumper for my next carb

 

[KartFab]

The flywheel spun. I was 70 degrees off. Flywheel was the adjustable one w/hub. I had it torqued to 65 ft lbs but still spun because I had to have a starter cup on it I believe. I torqued to 100 ft lbs and it runs like a beast. Wheelies at 1/4 to 1/3 throttle from Stop and wheelies at full throttle when going. It’s very very scary and I thought I was going to wreck big time. Got it just in time for the drag races. Will release a couple videos in the next week or two of the whole process plus flat track racing and the entire stroker build.

 

[gman3850]

Did u lap the flywheel to the crank?

 

[KartFab]

yes, i lapped it to the crank and the mating surfaces were good. wiped it off with clean rags, and sprayed with carb cleaner to get everything off. Wiped down dry as a final wipe.

 

[gman3850]

Hmm that is quite wierd. I have used zero flywheel keys on a couple of engines and never had a problem with spinning after lapping. But must be fun. I would like to see a dunk graph of that engine

 

[KartFab]

ill be getting a dyno before years end if i can make enough extra money. Ive been saving up foreeeverrrrr

 

[bob58o]

ill be getting a dyno before years end if i can make enough extra money. Ive been saving up foreeeverrrrr

That's some serious cabbage. Better start hustling these mini drag races. Are the videos ready yet? Wait what thread is this?

 

[KartFab]

That's some serious cabbage. Better start hustling these mini drag races. Are the videos ready yet? Wait what thread is this?

lol i dunno i keep starting new ones, videos are taking up a LOT of space on my hard drive so ill be editing all week long after work to get this stuff done. videos will come out something like this

stroker build
TOMB flat track racing
how to troubleshoot an engine that wont start
TOMB drag racing

I 'feel' like i would have won the drag race if i had wheelie bars, and geared even lower. I was pulling wheelies at the start, which kept me behind the wheelie bar guys, but then once i hooked up i was staying right with the guy that won maybe 2 mini bike lengths behind him at the finish line... so basically i wasted all the low end torque for half the race, then was at least as good the other half. I didn't feel like the engine had enough time to wind out. I could tell a noticeable difference between my rpms and their rpms. I was not even close to topping out the engine at the finish line. So im thinking ill change the 50t axle to either a 60 or 72, then figure out the jackshaft. Currently a 10, but i can go anywhere from 8, 9, 10 11, 12 or 17t. Im not willing to invest in a race clutch yet, just sticking with the 40 series for now.

 

[bob58o]

3/4" JS? Do they make 8T sprockets that fit around a 3/4" shaft?

 

[Kartorbust]

If they don't, one can always get one made at a machine shop.

 

[KartFab]

3/4" JS? Do they make 8T sprockets that fit around a 3/4" shaft?

5/8" jackshaft.

 

[gman3850]

I run a 10/60 on my 500lb (me and go kart) go kart gx390 and I wrap it out to 6000 in the 1/4 and that is with the stock flywheel. The biggest problem is the comet 40 shifts like ****, if I had a better clutch system it would rev better.

I am building my high rpm gx390 right now to mate to a shockwave 8.90 clutch setup.

 

[Whitetrashrocker]

Make some wheelie bars. If your going up, your not going forward.
The 'Cuda we raced would grow half a foot off the line and it would cause slow reaction times. The dart we had would pull the tires up. Same. Bad 60 foot times.
Put the power in the direction you want to go is the key.
And ya know.... Spinning ain't winning.

 

[KartFab]

Make some wheelie bars. If your going up, your not going forward.
The 'Cuda we raced would grow half a foot off the line and it would cause slow reaction times. The dart we had would pull the tires up. Same. Bad 60 foot times.
Put the power in the direction you want to go is the key.
And ya know.... Spinning ain't winning.

Yeah, wheelie bars are in that bikes future for sure. I think it will be extremely difficult to spin the tires, seeing as they are 19" and it just picks the front end up so fast its just too scary right now until i can get the bars on it.