Pocketbike Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
50 Posts
Discussion Starter #1 (Edited)
I've got my son's baja 49cc mini dirt bike rebuilt for time #3. 1st rebuild had a petcock issue and went lean resulting in a blown piston. 2nd try rocked out many hours at a OHV park until the air filter slipped loose and sucked dirt for the kill...
Here's the low down on build #3:

Case sides blueprinted


Ported intake & exhaust and hp treatment on the piston


Turbo crank mod

Lightened flywheel (ugly gouges due to glove pulling hard start last two builds??)

Any reason these builds have been this hard to pull resulting in breaking the pull starter metal shell twice now??
.......
 

·
Registered
Joined
·
50 Posts
Discussion Starter #2


Opened up the carb and velocity stack as well as the petcock delete and drilled the flow to 1/8"


Coil got the NGK wire and cap

High flow air filter


Squish from both sides
(Son helped smooth the piston face, he went a little lop sided...)
 

·
Registered
Joined
·
50 Posts
Discussion Starter #3
I forgot to photograph my reed plate and reeds but I'm running tin reeds (with tin reed cut black widow mod style for experiment), with the plate opened up and reed guide trimmed and smoothed to allow the reeds to open more. I think it likes it..


All assembled and rocking out the yard! I'm running 91octane non-ethanol made with xylene/marvel oil. Again the bike flies, it'll pull up hills, rocks, roots through the trails.

Only thing that bothers me is the glove required starting and the pull starters we've killed, destroying two in two weeks time. "Mr Kitty" mentioned glove pullers are wrist pin bearing killers, am I doing something wrong here??

Either way loads of fun, I'll try to post a video of this thing ripping up the yard if I get it back in business here soon..
 

·
Registered
Joined
·
23 Posts
nice job. I should pick up one of these dirt bikes.
Sometimes hard to pull start is high compression. Have you tested that??
 

·
Registered
Blata quadard. 2 xhinese copy quads. 3 lucky 7's. 1 lucky 7 trike. 1 cag.
Joined
·
32 Posts
My guess would be high compression as well mate, ive rebuilt two of these chinese engines recently. The one on my bike i dropped the head gasket and a few other bits. Runs superbly now but I have to pinch it between my knees or it will lift it off the floor. Have a big gauge on my ankle as a reminder when I didn't and dragged the rear brake up my leg.
Have you fitted the pullstart with the metal knuckle? I've not had any issues since fitting these?

Max
 

·
Registered
Joined
·
50 Posts
Discussion Starter #6
Unfortunately too high compression, I did my math wrong and need to add a gasket to get in the proper realm. The last build that was the case, I got a little rushed this last time.

I've destroyed two aluminum case, metal cog pull starts, I'll get another old style metal one ordered this weekend, the last two weekends killed the wallet..

The stock dirt bike was good years ago when our youngsters were smaller. Four years later it needed some power upgrades, we've put a little time into this thing and it'll pull an adult through the woods. Pretty good bang for the buck once you figure them out!
 

·
Registered
Blata quadard. 2 xhinese copy quads. 3 lucky 7's. 1 lucky 7 trike. 1 cag.
Joined
·
32 Posts
I'm amazed to be fair. Got a bbk and fcc for £28 delivered. Didn't expect a huge amount but followed all the guides, flowed the case, matched the two halves. Widened the exhasut port and matched the exhaust. Nothing major. Took it for its first proper test last night, still not more than 60% throttle and its a different machine. It flys, crisp throttle and feels like genuine power. This is in a quad as well, in a bike that engine would fly.
I'm 6 foot 1 and 14stone, mad how much power you can get from less than 50cc. Seriously impressed.
I'm going to buy a few heads, you can get them for 7quid if you look and get the dremel out and start experimenting. Keep us updated mate.
 

·
Registered
Joined
·
23 Posts
Unfortunately too high compression, I did my math wrong and need to add a gasket to get in the proper realm. The last build that was the case, I got a little rushed this last time.

I've destroyed two aluminum case, metal cog pull starts, I'll get another old style metal one ordered this weekend, the last two weekends killed the wallet..

The stock dirt bike was good years ago when our youngsters were smaller. Four years later it needed some power upgrades, we've put a little time into this thing and it'll pull an adult through the woods. Pretty good bang for the buck once you figure them out!
Nice find. Glad it's working out. Definitely awesome that we can spend a little money and have so much fun. Then yeah you definitely need more power the heavier you get. LOL
 
1 - 8 of 8 Posts
Top