Pocketbike Forum banner

1 - 20 of 30 Posts

·
Registered
Joined
·
50 Posts
Discussion Starter #1
Hey y'all,

We've got what looks to be a baja dirt runner 49cc that had a big bore kit that we blew the piston on. The pull start is about shot as well so we figured best bang for the buck would be just start over with a new motor.
There's two we're looking at on ebay and we were wondering which would be the best bang for the buck. Here's the two :

Can y'all give me any advise? Thanks much!

Keith in TN
 

·
Super Moderator
Joined
·
2,355 Posts
It's best to build a performance engine yourself pre-built performance engines are just slapped together I have heard stories from members that have purchased pre-built performance engines and sometimes they are okay and other times they produce less power than stock
 

·
Registered
Joined
·
50 Posts
Discussion Starter #3
I figured I would need to open it up and blue print and such to match the parts, and then swap more components as well. I was not sure about the high compression heads on these two, if one was preferable; the one piece vs.two/three piece head?? The other notable difference was if the carb with the plastic bowl was a problem for anyone who's used one. Not sure if there's a better base model engine to start from out there for this kind of build, these are within the budget, and would be direct fit.
The only major difference from the one I'm replacing and the two above is the transmission. My old one has the gear reduction box attached which I can use on the new one, but I want to see about the "3 bearing support" sprocket only transmission these have that I'm looking at on ebay. Any preference on gear reduction transmission box vs. sprocket only?

I do have a rocket key, upgraded reeds, and the racing clutch from my old motor to hop up my new purchase, and plan on a fatty pipe asap.

Any better suggestions? Thanks!
 

·
Super Moderator
Joined
·
2,355 Posts
Yes if you plan on opening it up and deebur blueprint check the end play and make sure the wrist pin bearing can't slide into a position where it's half on the connecting rod then it should be good
You wouldn't want to just purchase one and then run it right out of the box...
I would go with the one with the big blue head
Those clone carburetors with the plastic bowl are a piece of junk mine cracked and fell off before I was even able to tune it I would definitely go with a pumper or modified stock carburetor
You're going to definitely want to use the reduction transmission that you have the direct drive setup where it goes from clutch to chain works great on CAGs where it's on flat surfaces with low resistance but on the pocket dirt bike you're going to have hill's dirt and sand to deal with and the clutch will just fry without the transmission.
 

·
Registered
Joined
·
50 Posts
Discussion Starter #5
Great info on the transmission, that solves that question.
I had one other thought, I pulled the shot engine apart and the innards are blueprinted and bearings look okay, plus the pull start is salvageable. My stock carb has been modified with a high flow filter.


I was wondering if a 44mm top end rebuild kit would be any better quality/performance. Here's the one I was thinking with the big cooling fins on the head and includes a "performance crank"


It looks like the kit above has one boost port in the cylinder as opposed to the complete engine above with the blue head has two boost ports. Would one or two ports be better for dirt bike use? Would the full circle crank w. 12mm pin that I have be better suited or the half circle "performance" crank? I could get the kit comes without the crank if the full circle would be better.


Much appreciated!
 

·
Super Moderator
Joined
·
2,355 Posts
Boost ports give you torque and sacrifice RPM for a dirt bike I would definitely go with 2 boost ports and the half circle HD crank more torque will always be your friend
 

·
Registered
Joined
·
50 Posts
Discussion Starter #7
Sounds like a plan, I'll mull over some options. Thanks for your help, I appreciate the advice!
 

·
Registered
Joined
·
50 Posts
Discussion Starter #8
Here's what I got today..
The new cylinder has a ported piston but no boost ports, compared to the one I've got that died before it broke in the has two boost ports. The old one looks okay to use, I'm thinking about putting the new piston and rings into the old cylinder with the boost ports. Would this still break in properly with new rings and done properly on a stand before loading it up?
My HP half circle crank is on the way, I plan to grind it a bit to look like the turbo crank mod I've seen on the site... Is this the route to go?

 

·
Registered
Joined
·
50 Posts
Discussion Starter #9
Here's what I got today..
The new cylinder has a ported piston but no boost ports, compared to the one I've got that died before it broke in the has two boost ports. The old one looks okay to use, I'm thinking about putting the new piston and rings into the old cylinder with the boost ports. Would this still break in properly with new rings and done properly on a stand before loading it up?
My HP half circle crank is on the way, I plan to grind it a bit to look like the turbo crank mod I've seen on the site... Is this the route to go?

 

·
Super Moderator
Joined
·
2,355 Posts
It should still break in okay I have done it many of times where I've replaced just the piston and rings in a slightly worn cylinder you're definitely going to want to give it the fall test.

Take the new piston without rings on it spread a thin layer of two-stroke oil on the piston make sure you spray and clean the cylinder you want to test with brake cleaner and that it's dry now drop the Piston into the cylinder it should fall slowly through the cylinder if it stops and gets stuck and can't make it through on its own weight then the Piston is too big for the cylinder if it falls quickly then the Piston is too small for the cylinder
 

·
Registered
Joined
·
50 Posts
Discussion Starter #11
Piston passed the test so i gave it a go yesterday. Ran a good hour or more at idle then stalled out. Pulled the engine off the bike and the pipe is filled with black oil, which is likely the reason it stalled. Maybe I'm running a little too rich, I'm draining the pipe and adjusting the clip on the needle from bottom to middle slot and gonna go again this afternoon.
I'm breaking it in with the FC crank until the HP half circle crank makes it here, trying to break it in before maybe taking it out to some trails this weekend. I'm running non-ethanol 87octane 1:25 mix the first gallon (just idling), do I need to go one more gallon at that mix before backing down to 1:32/40 mix? Should I leave it idling a second gal/tank or is it ready for light varied throttle use? I'm not sure how much capacity people's "break in" tanks are, but this thing has a large fuel tank..
So far, so good it's looking, I've got a couple little ones chomping at the bit to have a ride!
 

·
Super Moderator
Joined
·
2,355 Posts
If you ran it for an hr it's broke in I do 3 10 minute heat cycles and call it good then mix 50:1 give it some half throttle rips for one tank then I go-to town
 

·
Registered
Joined
·
50 Posts
Discussion Starter #13
Right on! It went two cycles for me, 30+ mins or more per go with rich mix, I imagine that does it without the fail of our last attempt. Except a few minor details with leaky old clogged tank petcock restricting flow which might effect running, I'll have to test and wait for that replacement to arrive mid-week. Got some stuff to do tonight but will get on the bike tomorrow possibly and give it a go.
Thanks for the advise, (I'm following your budget build for extra tips;)
 

·
Super Moderator
Joined
·
2,355 Posts
When they get fuel starved and go lean that's when they want to blow when it gets down to 1/4 tank I stop for a cool down and refill most of my engines want to start overheating after 10-15 minutes
 

·
Registered
Joined
·
50 Posts
Discussion Starter #15
I ran the bike last night before dark a couple laps on the road. I didn't go much further then half throttle when the engine gets some serious vibrations. I mocked up the build to break in the piston/cylinder using my full circle crank since my HP half circle crank wasn't in yet. I'm wondering if that's the culpret.. the HP crank did make it yesterday so I'm doing the rebuild with the more balanced part today if I get the time.

The last thing this one needs is a good exhaust. I've a couple of models I'm looking at:
Dual snake

Or

Snake
(Would the "non j-hook" version work on a cateye 43cc? )

Here's what on there now..

And last.. would it be okay to put the rocket key into the fresh built bike (it may be part reason why our last piston blew before break in..oops)?

Thanks for any suggestions!
 

·
Super Moderator
Joined
·
2,355 Posts
The vibration is the FCC that's why no one wants to use them anymore the HCC will be balanced a lot better

Your last engine the one with hole in the top of the piston that's from detonation running fuel that has an octane rating that is way too low or it went lean either from a vacuum leak or bad carb settings or partially clogged jet.

I brake all my engines in with rocket keys.

3 10 minute heat cycles at 32:1 then one tank at 32:1 half throttle then 50:1 and go for it
 

·
Registered
Joined
·
50 Posts
Discussion Starter #17
The piston we killed was likely to improper petcock/carb function and lean effects, the plug was white and crisp. I got a new petcock and went bigger on my jetting. I'm going to source higher octane fuel or make some xylene mix of sort for future use anyways, I don't think it could hurt performance anyways.
The HP 1/2 circle crank looks good, it's got a little bevel from the factory, not a turbo crank but don't have the carbide grinders for the job.


I've looked at cleaning up my piston to get the casting marks from inside. Do I shave a little around the pin hole for better flow to the transfer ports (I marked with sharpie where I think I need to grind)?


The rocket key is going back on this last (hopefully) build and we'll give it a run this weekend. Just a nice exhaust pipe and maybe play with the gearing, but almost set.
Any suggestions for a good pipe, do the snake pipes give the low end power?

Thanks!
 

·
Registered
Joined
·
50 Posts
Discussion Starter #19
Darn, I'm trying to figure the best bang for the buck for a torquey cag dirtbike. I may try cutting up the stock pipe and bringing it down to the muffler shop down the road for this modification
 
1 - 20 of 30 Posts
Top