Doodlebug Build-Up (pg 1/2)
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We are taking a stock Doodlebug mini bike and modifying it in steps. Since the stock 97cc engine does not have the power to pull us up a slight uphill grade we will start with an engine swap. We'll take out the stock motor and replace it with a 6.5hp Harbor Freight clone motor. Follow along as we build this Doodlebug.
Here is our brand new, stock Doodlebug. The stock gearing makes the 2.8hp motor very sluggish and weak. Installing our jackshaft unit brings the 2.8hp motor alive and makes the Doodlebug fun to ride. We've opted to install a 6.5hp clone motor on this Doodlebug.
The first area of concern is the front forks. There is a good size gap between the neck of the frame and the top cross plate on the forks. This causes some undesirable slop in the steering.
This problem was addressed by installing a spacer. Now the handling is much smoother especially when going over bumps.
We start our engine swap by removing the stock throttle cable. Unscrew the wire stop and pull the cable out.
Disconnect the kill switch wire.
Remove the kill switch ground wire.
Remove the black portion of the chain guard. Two allen head bolts hold it in place.
Remove the chain.
Remove the four bolt from beneath the motor plate.
Pull the engine out of the frame.
Remove this spacer the stock engine was sitting on.
Line up the adaptor plate and drill the four holes per the adaptor plate instructions.
Here is the motor plate with the four 5/16" holes drilled. You can drill four holes to mount the motor on without the adaptor plate, however the plate is very flimsy without the support from the adaptor plate. The adaptor plate also provides front/rear and side to side adjustablity making the engine swap much easier.
Put the adaptor plate in place with the 8 bolts. Do not install any nuts yet.
Put the engine on the adaptor plate.
Note: The Harbor Freight engine we used has a billet rod, Honda aluminum flywheel, flat top piston, 14cc cylinder head, 265M cam, chrome moly pushrods, 18lb valve springs, 92 main jet, bored carburetor and a high grade bolt kit.
We removed the rear portion of the chain guard to make installing and aligning the chain easier.
Install the chain, square up the engine and tighten the four engine nuts and bolts. Now slide the engine forward to take the slack out of the chain and tighten the four nuts on the bottom of the mounting plate.
Note: We removed two links from the chain for a better fit.
Install the chain guard. The rear portion of the chain guard went back on. The front portion of the chain guard does not fit the 6.5hp motor. With the chain adjusted we noticed the chain was too close to the chain guard. To avoid the annoying sound of the chain smacking the chain guard, we moved the chain guard upward. This required us to make a little bracket for the front mounting bolt and drilling a new hole for the rear mounting bolt.
Connect the Doodlebugs kill switch to the coil wire on the 6.5hp engine. Attach the ground wire from the Doodlebugs kill switch where the ground wire from the engines kill switch was located. Now you can remove the kill switch from your engine.
Note: We used a new connector on the end of the kill switch wire because the stock connector was too small.
Hook up the throttle cable and throttle linkage.
Note: The throttle linkage we used on this motor was a bit awkward. This is not our preferred method of hooking up the throttle linkage. See our "Throttle Linkage" page in the how to section for more ideas.
Install the fuel tank. The distance between the fuel cap and frame rail is close, however, removing the fuel cap and filling the tank does not take much effort.
Install the header. This is our custom header for mini bikes.
Our engine swap is complete. Our Doodlebug has tons of power now. We used a 12T clutch and the stock 72T rear sprocket. With this gear ratio the Doodlebug had great power coming off idle and the top speed is in the 40 to 45 mph range.
Go to Doodlebug Build Up Part Two