baja doodle bug help

#1
Could someone give me a general idea on changing over engines on a baja doodle bug? It came with a 2.8hp but its not powerful enough so I would like to get a larger engine and change over...How do I choose a larger engine?How do you meausure for proper clearance on the frame for a new engine? what about clutches? how does one line up an engine to the rear sprocket?If someone could give me a general idea where to begin, what to look for, and the general procedures I will have to undertake changing out engines It would greatly be appreciated..Ive done engine work before but never on a mini bike so Im kinda lost, any pointers or suggestions would greatly help a lady out..heres a pic of the bike without the current engine on it..thanks, Kim aka Squeezle

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#2
Hi Kim,

I take a tape measure with me when buying engines, so I can measure the overall height of the engine in comparison to the distance between the engine mounting plate to top frame tube distance of the frame I need the engine for. There are many engine choices in the 5-6.5 hp category, available both new and used. I attempted to find the output crank diameter of your 2.8 engine, but it's not listed in the specs at bajamotorsports.com...you may need a new clutch for your new, larger engine. Aligning the clutch sprocket properly with the wheel sprocket is critical...I use a non-flexible length of steel bar laying flat across the face of the rear sprocket and slide the engine over until the face of the clutch sprocket touches the steel bar. I can make how-to pics if you'll find them helpful. You may also need an engine mounting offset plate to properly align the sprockets when you install a bigger engine. These plates add a half-inch to the height of the engine, so account for at least that much clearance when you choose an engine. An alternative is to drill new holes in your frame's existing mounting plate. The stub of a crayon fits nicely into the mounting holes of the engine, and marks where to drill the new holes after you've lined up the sprockets.
The members of this site can offer first-hand opinions on engine and clutch brands, and advice on the best locations to purchase at reasonable cost. We also have helpful tips on throttle linkage and other options like torque converters. Since you've already got your old engine out, I guess we can assume you're ready to get started...fire away with any questions.
 
#3
Kim, quick and easy engine upgrade would be to a Tecumseh powersport 4hp engine...looks to be a "bolt on". With the size of the bike, you sure don't want to over power it and make it a pain in the rear to ride it. You would need a clutch and chain and 4 bolts to mount engine and be ready to run!. The holes in the engine mounting plate should give you proper chain alignment. You just look down the chain, like a gun when aiming it....you want chain to run straight. Feel free to email with more questions...I am ALWAYS happy to help
Brandt
 
#4
Thanks guys for your input with my questions...yellowhand, my engine isnt off yet, I got that pic off ebay showing my frame.Right now I wanted to understand just how complicated it would be to switch engines and what I would need..It doesnt sound all that complicated for me to attempt, so my first aquisition would be an engine. I wasnt quite sure what size I should try and find, or just how much room to allow extra around the frame for the engine..thanks for that tip about the extra 1/2 to add for an engine mounting offset plate...Since Im not exactly a tiny woman, I was also wondering about the hp I should choose..what exactly IS too much power? How do I determine if that 4hp tecumseh is adequate or a 6.5 is too much?What does too much power make the bike do Rupp???? When looking for an engine I suppose shaft dia is important due to attachment of a clutch system..so what would be my best choices there? Do all horizontal engines run counter clockwise? How is the clutch attached to the shaft?would that make a difference in choosing which engine I get?Currently on my bike I have a throttle assembly thats your basic generic cable end attached to the carb, would that work on a new engine as well as long as its long enough? My braking system i Have on the bike as well (which the ebay picture doesnt show) is a disc brake system with hand lever....I can leave that attached as is cant I?Im assuming my current sprocket on the rear tire will be ok to use?Will I have to get a new chain to lenght? when looking for an engine what questions should I ask? I really appreciate all this information you gentlemen are providing me with, I feel confidient with your help this will be a fun and rewarding project.Thanks so much fellas :) Kim aka Squeezle
 
#5
Normally, switching an engine isn't all that complicated, but there could be a few issues involved in your particular situation because your minibike was sold by an American company, but the bike was made in China. There have been complications trying to fit other parts on and in some of Baja Motorsports other models because the Chinese don't follow American standards for things like engines, chains and sprockets. Worst case scenario would be you needing an offset plate or redrilling to mount your new engine because most minibike engine mounting plates have slotted holes to allow you to move the engine forward or backward to adjust the chain tension, but there is no side-to-side movement to allow for adjusting chain alignment. Sometimes, when installing a larger engine, there can be chain alignment problems because the new engine is wider, and the clutch position on the crankshaft is farther from the centerline of the mounting base of the engine. Your bike may also have metric standard chain and sprockets on it now, and they would not be compatible with a new American standard clutch designed to fit your new bigger engine. We have piles (literally) of Chinese minibike parts in our shop, waiting to be machined to fit American standard minibikes.
The Tecumseh engine Rupparts recommended is an excellent choice for your project because it is specifically made for minibikes, and already has a throttle hookup made to fit the throttle on your bike. It will save you the problems associated with converting a fixed-type throttle found on most other engines. Rupparts is a dealer for lots of minibike parts, and I think he sells that engine for about $200 plus shipping from Missouri. He might also have clutches, and chain and sprockets if you need to change yours to American standard. The Tecumseh Powersport line of engines also come in higher horsepower ratings, each one costing a bit more as the horsepower goes up. Most minibikes function well with an engine between 4 and 6.5 horsepower. 3/4" is the standard diameter for crankshafts in that horsepower range, and clutches and sprockets with a #35 chain pitch work well. All commercial-use engines have a counter-clockwise rotation of the crankshaft, and that is standard for minibike usage. If you buy a used engine, it may be an Industrial-use model with clockwise rotation...check it by pulling the starter and watching the crank...there are a few out there.
American standard clutches atttach to the crankshaft by one or two small allen head set screws in the hub of the clutch, and have a built-in 3/16" key to prevent slipping on the shaft. They should also be held in place by a 5/16" x 3/4" fine thread bolt with a lock washer and a flat washer at least one inch in diameter. This bolt threads into the end of the crankshaft, and keeps the clutch from moving outwards along the crankshaft, causing the shaft to bend or break. It's just one bolt, but it's the most important bolt on a minibike engine.
If your chain is #35 pitch, or your clutch is 3/4'' diameter and you can use it on your new engine, the rest of your drivetrain including brakes shouldn't need to be changed. If the phone book lists a dealer of engines in your area, they should be able to test a #35 drive sprocket of any diameter they have available against your chain and tell you if it's correct. If they just look at it and say yes,...don't use that information...many metric chain pitches are close enough to appear the same as American standard, but they don't function properly when parts are mixed.
You're the first person I know of to try an engine swap on this new model...it should be interesting. Keep us posted as you progress, someone else may benefit from your first-hand knowledge. If you want to ask anything else along the way, that's ok too.
 
#6
Your bike is short wheelbased....too much power is going to give a tendency to wheelie on take offs. If you like wheelies and speed....and can handle too much power, go for more than 4hp...if you want a nice pleasure rider then a 4hp is probably a good choice. If you opt for a used engine....use caution and be prepared that you may be buying someone elses used up junk. There are lots of worn out engines out there and if you cant witness it running, it is possible you may spend good money on bad.
Remember when stating engine rotation direction, to remember which side of engine you state that direction from. On recoil side engine runs clockwise(tip of the trade).
With your frame size...a few engines to shy away from will be the Tecumseh H50s, H60s, H-70. These engines are large and will most likely hang out of the right side of bike like the 68-69 Rupps did creating a very off balance bike. Tecumseh powersport 4hp flat head, Honda GX, Tecumseh OHV would be 3 good brands/styles that are relatively compact in size and all feature engine mounted gas tanks which will eliminate fuel tank issues. The 1st would be the easiest of all due to having no modifications needed to use on a mini bike
 
#7
Hey Brandt,
I thought I implied clutch side of the crank when I mentioned clutches in an earlier paragraph. Thanks for pointing out my omission.
I haven't personally encountered any wheelie problems with engines up to 6.5 hp when run with a centrifugal clutch, but I probably should have mentioned that also, in case Squeezle is considering buying a torque converter or a 'race-only' type of centrifugal clutch. I was assuming we were thinking about the standard $30 cheapie that is readily available where engines are sold, and I think Squeezle has mentioned she is not 'kid-sized' and wants something more powerful. I respect your opinion...an engine bigger than 4hp may be more than she wants, but she would need to ride similar bikes with different engines to determine what power output she personally likes best. I personally like a little extra power to be there when I need it for inclines and passing guys on smaller-engined bikes, and usually try to keep it under control when I hit the flat stuff, but that's just me. My guitar amp goes to eleven also, but I usually play at about five. It's just comforting to know I can rock out if I feel the need for speed.
 

minidragbike

Supporting Speed Nut!
#8
You will pop a wheelie with that factory rear sprocket. I have seen those before and they have a real large sprocket. And add the torque of a 6.5hp motor and blam on your but.

Now if she purchased a smaller rear sprocket she should do well without worry. Most of the kids I run with all have 12/40 gearing. I'm bigger and heavier so I use a race clutch with 13/45 gearing. And that's with a 13" tall rear tire.
 

mnm

New Member
#9
I just did this...

I just finished this project myself. I used the Tecumseh Power Sport H40-88516 4 HP. Bought it of of E-Bay $220.00 with shipping). It runs much stronger than the Lifan 2.8(2.5 HP) that comes stock on the Doodle Bug. I just had to buy a new clutch - 3/4 " bore 12 tooth 35 pitch. I initially mounted the engine in the factory holes on the plate but found after a day of riding that the chain began to jump off. Checked the alignment a little closer and had to remount engine ~1/4" more to the right. Yes that meant drilling new holes. Also the plastic clutch guard will not mount on the Tecumseh engine. I made a new one with sheet metal and it works just fine. Also had to mount the clutch with the sprocket inboard. It is much more fun now with the extra torque, but it's not enough to pop wheelies even with my 8 year old twins riding it.
 
#10
Thanks for the first-hand info, mnm...you answered a bunch of my questions. Can you tell what size crank is in the lifan?...is it 5/8" ? I'm wondering because we see a lot of used Briggs 3hp engines with 5/8" cranks for sale locally, and this might make a really low-cost swap for someone if these lifan motors don't prove to be reliable. The E-Bay bikes for $329 w/free shipping might make them very popular.
 
#11
I just finished this project myself. I used the Tecumseh Power Sport H40-88516 4 HP. Bought it of of E-Bay $220.00 with shipping). It runs much stronger than the Lifan 2.8(2.5 HP) that comes stock on the Doodle Bug. I just had to buy a new clutch - 3/4 " bore 12 tooth 35 pitch. I initially mounted the engine in the factory holes on the plate but found after a day of riding that the chain began to jump off. Checked the alignment a little closer and had to remount engine ~1/4" more to the right. Yes that meant drilling new holes. Also the plastic clutch guard will not mount on the Tecumseh engine. I made a new one with sheet metal and it works just fine. Also had to mount the clutch with the sprocket inboard. It is much more fun now with the extra torque, but it's not enough to pop wheelies even with my 8 year old twins riding it.
MnM..I also will be changing over my doodle bug motor and am very excited to see it can be done, hope you dont mind a few questions on the install..first, did you have to change over the new clutch to metric sprocket?and what do you mean about mounting the clutch "inboard"? was the original clutch hard to take off? could you tell me where you purchased your clutch?and did it come with a new sprocket? could you possibly post some pics of the sheet metal clutch cover?How much more would you say the power is from the orig engine to the powersport? twice as much? Im not a lightweight and the orig motor cant do small inclines on it very well with me riding it..Im questioning whether a 4hp will be enough...Thanks so much Im glad an engine swap can be done for these chinese imports:) Squeezle
 

mnm

New Member
#13
MnM..I also will be changing over my doodle bug motor and am very excited to see it can be done, hope you dont mind a few questions on the install..first, did you have to change over the new clutch to metric sprocket?and what do you mean about mounting the clutch "inboard"? was the original clutch hard to take off? could you tell me where you purchased your clutch?and did it come with a new sprocket? could you possibly post some pics of the sheet metal clutch cover?How much more would you say the power is from the orig engine to the powersport? twice as much? Im not a lightweight and the orig motor cant do small inclines on it very well with me riding it..Im questioning whether a 4hp will be enough...Thanks so much Im glad an engine swap can be done for these chinese imports:) Squeezle
By "inboard" I mean with the sprocket on the clutch towards the motor instead of the way the sprocket is on the stock set-up. In other words just turn the clutch around on the shaft so the gear and chain are closer to the engine. I purchased the clutch at Agri Supply a local farmers supply store. I do not think it is the best clutch set up but it was cheap and it was there so I got it. I forgot to mention that I also had to remove 3 links in the chain as well. The chain had already stretched right much since new and I'm sure I'll need to replace it by the spring. FYI, I have looked for torque ratings for this 4 hp engine but it must some kind of secret as I have not found any info but I would guess it is close to 2 to 3 times the torque of the Lifan. The Lifan is a knock-off of the Honda GX100K2 engine which from some info I found on internet has 2.7 ft lbs. I weigh 205 and it will get me up moderate inclines but it is in a strain. In the yard and on our dirt driveway it will fly and it gets up to speed quickly. I have to keep reminding myself that this mini-bike was for the kids! It is just to easy to fall into that ol' unfulfilled childhood thing!:rolleyes: I would post some pics but I don't have a digital camera.
 
#14
By "inboard" I mean with the sprocket on the clutch towards the motor instead of the way the sprocket is on the stock set-up. In other words just turn the clutch around on the shaft so the gear and chain are closer to the engine.

If using an inboard clutch, make sure it will work inboard when buying....some will not work inboard.
 
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