Hello everyone, this is my first post. Like a lot of people here, I had the joy of owning a mini bike when I was a kid, and I still ride at 63. I have looked here and no one else seems to have tried the combination that I have just built for my 4 youngest grandkids, age 5 to 12. I was given a 2013 Baja Viper, with a 2.8 horsepower Lifan engine. It had a bad carburetor and would barely run. Having rode a copy of this bike with a good running engine like I have, I decided not to waste a dime fixing it. Instead, I bought a HF 3 horsepower Predator and put it on. Choose the 3hp because it is for lightweight (45-80 pounds) inexperienced riders. Rear sprocket is a 70 tooth and the front was an 11 tooth clutch. To gear it down for more power, I bought a 10 tooth clutch. I want to thank Max Torque for their help as this is not a common item. The new clutch came with a light blue spring (1800 rpm engagement). Haven’t rode it yet but it sounds strong and will do 3720 rpm with me holding up the back of the bike. Has anyone else done this? How will the blue spring work out with the kids? Jay
Predator powered Viper
- Thread starter Jay 2504
- Start date
I haven't owned either engine but since no one else has replied I'll provide the little bit of information I know of, plus some speculation.
Like the Blitz, Racer and Dirt Bug, the Viper is a Doodle Bug with another name on it (there are many DB threads you can brouse). As far as I know, paint and decals were the only differences.
http://www.bajamotorsports.com/categories/minibikes
It seems the OEM 97cc engines did not perform well with the choke completely off. I assume this is due to an overly lean condition (jets too small). See @Fat Tony157 's post in the linked thread.
https://www.oldminibikes.com/forum/index.php?threads/baja-mini-97cc-governor-issue-need-help.168244/
You've replaced the OEM 97cc engine with a 19% smaller engine, don't be surprised if it produces less power. I imagine HF's 3 HP rating is rather optimistic. Most OHV 3 HP engines that I've seen are around 98 cc (the 98cc 3 HP Hisun/Coleman for example). I imagine the 79cc Predator would make 2.5 HP at best. It's torque that gets you moving and small engines generally lack torque. If properly jetted/tuned, I would expect the OEM 97cc engine to outperform the 79cc Predator. With the 79cc engine, you may need an even higher ratio (commonly referred to as lower gearing) than 7:1. A 75T rear sprocket is about as large as you can go with a 145/70-6 tire (a 75T Coleman CT100U sprocket might bolt up but I don't know that it will) for a 7.5:1 ratio. I think you may need a higher ratio (AKA: lower gearing) than that, though, with such a small engine, so you may be better off keeping the 70T sprocket and adding a jackshaft. Just guessing, I think you may need somewhere around 9:1 with the low torue output I'd expect from such a small engine. IIRC, @manchester1 has used the 79cc Predator with a jackshaft and may be able to advise you on that.
Like the Blitz, Racer and Dirt Bug, the Viper is a Doodle Bug with another name on it (there are many DB threads you can brouse). As far as I know, paint and decals were the only differences.
http://www.bajamotorsports.com/categories/minibikes
It seems the OEM 97cc engines did not perform well with the choke completely off. I assume this is due to an overly lean condition (jets too small). See @Fat Tony157 's post in the linked thread.
https://www.oldminibikes.com/forum/index.php?threads/baja-mini-97cc-governor-issue-need-help.168244/
You've replaced the OEM 97cc engine with a 19% smaller engine, don't be surprised if it produces less power. I imagine HF's 3 HP rating is rather optimistic. Most OHV 3 HP engines that I've seen are around 98 cc (the 98cc 3 HP Hisun/Coleman for example). I imagine the 79cc Predator would make 2.5 HP at best. It's torque that gets you moving and small engines generally lack torque. If properly jetted/tuned, I would expect the OEM 97cc engine to outperform the 79cc Predator. With the 79cc engine, you may need an even higher ratio (commonly referred to as lower gearing) than 7:1. A 75T rear sprocket is about as large as you can go with a 145/70-6 tire (a 75T Coleman CT100U sprocket might bolt up but I don't know that it will) for a 7.5:1 ratio. I think you may need a higher ratio (AKA: lower gearing) than that, though, with such a small engine, so you may be better off keeping the 70T sprocket and adding a jackshaft. Just guessing, I think you may need somewhere around 9:1 with the low torue output I'd expect from such a small engine. IIRC, @manchester1 has used the 79cc Predator with a jackshaft and may be able to advise you on that.
Thank you, Charles S for the reply. I too wondered about how a 97 cc engine is 2.8 horsepower and a 79cc is 3 horsepower. Doing what research that I could, the original engine is a flat head with a 6.6 to 1 compression ratio. The HF engine has OHV and a 8.5 to 1 compression ratio. The HF 79cc has more torque also, at least by rating. The first ride will quickly tell the difference! If only more of this snow will melt! Jay
I didn't realize they came with flathead engines. I assumed (never wise) that they were too modern to have flatheads. Just checked parts diagrams on the Baja site (amazing it's still up after so many years) and they definitely show a flathead. My bad. I still doubt the 79cc Predator is 3 HP, though, but Lifan has a 79cc OHV engine that's also rated 3 HP so who knows... IIRC, Monster/Mega Moto used to have bikes with Lifan OHV 79cc engines rated 2.5 HP and 105cc rated 3.5 HP...
https://www.homedepot.com/p/LIFAN-5...rizontal-Shaft-Gas-Engine-LF152F-3Q/203281925
https://www.homedepot.com/p/LIFAN-5...rizontal-Shaft-Gas-Engine-LF152F-3Q/203281925
Last edited:
Toomanytoys, that is my exact experience with the stock 2.8hp Lifan,from a few years ago. I am about 235 pounds,so it was probably worse. That was on a different mini bike. This engine would barely even run,wouldn’t pull me at all. I will report back how well changing the gearing from 6.36 to 7.0 to 1 helps out. The gear change might be ideal for your daughter. It takes about 2 mph off the top speed,that might be good for a 5 year old!
Charles S, I considered the 3hp Lifan that is sold at Home Depot, the throttle looked like what I had, but after experiencing 2 of those engines, that was enough for me! Plus the Predator 3hp is $10 cheaper and in stock. The snow is melting here pretty good, maybe tomorrow I can go out for a ride. Waiting is killing me! Jay
Charles S, I considered the 3hp Lifan that is sold at Home Depot, the throttle looked like what I had, but after experiencing 2 of those engines, that was enough for me! Plus the Predator 3hp is $10 cheaper and in stock. The snow is melting here pretty good, maybe tomorrow I can go out for a ride. Waiting is killing me! Jay
Coleman took over some of the Baja line and they actually make what you have built, they call it the CC100X. This being said, doesn't mean the engine is strong enough for an adult. I had a DB30 that came with one of the 2.8HP engines like the one you replaced, wouldn't pull me up the slightest incline and I'm in at 165lbs. The Predator 79/99cc engines are pretty much the same way, once they start bogging down, they don't have enough "umph" to keep the clutch engaged no matter what gearing I had
It has been ridden! First impression, true to their word, the Max Torque, clutch engaged at 1800 rpm. With my 245 pounds, the bike struggled, until it hit about 22-2300 rpm then it pulled really nice up to a governed engine speed of 3840 rpm. Riding on the street, I found myself keeping the bike moving to avoid as many take offs as possible. Dropping the gear ratio, from 6.36 to 7.0 really helped this machine out. On flat roads it will accelerate nicely to the limit. Next I tried a pretty serious hill (still on the street), that is almost twice as bad as a stock 2.8 hp bike would not pull me up.it lost sped but got much further than I thought it would. What a pleasant surprise! After I got back, I thought about ordering a silver (2200 rpm) clutch spring. Before I did that, I tried the old stock spring. It raised the engagement went to 1980 rpm. That helped. Then I has a stroke of genius, I asked my granddaughter, one of the recipients of the bike to “test” it for me, telling her the lie that it was for her cousin. The take off is much better when you drop 2/3 of the weight! Her reaction? “I want one!” Little does she know that it’s for her! She is going to help me “test” on the same hill tomorrow. I have also ordered a 75 tooth rear sprocket for more hill power verses the 70 tooth one that is on there now. I saw that 100 Coleman with both the Lifan and a copy of the Predator engine. I would love to see a drag race between the 2 bikes! Sorry about the long post, Jay
What a difference 150 pounds makes! Today my “test pilot” took the bike up both those hills I talked about. She went up both at 3840 rpm, never lost any speed at all! She even named it, and wrote out a birth certificate for it, all the while begging me to get her one! What a joy it is to ride beside my granddaughter, each of us on a mini bike. I explained to her what the 75 tooth sprocket would do, she said she didn’t want to lose the 1.5 mph! We will try it anyway. I think it’s running better now that it has 1.2 hours on it. She even managed to get it started by herself. Jay.
No not yet. She thinks it is for her cousin. The story is that his dad is working a lot and had me do this for him. In reality her cousins dad gave me the bike. I am putting it together for my granddaughter and 2 of her siblings. I told her I needed a “test pilot” in the weight range of her cousin. That way I can see how it works with the heaviest and oldest of the three of them without spoiling the surprise. I am surprised how much she likes it. It’s all I can do to keep the story going for another week or so. I told her to trust me and that I would talk to her parents about getting one closer to the summer. Jay
Another ride alone today. No it still won’t pull me up the second big hill and barely does the first hill with me on it. I moved the kill switch to where my granddaughter said she wanted it. Then using a piece of mechanics wire I made a tell tale so I could see how far the governor was opening and closing the lever on the carb. Pulling me on basically flat highway, I would say it is about 3/4 of full throttle to keep it going 3840 rpm. After the ride, I discovered that the governor would not open the carb all the way even with the engine off. I had to pull the tank and re index the governor arm on the shaft coming out of the block. Now instead of about 85% carb opening I get 100%. With only 3 hp I need all the help I can get! Jay
So here is an update. I got and installed the 75 tooth rear sprocket on the bike. Even with a 10 tooth instead of an 11 tooth clutch, this required me to add 2 links to the chain. Without adding the links, the chain is too tight even without the spring loaded roller! Started it up, and before I left the driveway, I can tell a big improvement. With my 240 pounds onboard, the bike pulls away from a stop much easer. Going around a street corner, it accelerates much better. I can tell that it is 1.5 mph slower on top end, I hope my granddaughter can’t! Then I took it up the big double hill near where I live. I made it up the first shorter hill with enough speed to not have to stop for the first time. When I hit the second longer hill I went up it twice as far before I had to stop. ( being careful with the clutch). Dropping the gear ratio from
7.0 to 1 down to 7.5 to 1 really helped this mini bike out. My “test pilot” granddaughter hasn’t rode it yet, but I would think that at her weight, grass would be no problem with this gear ratio. Just 1 week left until delivery! Jay
7.0 to 1 down to 7.5 to 1 really helped this mini bike out. My “test pilot” granddaughter hasn’t rode it yet, but I would think that at her weight, grass would be no problem with this gear ratio. Just 1 week left until delivery! Jay
Well my granddaughter has rode it with the new sprocket. She really liked the faster take off, but noticed the loss in speed. She also had her first accident. She lost it at about 5-6 mph right after takeoff. We have a bent foot peg, and engine scratches to show for it. She scrapped her ankle and chin and knee, but is ok. The important thing is that she learned something! She got right back on and we rode again. Then we went up the 2 big hills. I could see the wire that told me that the engine was a little past 1/2 throttle to pull her up the hill. When we got to the top she ran out of gas! So we walked home. This upcoming Saturday she will get it given to her and 2 of her step brothers. So the mini bike is a little bit gear bound at 7.5 to 1 with a 85 kid riding it, but that is ok with me. Next we will try some grass and see how it does. Jay
Yesterday was the big day. Loaded my CT200U and the Baja Viper into the truck and went to my daughters house. Talk about excited/happy grandkids! Wow! Even I was surprised. My granddaughter “test pilot” gave me a giant unsolicited hug, maybe the 3rd one ever. (She is 11 and is always messing with me). There is a huge new subdivision going up about a block away with brand new totally empty unused streets to ride on. My granddaughter rode just fine and I taught the next 2 youngest boys how to operate the mini bike safely. Of course my granddaughter prides herself on being able to start the engine. We put 2.5 hours of engine run time on the bike and used a full tank and a half of gas. The bike is so “gear bound” (by design) that the carb is only open about 1/3 with the kids riding on basically flat asphalt. The boys started out riding slow but we’re soon going “full throttle”. The youngest boy is only 5 and can’t ride a “2 wheeler bike” yet so I rode him with me. I got to see how happy I must have looked when I got my first mini bike. I ended up leaving both bikes there so they can ride together. My daughter wasn’t happy with me at first until she saw how the kids reacted. All the kids signed a thank you card to my son in law who gave me/them the Baja. I liked this little bike so much that I am considering building another one for me. Jay
That is what this is all about sir! You have succeeded in making memories that will last a life time. Planted the seed that'll carry on when we're gone. Thank you for sharing that with us. Can't imagine the smile I her face when she learned it was hers. 
Now sounds like you have a cpl more to build yes?
Now sounds like you have a cpl more to build yes?