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Yamaha BW80 Custom Build - 212 Engine Swap


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I found a complete BW80 roller for 150 bucks. I've always been interested in these fat tire bikes, so I began thinking of a way to fit a built 212 Predator engine in it. After a couple days I began the project. I will be posting images as the project progresses. Feel free to throw some ideas my way, or ask questions. Here we go!

So I began with drilling out the tabs behind the original shock mount just to give the shock a little more stiffness. I weight 170, and by doing this it kept me from bottoming out. I do plan on figuring out what size rear shock I can buy that will lift the bike a little more, you'll see why soon enough.

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While I was drawing up some ideas for the custom subframe, I built up a brand new 212cc Predator engine. I removed the governor and low oil sensor, I also added 18lbs valve springs, an upgraded header, clear valve cover, and 22mm Mikuni carb.

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I also installed a torque converter. I've done this before on my previous DB30 mini build and she reaches a max of 40mph with the governor still installed. So I'm thinking this one will do over 50 with the right gearing. Once I knew the engine would fit I used 550 cord to hang the engine from the frame. This allowed me to figure out what kind of subframe I would need to build.

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I decided that I was going to build a removable subframe, rather than welding everything permanently in place. I welded a small piece of metal between the 2 tabs on the underside of the frame and drilled a hole in the center, this gave me 3 bolts to mount the front of the subframe on. I removed the footpegs/engine mount, and I then created and drilled tabs to insert inside the lower box so I could begin making the actual subframe. The engine was somewhat tall, and the frame was a bit tight, so I had to drop the subframe down a bit for clearance. I also had to flare the subframe out so the engine would fit and clear the front tire during turns and fork compression.

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Edited by TheBigJustino
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Next I placed the engine on the subframe to see where I'd need to weld the engine plate. After doing so it looks like I won't need anything to overlap the edges of the sub-frame, so I traced a template on some cardboard and began cutting the plate out of a tv stand they were throwing away at work. I don't know about you guys, but I enjoy rescuing metal items from the trash and using them to build custom projects.

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4 hours ago, Scrubble said:

You've probably seen Mike Festiva's take on this but if not check out: https://www.youtube.com/watch?v=PQ48NXZpZoM

I have, I've watched the majority of his videos and I was happy to see how well these engines hold up over time. I also got some goodies in the mail this week like foot pegs, sprockets, main jets, and some other items I'm going to keep secret for now. Let's just say this beast is going to get much taller. I'll have more updates soon.

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I saw a video from TimeMachines on Youtube saying that xr80 forks will give a bw80 some serious lift, so I ordered a set off Ebay from a 92 xr80r. The shocks arrived this weekend so I began planning out the install. The xr80 forks were 1mm thicker than the stock bw80 forks so I went to work on the triple tree with a rasp file and a dremel. It took about an hour but I got the forks to fit properly. I was very impressed with the clearance I gained with the xr80 fork tubes. This will also keep the tire farther away from the engine and subframe. In the photo you can see the difference between the bw and xr forks.

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Edited by TheBigJustino
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  • 2 weeks later...

So I finally had a chance to work on the BW this weekend. I finished cutting out the engine plate and tack welded it in place. Then I placed the 212 on the plate and tried to aligned the sprockets, but I ran into an issue. The lower brace on the swingarm was rubbing on the chain, so I cut out about 8 inches. I will weld in a new piece that curves out, providing clearance to the sprocket and chain. Once that was finished I traced the inside of the holes for the engine mounting locations with a pencil and began drilling. After drilling I realized that I got it on the first try, usually my drill bits walk. Next week I plan on finishing/cleaning the welds on the subframe and adding some cross-members for rigidity.

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So I did some more welding when I got home from work today. I added some crossmembers, and will be adding two more for the engine bolts to thread through. That should give the subframe some serious rigidity. From the looks of it, I may be able to take this girl out on her maiden voyage tomorrow! I will keep you posted. 

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Ok folks, I could use some input/ideas. I finally took her out for the first ride, and holy $hit this thing has some torque. Unfortunately the chain gets far too tight when I sit on the bike, and extremely loose when I get off. Aside from welding the rear end into a rigid frame, I'm not too sure where to go from here. I was thinking about upgrading the rear shock to something much stiffer, and adding a chain tensioner. Could this be remedied by a jack shaft? If any of you guys have run into this, let me know how you approached the issue. Thank you in advance.

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Try substituting the rear shock with a piece of solid steel square stock the same length as the shock and see if the "hardtail" setup improves the chain situation and the ride. Your rear tire air pressure will then become your "rear suspension", so to speak.

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3 minutes ago, dirtbkr188 said:

Try substituting the rear shock with a piece of solid steel square stock the same length as the shock and see if the "hardtail" setup improves the chain situation and the ride. Your rear tire air pressure will then become your "rear suspension", so to speak.

I was JUST sitting in the shed thinking the same thing instead of welding the swingarm to the frame. Although I didn't think to use the PSI in the tire to soften the ride. I greatly appreciate the input.

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I spent the weekend fishing but came home early to work on the BW. I ended up making a work bench so I'd have something higher up to work on when I'm changing tires, bearings, etc. Unfortunately there was a hornets nest in the shed so it took a while to get rid of that with some carb cleaner. After the autopsies came back it turns out the hornets died from Covid. Who would have thought? Anyway, the new rear tire came in so it should give it some serious grip and a little more lift. The stock tire is 18x8x7, so I'm going to try to fit a 19x8x7. 

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I left work on time today so I spent a couple hours in the shop. I finished up some welds, and only have a couple more left. That hollow tube stock will be replaced with the cr250 footpegs, I just needed a quick place to put my feet for the test drives. I also pulled the old tire off the rim and got the new one on using the mini tire changer from Harbor Freight. It took some elbow grease, but it got the job done. The only issue I'm currently running into is seating the beads on the rim. I tried the ratchet strap trick around the tire, but no dice. I'll do some research at work tomorrow and see what I can find. I'm not crazy about the starting fluid and lighter trick though. Below are some pictures of the modular frame and the new tire.

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So I finished up all the welds which is great, but I realized I put the fing tire on backwards and I installed an axle bearing sleeve backwards as well. But the good news is that I have some plastics on the way, and my gas tank arrived as well. My buddy is coming up for the weekend, so we will be doing some riding on the nearby powerlines (picture below). I'll put some more time in on Sunday after he heads home, and we'll see if the rubber engine mounts work too.

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Edited by TheBigJustino
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So there was a lot of progress made this weekend. My buddy came up and we took the bikes out and had an absolute blast. I had some chain issues on the BW. It was vibrating and making some noise, so I didn't push it at first. The bike did well on the power lines and I decided I'd race my friend on the yellow DB30 (Doodlebug) that does 40mph. Well I hit about 50 and my chain blew right off. Luckily nothing locked up but I had to get towed home haha (picture below). My buddy left Sunday morning so I spent the day working on the bike. I decided that I would use the cr250 foot pegs for a different project, and I cut the original BW pegs off the bracket  and welded them on to the new frame. I also noticed some play in the swing-arm that may have been responsible for the chain snapping earlier, so I welded in some reinforcement tubing. Lastly I mounted the aluminum gas tank using some more pieces/tabs from the original BW setup. I took her for another test drive before the sun set and I was quite pleased with the results. I was able to really hammer on the throttle without any chain slap or swing-arm flex. So it looks like she's good to go. I'll be doing some more testing this week, and I will progressively push the bike harder and harder and see how she handles it. A top speed run will also need to be done. I'm positive she will be able to pass 50mph and maybe hit 60mph. Enjoy the photos and let me know if you have any questions or better ideas to throw my way.

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