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View Poll Results: What size engine?
Keep it as is: Restore the 70cc 3 50.00%
Just a bit more: 90cc 1 16.67%
More power: 100cc 0 0%
Come on!: 125cc 2 33.33%
Scooter... make it a motorcycle: 150+cc 0 0%
Voters: 6. You may not vote on this poll

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Old 06-06-2008, 12:24 AM   #1 (permalink)
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Lightbulb Honda c70 Passport Mod: Take 3 - Restoration

Ok folks here we go... my 3rd brainstorming thread for what to do with my completely trashed honda c70.

All kudos for this idea go to Ben (who is never short on good ideas). One of my issues with just flat out restoring the Passport is that the 72cc 4 stroke OHC engine may not be enough to pull my 6'1" 215lb frame up the hills of Ann Arbor Mi. Ben's idea... put a larger motor on it and possibly an aerodynamic fairing.

First the good: I never wanted a real motorcycle, and the first time I saw a picture of the Cub/Passport body design I immediately fell in love... this bike is classic, cool... everything. But I'm a big guy... and the bike isn't really designed for someone like me. So upping the anti to 90-125cc's is perfect (depending on the thoughts I get 150 might be going to big for me Ben). If I can find a reliable Honda engine that still pulls close to 100mpg and gets the kind of torque/top speed I need then perfect.

Next the bad: One concern I have is that these bikes were never built with more than an 89cc engine (i.e. the Honda c91m) and so would even putting better brakes on the bike be enough to keep it safe? Would a larger engine even fit on this frame with the leg shield? What about the clutch? The Passport is a 3 speed semi-automatic bike (not 100% sure how it works but I know it's different than most other bikes).

Other thoughts:
1. Sprocket: Could I adjust the sprocket ratios to play with torque vs. max speed to get the hill climbing and top speed to exactly what I need?
2. Hybrid: This is super out there, but what about some kind of hybrid technology (haven't done any reading so if this is totally ludicrous just go to no. 3 without thinking too much about no. 2)
3. The Fairing: How much does a good fairing setup affect the performance of a bike?
4. Degree of difficulty: I am going to be working with someone who has a lot of knowledge working on cars, tractors, trucks, and other big machines... and I think he may have fixed up a motorcycle once, but will this project be outside of our range (him teaching me and helping me do the work)?

Thanks for the thoughts I'm really excited about this, the more efficient, cool, eco friendly, and rideable the outcome the better!!

p.s. here's a link to pics of my bike (including the problems with the current engine casing)

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Old 06-06-2008, 10:23 AM   #2 (permalink)
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I don't really like the look of your cracked case, somebody had to be abusing it pretty good to do that. Is that shaft for the gear shift lever? So I'll assume that the cracked case allowed all the oil to drain out. Can you kick it over at all? I'd be worried whoever smashed the case wouldn't bothered to stop riding it until it siezed...
Assuming that it's fixable at all, I would just fix it to stock, try it out, if you find its to slow for you and you still like a motorbike, sell it and get Ninja 250. The 250 will get nearly the same in town mileage plus you can ride on the highway with out fear of imminent death. People take them across the country all the time. I don't think insurance will be any more for the ninja than the C70.

If you like the C70 and the motor dies, I believe many of the made in china crappy kids dirt bikes and atv's at walmart type stores have a knock off of this engine and tranny, I imagine there are 1000's of them not running due to simple malfunctions and they are not "worth" fixing, check garage sales etc...

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Old 06-07-2008, 03:55 PM   #3 (permalink)
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Yeah I haven't quite figured out how fixable it is. I'm going to get it looked at and we'll see what we can do... unfortunately in my budget I have not been able to find anything else, and I'm still reluctant to give up on this bike (despite it being so trashed).

I still need to check around for salvage yards and such to see if I can find anything there.

Edit: Ben if you are looking at this here's a picture of a honda c90 with a fairing (so it can be done... yay!)

Last edited by papaslides; 06-07-2008 at 04:58 PM..
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Old 06-08-2008, 07:43 PM   #4 (permalink)
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I'd just restore it. Parts are cheap and readily available from any Honda dealer. They are very simple and a dream to work on. Good learning material.

You'll be surprised how well these things pull. Back in the '70s my neighbor got a used one as a birthday present and we went everywhere 2-up. Combined weight probably 350 lbs and we didn't find a hill the thing wouldn't climb. That little auto tranny is amazing. As I recall the brakes were a little iffy, so make sure you have new shoes and keep them adjusted. He road that thing all thru college and then gave it to a frat brother. It had a basket on the front perfectly sized to hold a sixpack.

You won't be going fast enough for aero to matter. That windshield in the picture would keep the bugs off but would have added drag. Never mind what wind does to handlebar mounted windshields/fairings.

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Old 06-08-2008, 10:39 PM   #5 (permalink)
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Look for a Lifan replacement motor, they are copies of the Honda and should bolt right in. Less expensive than a Honda, I'm not even sure you can get Honda replacment motors anymore. I know of one person using one as a commuter, haven't heard any complaints yet. The Lifan comes together with the transmission. Although I am sure the original Honda motors are much better.

I would probably up it to 90cc or so. I have driven a 70cc Passport for many miles, you will have no problems with a 70cc but a little more power might be nice. 70cc will get you 45mph.

If you decide not to do it, let me know, I might be interested in the frame. The Honda 70 s are the ultimate low cost, high mileage transportation design. We never even tried to get good mileage, us kids used to drive it hard, but it just runs forever on a tank. Just for fun, my dad used to stop along the road and pick up pop cans for the deposit money, he could easily fill up the tank for his commute on the 70 just from the cans he found along the road. It was a game to see if he could commute for free.


We had windshields on ours, they were nice to have, the wind is a little tedious at 45mph. They keep the rain off too.
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Old 06-28-2008, 05:16 PM   #6 (permalink)
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I have a 1966 90 cc for sale right now.

eBay Motors: 1966 honda 90 ( cm90 c90m cm91 c91m trail c90 4 parts ) (item 190233060655 end time Jul-05-08 08:18:09 PDT)

check it out. It would need work, but it is probably in your budget. Starting at $10. The 90cc goes 60mph. I think the wheels are interchangable with the 70's too, but I'm not sure. If so these may have the upgraded braking system you are interested in.

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