1983 Honda C70 Passport from United States of America - Comments

I don't see what's so great about going fast; it's dangerous first off. It's more fun to have a responsive smooth riding refined bike that can corner. Sure if I had good money I could throw on a twin turbocharged Hayabusa monster motor, but making it refined as a Hayabusa takes skill and work.

Can anyone help please! I need a new headlight for my 1981 C70 - and second, does anyone how to wire the headlight (maybe a resister) to prevent future burnouts?

Hi, this is going back to a much earlier comment about someone's C70 being capable of 92mph. That is extremely optimistic. I have an 80cc derbi engined road racing machine. This is tuned as much as possible at 26bhp. This gets it to a true 98mph. The Honda C70 is 10kg lighter, but poorer aerodynamics. I think you'd need about 18bhp to get a C70 to 92mph. That is about three times the power of the standard C70.

One of Honda's greatest 4 stroke 50cc race bikes, the RC116 was only capable of just over 100mph. I have a C70 and a C90 that I use for off roading. I've had about 45mph out of the C70 and 50mph out of the 90. There is no way you could approach 92mph, even on the 90, without masses more power and ridiculous sprockets.

The power would need to be achieved by: much higher compression, serious porting, bigger valves, better cam, oil cooler, bigger carb, programmable ignition and a supercharger. You would also need to run the engine on methanol or nitro fuel. As far as I can see that is really unlikely.

Sorry to moan, but comments like that benefit nobody.

For people who are interested, this chap has a very fast engine that is almost the same to a Honda C50/70. It is a beautiful bike and very interesting. Thanks.

http://www.elsberg-tuning.dk/the%20bikes.html

Interesting comments, I agree on the whole that 90+ mph is a bit optimistic! I have a 1970 Honda C90 and my old engine is getting a little weak, having done over 18000 miles. I have however found a place called ooracing that specialises in pit bike engines (i.e c90 copies). They produce a lump called the 160 charger that produces 18bhp on the dyno, it revs to 14000 rpm, has a balanced crank, gas flowed head, big valves, race cam, strengthened valve springs, alloy oil spinner and an oil cooler! I reckon this would propel the old beast to somewhere near the 80-90mph mark. Not sure what to do about the bouncy old suspension though, any ideas anyone?

I think if anybody touched anywhere near 70 mph on a c90, they would have a 90 per cent of losing control, and the front brake on a c90 is reputably atrocious (bad).

These comments about 92 mph speeds are such utter BS that they are detracting from the value of the site here. It was mentioned that a larger sprocket was put on the rear to attain higher speeds... Except when climbing hills that tax your engine's horsepower to the limit, a larger sprocket on the back will result in LOWER speeds. Then again, a much smaller sprocket on the back would probably result in lower speeds with a 70cc or 90cc engine too, because there is no torque to back up such a gear ratio.

I think the only way one of these engines is going to hit 92 mph with any gearing is if the driver was laying prone inside a streamlined torpedo-shaped shell and had miles of run to get up to speed. These type bikes are at their best at 35 to 45 mph. Even 60 mph, while it might be attained down a steep hill, would be hard on the engine and stupid dangerous for the driver.

As for how to prevent headlamp burnouts, I had this problem at the beginning and fixed it by installing a full-wave rectifier between the alternator and the headlamp. This is a simple circuit to build that is just 4 diodes and a capacitor.

The reason the headlights burn out on these bikes is that they are driven straight off the magneto, which is an extremely choppy AC current. Installing a full-wave rectifier smooths out the voltage spikes and provides the headlamp with DC instead of AC current. You will have to separate the headlamp ground from bike ground if you do this because the rectifier output is not ground-relative to the bike. It is not a terribly difficult mod and I have never had the headlight burn out again.

I have a C70 1981, and it does about 47mph in perfect tune when it's possible. I really love all the exaggerations some of the comments record.

A friend of mine had two 1982 C70 Passport motorcycles behind his garage that had been there for 15 years. They were abandoned and left to rust in the endless Seattle rain.

Due to lack of time to restore, he gave them to me. He said he had purchased them with the idea to get them running. There were just enough parts between two of them to get one running bike out of both. One of the bikes had been in wreck and the front forks were bent. It took me eight months of picking away at those endless little restore jobs.

I have been riding the bike several hundred miles now, and the impression so far has been really good. I like running it around 30 MPH where it seems to be most happy. After new tires and fixing a shifting problem due to loose screws, I have lots of hopes to commuting to work on it.

I have restored four motorcycles, and what is it about spiders and filling a bike with their nest? The design of the C70 bike did well to keep the rain out of the electrical components while sitting, not spider proof.

When I got into the wiring, it was clean even after sitting outside in the rain. The seat pan protected the electrical components under the gas tank.

The C70 is an amazing feat of good design, and whoever the designers were, they get a bow from me. The only thing I need to finish the job is the rebound stoppers that go in the front forks.