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1981 C70 Passport Shenanigans

Ribrickulous

Ribrickulous

Veteran Member
Joined
Jun 8, 2020
Total Posts
1,852
Total likes
270
Location
Brooklyn, NY, USA
So an opportunity for a single owner “aesthetically beat up but mechanically sound” scooter came up.

Haven’t seen it yet, but I trust the guy who found it for me.

Should make for a great commuter bike around the city.

Of course, I want to tinker with it. First thought was swapping in a 125cc engine to get some more horsepower out of it and then… supercharge it :p

Anyone have experience modding these little single cylinder engines?

A resource I found so far: https://www.elsberg-tuning.dk/supercharging.html
 
And scratch this... no title, last registered in DC in '81 o_O.

Probably some way to title it in NY but not worth the time.

Mods, feel free to delete this one.
 
Ribrickulous said:
Mods, feel free to delete this one.
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Sorry it didn't work out, could have been fun. I built quite a few 70 and 100 engines for kids racing scrambles and flat track back when I worked at the dealerships, they can be made to run really well and they're durable.

No real reason to remove this unless you're adamant about it.
 
Meh, we can leave it up if it isn’t cluttering. Eventually I’ll stumble on a worthy candidate and then figure out the ins and outs of supercharging teeny li’l engines.
 
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We may be back in business. Another guy at the garage who doesn’t mind DMV bureaucracy as much as I do is willing to do the legwork for the title.

I’ve agreed to do the mechanical work which includes a top end job due to a broken head at the exhaust port.

Already welded the muffler back together (partially rotted from sitting on grass for a year) and now need to find the little flange that presses the muffler end against the gasket. It fell off and rolled into a black hole.

Bought a new top end on eBay and found a gas tank for, ostensibly a ‘72, that I believe will fit based on the photos.

Going to get it running stock (hopefully even with the cheap china carb) before going too wild.
 
Tackled the eBay gas tank today, how it started:

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How it finished:

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Started with evaporust for a 24 hour soak and then finished with muriatic acid at 50% dilution with water.

Not going to bother with the last bit of crud - will just be vigilant about cleaning the petcock screen and remember to flush once more after 25-50 miles on the tank.

By end of week the engine should be back together and re-mounted along with the tank.
 
I gotta say this thing is turning into a much larger pain in the butt than I ever expected.

Spent the weekend doing little odds and ends and some welding to get the exhaust pipe back into shape. Exhaust is mounted temporarily now until I can get the bike tuned in.

No matter what I try, this thing will not idle.

I can get it started at half to wide open throttle but as soon as I drop off, it dies out.

Carburetor (Sheng way replacement, the Keihin one is… well, toast. Someone let it sit with nasty gas in it for too long - every jet, passage, and surface is covered in this white gunk that left pits on the aluminum. Really unfortunate.

The Sheng Ways are apparently the most reputable substitute.

Timing has been odd as well, following the procedure in the manual, the test light never really went “off”. I got it to dim a bit, and then set the timing there, which has gotten it to run off idle.

Unfortunately I can’t really dynamically time it without someone else.

I also grounded the ignition switch directly to ground instead of through the neutral switch. I found it really odd and wanted to eliminate poor voltage as a cause of the issue.

Lastly, the petcock seems to only want to run off of reserve. Least of my worries right now.

If anyone has experience with these I’d appreciate any help.
 
I feel ya man. This 76 CT90 hasn't behaved right with the original Keihin or the DrATV replacement either. It idles but stumbles at throttle, ok in mid range but finally gets full power at speed. I use a spare 12volt to run the strobe.
 
Ribrickulous said:
Tackled the eBay gas tank today, how it started:

View attachment 32539

How it finished:

View attachment 32540

Started with evaporust for a 24 hour soak and then finished with muriatic acid at 50% dilution with water.

Not going to bother with the last bit of crud - will just be vigilant about cleaning the petcock screen and remember to flush once more after 25-50 miles on the tank.

By end of week the engine should be back together and re-mounted along with the tank.
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A thousand BB's in there and shake it up.
 
Ribrickulous said:
Timing has been odd as well, following the procedure in the manual, the test light never really went “off”. I got it to dim a bit, and then set the timing there, which has gotten it to run off idle.
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Jim's probably right about the condenser, but I've seen points do that when there's a little bit of debris left between the contacts after a light sanding. Wipe a piece of clean paper through them (or some contact spray and a blast of air pressure) and see if that clears it up.
 
Ribrickulous said:
Thanks guys - I’ll try pulling the condenser and I suppose sourcing a replacement if it’s gone bad.
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I wasn't even sure it had points, but when Jim mentioned the condenser I looked at the picture of the head above and saw the points cover off the engine in the background.
 
Ok - condenser is a-ok, and filing down the old points seems to have done the trick for getting it to rev well, but I’ve still got nothing at idle.

Gonna need to go back in to the carbs one more time to see what’s what there.

Timing is spot on verified both statically and dynamically. Throttle adjust and fuel screw have no impact whatsoever on my idle issue, so I assume there’s a clogged passageway somewhere.
 
I had an epiphany.

The slide from the Keihin carb is still what’s on the Sheng Way carb because it’s what was hanging off the bike when I got it.

I wonder if the slides are different

Still going to clean one more time, but this is worth a shot.
 
Ribrickulous said:
I had an epiphany.

The slide from the Keihin carb is still what’s on the Sheng Way carb because it’s what was hanging off the bike when I got it.

I wonder if the slides are different

Still going to clean one more time, but this is worth a shot.
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And for whatever reason, is the carb body different enough that it would cause the Keihin slide to be backwards (cutaway facing toward the engine)? That would certainly do it.
 
I don’t think so, but I’m going to do a deep dive between the two of them. I know the throttle ramp for the adjusting screw is slightly different but looks functionally the same.
 
Ribrickulous said:
I had an epiphany.

The slide from the Keihin carb is still what’s on the Sheng Way carb because it’s what was hanging off the bike when I got it.

I wonder if the slides are different

Still going to clean one more time, but this is worth a shot.
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And the needle too.
 
OK - was finally able to spend some time with the thing and after much swapping of carbs (another guy in my shop had a cheap(er) chinese carb on the shelf for these), slides, and jets, I've got it idling and revving on the stand, though it takes awhile to come back down after revving. I assume there's an air leak somewhere, maybe on the inlet joint (that metal elbow), though I'm going to try lightly lubricating the slide first to see if that helps. It currently fits tightly.

It also wants to rev away when I set the fuel mix screw all the way in. This screw is on the engine side of the slide, so I assume I'm leaning out the idle mix, making me believe that the middle setting of the needle is too rich, so after lubricating I may try playing around with the needle setting.

To give my brain a break though, I ran through and did the front and rear wheel bearings, and replaced any rubber I could source which included the bottom shock bushings on the rear, and though I have the swingarm bushings, they absolutely did not want to budge, so I left well enough alone on them. Another project for another day if the inspiration ever strikes me.

Before I head out of the country for a bit of vacation I'm hopeful to get the front end rubber and cables done as well as the steering bearings (I can visually see them right now and they look scuzzy), and hopefully get at least one good test ride in to sort out these carb issues.

I'm a bit disappointed in the Sheng Way carb, as that's supposed to be the true problem solver for everyone who's had their Keihin carb stolen (or in my case, horribly abused). I may try to fiddle with it some more, but again, just trying to get it running right now.

A nearer term over-the-top project for this bike is more likely to be setting up the wheels on the mill's rotary table to re-finish the brake drums, something like what this guy is doing:

Which will actually be practice for doing the same on the 350's drums... it's always bothered me that my front brake on that bike is weaker than the rear.
 
Still no dice on the little guy.

It’s got a hanging idle now, takes about 20-30 seconds to come back down, and runs like garbage under load. A lot of blubbering and wheezing.

Tried moving the needle clip up and down but no change. Same thing every time.

Also noticed that screwing the fuel mix screw all the way in sends the engine racing, like it’s lean? Maybe time for a few jet swaps.
 
Wow... I can't believe I've been working on this thing for three months.

Finally got things working well enough last night... involved another top end rebuild and rescuing a miraculously underpriced OEM carb from ebay.

The root issue was that the top end kit I bought used a flat piston in lieu of the OEM domed piston. .200" difference from top of the wrist pin to top of the piston surface. I was getting 90PSI of compression. Manual calls for 180. The aftermarket head has a smaller dome volume, so I assumed the piston was flattened to make the same compression with the difference.

Found a complete OEM head on ebay, lapped the valves, cleaned and resurfaced gaskets surfaces, slapped it all together with the original piston and new rings and it's got 155 PSI cold.

Rebuilt OEM carb, some fiddling with the mix screw (which has a significant impact on WOT operation on this bike, odd) and it pulls up to 35mph confidently. All I really need out of it.

There will be more fiddling in the future and maybe a 125 or 140cc engine swap, but I'm feel like I can finally put this project out of my head and just use the thing.

Still need to figure out how to get the e-start working properly (the bike didn't come with the original relay, and the aftermarket 6V ones haven't proven workable). That's not a critical issue, though.
 
Uplifting report on your carb issues. My buddy's 76 CT90 has never run right, only once rebuilt OEM and a Dr.ATV replacement still not right.
Good but weird tip on the mixture screw and WOT.
 
The OEM carbs are a bit difficult to work with - the pressed in idle jets are, in my humble opinion, obnoxious. Still better than the Sheng Wey carbs. I had a hanging idle no matter what I did with them.

Doing a side by side there are differences in the needle and slight differences in the slide. Make sure you replace that o ring in the, I don’t know what you call it… bit that the throttle cable passes through and screws onto the top.

Buddy of mine with the same bike is having the same issue. He’s frankencarbing an OEM one back together.

and of course, run compression, check the points etc etc
 
You know I was sort of hoping that I could get a little more power out of the passport by putting the right plug in… thought I had a C8HSA in, but the right plug (C7HSA) is in fact already in, and to be frank I don’t think I’ve seen a better looking plug. Black around the ring, light chocolate on the insulator and the end of the ground strap.

IMG_7372.jpeg

The bike spends probably 20% of its time at idle, 20% at half throttle, and the other 60% at WOT.

Put in a new plug because I had it anyway.

Remaining issues are :


1. Left blinkers not working - front or rear. Right side works fine when the bike is running.

When I signal left side, I hear the flasher clunking, but the only thing flashing is the rear brake light. Brake light also flashes a bit with the right side. Attributing that to crappy 6V system. Guessing either the switch in the handlebar is corroded on the left side contacts, or I have a short in the orange wire that isn’t bad enough to pop the fuse.

2. Starter Solenoid not working.

Original isn’t with the bike, and the only 6V one I’ve found (ostensibly… it’s an eBay special…) I can hear pull in, but it doesn’t trigger the starter. Starter works fine when I short the wire to battery positive.

Thinking either to wire up a 50A momentary switch and start it directly, or find a different switching relay and Frankenstein something in. Will probably add a 40 or 50A fuse to this in either case.

3. Brake switches finicky… rear brake works fine and just needs to be adjusted but the front brake switch doesn’t trigger. Need to get this and the flashers sorted to get it inspected.

Feedback on either of the above would be appreciated.

That being said, I’ve commuted to queens on this twice and took a little trip to Manhattan last night. Proud to say I got the Speedo to read 40mph on the downhill side of the Manhattan bridge 😂

It will juuuuust maintain 25-30 on the uphills and can reliably hit 35 on a flat stretch given enough runway. Waiting to trigger a speed camera to prove that out.
 
Unfortunately not. NY governs moped classifications by top speed, with 30-40 being a "class A" moped, requiring most of what a motorcycle requires. Since the C70's speedo goes to 50, it gets interpreted as a full blown motorcycle.

I wish they did it by CC, though I understand what they were trying to get to by classifying it by actual top speed.
 
Ribrickulous said:
Unfortunately not. NY governs moped classifications by top speed, with 30-40 being a "class A" moped, requiring most of what a motorcycle requires. Since the C70's speedo goes to 50, it gets interpreted as a full blown motorcycle.

I wish they did it by CC, though I understand what they were trying to get to by classifying it by actual top speed.
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Considering that you're required to drive it in the automobile lane,I might think to install a large bore piston,if it was owned by me. :unsure:
 
bilbikek411 said:
Considering that you're required to drive it in the automobile lane,I might think to install a large bore piston,if it was owned by me. :unsure:
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That or a 125/140 swap are in the works for the winter. I'll likely big bore it if all it is is pulling out the spare sleeve i already have, turning it down, and re-installing.

Thankfully the side streets I ride this thing on are usually jammed up with traffic, so I'm rarely actually hitting top speed. NYC dropped the speed limit on local roads to 25 unless otherwise posted, and during commuting times I'm usually doing 15-20 on busy streets. Occasionally on an open stretch it can hit 30-35.

The big bore would help when its time for re-inspection though - ever since the muffler laws passed in NYS it's an inspectable item. I believe I could get it to an inspection station and back with the stock exhaust, if not use it consistently.

Could certainly use some help on the bridges though.
 
Ribrickulous said:
That or a 125/140 swap are in the works for the winter. I'll likely big bore it if all it is is pulling out the spare sleeve i already have, turning it down, and re-installing.

Thankfully the side streets I ride this thing on are usually jammed up with traffic, so I'm rarely actually hitting top speed. NYC dropped the speed limit on local roads to 25 unless otherwise posted, and during commuting times I'm usually doing 15-20 on busy streets. Occasionally on an open stretch it can hit 30-35.

The big bore would help when its time for re-inspection though - ever since the muffler laws passed in NYS it's an inspectable item. I believe I could get it to an inspection station and back with the stock exhaust, if not use it consistently.

Could certainly use some help on the bridges though.
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I think that's a great way to get through city traffic !
3 speed automatic clutch? or 4 speed standard ? if it's a 3 speed,it really needs the larger bore.
I know the early Trail 70's had both;the best being the CT70H 4 speed standard clutch.
The lower-end from a CT70H would be a nice option to rebuild and install;they do come-up on Marketplace sometimes,like a 'Honda 70 parts' lot,etc.

Or getting a used OEM Honda S90 engine and rebuild/install to Go !
The stock S90 will pull 70mph tucked.
Grind-off smooth the '89cc' casting mark on the cylinder :)
 
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Blinkers and brake lights working again, so should be off to the inspection station tomorrow.

Blinkers were caused by a short at the rear left, just some wear and tear, that was enough to pull voltage away from the lights but not blow a fuse. Annoying.

Rebuilt the switch before I put it together, so at least the horn contacts well now.

Brake light switch was just a bit loose and needed to be shoved into the right spot. I like easy problems.

Starter solenoid will be another time. Kickstart works fine for now.
 
Wheeeeeeeelp, time for a big bore kit:

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Turns out riding two up going over a bridge does not work well on the stock setup.

Oil was still about 3/4 up the dipstick so I had lubrication, but my assumption is some combo of a leaner-than desired fuel mix, the heat of the day (about 85 today) and the strain of another passenger made the little engine give up the ghost.

Thankfully it was easy to pull off as it was dying (horrible screeching sound and some chattering) and roll back down the bridge.

Still made it to the beach while we waited for a buddy with a pickup to swoop us all up.

Big bore kit is in the way because at this point it’s no more expensive than buying a stock block and piston.

In the plus side the bike is inspected and I managed to find a 6V Bosch style relay to get the starter going.

Between this and the issues with the 350 I’m scared to take the interceptor out tomorrow!
 
I remember what happened to you before.. where the dome height of the piston gave you the wrong C. R.
I hope you get a good one this time.
 
Ok. Big bore kits in, but it’s got a bit of a knocking sound coming from the bottom end. If I use a screwdriver like a stethoscope its loudest when I put the screwdriver on the top of the crankcase, right behind the starter motor.

Occasionally wants to die when I decelerate quickly also. I don’t think that’s related - it’s running a bit lean at idle because the carb isn’t adjustable. I may swap the oem carb for one with a replaceable pilot jet.

I want to say the knocking gets a bit louder when I press on the back of the shift lever (“neutral” on the semi automatics).

I’m wondering if this is the piston banging against either the head or the inside of the crank? It moves very smoothly when rotated by hand, and I can watch it easily come up and down with the plug out.

I want to stress that it doesn’t sound anywhere near this loud/bad when I’m standing next to it, for some reason the cell phone cam exaggerated it.

Other than this audible issue and the one I described above, it’s got a strong idle, accelerates smoothly, and has just a bit more torque than before, which is very welcome.


Any thoughts? I’m leaning towards just riding it relatively gently and seeing if whatever this is… uh… “self clearances”…
 
Ribrickulous said:
Ok. Big bore kits in, but it’s got a bit of a knocking sound coming from the bottom end. If I use a screwdriver like a stethoscope its loudest when I put the screwdriver on the top of the crankcase, right behind the starter motor.

Occasionally wants to die when I decelerate quickly also. I don’t think that’s related - it’s running a bit lean at idle because the carb isn’t adjustable. I may swap the oem carb for one with a replaceable pilot jet.

I want to say the knocking gets a bit louder when I press on the back of the shift lever (“neutral” on the semi automatics).

I’m wondering if this is the piston banging against either the head or the inside of the crank? It moves very smoothly when rotated by hand, and I can watch it easily come up and down with the plug out.

I want to stress that it doesn’t sound anywhere near this loud/bad when I’m standing next to it, for some reason the cell phone cam exaggerated it.

Other than this audible issue and the one I described above, it’s got a strong idle, accelerates smoothly, and has just a bit more torque than before, which is very welcome.


Any thoughts? I’m leaning towards just riding it relatively gently and seeing if whatever this is… uh… “self clearances”…
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Ed,I might consider taking the top-end back off and inspecting everything before giving it the 'all done'.
Do you know if the new piston/cylinder has the factory approved clearance ?
 
Well, when you’re right you’re right.

I’m leaning towards some kind of issue with the cam chain or clutch at this point. Either way full tear down. Again.

Decided to ride it out to see what the worst that could happen is. Died three times, once at speed. Made *horrendous* noises - loud squealing, rattling that sounded like the whole damn cam chain came loose, and then…. Started up like nothing happened after a minute and happily chugged all the way home.

Still a bit of that tappy noise but not as much as there was before. Utter chaos.

Naturally this all happens in a part of Brooklyn I’d rather not break down in.

I’ll be going ahead and tearing this all back down shortly.
 
Ribrickulous said:
Well, when you’re right you’re right.

I’m leaning towards some kind of issue with the cam chain or clutch at this point. Either way full tear down. Again.

Decided to ride it out to see what the worst that could happen is. Died three times, once at speed. Made *horrendous* noises - loud squealing, rattling that sounded like the whole damn cam chain came loose, and then…. Started up like nothing happened after a minute and happily chugged all the way home.

Still a bit of that tappy noise but not as much as there was before. Utter chaos.

Naturally this all happens in a part of Brooklyn I’d rather not break down in.

I’ll be going ahead and tearing this all back down shortly.
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I'll be interested to see what you find. That engine is so durable, it's hard to imagine much that would fail easily.
 
It really is incredible that after all the drama this thing had in the 30 minute ride, it wound up getting me home without stalling or throwing a fuss.

I was riding with the choke about 30% out, because I knew it was running a tad on the lean side, but overall I can’t fault the ruggedness.
 
bilbikek411 said:
Ed,I might consider taking the top-end back off and inspecting everything before giving it the 'all done'.
Do you know if the new piston/cylinder has the factory approved clearance ?
Click to expand...

I think you were spot on here. Factory is 0.0002” - 0.0014”, Svc limit 0.006”.

I’m coming up with 0.0037” at best, 0.0064” at worst depending on what combo of bore and skirt you measure against. And of course, the photo evidence:

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The wrist pin was practically pressed into the piston, but felt right in the small end.

No discernible play up and down, a bit of lateral play, but within the standard spec.

Waiting for the eBay seller to get backs to me.

Hopefully they’ll at least provide a new piston and gasket set, if not block as well.
 
Freakin china,huh? :mad:
How many mm is the piston size?
I ask because you might see if an OEM Honda 89cc stock piston(S90?)could be bored into it and if the specs would allow fitment..?
:unsure:
 
It’s an interesting thought. I’ll have to check it out. I believe I’ve heard of people dropping S90 engines into these.

The bore did clean up with a few passes with a ball hone - there’s still a mark on the top and bottom of the right side, but my fingernail doesn’t pick it up.

Engine had 155PSI hot compression with a leaky compression tester when I brought it in today.
 
Ribrickulous said:
It’s an interesting thought. I’ll have to check it out. I believe I’ve heard of people dropping S90 engines into these.

The bore did clean up with a few passes with a ball hone - there’s still a mark on the top and bottom of the right side, but my fingernail doesn’t pick it up.

Engine had 155PSI hot compression with a leaky compression tester when I brought it in today.
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I understand.
The reason I mentioned OEM Honda piston and rings are because their quality;they last and perform very well.
Do you have a local Powersports machinist who could precisely bore that cylinder, or do other work ?
 
bilbikek411 said:
The stock S90 will pull 70mph tucked.
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Holy crap. Mine topped out at about 45, and that was downhill with my chest on on the tank. Maybe it wasn't in as good a shape as I thought! 😮

And Ribrickulous, that's a cute little bike. Here's hoping it stops giving you so many headaches.
 
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