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1970s Honda Monkey upgrade/rebuild/fix project

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So, this was the bike as i got my hands on it. (albeit, with the engine removed)




Fuel tank



Front wheel/brakes


49cc engine + Rear light + A big pile of bits!





Rear Wheel




Dodgy bars:


The bike was slowly rebuilt up to look fairly original with some improvements, before I really got stuck into it after a small fire on an aborted trip last year.

Before the fire it was looking like this:


The wiring was cooked, and I made a small loom to get it running again, and ended up using this as a start of a wider project to improve various parts of the bike, which I will detail below!

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I bought a new Regulator (2 wire) but was a bust, it worked, but it seemed to reduce the lights brightness. I guess I need to ensure the battery is on the lighting circuit to get proper light brightness. I was only working with the bike on idle, so its possible that the battery would take over at that point, and only charge on higher revs.

I also need a new multi-meter. Mine is a bit of rubbish, and seems very unreliable!

Wiring, tidied up a bit:



Done a bit of polishing on the engine block when it was out, along with a lot of de-greasing, but the flywheel cover is going to need some work, along with the exhaust. Otherwise, I think its getting close to be ready for a full strip down, and re-paint :) I even cut down the coil - plug cable length, and added a new NGK plug cap.

Oh, and some tank badges are needed! I also have a new rear brake cable coming, in black, to fix the rear brake light switch, and get rid of the old brown one!

I am still waiting on a fork-rebuild kit, with some up-rated guides from America, along with some. I already go some ct70 fork sliders to get rid of the top end fork slop, and the usual seals to replace on a fork rebuild. But, I think I might do that after paint, as I will be pulling the forks off for that anyway.

Update -

I FINALLY have the tank badges on, and have been requesting quotes to get the frame and forks resprayed.

I put on a larger carb with an offset manifold, as the 75cc kit was struggling with the stock carb. However, even with the upgraded carb, the kit is not breaking 30mph on the flat. It seems to be down to the limited bore size on the original head. Which I kind of think makes the 6v 75cc kit pointless? I am emailing the kit seller to ask if other people have similar experiences with this kit. It does run well, but its not really much of a performance increase!

Next up, I am going to have to learn how to polish/port the standard head i think!


So after some discussions with the vendor and fitting a rev counter for testing on some longer trips, I am almost certain this lack of speed is down to gearing. (27mph at the 8500 rpm in third)

Going to try a few larger front sockets. Standard is 13t, so going to get a 14t and 15t to try ASAP.

Yup, just ordered a 15t front and 31t rear. Should get me about 30% increased top speed, and a corresponding lower acceleration. But as first gear is almost useless, I hope this will be fine, and just make it seem a bit longer!

Should be able to get away with the chain already in use on the bike for testing purposes.

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Ok, so I have done a few things to the bike recently.

Firstly, I was not happy with the rear of the bike, so I wanted to clean it up a bit..

So, first step was to clean up the numberplate/light bracket. You can see in the pics earlier in the thread, it was looking a bit tatty. So, it came off, I stripped the paint, cleaned it up, and gave it the usual paint treatment.

So it looked a bit more like this:




The original part quality leaves a bit to be desired, and the welding is somewhat agricultural, but it should be a little better protected, and looks less rusty.

Then I was not happy with the rear lens. I did my best to clean it up with a bit of white spirt, and then it just fell to bits. :( I tried epoxy, and superglue to bring it back, but it just looked terrible. So i ordered in a part from the Philippines and it looked a lot better! here is the original lens before I killed it :(



Then with the new lens fitted, and back on the bike, with the metal parts newly polished:




I also had ordered a new "big bore" exhaust for the bike! he it is next to the old one:


Fitted!! I had to raise the seat up to its top height to allow the kit to bolt up, had a quick ride, and it actually feels a bit nicer to sit on - Just got to hacksaw off the extra long number plate bolts next...


I was hoping this might unlock a bit more power, and it does feel/sound a lot better, but still not as quick as I would like. Next up, points cleaning/adjustment just to check that is not the issue. I will also check the valve clearances again, just to be sure they are spot on. If not, i will do some carb adjusting again with the new exhaust, and see how it goes then. If no more speed, I need to replace the points/check the timing, then if no change... Time to find a 70cc/90cc head to drop on the top to increase the port sizes!

I also have a spangly new gold and black chain to replace the old one, as seen here...


I am just to lazy to fit it yet...


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Bike is running well, but just not as quick as I would have liked, and there were a few niggles that were annoying me!

Firstly, I suspected the throttle was not allowing full opening with the new carb, it was topping out at thirty, and the dropping revs on slight hills in top gear.

The right handlebar was rocking in the mount, and it had cracked the original socket it fitted into.

The throttle itself was also not returning correctly, as the slide in the bars was providing too much friction. Great for a pseudo cruise control, but not for safety!

The original brake levers are VERY rudimentary, and just have no feel, and limited throw and look terrible, bent and dull.

The speed issue is very annoying, and as such I wanted to add a new throttle and cable. However, if I am doing this, I wanted to try and fix the wobbly bars. So I decided to get a new yoke, top plate, handlebars, throttle+cable and brakes....



The new top plate is MUCH thinner than the original combined top plate/yoke, but I managed to use a small spacer on the head bolt to allow it to be clamped down fully. Not happy with how this looks, so will try to get a silver or red spacer, and clean up the top bolt/washer to look a bit nicer!


It took about 30 minutes to replace everything, and as the handle bars are 22mm I suspected I could mount mountain bike aluminium brake levers... Then, if I removed the adjusters, they would almost fit perfectly to the cable ends. Bit of adjustment with a big pair of pliers, and it was nice and snug for the front brake, and the rear needed a bit of filing out of the cable holder, but worked out fine.



I really should have checked what my camera focused on...

Still the brakes feel a million times better, and more powerful, the throttle opens fully, and returns correctly.

Best thing is that the bike is now feeling like it is gear limited. First and second gear are almost useless, they are gone very quickly, and need to be much longer. I had changed the front sprocket to a 15T from a 12T, and the rear sprocket is currently a 34T, so I have a 28T rear sprocket somewhere that I put away and cannot find.. So just in case I ordered a 26T to arrive in a few days, and a 17T front. This should give me a good selection of sprockets to adjust and play about with the gearing. I am waiting to fit the new chain till I know what gearing I am running.

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Back in bits again, awaiting some new sprockets, so dropped in the new chain whilst it was already in parts.


New chain vs old one, little bit of extra bling never hurt anyone :D


Rear sprocket removed. Its a 35 tooth, and I have a 24 coming (Really long!). Will give this a clean up whilst its in bits, its got a little bit dirty!


Here is the 15 tooth front I am currently using. It replaced the 13 tooth (i think) that was original. I have 14 tooth here also, and a 17 tooth coming. Just in case the 24t rear makes the gearing too long to use the 17t front. I will run it with the 15t to start with, and see how I get on...


I also gave the points a quick sand whilst the cover was off, and cleaned in behind the front sprocket.

I am a little worried that the 24t is going to be too small and make the gearing too long and will mean I need to pull some links out of the chain, but will see how we get on. They don't do a smaller rear, so I can always go up a size, they are only £12 each in varying sizes.

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I did some playing around with the gearing today. I have moved up to a 17T front sprocket (it will not fit any larger without fouling!), and a 31 rear. I am now hitting 40mph on a slight descent, and 36/7 on the flat.

Happy with that for now, the gearing is a LOT longer, and 1st and 2nd are actually usable, if not perhaps a tad slow.. So will do a bit of tweaking with the carb, timing and valves to see what I can get out of it, and will likely iron out any flat bits nicely. I will leave the 24t rear in case I get a LOT more power out of it.

Some of my spare sprockets, new tank emblem bolts (Honda package each tiny part idividually!!) and front sprocket retaining bolts:


Couple of things happened in the testing... The wiring starting playing up and it would not let the bike start. So I pulled it all off, and continuity tested the lot, and could not find any problems... Next step was to just run the Coil wire direct off the stator (effectively hotwiring it) and it jumped straight into life again.

I figured it might be the ignition switch/key that was screwed up. So I pulled it apart, and it looked VERY secondhand. With some corrosion:



I continuity tested everything, and it mostly made sense, and matched to the wiring diagrams I had. So I supposed it could be the corroded case wire, i removed, and re-soldered in a new one to no avail. So i think this part may have more corrosion inside and i didn't want to delay stuff more, so I got fed up and put the part on the shelf for now.

Ok, so as I could not find the issue, I decided to make a new "loom" (it's so simple once the key part is out of the equation, it should not really be called a loom!). For testing I hooked up the lights direct to a hot wire from the stator (using spade female connectors), then the coil, and switched brake light (which runs on its own line). I don't think I really need the no-lights option on the ignition, so will just have an ALL ON setting for everything. I also changed some connectors in the headlight/front loom, and re-wired it/wrapped it all to make it a little tidier in there.. See below in a state! Also note the missing tank badge ready for its new bolts/screws:


You can see the empty ignition/key ring under the front of the fuel tank. Plan now is to replace that with a standard (generic) ON-OFF key controlled switch which I am sourcing now.

Rear lights wiring in progress:


Once that was knocked up, and tested as working, I took it out for a ride, and it rode perfectly. GREAT!

So I fabric wrapped it, and hid it away where possible to make it look a lot neater. Its a bit dirty from its test run in the rain!




Next is a plan to replace all the spade connectors with waterproof modern plug connections (already arrived), and fit a 4 point plug to the new loom to fit into the original stator plug a bit more neatly than my individually crimped spade connectors.

You can also see the new bars properly now in these last pics!

I have also decided to try and fit a regulator and rectifier to the new loom, so I can run a battery for the lights. It should keep them brighter at low throttle, as the voltage varies on the hot lines from 3.7v up to about 9.5v at full thottle. The Regulator dumps any voltage over 6v to earth, and the Rectifier changes the AC to DC to keep the bulbs from blowing. Mine don't actually blow at the moment in fairness, as some bulbs are now LED and the remaining incandescent bulbs are 12v rated so should survice the 9+ volts on thr 6v line. This will also allowing it to charge the 6v battery I am planning to fit.

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Plotted out the cabling, and cobbled it together with new wires and connectors off the stator. I then wrapped, and neatened it up. I still have 1 live to play with on the brick connector, which should allow me to do battery charging/light power without changing the main loom for now.



Overall it looks a lot neater under here. It also shows clearly the little fire damage the paint had under the tank, where the carb caught fire once! I also need to work on my fabric wrapping skills :D



I still need to find and add a key that will fit to do two things, bridge the ignition live to earth when off (acts as a kill switch) and then have 1 position (ON) for powering up the lights. I still also have to figure out how to add the regulator and rectifier when they turn up, and make it look neat, so I can add in the battery. Under the seat, or behind the engine mount look like good options when I get around to it :)

I will shrink wrap any ends when I do the final loom, just to keep it tidy, and I decided to use a rechargeable horn which arrived this morning to keep the wiring to the bars more simple!

Getting closer to pulling it all to bits, so I can get it shot blasted and re-sprayed, and rebuild the forks! Should have a new project in the garage very to help me keep occupied whilst it is away!

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Ok, bit more work done.

I added a remote kill switch to the current loom so I can kill it from a flip switch on the top of the headlight whilst it has no key kill switch. Its temporary, and uses the existing high beam indicator hole, so not a final item, but it looks cool :D


You can also see my temp headlight for test riding at night whilst the old lights are too pants! :D

Anyway! The 2 wire regulator I had was a half wave, so was producing 3v, so I found a full wave 4 wire which claimed 6v Honda compatibility (albeit not the specific model I have).

4 wire:


The pink and yellow wires are AC feeds from the stator, and the red is a 6V+ out and the green goes to the battery negative (6v-)

This is then fed into a 5v-11v DC converter to provide a solid 12v, in theory. (seen here with the now redundant half wave rectifier)


This SHOULD allow me to run at least a full power 12v headlight (maybe an LED one if needed). I will then look to run all the other lights from the 6v+ from the battery meaning I can use LED bulbs all round, as the existing variable AC setup makes them flicker/dim too much at idle

I mocked up a temporary wiring loom from the old loom I removed previously just to test my plan:


The bike started. and the regulator/rectifier was producing 4.8v at idle to 7.2v at fullish throttle (was testing indoors, and didnt want to be anti-social). the DC converted was producing a SOLID 12v at any throttle position. I ran an LED bulb from it as a quick test, and it worked well.

So, my theory works (at least for now!). I just need to place the battery into the bike frame somewhere sensible, hook up the 4.8/7.2 line to the battery with a fuse of some value (not figured this out yet). I could do this without the battery in theory, and just run the lights only when the bike is on, like the original, but I might as well try to do it properly :)

Then, run the 6v lights direct from the battery on 6v, the converter off the battery to give me a 12v headlight (and maybe a USB socket).

Once this is all working, then its wrapping the loom, changing all the bullet and spade connectors to waterproof automotive connectors, and work out fitting in a key powered ignition/kill switch.

I have also ordered a set of 6v/12v indicators, so I can decide where I am going to run them from. They are a slim, chasing LED type, to keep the power consumption down. Further to this, I have also got a 22mm bar hanging switch unit with an indicator switch, horn and light on/off switch. To work with that switch and indicators, I have ordered an adjustable flasher relay, so i should be able to adjust the flashing rate on 6v and 12v to suit.

I managed to find a decent key/kill switch unit that should fit my existing mount, so that is on its way too! Oh, and a 6v horn, as the bike horns seem to pull quite a bit of current for what that are (~1.5amps) so I guess with my 12v dc converter maxing out at 4amps, I want to keep as much on the 6v live from the battery as possible to give the best headlight power available.

So, the next couple of weeks will be putting all these bits together, working out how best to make the loom work in as few a wires as possible, adding proper waterproof automotive connectors where needed, and generally tidying up.

After this is all done, its likely going to be the final strip down, to get the frame and forks sent off to the painters :)

I also happen to think my existing LED array bulbs are less than efficient, but I have found a halogen 35w BA20D bulb that should only pull 3amps, so will give that a go on the DC converter.

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Started on the franken-loom.

Hooked up the battery side on the bench with all the new bits with clip connectors to test it all out, and work out what is the best way to wire it.


So, we have a little spot light working as the headlight off a relay (just for mock up to test the relay), Orange unit is the 12v flasher (would not run at 6v, so going to hook it up off the 12v line), then we have the 4 indicator (chasing LED type) on the right. I have a 6v horn on the middle left, just below that is the igntion/kill switch/key unit. 6v battery is at the top, with the 6v - 12v adapter below it. Then the control unit at the top right. Finally, a volt gauge to keep an eye on the converter's output.

Wiring was just done with small scraps for the mock-up, and clearly is not for real usage! But I wanted to get my head around it all before I started attaching to the bike.

Next was to mount the battery to the bike. I decided to mount it below the seat, as it would be mostly hidden, and it has room for the adapter and other components, if needed.


I drilled a couple of holes, tapped them for mounting the adptor, and the battery is held in with a thick zip-tie, and it can be removed and replaced easily enough. I have a sealed battery unit so as to remove mounting angle restrictions, and allow me to lay it horizontally.

Installed the controls:


Seat back in place, with battery and adapter hidden away. You can see the 12v lines (yellow and black) to head to the front of the bike for lighting/accessories duty and the 6v charging lines (Red and black). I have put the Halogen bulb plans on the backburner for now, and have ordered a better LED bulb for the headlight, and will run it off the 12v supply. I did this as I am also planning to run all the other lights off the adaptor for now to see how the battery/charging holds up from the new regulator/rectifier.


I also have started testing out waterproof automotive connectors. So, used one of those to connect the 12v line, which you can see here:


I also mocked up the charging circuit, and tested the output. It seems to be working fine, and I am planning to try and put it all together tonight on the bike with the temp connectors, then use the proper higher grade wires to get the correct lengths and connectors if I have time. So it should be tidy, easy to remove (for painting) and a lot safer/cleaner/reliable than before :)

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Ok, so here was the loom being mocked up to the bike with crappy connectors, and long wires to work out the routing. This was when I decided to try and mount as much as I could under the seat to try and keep it looking a little bit more tidy!


I also managed to get a decent 12v Led headlight with proper dual led function for high and low beam.


Then i found a small LED equipped fuse box. It's great! when a fuse goes, a red LED, lights up next to it, managed to hide this away under the seat, and this makes equipping the bike with most of the 12v gear easy. You can also see the black finned regulator/rectifier bolted to the front seat mouting point in this pic, and later in the completed pics.


I then took two of the variable AC feeds from the stator connector, and fed them into the new 6v regulator/rectifier. This is then run to the 6v sealed battery, and this in turn feeds to the 6v to 12v adaptor.


I set the headlight main bulb to come on only via a switch when the ignition is on (red switch on the indicator/horn controls) so it would only run off sidelight unless I specifically wanted the lights on. I have decided I now need to add LEDs on the top of the headlight housing that light up when either the indicators or main lights are on, as its hard to see during the day, and the switches are easily knocked accidentally when reaching for the horn, indicators or lights.


The wiring was all shortened, wrapped up, and the horn was added. Then it was ready to ride! here is the "cockpit" all lit up. I added and LED t5 bulb to the speedo to brighten it up a bit, and its a clear improvement, and works fine in the dark :)


Fresh back from a rainy shakedown the night before the run itself!




I rode about with a guy I met before from a C90 club, and also another monkey aficionado and some other riders of proper sized bikes. One chap on a massive Harley rode alongside me all the way down an NSL dual carriageway, and kept a big gap between me and the cars! He was a great help. My monkey mate and I avoided the main roads all the way home!

Take aways from the ride..

1 - The bike needs to be faster at the top end.

2 - Electric start would be superb

3 - Lights didn't last a full day

4 - More brake performance would be nice

5 - Paint needs doing!


1 - New engine and carb are in the works, just waiting on a quote for a newer engine with more capacity!

2 - As above - electric start should be a possible with this new motor

3 - 12v system would come with the new motor, allowing a straight 12v battery and 12v regulator/rectifier, with little adjustment of the custom wiring loom needed!

4 - My new c90 cub friend is helping me source a new set of forks, with a disc for the front :D

5 - The Cub was sporting a superb powdercoat job in the correct colour. So numbers have been exchanged and she will be going in soon, likely after the new motor.

Not sure when this will all happen, it depends on the availability of the new motor. Its going to be fun!!

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Today I should have all I need to get this up and running:

Brand new CDI type engine, of a certain capacity...

A new 12v battery

A wiring loom from a CDI equipped c90 cub

A new 12v regulator/rectifier

A new matched carb

A CDI coil/wiring kit - Due today in the post

A new low exit exhaust - Due today from courier.

One engine:


I actually already have the engine in place, but had clearance issues with the exhaust i was using before. Mainly around the electric start. So I decided against bending pipes to fit around it, as i was not sure how well it would survive such high temps. So found an under slung exhaust, that will change the exhaust routing totally.

The new manifold and carb are fitting, but I have had to use a carb spinner to get it to fit nicely. Still, the carb spinner is a nice shiny red thing, so looks pretty cool.

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  • 2 weeks later...
  • 3 weeks later...

top stuff! although the OH pointed out we could carry a monkey bike up the stairs and work on it in our kitchen so you may have started something... :lol: :lol:


Good! Everyone should ride a Monkey :D

Had a couple of rides, new engine is fitted, and working well. Exhaust sounds great!



Just got to add the new chain, 5 mins, job, and then it needs a clean after the wet weather!

Its due for an MOT in April, so will get that booked in asap, and might have a bit of a prod of the carb to tune out some flat spots. But otherwise, it's riding well, and everything is working :D

I have all the electronics fitted under the seat, its now 12, and using a larger 12v battery. Its got Electric start, and working dials. I am toying with the idea of adding a gear selector display, but I am not sure it really needs it. I also am planning to make up a small battery box and tidying up the wiring under the seat to make it look a bit neater, but not much more I want to do with it really!

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