Welcome to CubChat - The Forum For Honda Cubs (and Similar Motorcycles)
Att'n Bogger, Migs engine
Posted by : Nut and Cheri - Cheltenham (79.75.87.97) 
Inappropriate or SPAM?
On Tuesday 26/02/13 at 00:15:32
Hi Bogger.
We haven’t raced in the Open Class yet, so our first test of the new improved Big Mig will be at Anglesey in April, At Mallory we ran in the C90 class using a standard Cub90 engine, carb, and air filter, and to be honest we sucked lol. Our main problem was grip, it rained hard and our cheap balloon tyres just drifted as if someone had rubbed the track down with a bar of soap. Anyway it has since been ruled that Miguel can no longer race in the C90 class and therefore we find ourselves up against the Open Class machines. In a desperate effort not to suck even worse than we did last time, we have decided to modify Miguel to take advantage of the Open Class regulations.
The list for Open Class conversion consisted of 4 speed gearbox, larger carb and 107cc.
We decided to start with the 4 speed bottom end. A hunt round the shed rewarded us with an incomplete Chinese semi auto bottom end of unknown manufacturer with the ability to change from 4th to 1st whilst going along! The purchase of a 4.2mm drill for £1.50 and a modification to the selector drum, and the potential gearbox destroying/launch over the handlebars flaw in the design was fixed. The clutch actuating mechanism was missing, so I used Cub90 parts (also from the shed) to complete the assembly.
The bottom end came furnished with a serviceable iron 110 barrel, but alas no piston or head which I had scavenged some time ago to upgrade my 72cc Monkey Bike to 107cc, so I ordered a 110 pit bike piston kit off ebay for the princely sum of £13.
Chinese crank cases in my experience are made of the same stuff used for thermalite building blocks, and have a similar strength when it comes to putting screws into them. So to avoid future problems with the footpegs, I cut 4 studs from some old M8 bolts and fitted them to the crankcases with loctite. The footpegs can now be fitted onto the studs and secured with Nyloc nuts.
Whilst we are on the subject of Chinese metal, unbranded (or even branded) Chinese gearboxes don’t have the greatest of durability records and therefore it seems reasonable to assume that in the hands of Bowman (who broke his Baotian gearbox in less than 24 hours from the point he purchased his bike) at least part of our gearbox was going to end up lying in the bottom of the crankcases. So a search through my old Quality Street tin of nuts and bolts rewarded us with a magnetic sump plug. The addition of a copper washer and the job was done.
The new piston included a bit of old gas pipe pretending to be a gudgeon pin, so this was replaced with a lighter gudgeon pin I had knocking about and the circlips provided were also replaced with genuine Honda items which are designed to be forced into their groove by the chamfer on the end on the gudgeon pin, rather than out of their groove in the case of the Chinese items.
The original 12v 72cc Jincheng engine in MIG was a strange thing; the flywheel was the same depth as a 6v flywheel. When we fitted the Cub90 engine for Mallory Park we tried to fit the original Jincheng Chaly style cover, but the Cub90 flywheel was too deep to fit inside. I tried swapping the flywheel and stator, only to find the Jincheng flywheel strangely used the smaller 6v taper on the crank, which meant the flywheel taper was too small to fit the Cub90 crank. I gave up and fitted the Cub90 cover. So with a little time on our hands and a new engine to fit, I decided to buy a second hand set-up on ebay from a Lifan 150. Lifan 150’s still use an outer rotor and even have a charging/lighting coil, but the flywheel is a much smaller diameter. The smaller flywheel fitted into the recess in the Jincheng cover which has the logo on the outside. I still had to shave a bit off the flywheel rivets in the lathe and a bit off the inside of the cover with a die grinder, but eventually the cover tightened down without anything touching.
One point when fitting a Lifan 150 stator plate and flywheel is that it has adjustable advance, but don’t get excited because you have to advance it as far as it will go to achieve the same timing as a standard Cub90. The other thing about them, is that they do not work with a Cub90 style CDI, and colours and connectors are different too. Fortunately our second hand item came with matching loom and CDI, which only left us with a matching kill switch to buy, which was a probably a good thing as our existing wiring had shorted out in the rain at Mallory and caused us to retire early as we were unable to find the fault. I suspect now that it was the kill switch earthing onto the handlebars due to water ingress, although strangely at the time we removed the kill switch and we still couldn’t get a spark 
Next it was the turn of the cylinder head. I decided to use our existing Cub90 head as it worked fine at Mallory and is the late type with ball bearing cam. The head has been through some kind of trauma at which point an earlier type inlet valve, guide, springs etc have been fitted. It’s a bit odd because now it has one 4mm valve stem and one 5mm, but like I say; it works, so a quick clean up of the ports with a die grinder (no idea what I’m doing, but I took some metal out) and it was good to go.
Big Migs Chaly style frame restricts air flow to the sides and upper surfaces of the head and barrel and as I’ve already melted a 110 piston when using a Cub90 head, I knew the importance of keeping things cool . I already had an old oil cooler which was used on Scruff for a while, so decided to invest in a second hand take-off plate from ebay to complete the set-up.
We’ve decided to fit a go faster (yeah right) cam, it’s never worked so far, but hey I’ve never let that minor detail stop me before. Finally the tappets were adjusted and the head bolted to the new engine.
To complete the package a 20mm monkey bike manifold was added and a Sheng Wey pit bike carb (also from the shed). To fit the carb we had to remove the choke mechanism and also the Cub90 style fuel tap as both hit the inside of the frame. Fortunately the Chaly has a separate fuel tap. Finally a PM tuning filter which I found on the side of the road was fitted. The engine started easily but died at heavy loads at lows revs when the throttle was opened fully (throttle full open in gear with brake on) and so the carb was stripped to fit a larger main jet. The jet in it was 0.85 and not the original as I had pinched it for something else. Oddly the carb uses the same jets used in Honda GX utility engines which fortunately are also used in go karts, and so a new 0.92 jet was purchased from a Kart tuning company.
The needle was also not the original as I had obviously pinched that too, but on its highest setting the engine now runs fine, although with nowhere to test the bike, the real proof of the pudding will be on the day of the race.
One final pain in the arse was the unbranded Chinese clutch. Firstly it slipped when trying to kickstart. The bike was stripped down and the parts inspected. It turned out that Chinese machining tolerances meant that it was unable to clamp the plates when the kick start was operated. I mixed and matched parts from another Chinese clutch until the kickstart operation clamped the plates. At this stage I had also swapped the clutch plates. On reassembly it kick started perfectly but dragged horribly. I stripped it down again and blocked one of the holes in the oil spinner which allows oil from the pump to pass between the clutch plates. I reassembled the bike again and this time the drag was better but still present. I stripped the bike a third time and measured the clutch plate stack and found it was 0.5mm thicker than a new set! Chinese tolerances had struck again. I fitted the new set of plates and this time everything worked as it should, no slip, no drag.
So that’s the story of our engine/carb, a collection of spare parts which I hope will work well together. But that’s the fun of it for me, and it maintains the character of Mig 
Message Thread:
Back to main forum
Sorry, this post has timed out and can no longer be responded to (feel free to blame spammers).
If there are more recent replies, you might still be able to reply to those!
Just a counter down here...
