The engines are very heavily restricted,
Sorry, but they're not 'very heavily restricted'. The H100 used a bigger version of the MT/MB5 engine and produced about 10.5bhp from 99cc. The H100 had no restrictions in it whatesoever, and could have produced 12bhp and still fallen within the learner law horsepower restriction.
So if 99cc produces 10.5bhp in unrestricted form, then 49cc will be producing approximately 5.25bhp. If I remember correctly, mopeds are supposed to produce no more than 3.5 bhp or a figure near to that. Therefore 'de-restricted' they'll gain an extra 1.75 bhp!
seem to recall 'mechanics and the biker' (magazine evolved into performance bikes magazine in 1985), or it may just have been called 'mechanics' then did a full on tuneup of the mb5 engine. ,
You can 'tune' two stroke engines fairly easily mainly by porting and increasing the air/ fuel mixture (carb modifications and free flowing airfilers). Certain expansion chamber exhausts can help, but they also tend to move the powerband further up the rev range.
seeem to remember main restriction was very mild porting on them,,
The porting is not really restriction. Porting changes the point at which the exhaust gases leave the combustion chamber after the firing stroke. If you raise the exhaust port then the gases leave the chamber quicker, and the scavenging effect draws in more air/ fuel earlier. This produces more power but also raises the powerband further up the rev range. Raise the exhaust port too high and the engine loses all tractability and you end up with a knife edge powerband. Great fun when it kicks in and pulls the front wheel skywards, but a pain in the arse for driving every day.
Lowering the exhaust port does the opposite by lowering the power. What it does do is makes the engine more tractable and the power is spread over a larger rev range. Thus manufacturers would compromise by having the exhaust port height give the engine the power required and the tractability needed to make it suitable for every day road use.
There is a way round this, and that's to use a 'powervalve' system. This is a valve that electronically raises and lowers the height of the exhaust port depending on revs. Yamaha were one of the first to use this - 1978 on their F1 race bikes and then in 1983 on the road going RD 350 YPVS (YPVS stood for 'Yamaha Power Valve System'). By having a variable height exhaust port you could have a tractable engine at low revs with high power at high revs. To put it in perspective the original RD 350 LC (no power valve) produced aroun 46bhp, then the RD 350 YPVS that came after it could produce around 60bhp AND it had more power low down compared with the earlier non powervalve model.
However, you
can't retro fit a powervalve system to an engine that was never designed to have one. You
can buy a powervalve kit for bikes that were designed to have them but didn't have them fitted because of learner legal restriction reasons. Examples of this are a lot of the 125s such as the Yamaha RD125LC/ TZR125, Kawasakia AR125 and I think the Suzuki RG125. Honda had the NS125 but from what I remember it was very hard to derestict it.
cant remember about the card, there may have been a collar in the exhaust.
Some exhausts had a washer welded on the mounting flange reducing the diameter and therefore reducing gas flow. Carburetors sometimes had a plastic washer placed between the throttle cable and the throttle slide to stop wide open throttle (WOT) from being achieved. Only way to find out is to remove the exhaust and carburetor and look. In the case of the MT/ MB5 you'll probably find that it's simply being fitted with a smaller venturi carburetor.
And be careful of upping the jet sizes too much as you'll change the air/ fuel ratio.
I can remember however that on the inlet side, there were ribs cast on the barrel to prevent you opening the reed valve stops past 5mm........
The ribs in the barrel are to guide the piston rings and stop them from catching the ports of the barrell as the piston moves up and down. The reed valves fit into a block that can be removed from the the barrell and carburetor. Bolted to this will be solid metal reed valve stops and it is these that can limit the reed valve lift. All you have to do is bend the stops back a bit and the reed valves would have greater lift.
have a look on ebay for bike magazines of the early 80's, theres a slim chance you may find the issues, sadly i car booted mine about 8 years ago. To derestrict a Uk model is a lot of work, but im sure they got the mb5 upto around 65 mph.
They didn't de-restrict it, they TUNED it! Most two stroke engines can be tuned to give more power but at the expense of low down power and also ending up with a knife edge powerband. An 49cc moped engine with no powerband and that will do 65mph will be almost impossible to ride under normal road conditions. It will in effect be race tuned and will have similar characteristics (all or nothing powerband).
Remember the H100 well too, very nearly had one, but bought a 2nd hand s reg yam RS100 instead, no airbox, micron, managed to get it just off the clock (a whole 80 mph, my old cavalier V6 did nearly that in second gear
)
I loved my Honda H100A. It did 60mph all day every day, 65mph with a tail wind and I once saw 75mph on the clock on a long downhill stretch. I managed to do 12,000 miles in two years and went EVERYWHERE on it!
I never did any 'tuning' to it, other than removing the baffle to see if that would make it go faster (it didn't, just made it sound fcuking loud!), enlarging the holes in the baffle (again, sod all difference) and enlarging the main jet with a drill (massive flat spots and and DECREASE in performance). You know what? It always performed best when left just as Mr. Honda had intended it
Good luck, and use decent 2 stroke oil, I ber belray Si7 and castrol A545 are things of the past now.........
Modern two stroke oils are fully synthetic and make a huge difference. Better engine life and less of the infamous two stroke haze coming from the exhaust. However, there is still nothing like the smell of Castrol 'R'
havent read the threads so sorry if this has been said already, but on the MT5 the ignition dies at approx 6500-7000rpm (iirc) due to a restriction in the CDI unit. you can buy derestricted CDI units, well you could once upon a time, i dont know if you still can. thats the main thing to do on an MT5.
That will allow the engine to 'rev out', but won't make it produce any more power. The ignition rev limit is probably what mostly restricts the bike to 33mph. Nice and easy to calculate - engine revs x gearing x wheel size = speed! Peg the rev limiter and you restrict the top speed of the bike.
another problem with the MT5 is the speed at which the exhaust gets blocked up inside. you need to take it off and soak it in acid or petrol or whetever you think is best to dissolve all the crap. you need to do this fairly regularly to keep it running well.
Caustic Soda was the thing to use
Bung up one end of the exhaust, stand it on its end, pour water into it until it is nearly full then
gently add caustic soda crystals. DON'T put the caustic soda in first and then add water as you can cause an explosion!
The caustic soda mix is highly corrosive and will bubble away like a witches cauldron as it dissolves the carbon build up.
BTW, 'coking' of the exhausts is less of a problem these days due to the better quality of two stroke oils used as mentioned above. However, the exhausts will get coked up over time so yes, regular de-coking is part of the service schedule.
I remember there was a mod you could do to the exhaust if you chopped it open to stop this happening as quickly, and to give more power. iirc there is a narrowing part inside near the front of the exhaust, which you can grind off and then weld it back together. if you have an aftermarket exhaust i would guess this isnt neccessary.
I've never heard of that one!
if you get a derestricted CDI, clean/mod the exhaust, rejet the carb (only next size up on main unless you make major changes), fit an airfilter, then if you're lucky it will keep up with a DT50
i woudnt change the sprocket sizes unless you get a derestrcited CDI unit. if you do derestrict it, get a back sprocket with 1 less tooth, it makes a big difference.
I wouldn't even bother with that. Add up how much the bits will cost second hand and compare that with the price of a second hand H100 engine. I'll almost guarantee that the H100 engine will cost less than the 'tune up' bits for the 49cc engine, and if you tune the 49cc engine it will no longer be classified as a moped so you might as well fit the bigger engine and be done with it. Plus even with the 49cc engine in a high state of tune it still will be unlikely to be as powerful as the H100 engine. It will also have a greatly reduced lifespan.
Ironically as well, tuning the 49cc engine and bolting on other parts is going to show up far more than swapping in a standard H100 engine (grind the engine capacity off first though - not that I condone such behaviour
) You WILL have to tell your insurance company if you have made substantial modifications otherwise you risk riding without valid insurance.
To conclude, you should really stick with the standard engine for peace of mind, or wait until you are 17 and then buy something bigger and more powerful. Or fit the H100 engine and re-register the bike, but it will no longer be classified as a moped and you'll have to wait until you're 17 before you can legally ride it.
Two stokes are great fun, I've had a Honda H100 and three Yamaha RD's (2x 350LC's and 1x 350YPVS). Wish I still had them
