Faster Yammy!, Faster!

[wild foamy]

hi everyone

ive got a 1990 DT50 that i want to get a bit more speed out of, i don't particularly want to start moddingthe engine as it is vintage and i would like to keep it fairly stock. (am already running an upgraded exhaust can though)

what gearing would be better for more speed?, im not particularly heavy (about 10st) and dont mind sacrificing a bit of acceleration for a bit more top-end speed

have attached a picture, not sure why, but i like the bike

thanks and regards

Steve

[Airhead]

Hello Wildfoamy and welcome to the club, Nice looking little DT youve got there. To get slightly faster top end at the expense of acceleration you will need to go for a front sprocket with more teeth, I suggest no more than 1 tooth. Theres always the danger that this low powered engine wont pull higher gearing if you overdo it!

[wild foamy]

hi again.

will keep a lookout for a slightly larger sprocket and see how i go from there.

what about things like air filters and carb jetting? ive found some decent air filtes for £6 a go which doesnt seem to bad if it will help the little peashooter breathe better

Air Filters

[Airhead]

hi again.

will keep a lookout for a slightly larger sprocket and see how i go from there.

what about things like air filters and carb jetting? ive found some decent air filtes for £6 a go which doesnt seem to bad if it will help the little peashooter breathe better

Air Filters

No harm in putting a cartridge air filter in no, As for jetting, only jet bigger than standard, jets are cheap so try the next two sizes up, Have you had the bike from new, can you say its got the original jet in there, If you dont know then find out what size is standard and look at yours with a magnifying glass, size is stamped in the jet, next to the hole. Also you will need to plug chop when jetting

[wild foamy]

im not sure i understand a "plug chop"

i havent had this from new but the engine is the same as it was in 1990 (apart from the rebuild), will have to look up the stock size and go from there

will new jetting do anything to the speed/power output of the bike and is it wasted without buying a big-bore kit to use it with? (which i dont really want to do because then it wouldnt be a 50cc)

[Airhead]

im not sure i understand a "plug chop"

i havent had this from new but the engine is the same as it was in 1990 (apart from the rebuild), will have to look up the stock size and go from there

will new jetting do anything to the speed/power output of the bike and is it wasted without buying a big-bore kit to use it with? (which i dont really want to do because then it wouldnt be a 50cc)

When the bike was sold new, it had the correct jet for the engine with standard exhaust, silencer and air filter, It may now have an after market (Not Yamaha) expansion pipe and silencer, you are also talking of a different filter. These things are changing the bike from how it was, the danger is in running too lean, too lean can cause overheating, in extreme cases ahole in the piston crown or possible seized engine. This is why you need to jet when you start changing these items, although i must add that I doubt you need to make big changes, thats why I said go for the next two sizes up from standard

BTW Markyboy27 is looking into jetting on his 125 Dragstar, have a look at his recent posts, you will learn a bit from them

See plug chop

[wild foamy]

having read-up on the dragster posts i now understand the concept of jetting and plug chopping.

im familiar with fuel mixtures and the like as ive been flying i.c. powered model aircraft for a good few years and did i.c. powered monster trucks several years before, we often checked the glow plug for a healthy dark-grey head and examined the coil to see if it had gone white (indicating a lean run) and adjusted the needle-valve mixture accordingly.

another way i used to tune my planes was to set the mixture slightly rich and run the engine up gradually to full power, then cut the throttle and immediately open it again, if the engine hesitated i would lean it out and try again, if the engine cut out or maxed out RPM before full throttle it was often too lean.

also i used exhaust smoke as an indicator, excessive smoke would usually be caused by a rich mixture and little/no exhaust smoke meant it was too lean,

do these same theories apply to petrol powered two strokes? (all mine were methanol powered two strokes)

- Steve

[JimR]

having read-up on the dragster posts i now understand the concept of jetting and plug chopping.

im familiar with fuel mixtures and the like as ive been flying i.c. powered model aircraft for a good few years and did i.c. powered monster trucks several years before, we often checked the glow plug for a healthy dark-grey head and examined the coil to see if it had gone white (indicating a lean run) and adjusted the needle-valve mixture accordingly.

another way i used to tune my planes was to set the mixture slightly rich and run the engine up gradually to full power, then cut the throttle and immediately open it again, if the engine hesitated i would lean it out and try again, if the engine cut out or maxed out RPM before full throttle it was often too lean.

also i used exhaust smoke as an indicator, excessive smoke would usually be caused by a rich mixture and little/no exhaust smoke meant it was too lean,

do these same theories apply to petrol powered two strokes? (all mine were methanol powered two strokes)

- Steve

Hi Steve

Pop down the libary and see if they have a copy of Tuning for speed by Phil Irvine, although this book is old it will explain the principles of trying to get "a gallon out of a pint can". I used the book as reference for my time at Merton tech collage (the only place, at the time, that did the city & guilds in motorcycle engineering) when doing projects on making Bantams, YG1's , YL1's and B100p's go quick.

Regards Jim

[wild foamy]

Result, polini 45mm 70cc Big bore kit £158 posted 60cc is only £57 though... but theres no replacement for displacement

ive got £200 to spare on upgrades at the moment though (maybe more if i can sell some of my stuff off), so i will have to spend wisely...

[JimR]

Result, polini 45mm 70cc Big bore kit £158 posted 60cc is only £57 though... but theres no replacement for displacement

ive got £200 to spare on upgrades at the moment though (maybe more if i can sell some of my stuff off), so i will have to spend wisely...

Are those kits including the cylinder head ur going to need in order to release the extra displacments power, so perhaps a 60cc with the correct cylinder head will go better than a 70cc with the 50cc head on it. Another consideration is the head gaskit DT/TY/RD 50's in the UK used a 'thick' head gaskit in order to reduce the compression ratio, so you will need a non UK head gaskit. Have you researched the port timming on the 70cc barrel this will have a bearing on the correct exhaust system to fit to the bike... the std is junk with its inner sleeve so u will need to match the port timming of the barrel to the exhaust. Then the carb throw it away and buy a dutch spec one but u may have to rejet from there (2mm bigger venturi). As I said in other posts the FS1 has cost about £500 in order for it to get to 70mph ... CDI fitted, 45mm bore kit, carb bored out & disc valve cut about, gearing altered to suit the extra bhp. At present playing with the main jet in order to get the full bore mixture sorted, with following projects .. i.e the front brake ... slowing the FS1 from 70 is a bit of a bother but I dont want a disc as it will increase the unsprung weight. The YG1,s & YL1's slowed up ok.

Regards Jim

[wild foamy]

emailed the site owner and he says it comes with the head and gasket (everything together).

as for things like exhausts and carbs, i was considering replacing the stock ones with DT80 parts (as i understand it, they are similar engines) but finding said parts is a different can of worms altogether...

but now i think of it, by the time ive bought all the right kit i would be nearing the £300 mark before its all complete-ish, on a bike this old i doubt it would be really worth it seeing as i will be going to a 125 next year, 60cc will probably do just fine (and its £100 cheaper )

[JimR]

emailed the site owner and he says it comes with the head and gasket (everything together).

as for things like exhausts and carbs, i was considering replacing the stock ones with DT80 parts (as i understand it, they are similar engines) but finding said parts is a different can of worms altogether...

but now i think of it, by the time ive bought all the right kit i would be nearing the £300 mark before its all complete-ish, on a bike this old i doubt it would be really worth it seeing as i will be going to a 125 next year, 60cc will probably do just fine (and its £100 cheaper )

Not a hope in hells chance Steve .. Jog R is totally different... ask the supplier to give you the correct carb specs ... just throwing a DT80 unit on will not do the job .. same with the exhaust.. a DT80 may be less restrictive but may not match the other mods, but if ur doing a 125 soon why bother just use the little bike to learn how to ride ready for the next big step

Regards Jim

[wild foamy]

ah you know how it is, when youve got the cash to splash you start to get these ideas...

even for just learning i would like to get a bit more speed out of it, i think it would probably be better for me to stick to small things like air filters, up a tooth on the front sprocket and maybe up a jet size to compensate for the increased airflow (prevent lean running).

which leaves me plenty of cash left over to start saving for a 125

[racerd]

you could try putting a dt 80 mx engine in then put a big bore on the 80cc motor then you could port and flow the 80cc big bore as well also you could try a bigger a carb, open out the reed stops and port and flow the reed block, also you could try using the 80cc exhaust, also you can get the cylinder head skimmed and get the squish recut plus more as well

[wild foamy]

ive considered that option and should a DT80MX turn up i will probably do it, but for the moment its only a 50 so i wont be doing any serious modding.

will keep a lookout for a DT/RD 80 engine though...

[Cynic]

As i remember from the stone age on my old DT50m the 60cc kit will run with all the std guff, carbs pipe the lot and will give you some usefull extra shove, with some cute gearing you should be looking at 45 maybe. £60 for some usefull go would be a pretty good spend and the carb from a dt50m would be perfect if you can find one, cos they wern't factory restricted (bar the naff std pipe)and have a nice big carb actually designed to run a 60cc motor, try and get the reed block with it cos its the same as yours but with bigger holes.

You are better off with smaller sprockets overall, stay with a small rear and a small front, less torque for the engine to deal with at the output shaft, better sprocket to wheel ratio giving the gearing increace and of course less chain to drag around. If you have the smarts (no disrespect)use the gearbox and primary ratios to calculate the ideal gearing with a target max speed.

I was clocked at 72mph drafting a transit and thankfully back in the day the plod was more interested in how i did it than nicking me, not a lot of point in pleading innocent so i told him and he ended up let me off with a token chewing.

That was on 60cc, tuned so far it was a pig to ride, noisy, expensive (little ends lasted a month maybe 6weeks as it revved a little high ahem) and needed constant tweaks on the mixture. Hot and sunny drop the needle, cold and damp lift it etc. It needed reving to around 3500rpm just to get moving and revved to around 8 or 9. The ig was advanced into the next decade to rev (so it was a sod to start) that high and political promises last longer than the points used to. As far as it goes i recon it had 8maybe 10 bhp on a good crisp day in a power band as wide as a gnats a#se. Fastest thing in Northampton excepting the Kwacs for a while till it got nicked, god i miss that bike.

[wild foamy]

well i want it to be a good bike (cheap to run, reliable, long lasting e.t.c) so i'm not too sure about going that far with modding.

i don't mind having to use t'box more if needs be but on the flats i'd rather have a bit more speed. on a two-stroke like this one i doubt there is much i can do in regards to getting a good low-down power-band but i'd rather keep t'revs low to try and make t'engine last a bit longer.

ive got a K&N style filter coming in t'post, will order in a few carb jets to experiment with to help t'engine breathe a bit better (plus i have a "Big One" exhaust can ) and ordered in a bigger front sprocket and slightly smaller rear one (and a new chain, may aswell change it as a set)

oh, and i'm dieting, could make t'difference when going uphill

[Cynic]

No you don't want to tune it that far. It was evil, none of my mates could ride it worth a damn, it was as unique to me as the spots on my b/side.

The C/S will be a good move and as there is not a lot of go to start with make sure you are not wasting any with sticky brakes and naff tyre pressures.

[wild foamy]

brakes are fine and i check t'pressure regularly

i think i will just have to be happy with modest modding, open the reed blocks and some fibre reeds, MAYBE a big bore kit for a bit better hill-climbing ability and thats it.

oh, the gearbox, i want to find a good five-speed box so i have better selection for going uphill and the like, maybe it will give me a little more speed but who knows

[Cynic]

oh, the gearbox, i want to find a good five-speed box so i have better selection for going uphill and the like, maybe it will give me a little more speed but who knows

Er i don't know who has been giving you the idea you can swap gearboxes. The one you have is it. Unless it has all of a sudden turned into Rossi's M1 or a WSB/BSB racer you are stuck with what you have, the chain and sprockets is as far as you can go.