Rpm Limit on a 50cc
Discussion
We have just aquired a 56plate 50cc Derbi Senda SM for my 16year old (£150), I am in the process of going through it with him (father/son project) as bits do not work on it etc, the exhaust is aftermarket (un-restricted), I will be checking the jets this week to see if they have been changed etc.
He has asked me to re-restrict it so it will do no more than about 35-40 in top to extend the life of the engine so that he does not thrash it all the time (as the PO said it would do about 50)
, so I was thinking, what about adding some slack into the throttle cable or adding a washer to the carb to restrict the slide movement to limit the max rpm, would that have the desired effect?
I agree that a decent top end would possibly be safer on the open roads, but he needs it to last and with the way insurance is etc I need it to be 'legal', also one of his mates got pulled by the plod for acting like a bellend, going to fast etc, he just got a warning.
He has asked me to re-restrict it so it will do no more than about 35-40 in top to extend the life of the engine so that he does not thrash it all the time (as the PO said it would do about 50)
, so I was thinking, what about adding some slack into the throttle cable or adding a washer to the carb to restrict the slide movement to limit the max rpm, would that have the desired effect?I agree that a decent top end would possibly be safer on the open roads, but he needs it to last and with the way insurance is etc I need it to be 'legal', also one of his mates got pulled by the plod for acting like a bellend, going to fast etc, he just got a warning.
Mechanically limiting the throttle would be more likely to cause the engine to spend extended periods at or close to a particular RPM, which is undesirable (in four-stroke engines at least, anyone care to comment on the validity of this factor in two-stroke?).
Your son is being quite sensible but I suggest you tell him to acquaint himself with the risks and learn to manage risk himself using his brain.
Your son is being quite sensible but I suggest you tell him to acquaint himself with the risks and learn to manage risk himself using his brain.
The most common way of restricting a 2T is to either put a restrictive washer into the exhaust, either at the exhaust port flange or (less desirable) in the stinger.
Also restricting throttle slide lift will do it but you'd be surprised how much is needed as the motor will gain rpm even with a half open throttle. Don't put slack in the cable as it'll be horrid, making the throttle loose and baggy.
Other options are to limit the amount the reed valves open (if fitted) by closing the cage down or adding a few or much thicker head gasket to reduce the squish and CR effectively reducing power.
Also restricting throttle slide lift will do it but you'd be surprised how much is needed as the motor will gain rpm even with a half open throttle. Don't put slack in the cable as it'll be horrid, making the throttle loose and baggy.
Other options are to limit the amount the reed valves open (if fitted) by closing the cage down or adding a few or much thicker head gasket to reduce the squish and CR effectively reducing power.
Ignoring the boring bits of the post,
The engine would probably be fine revving to 12k rpm, perhaps a twice a year rebuild would be needed, you would need a decent crank / cylinder / exhaust and carb set up and definitely an inner rotor ignition system to attain the rpm required.
Putting a load of slack in the throttle is dangerous.
Restrict it by reducing the exhaust port size by welding a washer into the exhaust header, as previously mentioned.
The bike will be slow as fk and hell only have it round his mates house in bits to find out why he's the slowest. Who cares if it blows up let him fix it and he will learn more.
The engine would probably be fine revving to 12k rpm, perhaps a twice a year rebuild would be needed, you would need a decent crank / cylinder / exhaust and carb set up and definitely an inner rotor ignition system to attain the rpm required.
Putting a load of slack in the throttle is dangerous.
Restrict it by reducing the exhaust port size by welding a washer into the exhaust header, as previously mentioned.
The bike will be slow as fk and hell only have it round his mates house in bits to find out why he's the slowest. Who cares if it blows up let him fix it and he will learn more.
De-restricting your Derbi Senda >>>> https://www.youtube.com/watch?v=8i9vM2lWDAU
[Haynes]Restriction is the reverse of de-restriction[/Haynes]
[Haynes]Restriction is the reverse of de-restriction[/Haynes]
bass gt3 said:
The most common way of restricting a 2T is to either put a restrictive washer into the exhaust, either at the exhaust port flange or (less desirable) in the stinger.
Also restricting throttle slide lift will do it but you'd be surprised how much is needed as the motor will gain rpm even with a half open throttle. Don't put slack in the cable as it'll be horrid, making the throttle loose and baggy.
Other options are to limit the amount the reed valves open (if fitted) by closing the cage down or adding a few or much thicker head gasket to reduce the squish and CR effectively reducing power.
Bending the reed stops to close them up might be a good idea, I think if you mess with the squish it may lose all punch all together as well as being less economicalAlso restricting throttle slide lift will do it but you'd be surprised how much is needed as the motor will gain rpm even with a half open throttle. Don't put slack in the cable as it'll be horrid, making the throttle loose and baggy.
Other options are to limit the amount the reed valves open (if fitted) by closing the cage down or adding a few or much thicker head gasket to reduce the squish and CR effectively reducing power.
LiamB said:
Might have a standard exhaust here if you want it?
Yes please Liam, PM me if you can please.Yazza54 said:
Bending the reed stops to close them up might be a good idea, I think if you mess with the squish it may lose all punch all together as well as being less economical
I will look into this, a set of reeds probably will not cost much, I could just have a play. I think it would be better to leave as it is, but as I said, he has bought the bike and has got a Saturday job and starts college in September, so he has not got a lot of money so he wants it to be reliable, the insurance thing is my main concern as it would screw him up totally for his next vehicle if anything happened.
All in all , thanks for the posts, it has given me something to think about.
evo8 said:
LiamB said:
Might have a standard exhaust here if you want it?
Yes please Liam, PM me if you can please.It's off an 08 plate, but I'm pretty sure they didn't change from 04-05 onwards.
It's changing times when your son asks you to RESTRICT his bike for him. When I was 16, I had a brand new bike.. Job number 1 (around a week after buying the thing) was to work out how to de-restrict it and then squeeze every 0.001 bhp I could out of the thing...
Ultimately wrong but was a valuable part of my biking education
Ultimately wrong but was a valuable part of my biking education
Yazza54 said:
I'd avoid putting anything in the exhaust stinger as it could hold heat in the cylinder, someone had this problem with a rd hybrid where the pipe builder made the stingers too small
Some form of restrictor on the inlet would be best?
I bought a restricted Honda NSR125 brand new in '89 and this was their method of restricting it. I'm sure that if it's good enough for Honda it's a perfectly good way to restrict power output but I guess it depends on how efficiently the head can dissipate heat. On the flip side; a racing head is likely to get far hotted than a restricted one as it is making a lot more power I would have thought.Some form of restrictor on the inlet would be best?
Fire99 said:
It's changing times when your son asks you to RESTRICT his bike for him. When I was 16, I had a brand new bike.. Job number 1 (around a week after buying the thing) was to work out how to de-restrict it and then squeeze every 0.001 bhp I could out of the thing...
Ultimately wrong but was a valuable part of my biking education
As you say, changing times indeed. My first bike had pod filters, alpha exhaust, re-jetted, etc........mum knew when to unlock the door when she heard my bike start up at the pub about a mile away Ultimately wrong but was a valuable part of my biking education
If I had bought it for him, I do not think he would have been as 'sensible'.
LordFlathead said:
Yazza54 said:
I'd avoid putting anything in the exhaust stinger as it could hold heat in the cylinder, someone had this problem with a rd hybrid where the pipe builder made the stingers too small
Some form of restrictor on the inlet would be best?
I bought a restricted Honda NSR125 brand new in '89 and this was their method of restricting it. I'm sure that if it's good enough for Honda it's a perfectly good way to restrict power output but I guess it depends on how efficiently the head can dissipate heat. On the flip side; a racing head is likely to get far hotted than a restricted one as it is making a lot more power I would have thought.Some form of restrictor on the inlet would be best?
I wouldn't mess with anything internal, just try and limit the amount of throttle or potentially a restrictor plate after the carb. I'd rather restrict what's going in than what's trying to get out if it were me.
I think most of the manufacturers restricted the pipe because it was more difficult to de restrict tbh
Bit of an update, new cylinder and piston fitted yesterday (£42 delivered from Poland), she fired up second kick. Adam (my son) then spent an hour riding round the back garden (he stalled it about 50 times!!) at least he did not drop it or smash through the fence!!
Fork seals need replacing and the water pump leaks, but the bike only cost £150!!
Anyone got any spares for a 56plate Derbi Senda X-Treme SM??
Fork seals need replacing and the water pump leaks, but the bike only cost £150!!
Anyone got any spares for a 56plate Derbi Senda X-Treme SM??
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