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Suzuki AY50 Katana 1997
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Discussion Starter · #1 ·
Hello. I just installed a sport exhaust on my moped but the RPM limiter is really holding it back. Is there anyway to remove the RPM limiter on my CDI or do I have to buy a derestricted one?

Also the only derestricted CDI I found that looks like mine does not specifically say that is compatible with my moped. Is that a problem?

Here is a picture of my CDI:
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And here is the derestricted one I found: Derestricted CDI Unit Aprilia RS50,SX50 06-10 D50B Derbi Senda DRD Rieju MRT 50 | eBay

Do you think that it is interchangable with mine?
Thanks.
 

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You are wasting your money, the rev limiter is on there to stop you from blowing up your engine and you have done nothing to the engine to make it capable of higher revs, if you have power available to make it go faster you need to change the final drive gear ratios or put on larger diameter wheels to make use of that power, not rev the engine faster.

Are you certain your scooter is now hitting an ignition rev limiter? You would know it because when you hit top speed it won't just quit accelerating, it will start bucking and jerking and missing like somebody is turning the key on and off repeatedly while you are traveling at max speed. Scooters usually have speed limiters built into the clutch and drive train.
 

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Suzuki AY50 Katana 1997
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Discussion Starter · #3 ·
The scooter didn't have any restrictors except a cone in the intake tube which i removed. I changed the final drive ratio from 15to1 to 12to1 some time ago. Now I have put a sports exhaust and have setup the needle and main jet. The problem now is that the power is being made at an RPM I can't reach.

When I take off at full throttle, the bike shoots forward for a moment then the engine starts making sounds like a rev limiter and the RPM is stuck at ~6700. After that the CVT does its thing, the RPMs drop a little and I start gaining speed. When riding at higher speeds downhill the RPMs can reach a little over 7000 at full throttle.

Lastly in case it's relevant I have a Keihin PWS 12 carb.
 

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The carb might be one answer to making it pull higher revs, possibly even a requirement.
If you were out to set a land speed record for scooters carb would be one thing you would want to make larger. 12mm carb is chainsaw size.

This is a 2-stroke with a reed valve correct? You can make upgrades when there are reed valves. That would be the first place to go on a small budget.
pictures would be nice. (y)
 

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Discussion Starter · #5 ·
The carb might be one answer to making it pull higher revs, possibly even a requirement.
If you were out to set a land speed record for scooters carb would be one thing you would want to make larger. 12mm carb is chainsaw size.

This is a 2-stroke with a reed valve correct? You can make upgrades when there are reed valves. That would be the first place to go on a small budget.
pictures would be nice. (y)
I'll try to get the best out of my carb, but I think the RPM limiter has to do something with the CDI box. Do you know how the RPM limiter works in the CDI box? Could it be a resistor or something?
I'm not out to set any records, I just want the scooter to be tuned right.
Yes it's a reed valved 2 stroke, I'll try to check them out tomorrow but some of the bolts that hold the inlet tube are "factory rounded".
I don't have lots of pictures right now but here's 2 of the outlet of the carb and the inlet tube.

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That intake manifold in the lower photo is square on the bottom because it has a reed valve cage under it hopefully, and no part of it should be smaller then the carburetor output if it is roughly cast and undersize anywhere you should fix that.
 

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Rev limiters are typical on fuel injected engines but if there is one on your simple little scoot I should think Suzuki would make that a sales feature and it would be documented in the owner manual.
I don't think they put a rev limiter on your ignition and if they did there should be lots of other CDI ignition parts that would work around that. I guess the true test would be to go down a big hill and when your engine hits that 7000 rpm is the spark plug still sparking, do Not reach down there to check because I'm betting it's still sparking.
 

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Discussion Starter · #8 ·
Would a 14mm carb be a good replacement? My scooter comes stock with either a 12mm carb or 14mm, in my case 12mm but I'm thinking of buying a stock 14mm second hand one.
 

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Discussion Starter · #10 ·
I currently can't, there are some factory rounded bolts or whatever they are. Are theese things threaded, how do I remove them?

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The problem is the steel brackets they are holding down, they corroded and now your fasteners are damaged, clean it all up with some penetrating oil and an old toothbrush, then find some wrench that fits the bolts very well to remove them.
at the moment your bike is dirtier then my dirt bikes, and you don't want any dirt around that part when you remove it or dirt will fall into the engine.
 

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Discussion Starter · #12 · (Edited)
Nothing is damaged, theese bolts or whathever they are have rounded heads from the factory, they are by design not meant to be removed. I'll try to cut a flathead screw driver slot on them or grind them away tomorrow.
 

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I would get them off even if I had to cut a slot across it with a hack saw and take it out with a straight impact screwdriver and then I would replace them with a decent hex socket fasteners of the same size, but that's just me.
There's stuff in there you need to service eventually, have you checked for a parts diagram for this engine? it would help a lot.
 

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Discussion Starter · #15 ·
Thanks for the advice so far. Ground the fasteners away today and removed the manifold. The manifold is a bit restrictive so I'm going to port it a bit with a dremel (forgot to take pics, will upload soon). The reed valves are metal and on one side have a slight gap so deffenetely replacing, otherwise they were in tact nothing was broken. Now the question is, is there a benefit in getting carbon fiber ones or should I just get normal metal ones?
Something I noticed with the manifold and reed valve removed. When I turn over the engine by hand I can hear a hissing sound during the compression stroke. Would that mean that my piston rings have gone bad or is it maybe the lack of oil when the engine is not running? Also the rod has surface rust on it (forgot to take pics of that too, will do soon).

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Hello. I just installed a sport exhaust on my moped but the RPM limiter is really holding it back. Is there anyway to remove the RPM limiter on my CDI or do I have to buy a derestricted one?

Also the only derestricted CDI I found that looks like mine does not specifically say that is compatible with my moped. Is that a problem?
Do you think that it is interchangable with mine?
Most of the CDIs for Chinese/Japanese mopeds/underbones and smaller capacity bikes have rev limiters built into the unit to prevent, as Trials correctly states, you over-reving the engine and damaging it. Removing the rev limiter by replacing the CDI doesn't really achieve much for a stock bike.

However, aftermarket 'racing CDIs' with no rev limiter are widely available and do help with a performance increase - not because of the removal of the rev-limiter, but because they generally advance the ignition a few degrees, which can give you improved performance in certain sections of your power band. Some of the generic racing CDIs even have adjustable advance. I have run four racing CDIs on four different smaller capacity bikes, and combined with a high performance coil and iridium spark plug, there has a been a noticeable difference in performance on all bikes.

Most of the aftermarket performance/racing CDIs will pretty much fit any bike as long as the voltage is the same as your bike (12v or 6v) and the connectors fit - even then you can make up your own connectors by buying parts from the local electronics store as I did with three bikes.

Keep us posted on your progress.
 

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Yes to the carbon fibre reed valves being better, they flex easier. You can sometimes even get them 2-stage where they stack a smaller reed on top of the full size one.

You see the metal brackets that limit how far the reed valve opens? <- if you take those off and carefully curve them just a little more, the reed will open more and performance will enhance.

If there was a visible gap between the reed petals and cage when you took it apart, you found a serious problem if not 'the' problem.
 

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That hissing noise you heard when the reed cage is removed won't be happening once the reeds are fixed and in place, that's what they do! Reed valves are a one way valve to stop air pressure escaping back out the intake port.

That part right there is critical to your tiny 2-stroke engines performance and the easiest cheapest place to go to improve it.

Note: Make certain none of those screws come loose or break off, because it will go right through your engine and destroy it.
 

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Discussion Starter · #19 ·
Progress update. Today I installed the carbon fiber reed valves and replaced the clutch springs with stronger ones, I still haven't installed lighter rollers. There is a performance bump with the mods I installed today. I still haven't ported the intake manifold since the dremel bits I'm using only seem to polish and not take any significant material off. I'll try to find a carbide bit.

There is no longer a "rev limiter" at 6800 RPM when taking off. Now when I take off at full throttle, the bike reaches at most 7800 RPM and there is no longer a "RPM limiter like" sound, then the rolles fly out and the RPM drop a bit and then it reaches over 7000 RPM again at high speed. The bike feels like its having a hard time reaching and maintaining RPMS over 6500, but it does reach 8000 on a long enough stretch. According to the manual the ignition is advanced 14 degrees at 4000 RPM, doesn't say anything about an RPM limiter or ignition advancement at other moments.

Would a bigger advance help in thoose high RPM? Also the bike is running rich which helps with cooling so I'm not worried about the high RPM. I'm fine with the current top speed of ~70 KM/H, it just feels like it could run better in thoose high RPM, so thats why I'm thinking that it might be due to the lack of sufficient ignition advancement. The engine reaches 6500 RPM in the blink of an eye but has a hard time after that.
 

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The ignition advance would need to be either automated of manually adjusted on the fly, otherwise performance at low speed will suffer at the cost of improved top speed performance.
What is on it for exhaust and what are you allowed to change on a motor scooter in Bulgaria?

If it pulls strong up to 6500 rpm then gear it higher or put taller wheels on it and it will go faster, but speed that you can only attain by going down a hill to make an engine over-rev is not untapped power, it's only an indication that it can spin that fast without flying apart. At some yet undetermined speed it will fly apart, I hope you realize that, & when that happen it could result in the rear wheel locking up solid at maximum speed.
 
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