Hyosung Prima Racing SF50 ultimate build


New Member
With some help from a local I will got him to install my AirSal 70cc kit with a torque spring ,kevlar belt, and the new 19mm Arreche Carb.This was the start of the mods which have turned out to be long to say the least.

I could do all the mods myself (I am a machinest mechanic) but we have a familty memeber in ICU in the hospital and I would like to get the mods done before my insurance runs out and I just have no time for some time now.

Well after he installed the kit and went to set the Hebo variator I had installed before i took the scooter to him.He stripped the threads on the crank as the Hyosung variator uses a reverse thread.
Well he ordered a piece of the crank and then pressed them together.Well he broke that crank.

So he never said what he did but I got the scooter back from him after 3 months,running but not really working great.

AS we had the family member still in intensive care on life support my time was more directed away from the scoot and I just put it in the garqage.Well I contacted a shop which would look into the issues and fix what ever it needed.

After talking to them I decided to just pull off the AirSal kit and put on a Malossi 70cc kit.Got me lets do it.

So I took over the scooter to them via a 2hr ferry ride each way.Yes no other shop local would work on the Hyosung.

So i dropped it off and we talked , new 70cc Malossi kit and check the crank at the same time.As I had installed a Hebo Variator and also brough over 12 sets of 3 weights for the variator plus the new Technigas R exhaust I had also installed not much was needed to get this scoot tuned.

Well as we looked into the scoot that day we talked about replacing the front forks were well worn out (6100k only)

So lets do a new front fork as well as brakes.

Well after several months and I do mean months here is the progress for the 2005 Hyosung Prima Racing now 70cc.
Hebo variator below

Stock exhaust

The stock intake on a Hyosung SF50 very small opening so changing to a larger 19mm or 21mm carb is still restricting the engine greatly.

Here is what the front fork looked like with 6100 k on it remember.

Here is the new Malossi front forks which were sent to a machine shop to have them machine the shaft to match.

Here the new Malossi front fork is installed with spacors as well as we also put on new Dunlop treads.

Also notice the new brakes need to stop this new ride quicker.

Well I ordered a NEW Yasuni R exhaust as the Malossi cylinder kit we picked also was selected for altered timing and was not a bolt on.So that 70cc kit was sent to a machine shop for the outer coller of the cylinder to be machined down to fit into the block.

So the Technigas R exhaust only weeks old

was removed as well and the Yasuni R (for a Katan) was ordered with several other parts from scooter attack in Germany and sweet parts arrive here in Surrey BC close to Vancouver in 3-4 day.I have ordered from them several times and all orders are here in 4 days at the latest.

Custom bracket was made as well the exhaust was cut and modified to fit as it was ordered for a Katana so it was not direct bolt on.

Below is the frame during the mods

Here is the stock intake and look at how small the port is very restrictive.

I even tried to port and polish the intake but really the stock intake is very restrictive

and the best mod is to make a new one

Scooterattack doe offer a intake but is never instock here you can see how small the factory intake is.

Finally here is the scoot on the lift with just the final stages of tuning from them.

I will put the gears and new rear Bitubo rear shock in myself.

Well after 8 months I still have no scooter and I have been promissed it over and over every week now for almost 2 months.

So I have about given up on the shop plus if my scoot will be done correct, yes I have even went to the shop owner as he also owns the Mazda dealer above this shop.

So I was about to walk away and meet them in legal action.

At that point they spent every minute on the scooter and finally are getting it done.

I may even have it next week.But really with dates being told over and over it will be ready I will believe it when I see the scooter sitting in my drive way.

Once I get the scoot back I will put the gears and other parts I have in as well as update the complete parts list which went into the scooter.
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New Member
its not the nicest looking scoot imo , but its getting there, ecspecially the forks and disc's they look very nice!

Yes the process has been frustrating to say the least with the build taking so long I had asked for the shell pieces to do body work and I could not even get that from them.So once I get it back i will start the body work also a new front with twin lights not the single light.As I drive at night even with a 35/35 halogen bulb the lighting is poor.

Yes it is coming along


New Member
Not sure if they removed the restriction on the Yasuni R.

The place that has done all the mods has taken months and I do not dare ask to check it out as they will have it till next Christmas.

Once I get it back I will look into a lot of things.


Need tuning? PM!
haha! You can do that yourself. Remove the exhaust and check at the header from some kinda restriction.

Also in the in muffler there might be a restrictor. Proably still in.


Sprinten of Racen
good project

but one hint, get rid of the oil pump , mix your own gasoline

the oil pump gives you oilstains on your engine and its parts, i removed it on my honda mb aswell, never had to clean my engine again(only sand and regular filth ofcourse, but no greasy stuff anymore.)


New Member
I will get more of an idea of what has been done when I see it.


Here is my scoot and as they made mention today via email.

The front fender needs to be fit, the speedo they can not get to work now! plus they are now installing a digital speedo (what brand I have no idea)

Once I get it back I will be checking out a lot of things on the scooter including the exhaust.

But since it has taken almost 5 months to get to this point I do not dare ask for them to look into one thing??

It is like this once a single issue is found anything they stop work on the scooter till that is fixed.
See only one item at a time till that item is fixed.:rolleyes::rolleyes::rolleyes::rolleyes::rolleyes::rolleyes:


New Member
:r No they just got it going.
As a fact my gears are sitting here once I get it some time in the next month or so.

Seems that they only work on a single item at a time till that item is done.If they get held up with one thing all works stops completly.
This is why well over 4 months and still going.


Need tuning? PM!
That's really crap :(

Don't know where you at? But couldn't you find another dealer?
2stroke scooter = 2stroke scooter. Must have cost you a fortune about now :/


New Member
Well I have threatened them with court this scooter mod has gone on so long.

Still they drag it on week after week.

Like i said ,I have no idea what the heck is going one.But if one thing stops during the day,then it is held up completly.

I have been more than patent and till last week never seen a picture nothing.

Would I recommend them heck NO.Will I see my scooter well in the next few months
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New Member
Update as of June 6, 08

I have had it with the scooter

Well more insult to it.They shop who did the mod dropped it off this am.

First I go to attach my carrier and they removed the mounting bracket and it is not there plus mounting hard ware for my windshield as well.

I fire it up after going to get insurance for it.
I look NO OIL. you got it.
I shut it off drove to wester power sports got 1 ltr well grab 2 ltrs of 2T and drove home.
Pored away 1 ltr and 300ml into the oil res.

What damage is done I have no idea.

So what to do now???

Tear it all part.!!


New Member
Update as of June 6, 08

I have had it with the scooter

Well more insult to it.They shop who did the mod dropped it off this am.

First I go to attach my carrier and they removed the mounting bracket and it is not there plus mounting hard ware for my windshield as well.

I fire it up after going to get insurance for it.
I look NO OIL. you got it.
I shut it off drove to wester power sports got 1 ltr well grab 2 ltrs of 2T and drove home.
Pored away 1 ltr and 300ml into the oil res.

What damage is done I have no idea.

So what to do now???

Tear it all part.!!
mechanics like this anger me

you should be alright if the light had only just came on and you stopped riding, the light is there to warn you, you have little oil left
and considdering you stopped it should be alright

i hope for you that it is, judging on the place you've been taking it, it's gonna take another 3 months to sort that out if its broke :/
Mate this guys are just ripping you off
its sounds like a piece of shit in the video
Its quite clear they have no idea what they are doing


New Member
Mate this guys are just ripping you off
its sounds like a piece of shit in the video
Its quite clear they have no idea what they are doing
Well I got it on Friday and went over it on the weekend just a bit I have not opened it up completly but will now.

-exhaust mounted to the head with one bolt
-pinched fuel line
-cut vacuum line and i supplied all new fuel line,vacuum line and filters but they left all the old lines in place
-linkage hitting on the seat bucket
-plastic panels missing screws or not tight
To boot
I supplied
-all new lines plus new filters like I said above(not used)
-gears (did not fit)So I have 4 sets waiting till Wed when i am going to see if any of these fit then sell the rest on Ebay.
-new Arreche 19.5 carb they used another brand I have not seen but will post picts tomorrow
-Doppler filter they did not like so put on another Doppler!!
-12 sets of rollers
-Kevlar belt
-Clutch 110mm did not fit to small they said (so I have a 112mm sitting now to install now I have my hands on the scoot)

I got the scoot Friday dropped off, they did not have a box with all my parts old and new, screws for my windshield, rear carrier mount nothing.

Not even the instructions for the new digital gauges.

Just here is your scoot good luck!!

Why well I threated them with court and i ment it.

So I got it back after almost 5 months and no invoice.

Yes they did all the mods I purchased parts , never paid for a lot and will not pay a thing as the owner has said to me.

So that I can swallow but finding issues like these has made me shake my head.

And yes you are all right.It sucks!

Off the line walk for 10-25 feet then it pulls like a bugger, but at stops it has a large hesitation to say the least.

So I will get into the set up once I see if the gears fit plus the rest of going over on Wed.

Will I recommend them. Sure I will
Is that your bike in the Youtube video? If it is, it looks and sounds a bit like something isn't right with your variator or clutch. It seems like the weights are too heavy or the clutch springs may be too stiff in that video clip. It sounds like it's getting way too many RPM before leaving, unless the drivers are just playing with the throttle and brakes.

I live about 10 blocks from a Hyosung dealer. I'm not sure where you are, but if you are ever having a difficult time with your scooter, I may be able to help you with a few things. I may even be able to get you a factory manual for it.

That's a total bummer about the mechanics. I restore cars for a living and there isn't anything I can't do to a car or a bike. After working on my Aprilia, I can't understand how a mechanic can procrastinate when doing anything to a scooter. They are so simple, it's almost scarry.

It sounds to me like they just didn't have any respect for you or your scooter. And weather or not anyone else likes Hyosung bikes, it's your bike and people should respect your wishes at a shop.

The list of modifications you mentioned would have taken me about two days to do, if I was just poking around and messing with other things. If it were my job, I'd have it done as soon as I got the parts and ready to deliver the next day after testing everything.

If I were you, I would demand that they pay you for the damage and lost parts done do your property. And if they try to throw their "free" labor in your face, you can tell them that you wouldn't pay a starving woman with a crescent wrench to do that kind of damage to your bike.

Never take your bike back to those people and go through every single thing that they did yourself before accepting any of their work as being okay.

If they left bolts out of your exhaust and everything else, who knows what else they forgot to do. I would take everything apart that they did and inspect everything.

Let me know if you need anything for your bike.
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New Member

You are 100% correct and I find things day to day to add to the screwed up list.

To make this all the worst.I am a mechanic,machinest but when this started I had my family member who lived with us on life support for just over 4 months and I just said here is my budget to it RIGHT.If we go over the budget with in reason call me.
They never called me once in 6 weeks so I called to be told my scoot needs a kit!!The Airsal kit had been installed wrong by the previous person not myself.WEll that was what got me to drop off the scoot and was our first list of mods to do was the Malossi 70cc kit as I was going to order a Polini kit from scooterattack and they said no to use a Malossi.

At that point i just said use the Malossi as I did not have the time nor mind set to look for parts.

My family memeber is now in a nursing home as of April but we have all those issues to get in place and the scoot was low on my list of wanting it back at the time.

So I got in the mood for the scoot so I started asking when it was going to ready and I was told in 5 days it would be ready.So I call back in 7 days to find out they had not started on it.And this just played over and over for months.

Add more to this i tore my lower back out from 3 accidents and can not do any work on the ground any longer.

So I threated court to the owner who owns the Masda dealer as well which this shop is below.

I am contacting you again about my 2005 Hyosung Prima Racing scooter which was having a lot of modifications done at Scooter Underground, which we talked a few weeks back in the store about.

With the last email from you, T*** and R*** had given me a time frame of when the scooter was to be finished and then the additional week for testing and tuning to make sure the scooter was ready for pick up.

I hadn’t heard from T*** or R*** so I called on the Friday before the date I was supposed to pick up the scooter to find out that it still wasn’t ready. Had I not called, it would have been a $125 ferry trip there and back for nothing. If a time has been arranged for a customer to pick up their scooter and it wasn’t ready I would expect to have either a phone call or an email to advise of this so I don’t waste my time or money. This has happened on 3 previous occasions where I have called before I come over to find out that the scooter isn’t finished yet. And it seems that each time I call to make sure my scooter is ready I get told that it’s just going up on the lift. I’m starting to wonder if my scooter has been worked on at all.

And to top it all off, yesterday I got a call from R*** stating HE broke a part on my scooter, AND HE IS SICK OF LOOKING FOR PARTS for my scooter, which do not fit. So I am supposed to find my own replacement part that HE broke and let him know what it is. I find this attitude completely rude and uncalled for. If I was to take my vehicle into a dealership to have repairs or work done to it, and the mechanic broke a part, and then phoned and told me that he broke it and he was sick and tired of looking for parts for my vehicle and told me to find it and let them know the part, I would expect that mechanic would be fired. This is very poor customer service.

Now, I am not saying to fire R*** but I have just about lost my patience with him and Scooter Underground. I took my scooter to you to have some modifications and repairs done. I know that these types of modification can take time, but you’ve had my scooter for over 3 months and the lack of customer service is causing me to wonder about my decision to take it there.

I feel I’m being lied to each time I call to find out the status of my scooter. And then when I do call I get barked at about wrong parts which do not fit, that R*** has asked me to source for him. I’m not the scooter mechanic, he is. He should know what parts are required and where to get them or should be locating them himself.

From the day I dropped off my scooter I said to R*** fix it and make it right using some parts I had already purchased and a budget I had set in place. I understand that some of the parts that I dropped off may not have been correct and that the last mechanic had made some modifications that were incorrect as well. I told R*** all about that. I have been upfront with R*** regarding all previous work that was done on my scooter. He knew it was wrong as that was why I was taking it to him.

I would have expected that he would have gone over the scooter, let me know what parts will work and what parts won’t and then discuss how to proceed. It seems that weeks go by and I call back (notice it’s always me calling about my scooter, no communication from Scooter Underground what so ever) and I get told that, for example, the cylinder kit on my scooter which was installed previously had poor port timing. OK, but when I dropped off my scooter, R*** said we were going to install a Malossi 70cc kit, so I don’t know why he is chasing down what’s wrong with the old kit several weeks into having my scooter. Just install the new kit and get it going.

I also made note when I dropped it off if it needs a new crankshaft to do it, as the other person who worked on the scooter before R*** pressed a new piece onto the old crank. This never got taken in to account until it was too late and the motor was assembled. When I contacted R*** to see how the scooter is going we discussed the crank again. He said sorry too late I have it all assembled but I think the crank will be ok. NOTE THINK IT WILL BE OK.
I had said in the first 15 minutes at the shop if there is any issue with the crank to REPLACE IT. I ride the scooter at 11 pm at night and I want no issues. And being told the crank “should be OK” has left me with concerns about how my scooter is being put together. R*** even made comment saying he was not sure how it will do as the other person messed it up. SO WHY was there no call to myself the customer, when the scooter was at that point. Again, a mechanical shop would call the customer at this point and discuss that the part should be replaced.

I now feel that as of today my scooter is still in pieces and far from being done, let alone having any time put towards testing it for a few days before I pick it up. I was given a date of May 10th to pick up my scooter and again, this date will pass. So another weekend goes by were I was told the scooter would be ready and it is not.

I have now been without my scooter for over 3 months. I have been barked at, lied to, and now am questioning the condition and the quality of the work being done to my scooter. I don’t want R*** to rush and just slap my scooter together so I can pick it up tomorrow. I want my scooter fixed right.

I’m asking you to please advise me as to when I will finally be able to pick up my scooter and also to monitor the work going into it and ensure it is to your satisfaction as the owner.""

Response from the OWNER on May 9

I have met with T*** and R*** this morning. I can assure you that your scooter is at the top of my priority list. We are having the part that was broken custom machined and should have it by the end of today. R*** will then assemble the scooter and test it. Once it has been tested to our satisfaction, we will deliver it to you in Vancouver. I will let you know on Monday when I expect this will happen.
This process has been frustrating for everyone and I appreciate your working with us to get to an acceptable resolution.
We are very good at what we do but sometimes we do not properly anticipate the scope of a job.

We have spent far more time working on this scooter that we anticipated and you have spent far too much time waiting for it. In this period, there have been lapses in communication and I am here to assure you that there is no inaction on this matter here at Scooter Underground.

I want you to know that I have reviewed your concerns in detail with my staff.""

My response on May 20

OK as I never got a email last week from you about the progress which you had said would happen and I believe T*** called me on Thursday to say that they were working on getting it over to me Tuesday. Again Tuesday passes as does the time with no contact to say if my scoot is coming over Tuesday or Wednesday.

I am done.
Your company has roasted me inside and out. I have crohns and have spent 4 days in the hospital over the last month due to the stress this has caused me plus 5 days of lost wages as I was bed ridden with my crohns from the stress of this. Let alone a financial loss for my scooter plus all the parts and new parts I had ordered as well.

I am not going to contact you again about my scooter as it is a bloody waste of time with promises which never see the light of day.

Scooterunderground can put a lean on my scooter for the repairs and let me know the court date will be. I will be there along with a ton of emails plus notes I took ever call I received from T*** and R***.

I have been more than patent with the process but scooterunderground has ripped me off for a scooter with my modifications.""

And th


New Member
As far as the help yes Dave i could use a little as i said I am a mechanic but setting it up once I go threw it will be 1st time and the list of mods less a new crank is very long.

I am in Surrey BC Canada
That sucks about you going back and fourth with those people over something as simple as this bike. That also sucks to hear about your back and your relative in and out of the hospital. I hope everything goes smoother for you in the months to come.

I would definitely replace the crank. I'm with you on that. I'm sure you are familiar with thread repair, but I'm not exactly sure how well something can be pressed on to a crank that spins that kind of RPM and still hold a decent balance. The variator is only held on as good as that repair was made, however it was executed.

Who knows how accurate that repair was made and how accurate your variator part spacing and alignment is now. I would almost put money on the excessive RPM being something fishy with the variator and/ or crank, but I would definitely check those clutch springs as well as the belt.

If you'd like, I can call the Hyosung shop down the street from me and see exactly what they recommend or have for that bike, as far as a crank or any other parts go that you need. I know they have variators, plugs, kevlar belts, exhaust and all kinds of maintenance parts in their case for those bikes and he had more in the shop area.

Even if it's a piece of missing hardware you need, I might be able to get it for you from these guys. They were very honest with me and very down to earth when I went in.

He didn't try to sell me a bike, even though he was the salesman. He did offer to sell me a set of variator weights if I came back in with my setup to get the correct stuff for my bike and he gave me a few random springs and other parts that I was missing.

I'm not sure if it matters weather or not you have a standard or reverse thread on that crank, but I'm certain that these guys down the street would know what to get for your bike and what other parts (and part numbers) would work for your bike.

They had that exact model in that color on their showroom floor. I recognized it the second I saw the pics you posted.

I can see about getting a manual for you, or if nothing else, at least one on CD that you can use. They would probably have one and if nothing else, I know they can get you one.

I'll call them today (wednesday) and see what they recommend for a crank or any other parts you may need.

Let me know if there is anything specifically that you need and I'll see what I can't get for you, including that manual.
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