Aprilia SR 125 to 172 (Malossi) also, vibration issue and Mikuni VN20 carb?

Hi all,

My dad has an SR 125, been trying to convince him to 172 it but he wanted to leave it standard. For that reason I bought my own and plan to 172 it.

However I have a few questions, I thought all SR 125's had the 20.5mm Dell'Orto carbs on them but turns out mine has a Mikuni VM20 360... I believe I will need between an 86 and 92 main jet but thinking of plumping with an 88 (stock airbox, air filter element and exhaust). Is all I need an 88 Mikuni jet and I'm good to go once the 172 Malossi kit has been fitted or does the needle need adjusting too?

I plan on getting a new air filter element, new plug, new 12.5g stock rollers and a drive belt for maintenance purposes before I buy the 172 Malossi Kit and have it fitted.

Finally, I have an issue at the moment with the moped violently vibrating when ridden / rev'd. It is fine on the stand at tickover but when you ride off the vibration through the handlebars is absolutely horrendous. It would be tiring to ride for more than five or ten minutes due to how much the vibration fatigues the arms, that's how severe it is... It's like riding a different ped compared to my dad's SR. Any ideas chaps?

Disclaimer: I do not require a lecture on legality of 172 kits, I have had a full bike licence for over seven years now and the kit will be declared.

 

Check the silent block and engine mount bushes in the engine cradle end. Typical tell tale is as you start to pull away you can feel the back end start to kick to one side.
As for jetting, see how it goes first with the kit, then re-jet to suit. There is no one sure fire answer to jetting as although all bikes are the same, the seem to respond differently.

 

I always ran with a 90 main in the Mikuni carb. As Stevep says, check bushes. Not sure if it uses big spring on engine like the typhoon?

 

Cheers chaps,

Can anyone tell me why some of the Malossi kits come with no cylinder head..? Does this mean you can bolt the barrel to the stock cylinder head..? If so are there any issues with this such as inferior cooling / heat dissipation or will it be fine?

Ta

 

DON'T fit 125 head to 172, there sold when they've been seized and only barrel needed, compression will be to high and piston may hit head.

 

I'm not sure I understand what you mean matey? I wouldn't be using the cylinder head off of a different bike, I'd be using the cylinder head off of my own Aprilia which runs just fine, then I'd be using that with the new Malossi Cylinder. Does your advice still apply in this scenario..? The Malossi cylinder head and the stock Aprilia cylinder head look very much the same?

 

if it was me I would buy a cylinder kit that comes with a head.

i dont know if the cylinder kits without heads are cheaper, but remember the old saying... "buy cheap buy twice"

 

The 125 is 55mm diameter, the Malossi is 65mm. it will fit but it's compression will be to high. Buy a complete kit.

 

Thanks very much matey, I will ensure to get a full kit in that case..! Is there anything else you'd recommend doing whilst the cylinder is off?

 

Check main bearing (up down) on crank, fit new mains if needed, carbon reeds and crank seals, oil belt, drive belt. If your on std 125 gear box you will need to replace with a 1800cc box or a malossi gear kit. It will run perfect with a std exhaust/ airbox !

 

Appreciate the check-list it's greatly appreciated. Some people have said stock gearing is fine for 70mph or is that misinformation? Seems totally ample to me considering I'm sticking with the stock carb? Surely getting a Malossi secondary 18/41 would be more suited to putting a PH 25 framed carb on as well..? 75mph is way more than ample for my use (it's going to be a bit of a toy). I'm more concerned about the 0-40mph performance which I imagine will be faster with the stock gearing? Please let me know if I'm misinformed.

 

Stock gearing will be fast 0-60 for sure but you will find it revving it's bits off ! std carb will work fine, I think you will want to fit a 180 gear box after a while, it will pull it easy and you'll pass 70 on 125 gears with ease. If its purely town the leave std gears and carb, pipe. I had a 172 typhoon, 180 gears, everything else std and it was superb !

 

Hi all,

All the parts have arrived and I'm ready to take it to my bike mechanic tomorrow.

I have a question, the Malossi kit came with a single set of carbon reeds... Do these bolt on in place of the standard reeds? Does anyone have any advice for the reeds as I am yet to look at the reed black as I know sometimes that are multiple sets... Are all sets replaced by the single Malossi set?

I know this sounds a little confusing but for example in my younger days I had an RS 125 and that had 2 sets of reeds which is where I'm getting my info from?

Cheers lads, very excited..!

 

Just fit reeds supplied, heat cycle the barrel a couple of times before giving it some ! Piston rings fit one way, it's easy to get them upside down, give the crank case a good glug of 2t oil to help it on it's way. 90 main jet, have fun !

 

Cheers Marsh, sorry for the awful sentence structure but I was rushing. I'll relay the info you've gave to my mechanic.

Bit off topic here, is it just my experience or do people in the bike scene, particularly mopeds seem to lie about 'top speed' a lot? It's not something that interests me a great deal but I find it odd how people claim such crazy figures.

For example... Both mine and my dad's SR 125 do 62mph indicated on the flat (I only weigh 11 stone piss wet through)... That's about 55mph real time tops. Not sure where people claim they get 70mph+ out of a stock 125, super confusing... I'll be happy to be getting a true 70 mph once the new Malossi 172 has been fitted yet some people with rollers and 70cc kits claim to be doing this tough speed which is just bizarre when my engine is going to be two and a half times the size just about getting that kind of speed..!

It was the same with my RS 125 when I was 17, I had mine full power (power valve and unrestricted CDI), swapped to the VHSB 34mm carb, airbox mod, large reed block and carbon reeds etc, did 95mph indicated, which was probably 88 yet people were claiming 120mph on their RS 125's and Mito's yet I'd pull away from them slowly across the whole rev-range (they had 28mm carbs)... Really don't get why people lie about top speed so much.

 

AH yes, the Malossi tuned speedo ! The sr and typhoon 125 were booked at 62 mph like most 125's, my std 125 would just do 60 ish on a nice damp day, slower when it's warm obviously. Your 172 will do 70mph easy. As you know the wind resistance is expenecial to power, I'm not sure exact figues but eg , 1-2 mph faster 4x the power needed ! Someone on the site will know. Ask around for a 180cc gearbox as you'll be able to cruise at 60 without trouble. See how you get on with it.

 

Okay so he's started taking it apart and he's said my exhaust is rotten and I need a new one...

Do Gilera runner exhausts fit the SR?

I've found this Technigas one for a reasonable price, are they any good and are they stupid loud or..?

https://www.ebay.co.uk/itm/Exhaust-...926432&hash=item43f8d87f67:g:WZkAAOSw1uRZr2bH

Please let me know

 

Hmmm, guess what I have sitting around doing nothing at present................

 

Also, where are you from? Just in case you're in the same region or not too far away, I'm in the North West.