T5 Mallossi 172 - Standard Jetting?

Hi All,

I finally coughed for a new Malossi 172 kit for my T5 and installed it this week. It is just as noisy at idle as one i bought second hand a few months ago so seeing as this one is brand spanking new, i’ve just consoled myself that these kits are just a bit rattle-ey!

The question concerns the jetting.

I setup the piston and barrel, changed timing to about 13.5 deg before TDC and installed the main jet that comes with the kit (122).

After running in for a few hours i started to open her up and the motor stated missing pretty badly and basically wouldn’t rev up to 5000 rpm smoothly under load. It would stutter and then finally find it’s feet at the higher revs.

I swapped the main to a 118 that i had in my kit and it seems a lot better … pretty much spot on. The plug looks much healthier too (it was pretty black with the 122.

So… why would my motor not run with the recommended jet for the kit? Does that mean there is something wrong with my carb? Should i be concerned?

Thanks a bunch.


T5 Mk1
Malossi 172
Standard Del’orto 24/24g
120 air, 50/100 slow running, 60 cold start, Be4 mixer.
Simonini pipe

Hi Diablo, thanks for your input re this post.

Just wondering… what do the Taffspeed T5 pipes go for? I think their website is down so i can’t check it.

also… with the ramair filter… which size fits the air inlet to the carb box the best? Do you have to use the ring that is listed in Sip’s site ‚air filter adapter W5‘?

Thanks a bunch.


Anyone got any ideas about the initial post?


Thanks for the tips. I’m back on the 122 which is feeling much safer (!).

What do you mean about setting the timing back to ‚standard‘? Do you mean standard t5? (16deg btdc). It’s currently about 14 deg.

The head has been properly machined by chiselspeed.

Cheers and thanks

Nice one… all this info is really good. Just to clarify… what do you gain by timing your motor at 16 deg rather than the 13-14 specified by malossi? How does it affect the burn and why do you do it?

also… do you think there is much to be gained by going to a 26mm si carb on a malossi 172 with simonini? Would i notice much difference from the standard 24mm carb?


have you got the scooter sorted yet

Hi Diablo,

Just chasing up on the details of the Ramair mod you were talking about (fitting to T5 carb box and junking std air filter).

The biggest size Ramair i can find is 55mm. The box inlet looks about 58mm ish. Do you reckon the 55mm is the one you’ve used?

Thanks mate.


Have had the cylinder head reprofiled to go with the kit as it needs to be done

what set up did you have on your last malossi kit

Thanks Diablo, any info would be great!

Luckily i’ve got an extra air box lying about so can mess with one safely.


I have same kit as you but exhaust is a scorpion i was useing a 122 main i know some people with 120 main and 125 goes to show not all engines are the same. If you are happy with how the scooter goes should be ok but keep an eye on the plug have you tryed a 120 main

All malossi kits seem to rattle. I have measured them out of the box with a 5thou piston to bore clearance. I have heard some of the germans replace the std pistons with cgrade pistons to cure this. You will blow your bike up with 118 main jet. Make sure you do a proper plug chop to test colour of plug. This means holding it at full throttle for a long straight and cutting the ignition. This way you see what the bike was running at then not what its like on mid range or tick over. Put the timing back to std. if your head has been machined properly it will take this 13,5 degrees b4 tdc is well retarded.

I have one at work which i will measure tomorrow and get back to you. You need to remove the box and grind away the lip that secures the original hose though.

yes, thanks Gaz… head sorted by Chiselspeed.


Another thing is try not to let your tank run low if you use a std vespa tank. They dont flow very well at low volume which can lean your mixture on the motorway. As youv’e probably found T5s can drink some!

Ignition timing is complicated and I dont claim to be an authority but generally because vespas have no ignition advance/retard system you have to strike a balance between having lots of advance during accelaration and retarded ignition at top revs. 16 degrees btdc is already reasonably conservative. As an example my lammy TS1 runs 16 degrees. Im not sure but running such retarded ignition could be the reason you were able to run a 118 main jet. Once the bike heated up on the motorway or fast a road it definitely would have blown up.
I tried a 26mm carb on my bike and it seemed to make a slight difference when riding but there was virtually no difference on the dyno. The new vortex carbs do make a difference though.
The best thing you can fit to your bike is a taffspeed pipe. These are the best bar none. You will notice power throughout but mostly mid range and top end.
Another mod I found useful was to junk the std filter and fit a ramair filter where the rubber hose attaches to the carb box. You have to take the box off and take the lip off where the hose attaches. This also cured an annoying lag when rolling on and off the throttle. Once you have done all this thrash with confidence-T5s are the dirty sluts of the Vespa range and love it!

Its my pleasure. I think the Taffy pipes are about £230-240. Not sure about the diameter of the filter but will get back to you on that.
Just got back from Cleethorpes great rally but my brain is broken!

Hi I knoiw it may seem well advanced but you have to realise the std T5 motor was a sports motor. This is why they are so good to start with. I have two T5 and run them both on std ignition timing. One is 172 with simmonini the other a full blown reedvalve. I do a lot of motorway miles and have had no problems. The only thing I found with std carbs is they tend to make the engine pink for a second or 2 when you roll on and off the throttle. My answer to this is don’t come of full throttle.
You can sometimes experience the probs your having if the head has too little compresion but if chiselspeed have done it its unlikely. Have you measured your squish clearance?

should have read have you had-----

Didn’t really run it for long enough to try it out properly cause i was really worried about the noise it was making (!). (i bought it on ebay and thought that it might have been damaged or sommit).