T5 performance problem

Hello all, new to here…
Have just brought a T5 (mark 2) with a SIP performance exhaust on it. Problem is its running really badly. In gears 2-4 it losses power at higher revs and top speed B4 it looses its will power is 50mph. At moment mainjet is a 120. Any advice would help. Thanx in advance

had same problem with my t5 with same set up change to 114 jet also check your carb base is air tight sounds as if you have a air leak other than these problems i had cant fault the pipe


if you are sure everything is bolt well together to avoid air leak, this is what you should have to have a convinient set up:

  • air calibrator 120 is correct,

  • check the mixer tube. Should be BE4.

  • I would try out a Main jet 114 as suggested from somebody else.

  • Main jet 120 for standard T5 barrel with SIP exhaust is way too big.

  • Spark plug Bosch W 3 CC, or Champion N-2C or equivalent.

  • Ignition 16 BTDC.


Ok have taken it for test spin. 43in 2nd and 55 in 3rd. When i go to 4th gear it looses a bit of power and won’t go any better than 50. could this be jets?.

Ok thanx [:)]. Will try that out and get back .


sorry for the exhaust I saw you have a SIP performance.


Just checking carb this morning and had a look at the air correction screw above main jet. Is it supposed to be a 120 or should i change this??. Sorry for so many questions but my last scooter was a 150 sprint and never had any problems apart from pesky cables snapping.

Righto here goes,
Spent an hour looking at scooter. Took out jets and made sure they were clear of dirt and changed main jet from 120 to 118. The carb over all was fine apart from the rubber going around the housing could do with replacing…seems a bit bent out of shape. Changed spark plug as it was black and oily…put in a B8. Took it for very quick test ride. No difference basically. So i did what most would do…I sat next to scoot and had a ciggie. While i was pondering kicking the T5 i noticed the plastic engine cover wasn’t secured on at all. I put some screws i had lying about into the 2 holes and what a difference this made. In 1st and 2nd gear the high RPM problem has disapeared and seems like theres a power increase. I think some of that problems was the plastic engine cover rubbing against the spark plug cover…could this cause such a crap problem?. When the exhaust does smoke its a white smoke. Anyway will go take it on a road i can test 3-4 gears and see if it is any better.

Okies will try both those out. Am picking up a 112 and 114 main jet tomorrow from local shop. Will get them to make sure carb is tight enough etc just in case I haven’t put it back correctly [:rolleyes:]

118 sounds a bit on the large size if its a standard motor and pm pipe. would also be the cause of your over rich plug.keep knockin it down id say to around 112, 114.a t5 atandard is a 110 usually a sep up of 2 maybe 4 sizes up is sufficient.check timming also 16deg. a standard 200 would be running a 118 ,120 with a pm pipe on it!this could be the reason it wont (kick) in 4th gear.common on t5s and goin up hills 2 up;D


as far as the plastic cover it is not possible that will make this kind of difference, but sometimes woo-doo things happen.

I would need to know the pipe you are using, and all your carb set up not only main jet.

This kind of problem you are experiencing on 4th is sometimes and I repeat sometimes, a problem of the carb not set perfectly.

It means for example :
your main jet is ok but you are lean or rich on 3/4 of trottle.
When you are in 3 is ok but when you go to 4th your engine will see that range were it can’t give the best power to go beyond and get to that rpm where the main jet is most effective.



please try to give more info.
It looses power at high rpm, but it is OK at low and medium range?
When you kick start compression feels OK or you don’t find any resistence?
Could be many things.
Start checking :

head bolts are tight.

spark plug is good condition and tight in its place.

carburator fixing screws.

have a look if comes out some smoke from the exhausts.
Is it dark? white ?

After all this let us know.