T5 MK1 tuning problems




Recently put my 1988 T5 on the road. But having problems with the tuning. I have never set up a carb before and also had issues with setting the timing. Any help would be much appreciated as now confused as to the next steps.


The bit below are what I have done and the basic set up.


Polini 152.


Simonini Exhaust.


Standard crank.


Fast flow fuel tap.


Original CDI unit (Have tried a different one no change).


New plug HT lead and spark plug cap (Tried alternatives and no change).


Original stator plate.


R9357 Dellorto VHST 28BS carb with the following standard jetting.


  • 55 Slide
  • D41 Needle
  • 269 HE Atomiser
  • 142 Main jet
  • 35 S Idle jet
  • 60 Choke jet
  • 270 Float valve
  • 15760.5 floats – 3.6 grams


The scooter starts fine and idles ok although a bit lumpy. Changing the mixture setting seems to make very little difference unless two or more turns are done. Acceleration is good but does misfires a little. Decelerations is a big problem as it backfires and also looses power until at a slower speed and revs are back up. I can lessen the problems by keeping up the revs slightly when slowing but never had this issue with any other scooter i have had.


I attempted to set the timing today but not sure I have it right. I tried to used a stop screw to get a reference point either side of top dead centre. I then halved the measurement and measured out 16 Deg. Used a strobe but the markings were way off (stator was aligned to markings at this point. I then used a Vernier to get top dead centre and marked out 16 Deg. Checked with the strobe again markings still out and to get them aligned i had to move the stator ten millimeters advanced past the casing/stator markings.


Tried a run with the new timing setting and found the back firing was worse than ever. The fuel consumptions seems exsesive but a long time since i have been on the road also not sure i can trust the gauge yet.


It feels like a carb issue but don't know where to start with the jetting (the jets arE standard when i bought it from SIP).


Sorry for the long winded post but thought i would get every thing down.




Appreciate any help Mark







It is the best to call the sip tehnical service to give You the update on what jets to use.

there are not many combinations with vhst 28 carbs so it is very hard to find some reference point.

 142 is way too big for this setup, and also the atomizer seams big.



Thanks for the reply I will give them a call. The plug is wet so definitely need to down size the jets Etc. I just need to get a reasonable place to start as never tried to match different combination before so unsure what influence one change will make on other jets in the carb.