Urgent Jetting Advice on T5 + Polini, ta

I’m running a Polini top end on a T5 motor with a flowed crank, Simonini and 21 tooth Clutch. I’ve the 24mil carb on and it’s way too weak top end. I’ve now got a 130 air corrector, be2 atomiser and a 150 main jet, I’ve tried a 120 ac but it’s way too rich and I just don’t get the delivery.
I know it’s a huge main jet but it seems to need it and it’s still to lean top end as it likes to seize.
Timings on factory setting and the plug is well hot.

I’d appreciate some help as Hollands getting way too close.

Thanks, Billy

Apologies, yes a PHBH. Probably go for a 28mm, if your sticking with the simonini, if your thinking of changing to a good expansion, then go for a 30. Possible starting point on jetting something around a 125 main, 264 atomiser X2 needle, maybe an X7. Obviously youll have to play around, I would expect the atomiser to be rich at that, also the main, but it wont be too far off, down one size or two on each. Come back to the board and let us know how you get on. On the subject of a fuel pump, Iwouldnt expect you to need one. The reason a pump is used is because as the fuel level goes down and reaches half tank, the level of the fuel is equal to that of a big carb sat up on a reed. and therefore flow stops. Flow RATE is possible up to and beyond your state of tune.

Hi Billy, if you want save a little cash, a Cagiva Mito Evolution (possibly earlier ones too) uses a PHBH 28mm Dellorto, so ring round some breakers, all you would need is a manifold and rubber clamp then, seeing as you will need to change most of the jets inside the carb anyway. I compared it to the 30mm on my 210 and it is the nearly the same as you will buy in a scooter shop.

Sorry mate, I was a bit insensitive there as I had read on the board you’d had an operation. I hope you’re sorted soon as and back on them 10" wheels. Thanks for your advice, much appreciated (and Adrian…), I will undoubtedly be back…


(It was very fucking good though [:D] )

Cheers Beerace, theres no autolube on as it’s all been nicely sealed off.
I was advised to leave on the standard carb just alter the jetting and also to leave the timing well alone.
Hmmmm… After all the f*cking around I’ve been doing I may as well go for the bigger carb, sort the timing and see how it goes.
Thanks again, no doubt I’ll be back soon.

After a lenghty consultation I ended up ditching the Simonini and getting a pipe made up to suit the set up(luckily my mate does it so it didn’t cost an arm and a leg). On his advice I kept the 24mm carb. I’m now running a 120ac Be3 1/2 (an LML one) and 145 main jet, coupled with the pipe I got to Holland and back, a round trip of about 700 miles with no problems at all, It was sitting at 60mph all the way with loads left in it and it certainly shifts.
The timing again on his advice was left as is, he’s a similar set up but with timing at 17degrees!(He touches a genuine 80mph in a PK frame and it’s very reliable)
The pipe seems to have cured pretty much all the difficulties, one thing I’ve therefore learnt is most certainly don’t skimp on this.

Anyone want to buy a used Simo?

I’d be interested to see what you think about the timing set up as I’m still not 100% despite the sucessful run.

All the best,

Oh yeh! Holland was superb…

Right 28mm it is then, I’ll let you know how I get on.



Do you mean the 26mm shb or a phbh 28mm/30mm? Would the 26mm offer enough in the way of cooler running? If you’ve any pointers for a starting point for the jetting that’d be great. I’m lost as to how I got up to a 150 on the 24mm but as I say it was still lean, and I can guarantee it’s a solid rebuild with decent parts so I don’t imagine any seals are duff.

Will there be any need for a fuel pump, I thought my problems may have been fuel starvation although theres a P-range tap on it and that should be adequate.

Anyway, any help is much appreciated…


I’m seriously considering a better pipe in the near future(Autumn/Winter) but for now it’ll be the Simo, the wifes going mental as it is and as she does my accounts theres no escape. So with that in mind would the 30mm be too much for it or would I have to use the Simo with the 28 then upgrade to a 30 when I get a pipe made?

Oh yeh! The ports have been polished etc… but not drastically altered.


Wish I could do the likes of holland. If you got to holland over 700 miles return, you should be ok on all settings. If it aint broke, dont fix it. From that sort of run, Im not going to suggest anything, cos its obviously ok. [;)]

This set-up running on standard timing settings will sieze/hole a piston. Retard timing by min. of 3 degrees, probably need 5. Then retry. Sorry , no experience with this type of set-up using a std. carb so cant help with figures, but I would have left air correctors, pilots etc. well alone until sorted the main jet and timing. Unlikely to do damage with the minor jets incorrect whilst setting up the serious stuff. It’ll take a lot of sorting out using a standard carb. You may get more accurate carburation, and a quicker scoot if you junk it and go for a shb carb 26-30. No doubt though you want to keep the autolube. Decisions, decisions

IF you are going to get an expansion then you can run with a 30mm. BUT , you will find that youll lose a fair amount of ‚grunt‘, and be more revvy. A 28mm should be able to fuel even a wild tune, and retain grunt, so Id probably opt for that. With the right pipe, you should get your ‚grunt‘ back if using a 30mm. You may be happy with the simo but there are better out there, and thats down to personal choice.