Polini 130 TwinInduct. w. Zirri

[MiNiKiN]

I spent a couple of hours on the A43 yesterday F**k

I have a Prima with 130 Polini twin induction and 19mm Dello SHB (sub-optimal I know) plug and play. Zirri Silent on the other end.
SH config: 47 idle jet, and 95 mainjet. No airfilter! Ignition is 19deg advance.

It runs quite good up to 3rd but it decelerates in 4th. Started missfiring and knocking in 4 th yesterday before she gave up

Where shall I start to patch the problem before I fit the 28mm Mikuni (havent got a manifold handy) and do the milling on the spare engine casing?
BTW: Squish gap is 1.2mm

I thought a bigger main and idle jet might help (105 / 50) and setting the advance to 17deg. Latter will probably worse the lack of power when shifting in 4th

Your Hints, please?

Cheers M

[Juan]

The jetting does sound a little lean to me.

The 4th gear problem is almost certainly down to the fact that the Zirri pipe is to revvy for your set-up,perhaps try a standard Banana to see if that improves the drivability.

And what sort of knocking?Is it from the top or bottom end?

[MiNiKiN]

No - I stick with the Zirri - just got rid of that gay banana!

Knocking was from combustion chamber - don't know the english term - means explosions instead of proper burning of mixture.

I agree with you - the mixture was way too lean - think it was the sum of the too small jet and the fuel level in the float chamber sinking too much under full load. SHB cannot cope with that massively sucking pipe

I have removed the barrel yet - no damage. Did some porting on it and my spare engine. Cut the manifold so I can weld an adapter on it, to take the Mikuni VM28. I will come back to you when it comes to jetting. Have never done one like that!

Cheers M (covered in aluminium swarf)

[Juan]

Knocking was from combustion chamber - don't know the english term - means explosions instead of proper burning of mixture.

Pre-detonation.

Usually caused by over advanced ignition,overly high compression and/or fuel starvation.Although I'm expecting Oz to get technical on my arse and make me look like a Muppet again. ;D

[mikea565]

i think there may be some similar 130 and 28 Mikuni setups for you on this site.

sfs.alexander-hepp.de/

[MiNiKiN]

Thx mate,

I've checked that SF-Setup page already - unfortunately they all use Mikuni TM whereas I have a VM from an RD or sthg like that. But it seems that my current 240 Mainjet is good to start with. I now have to get some mainjets, idle jets, 3.0 slide and different needles.