[Mikko Nassi]

Has anyone tried fooling around with different carburetors on the Rotax Max? Of course not for spec-rotax racing but for example in open-125 categories or just for fun on practice days.

Apparently the Dellorto VHSH 30mm carb for example which I think is used in the Vortex ROK engine should make the engine's performance improve. Also there's a very similar carb to the VHSB QD/QS 34 (Rotax spec carb) - the VHSB LD, which also supposedly improves performance noticeably.

There's an even more similar carb to the VHSB which is the VHSB Racing.. but the main difference I think is that the venturi is polished and the outside is a darker color, so not much there.

Then there are different airboxes - supposedly if you change the airbox to a K&N filter and throw in a 200-250 main jet there's a lot to be found.

Of course all there changes would bring down the reliability of the Max which is pretty much the basis of the concept, which is perhaps why Rotax didn't choose a better performing carb in the first place.

[gerald]

Hi there Mikko, whats the difference/advantage with the vhsb dl carby?

[Mikko Nassi]

Hi gerald,

The main difference in the LD is the atomizer, on the Rotax-spec carb this is marked "FN266", on the LD it's a DQ or DP. I think it also has an oil line going into the float bowl which wouldn't be used in a Max. The VHSB LD 34 is actually popular on the Aprilia RS125 bike - the engine is very very similar to the Rotax Max. Rotax use the VHSB QS/QD in the max which has the FN atomizer which in the Dellorto manual is referred to as a four stroke atomizer.

Here's the particular part in the Rotax Manual that mentions it:

THE ATOMIZER AND ITS EMULSION HOLES
The atomizer, in its simplest shape, is a tube that connects the main jet to the venturi. For this element there are two possible configurations that, traditionally, the engineers call "two stroke type" or "four stroke type".
Some have with a series of holes placed along the whole area and in
communication with the main circuit channel (four-stroke type).

If you take out your rotax-spec FN266 out you'll notice that it has holes along the sides. The dimensions are different though so you can't just throw in a DQ or DP atomizer to see how it goes.

[gerald]

Hi Mikko, thanks for the reply, i already have DQ tube upgrade fitted and it works well ( see tom barth at greenflag motorsport usa ,its called tag uprade). Im using DD2 cylinder slightly ported by madman engerneering usa, higher compression, its running 35plus hp. I have tried 30mm vhsh carb ( race version) it responded better down low but it needed jets in the 150 range which i didnt have, tried open class exhaust but got too loud , im trying to keep it looking original so when i go by someone 20 kg lighter they are scratching there heads. O verall its quick

[RocK]

its running 35plus hp.

wanna sell the engine.

[gerald]

not yet! its taken a lot of R&D work by me, i build my own engines and im going to enjoy it. sorry some blow ups as well, its very hard on your body though with open class rubber

[Mikko Nassi]

gerald is there a category you can race it in or is it just for fun practice days?

[JohnKing6]

gerald is there a category you can race it in or is it just for fun practice days?

My "funny" Rotax (Bernd's german one) puts out 35hp. Alas, no way to run it any more in Malaysia

[Mikko Nassi]

No wonder it flies! What're the main mods on it John?

[gerald]

Just for fun, i used to win in rotax heavy a couplke of years ago, now just tearing up the local tracks

[JohnKing6_]

Different bearings, reported, the piston is reshaped to allow for the porting. Different gaskets. The head is skimmed to increase the compression.

Before anyone gets confused, this isn't the engine I use in the RMC/AMC. It's easy to tell the difference - the funny engine's got a black head.

Driving it is weird compared to a standard Rotax or Parilla. Bottom end isn't to much different from a standard Rotax nor is top end but mid range is amazing - on MRF's it generates wheelspin out of turn 1 at Elite.

Alas I've never used it at Sepang. When he had it Bernd was about .5 faster than me at Elite but he absolutely murdered me at Sepang with it.