Balancing Carbies on the Rotax 9 Series Engines

[Perry]

Folks,

I love tinkering to the point that I get something right. I have long being attempting to properly balance my carbies on the 912S. I have tried pairs of Vacuum Guages attempting to balance these with bleed valves etc. only to find that they continually pulse with the engine, especially at 4,000 plus RPM where you want proper balance. I am located far from a Dyno Machine...

Well my days of searching are now over as I think that I have stumbled across the correct gear for a not too unreasonable cost...

I found a 2 or 4 port vacuum manometer manufactured in Northern Ireland of all places. This unit is called a Carbtune II and the 4 port unit cost around 55 pounds plus 3 pounds for postage. The unit works an absolute treat with the original mercury indications now replaced with more modern stainless steel rods - really innovative?

The price may mean that purchasing into a club or pool of guys may be more affordable considering it is seldom used.

It took me about 5 minutes to pneumatically balance my carbies on Sunday with this unit and I managed to get it perfectly done through the range from idle to full rpm (watch the water temp as it climbs steadily through the process and it should be carefully watched).

Details of the unit could be found at:

http://www.carbtune.com/carbdtls.html

The bloke who assisted me without issue was a chap by the name of Sean Morgan (the inventor of the unit)

An interesting note on balancing - I have run the 912S engine in a DTA for almost 150hrs over a period of 18 months since leaving the factory and only relied on a mechanical check at 100hrs. I found that just from throttle cable stretch over this period that the carbies were almost a full 1cm of Hg out of balance. The result of rebalancing has been an increased climb rate of between 150 and 200fpm (on an average of 1,300fpm) and I would shudder to think of what the improved fuel consumption is now.

I found the method described in the Rotax Line Maintenance Manual easy to follow and utilised the normally blanked inlet nipples (next to the idle mixture screw) to attach the tubes of the Carbtune to. NOTE - if you use the nipples on the inlet, do not forget to split the carb balancing pipes and blank off each end - I found prestic to be the best plug for such.

I have also purchased a couple of Colortune Testing spark plugs to see the combustion flame in each cylinder so that I can set mixture appropriately - watch this space...

Regards

[Perry]

Demon,

I have been playing heaps - especially since Paul sent me the new Headsets and Intercom...

Have a word to Sean Morgan at Carbtune - he is a really good guy.

I have agreed to send him some photos and a copy of the Rotax Line Maintenance Manual referring to pneumatic balancing for his site.

I will attempt to take some pics this weekend and post for u so that u can see what we did - really it was a piece of cake and takes minutes to sort out.

Regards

Perry

[Perry]

Demon,

Please find photos attached of the use of the use of the new Carbtune. Use the Rotax method of using the nipple under the inlet manifold and plugging the balancing tube.

[Perry]

More attachments:

[Rudix]

I have been balancing carbs for years on racing bikes.

The equipment (Vacuum) to do it is available from most decent bike shops. There are various models available, some better than others, make sure it has a way to smooth out the pulsing of the vacuum but I have used 3 different models and they all provide for that.

It will make a huge difference to your power, fuel consumption and smoothness of the engine !

Regards,
Rudi

[Massimo]

anybody keen to get one of these for microland? seems logical to buy one that can be used by all. maybe we can store it with the pressure washer. or in thatr cuprboard somehow.

[Perry]

Agreed,

Vacuum units are globally available, however, I have struggled to get a proper manometer for a decent price, hence my original posting. The guages have two distinct disadvantages - pulsing leading to erroneous reading and the contiunual need for proper calibration.

With this unit, they found a really simple means to get around the pulsing - cut the tube close to the mounting on the carby and insert a smaller diameter tube to join the cut section - this makes a huge difference! Note that the cut must be as far away from the gauges as possible. I think the supplied tube is approx 4mm inner diameter and the insert is approx 1 to 2 mm inner diameter.

As discussed earlier with Demon, it is perhaps best to pool money and purchase a single unit for a marauder/murder/squadron/gaggle of trikes. Entrust it to the most perfectionistic member of the team!

Regards

[ZULU1]

I can remember that these were the only toys to buy for Weber 40 DCOES in my youth. The entire range was available at Halfords nationwide, they also made some good timing lights which I still own. The colortune (spelt correctly) were brilliant for SU carbs on Minis.

I have family visit next week, will get prices as they live near Halfords in Telford and post prices etc.

Now one wonders who is going to have most toys:

Alan McKenzie ??

or

Perry ??

Alan couldnt fit a Vee rabbit antenna as he has a topless wing..
Perry has a VXA 710
Alan has a ipac

Perry 2
Alan 1

Ciao Paul

Alan will buy a Carb tune kit...

Perry wil get efi first...