New engine

[banaari]

Hi all,
sometime in the next few months I'll be installing and commissioning the engine (a brand new Rotax 503 DCDI) in my Gyrobee. So, some questions for people who've been there:

Apart from plumbing, wiring and control installation, what sort of carburettor setup & adjustments need to be done prior to first start and run-in?

Any hints, tips or tricks around balancing twin carbs?

I assume you're supposed to do the run-in as soon as possible before doing anything else, other than getting it to run in the first place?

Anything nasty lurking in the undergrowth waiting to bite me I should know about?

The beast is intake-silenced and after-muffled, turning a fixed-pitch Prince p-tip propeller via a 2.58:1 B box. Planning to use primer, not choke.

cheers,
John

[mceagle]

Re balancing Rotax twin carbs.
It is definately accurate enough to do it without any special tools. Ensure that the cables are slack, back off both idle stops so that they cannot hit the slides and make sure that both slides are sitting right at the bottom. Then adjust both cables until they are just touching the slides at the same time. The slightest throttle movement should see both slides move at exactly the same time. Then open the throttle and both slides should touch the top at exactly the same time. (If not then the cable splitter is faulty) Move it through its range a few times to make sure that the cables are fully seated and recheck and adjust if necessary. Finally, turn in the idle speed screws until they just touch the slides, such that any further turn starts to lift the slide. From this point on both the idle speed screws should be moved exactly the same amount. Turn them both in approx one full turn. Start the engine and allow to warm up to operating temperature. Adjust both screws the same amount to achieve the required idle speed. (Approx 1800rpm or just in the smooth band above the gearbox chatter). The idle speed may have to be readjusted after flight.
The Idle mixture screws should also be adjusted equal amounts out from their seated position (Approx 1 1/2 turns, or as required to achieve the smoothest idle with a hot engine).
Carby vacuum testers are not practical for tuning Rotax twins because there is too much blowback to give accurate readings.

[Chuck Roberg]

Here's a list of technical articles on Rotax engines and Bing carbs.

Hope it helps.

California Power Systems (http://www.800-airwolf.com/articles.htm)

[gyropilot]

John,

First, I would highly recommend you buy the Bing carburetor manual. It goes into great detail to explain the theory, operation, maintenance, and tuning of your carburetors. You can order it direct from Bing Vergaser. See their web site at: http://www.bingcarburetor.com/ for the telephone number / email address.

The correct carb tuning basics are as follows:

Set the basic idle mixture and idle speed screws as recommended by Bing.
Insure the throttle cables are slack and some play exists in the cables when the carbs are at idle. You should be able to lift up slightly on the cables where they enter the top of the carburetor and feel the slack. This is important and you're verifying the carb slides sit on the idle speed screws and are not held open by the throttle cables.
Start and warm the engine.
Adjust idle speed and synchronize between the two carbs. While measuring the carb slide heights using various means (drill bits, telescopic gauges, etc.) is better than nothing, it's not the best way. The proper way to verify both engine cylinders are receiving the same air/fuel mixture is by using either vacuum gauges or an airflow meter. The later is what I use. You can buy one from J.C. Whitney online for $29.95 here: Schleyer Carburetor Synchronizer (http://www.jcwhitney.com/autoparts/Product/showCustom-0/Pr-p_Product.CATENTRY_ID:2008076/c-10101/p-2008076/Ne-600003570/N-111+10201+600003570/tf-Browse/sct-body/s-10101/th-StoreCatalogDisplay/ss-10101) I use this one, which is a bit more expensive: Bugpack Carburetor Synchronizer (http://www.jcwhitney.com/autoparts/Product/showCustom-0/Pr-p_Product.CATENTRY_ID:2005371/c-10101/p-2005371/Ne-600003570/N-111+10201+600003570/tf-Browse/sct-body/s-10101/th-StoreCatalogDisplay/ss-10101)
Adjust idle mixture. This is done by very *slowly* opening one idle mixture screw at a time. The engine rpm should initially increase and then if you continue to go far enough, it will start to run slightly rough. Open (unscrew) the mixture screw just until the idle speed is at it's highest and go no further. Close the idle mixture screw 1/8 turn. Repeat the same procedure on the other carb.
Readjust idle speed and synchronize between the two carbs again.
Recheck that you still have slack in the throttle cable with the engine at idle.
Increase engine speed to approx. 3000 rpm. The exact speed is not critical. At this point the carbs will be held open with the throttle cables and the idle speed screw is not a factor.
Synchronize carbs at this higher speed by adjusting the length of the throttle cables where they enter the top of the carburetors. Do not touch the idle speed or mixture screws for this operation as they have no effect.
Return the engine to idle, recheck to see you still have slack in the throttle cables at idle, and recheck the idle speed.
You're done!
Good luck,

John L.
PRA Chapter 30
Western WA state

[banaari]

Thank you muchly people... gotta love the forum. :)

[Aussie_Paul]

......you cannot do any better than that.
Bert Flood, the Oz Rotax approved dealer told me to do it exactly like that.

I have done a few, but I bet you have done hundreds Tim.

Aussie Paul. :)