View Full Version : Dual carb adjustment

profqwerty - n/a
2-Mar-08, 08:35 PM
Hi all,

I'm just trying to balance my carbs on the 503. I'm using a home made splitter - the throttle cable is connected to the carb cables with an electrical wire block.

It seems one carb is always picking up slightly before the other - it'll rise ~1mm before the other one lifts - Is this an issue?

Also using the throttle they'll rise to about 5mm from the top of their travel. There is still a small amount of carb piston sticking down, on the engine side of the piston. The air filter side is completely open. Is this enough?

Also I've been hearing lots of different opinions:

With the exhausts, which would produce more power - a completely curled up one (90 extension, 180 expansion pipe, 180 tip) or (straight extension, straight expansion bit, 90 into the muffler, straight tip). This is bearing in mind the engine came with the curled up one...? (off a microlight)


nickyd - n/a
3-Mar-08, 08:26 AM
Hi Toby,

I would try your best to get them opening at the same time. Can you not use the adjusting screws on the top of the carbs? The idea is to wind them out until it just starts to take the tension up in the cable. if you get both to the same point they should raise at the same time.

You may have to shorten one of the two cables to help a bit.

Check the problem is not in your splitter box. Does it definately pull both cables at the same time, through the same distance?

Also you need to get the sliders right out of the way for max performance. You may need a longer travel lever to achieve this.

What carbs are you running?

In theory, the curves in the exhaust should not affect power delivery. However, I don't know this from experience, I read it in a book.......


sixpackpert - n/a
3-Mar-08, 10:13 AM

Get a proper throttle box, about a tenner, and some solder nipples. This will ensure that the cables are pulled up uniformly.

Other than that, what Nick said! http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

jon_curtis - n/a
3-Mar-08, 06:15 PM
hi toby

well this is how i setup my 503, it might not be the correct way, but it works for me!

firstly there are a couple of things you need to understand regards the bing 54 carbs!

the idle jet, (small screw on side near throat, same side as idle screw) is for adjusting idle spead only realy, as is the idle screw (which lifts the piston in the carb) these will have no effect when the slides are right up as then the main jets come into play.

i tried setting up the carbs with vacume balencers etc and it was a waist of time! its very important that both slides open and close at the same time, and the piston/slide needs to open fully, (none showing) in the carb throat!

i setup my carbs by getting someone to open the throttle till both slides are fully open and both INLINE with each other, line the bottom of the slide up with the top of the throat aperture, with the cable tensioning screws ontop the carbs. then both carbs are opening and at the same level. they should close together aswell, once this is done.

look at a rotax jetting chart and find the correct position of the idle jet (in turns open) and then with the engine running have a fiddle with the idle screw and maybe the idle jets, you can use a vacum gauge at this point to fine tune the tick over! but its not really needed you can do it my ear and also by looking at the egt's

the rear cylinder will run a little hotter than the front and you can put a larger jet in the rear, but this does depend a little on how much free air is flowing into the cooling fan, the front cylindet gets the air first!

at tick over the egt's will be further apart and as you rev the engine and it gets up on the main jets, they will get closer together.

before you start the thing, get a rotax jetting chart and consult, so your happy you have the correct needle postion and idle screw postion, not to mention the correct main jets! if memory serves, there is a pair of 158, i put a 165 in the rear, as i was operating my engine closer to half throttle most of the time and the egt's were closer together! jet size depends on many factors and also on the type of air filter!

the engine should be warmed up before going full throttle, first time i run a rotax i rev higher in increasing steps and amount of time at a higher speed and take out the plugs and have a look before opening it up all the way!

hope this help, but its always good to go onto ultralightnews.com and read the charts and manuals.



sixpackpert - n/a
3-Mar-08, 06:24 PM
All the operator manuals and maintenance manuals etc can be found here if you need them as well...

Rotax Site (www.rotax-aircraft-engines.com/a_searchdoc.aspx)

...you don't have to put in the engine numbers by the way.

profqwerty - n/a
9-Mar-08, 06:45 PM
Sorry for the lack of replies!

Apologies for the bad pictures. It was getting low light - when it's a bit brighter I'll take some decent pics of the craft (polished of course!


I've managed to fiddle this afternoon and this is the cable splitter I made:


There is plenty of room for it to slide and the joints doesn't get near its limits. I will be putting a cover over it once it's set properly.

I know people have been recommending buying the proper one, but to be honest I see no reason why I can't just use this? It now works fine with a quick modification of the throttle lever:


This pulls the carb slides to the very top of each ones slide (I can't push them any higher with my finger), and has plenty of head room to let them go lower if need be.


I'm going to test each exhaust, so this is just a temporary way of holding this one on.


Quick shot of the carbs - I have no idea what that red stuff is!

I'm going to need to put something over the ends of the bowden sheath [gaffa tape].

Also the ignition coils are actually mounted below the carbs as this was originally an free cooled engine. I might rearranged these to by the cooling fan at somepoint!