Carb balance.

by andrew owen in 912 / 914 Technical Questions 9 years ago

Hi, had a go at balancing the carbs on my 192 , at 1700 rpm it was 3inc hg out this error on one carb this went through the Rev range to 4500RPM, is doesn't seem to be a lot out but it's made a big difference.

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Re: Carb balance.

by Roger Lee » 9 years ago

Roger Lee

126 5.13k 4.58k

3" is a lot. Just make sure they are synced at your idle set point (1700 if you like). I prefer to set the higher sync point at 3500. In know the manual says 2500, but I think this is too low and not moving enough air/fuel to give a good representation of flow plus we don't fly that far down the scale. Usually if I try to set it at 2500 and then run up to 3500-4000 rpm the sync is off again. When you are done with the pneumatic sync the carbs should be dead on at idle and the higher rpm sync. Even 1 needle width on the gauge is too far for me.
I do check my gauges against each other every so often under a vacuum. Even new gauges can be 1/2" to 1" off between the two.
I just tested 6 gauges. 4 were equal to each other and two were 1/2" off.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Carb balance.

by andrew owen » 9 years ago

andrew owen

13 17 3

'Yes before I started the gauges were coupled up to a tee connection and vacuum applied, both gauges had the same reading on them. I attached the gauges by removing the aluminium crossover pipe, this pipe is connected using a short length of rubber pipe on each end, I found that the rubber pipe had clips clamping it to the engine manifold but no clips to the crossover pipe ,just relying on push fit, I did fit clips to the pipe on its return as I'm sure this could have been a source of air leakage.

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Re: Carb balance.

by tom oatman » 9 years ago

tom oatman

5 28 0

roger;
I want to purchase the gauges , hose & fittings to do a carb sync. What sizes do I need ?. I'm thinking of getting a liquid filled gauge with a 1/4" npt male fitting on it. But unsure of the manifold size fittings, or the crossover tube fittings size.

thx
Tom O.

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Re: Carb balance.

by Roger Lee » 9 years ago

Roger Lee

126 5.13k 4.58k

Hi Tom,

Here is a picture of an easy to build carb sync tool. Overall I think gauges work so much better than the electronic setups because with gauges you can diagnose carb problems. You can tell which carb you want to adjust and by how much, tell which carb has a clogged idle or main jet, tell which carb is flooding, know before you even turn a screw if the cable on the throttle arm needs to be lengthened or shortened. To me you can sync much quicker because you can see all this at a glance.

These are 2.25" liquid filled gauges that read 0-30 " vacuum. You could get them locally where you live or from the internet like Amazon.com.
There are two needle valves that you can close down and get the needles to be steady.
I wouldn't use the clear plastic tubing like in the picture and buy some regular rubber vacuum hose from an automotive store or even Ace Hardware carries it. Other than the gauges everything you need can come from Ace Hardware.
Buy a brass fitting to put on the end of one hose so you can attach a 3/8" piece of plastic tubing to go over the 90 degree fitting on top of the intake manifold. Use a clamp on this hose.
Buy another male barbed brass fitting to go into the end of the balance tube rubber hose. Sand the barbs off the end of this fitting so it is smooth and won't tear up the inside of the rubber hose. This just takes a minute if you have an electric belt sander. Use the existing clamp on the balance tube hose.

When you separate the balance tube of an intake the carbs are now completely isolated.

This is an easy tool to make. Cost will be around $80 depending on the quality of the gauges you buy.

Not all tools or jobs have to be high tech or complicated.

carb_sync_150.jpg (You do not have access to download this file.)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: RotaxOwner Admin

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Re: Carb balance.

by tom oatman » 9 years ago

tom oatman

5 28 0

Thanks Roger.
Off to Ace hardware I go!

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Carb balance.

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