Carb Synch doesn't hold thru RPM range

by Robert Hirschfield in 912 / 914 Technical Questions 12 years ago

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Re: Carb Synch doesn't hold thru RPM range

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

Hi Mark,

What problems can be mask by setting the carb sync at 3500? On the reverse side of that question what issues might you not discover by syncing too low and not advancing the throttle enough to see how each carb reacts?
I find setting the sync at 2500 rpm for the high side sync is too low and it doesn't flow enough fuel and air to be in sync at the higher rpms. The accuracy level at the low fuel and air flow isn't as good. Syncing at the higher rpms keeps the needles better in sync as rpms increase. I find if I sync at 2500 and advance the throttle over 3500+ rpm the needles will split again. Using gauge you can diagnose
The balance tube can only correct so much and isn't a cure all for out of sync carbs at high rpms. Using good dial gauges over the electronic devices allows you to see what's going on with each carb independently and nothing should be mask.

Syncing at too low an rpm is like using a torque wrench in the low 10% range and the highest 10% range. It shouldn't be done because it isn't accurate any more.

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

Thank you said by: Robert Hirschfield

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Re: Carb Synch doesn't hold thru RPM range

by Robert Hirschfield » 12 years ago

Robert Hirschfield

2 16 0

Thanks for all the great input. I switched back to my dial gauges (any body want to buy a carb mate? :) ). I disconnect my throttle cables. Synched the carbs at idle then advanced the 'chokes' and noticed the synch held through a pretty wide range of rpms. When I get the new throttle cables I will take everyone's suggestions into account and try different things until I am comfortable with the result. Thanks again.

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Re: Carb Synch doesn't hold thru RPM range

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Hi Roger

The best way to describe it Roger would be if you imagine that the Carbs are 1% out of sync. If the engine revs are high and the airflow through the carb is around 75% then the difference is only a small amount, which the balance tube can take care of.

If you then reduce the revs so the airflow is only around 10% and the carbs are 1% out of sync then the difference is massive.

As the vacuum is higher at low RPM and reduces as the throttle opens, it makes sense to balance the vacuum when it is at its highest and will have the greatest affect. As the revs increase and the vacuum reduces, you need to adjust the cables a lot to have an effect. as you then reduce the revs, that large adjustment will have a large affect, and at low revs the carbs will be out of balance by a long way.

If the carbs are out by 1% at high RPM it won't be noticeable, 1% out of sync at low RPM it will be very rough and very noticeable.

Mark

Thank you said by: Robert Hirschfield

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Re: Carb Synch doesn't hold thru RPM range

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

Hi Mark,

The problem lies in the flows that are not linear or on a straight line through the rpm range. If they were truly linear and cables advanced equally then we wouldn't need the idle stop screw and the Bowden cable adjustment combination, one would be enough. Cable and throttle equality figure into this also since they are not always equal down by idle and up by cruise so you want to make sure that when you are closer to the cruise rpm that things are equal and stay that way. This can and has fooled a lot of people including me in the early days. The carbs may be synced at the low rpm, but because throttles and cables don't always work in perfect unison as we hope the carbs become un-synced. Getting a throttle to stay the same in a very small movement (1700-2500) is easy. Getting the throttle to be equal with a large change (1700-4000) may not be so easy and that's where you need to make sure things go back in to position every time because that is closer to where we fly.

It shouldn't matter if the carbs/gauges show 1% or 10% in your example. If they are synced at 1% or 10% they are still synced. Your gauges should be accurate enough to show this and you can watch it as the throttle is advanced during the sync. By running the rpm up the scale and watching the vacuum you are watching the flows as it happens through each carb. If they are synced at 3500 or higher and you advance the throttle even more and the needles on a gauge stay equal then the carbs are synced no matter what the percentage of flow. We only need to know that the carbs are acting in unison and that flows stay equal. We tend to idle closer to the 2500 (1650-1800) especially during warm up. The carbs flow a reduced amount of fuel and air at those rpms compared to let's say 5100-5500 rpm where we normally cruise. Flows and pressures within the carbs can change and change to a point that the balance tube can take care of some of the imbalance, but if the imbalance is too wide then the carbs will still be out of sync at the high rpms. Where do we spend 90% of our engine on time, in idle range on the ground or cruise rpm in the air. We aren't usually flying at 2500 rpm except on approach. By syncing the carbs at the higher rpms then we get to check if there are any discrepancies between the carbs and watch the actual flow to make sure they stay equal. (part of my normal diagnosis. Air leaks tend to show better at the higher rpms too. Last, but not least I want to diagnose at each carb sync and only using 2500 limits what I may see as the throttle is advanced.

Plus the main jet at 2500 rpm is not in as full use as it is at 3500-4000 rpm. We don't want any part of the idle circuit influencing our sync for the higher rpm sync. I admit I do not know the exact rpm that idle is fully out of the equation, but you don't want it's influence at the high rpm sync because it isn't there when you cruise.

Here are two examples of testing in the non operational mode. I know these may be poor examples, but it's what I thought of at the moment.

When balancing a set of auto tires would you want them balanced at 10 mph or highway speeds at let's say 70 mph. Which will do you the most good.
If you do a heart EKG stress test do you want to test/stress the heart at a normal walk or at the jog and or run many cardiologist make you perform to stress the heart (blood and O2 flow).

My point being it is better to check and or test things where they normally perform because anything else isn't usually where we think they are. So if I set my sync at 3500-4000 rpm am I more likely to stay in sync than 2500?

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

Thank you said by: Robert Hirschfield

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Re: Carb Synch doesn't hold thru RPM range

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Hi Roger

This is exactly what I mean when I say that there are underlying problems that are being masked by setting the carbs at a high RPM. If you set them correctly at a low RPM and they become out of balance when the throttle is opened, the fix isn't to set the carbs at a higher RPM and accept that it will run rough at a lower RPM.

This may happen a very small amount and this is why the balance tube is fitted, but if it is too excessive, then the problem lies with the throttle cable set up and you shouldn't have to balance the carbs in this way.

Mark

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Carb Synch doesn't hold thru RPM range

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