Does it worth balancing sidedrafts off idle?

[FreeLSD]

Hello!
As I've mentioned, making sidedrafts balanced at idle doesn't guarantee that synchronization will keep along RPM range. Manometer readings sometimes begin to differ in some single carb barrels, as throttles opened further (2000-3000RPM). Adjusting with air bypass screws helps to equate pressures at off-idle modes (when progression system comes into play), but that makes pressures different at idle.

Every tuning manual, I saw across the net, insisted on balancing carbs at idle speed only. Is that actually correct?

[Guy Croft]

You can balance 'off-idle' but strictly speaking it can only be done effectively under load ie: on a dyno at steady-state.

The reason for this is that if you open the throttles too far 'off-load', that is to say higher than idle speed (750-850rpm or so) the carbs start to draw fuel off the progression drillings. This can cause the plugs to foul and the engine misfire - quite quickly.

GC

[FreeLSD]

Thank you a lot for reply!
Does it mean, that intake vacuum difference, seen at increased RPM with no load, doesn't reflect real conditions things state and can be ignored?
If I load engine with brakes and gear engaged (wheels in air), and vacuum difference seen - does that mean different torque output across cyliders? Can I smoothen torque output by equating vacuum readings using air bypass screws, in this circumstances?
Thank you again.

[Guy Croft]

When balancing carbs like IDF one air bleed screw must be shut - the barrel drawing the most air. The other on the adjacent barrel of that carb is opened to equalise. What you're doing there is compensating one barrel against another for fractional differences in depression across the throttle plate. To a great extent all you are correcting the carburettor's response rather than feeding more air to the engine, if you follow my meaning.

Let me put it another way: You cannot make a cylinder that is working 'inefficiently' - draw more air (an become efficient) by adjusting the carb. You tend to see this is there is a bent valve giving very low compression on one cylinder only. In that case the use of Colortune ( a plug with a window) will tell you the one at fault. Naturally a vacuum test cylinder to cylinder at any speed will tend to show that up as well.

Beware by the way, of the effect of a brake servo (booster) line if it hasn't got a non-return valve, because that will rather off vacuum and idle tuning effects.

I hope this helps some,

G

[FreeLSD]

Thank you!
Does is mean that cylinder, which is drawing more air, have weaker vacuum signal? And, otherwise, less effective cylinder creates more vacuum?
I just thought that with air bypass screw we weakening stronger cylinder to the vacuum level of adjacent - to get equal fuel response from carburettor.

[Guy Croft]

well, I'm talking about setting up via airflow (not vacuum) and the influence of the bypass screws on that, say typically measured with an airflow meter on the rampipe. The cylinder with the highest airflow there gets its screw closed. However the highest airflow measured by that means will be the lowest vacuum reading if measured on a manometer. You can picture this by imagining how high the depression (vacuum) would be if the throttle were closed completely.

The bypass screw takes air from upstream of the throttle plate and allows it to bypass the plate and go straight to the cylinder. Naturally this increases airflow thru the rampipe but reduces the depression/vacuum downstream of the throttle plate.

GC

[FreeLSD]

Thank you!
One more question, if permitted! As you have said, we actually modifying carburettor responce, rather than volumetric efficiency of given cylinder, by rotating air bypass screw.
Does it mean, that we want to feed (and burn) equal amount of fuel, even if adjacent cylinders have different inhale abilities?
The bent valve - is a quite extreme example. What if valve lash differ a bit, and single port shape is more turbulent or frictional, or one cam lobe weared more than another? I'm suggesting in a theorethical way, due to lack of real experience.

[Guy Croft]

That's right, we are modifying the carburettor response.

You can't change the engine's volumetric effy by that means - it is a function of port, manifold, valve & seat design, bore, stroke, engine speed etc etc

G