Carby Tuning

[Admin]

Hi all

 

This doesn't seem right to me but that I thought I would ask that if your carbies were out of balance/tuned with each other would that effect your air speed or just limit the rpm of the engine?

 

In other words if you are cruising at an rpm of say 5,200 would your airspeed be any different whether your carbies are tuned to each other or not as the prop would still be turning at the right revolutions and it will move the same amount of air which gives you your airspeed - the engine on the other hand would be using more fuel - that is all or is it?

 

 

[hihosland]

My understanding is that engine will run rough and if well out of balance the cylinders supplied by the leaner carb could over heat.

 

Davidh

 

 

[Guest brentc]

You are 100% correct Ian. 5,200 RPM is 5,200 RPM as measured by your tacho. The airspeed shall remain constant at those RPM (of course), HOWEVER, you may be down significantly on power and as such you will lose more airspeed in a climb than you would with a better tuned engine when the engine is placed under load. You'll also lose takeoff performance and your engine will feel less responsive and possibly rough. As David says also, the leaner ones will run hotter. Remember though, more RPM = more airspeed (with a fixed pitch prop) so the better the tuning the better.

 

If you're referring to your Gazelle, when I bought mine it was flat out at 5,200 rpm. I had it tuned (carby synch) prior to purchase and it was revving at around 5,500 rpm. That made a huge difference on climb and cruise speeds.

 

 

[Admin]

Thanks Brent but as you say that you had better cruise after you had the carbies tuned giving you 5,500rpm but if you reduced it back to 5,200 by the throttle am I right in saying that you wouldn't get any better cruise airspeed wise but you just had an extra 300rpm up your sleeve if you wanted to go faster - does that make sense

 

 

[Guest brentc]

Perfect sense yes. Technically your question was would you get the same airspeed from 5,200rpm and the answer still remains yes. The more RPM and power the better which equal more speed and more room to throttle back whilst still maintaining a good cruise speed.

 

 

[Guest Teenie2]

Ive balanced carbys hundreds of times (motorcycles ,cars ,aircraft).Generally balancing only effects the engine at idle.Now if your talking about rejeting or even adjusting float height thats another matter.

 

 

[facthunter]

Balancing Carbs.

 

Dead right teenie2. The normal balance adjustment with throttles fully closed would be achieved on the idle stop screws, and would affect the smoothness of the engine on part throttle and idle. Unless there was a large amount of slack on one cable causing one throttle to be fully open, and the other to be considerably less open. In this case 2 cylinders would be developing less than full power, so you would notice less than normal net power , evidenced by not achieving full revs, with full throttle position of the throttle knob.. I cannot see why the 2 cylinders which are "loafing" would get hot as they are being fed fuel by the needle's taper appropriate to the position of that particular throttle, and the airflow in that carb. The other 2 cylinders would be carrying a bit more of the load, but would also be receiving the correct mixture from its carb. relative to the throttle position on that particular carb. Propwise, 5200 revs is 5200 revs, if the plane is the same & the air is the same and the plane is not climbing, the speed will be the same. Hope this makes sense. N.....

 

 

[Guest brentc]

You'll find the 912 carbies are very intolerant of inbalances. They need to be well tuned to achieve maximum RPM. Idle is not so much of an issue on 912's in general.

 

 

[Guest micgrace]

Hi All

 

As some may know, most carbies are balanced for idle only on ground. Step one, set up a rough idle speed using the stop. (from carby specs)Then play around with the idle air mixture to where it runs the sweetest, then conitunually adjust the stop/ air adjustment. It is a bit of an art.

 

A fairly crude airflow meter device used to be available for such a purpose, where they are available now, who knows?

 

The main jets will have to be adjusted through use of EGT/ rpm (providing the prop is right) Till correct rpm/ egt is acheived. NOTE, egt very important on 2 strokes as well.The fuel is also used for cooling/lubrication purposes even if oil injected.

 

Different needles are available as well as different main jets and a needle MAY have a tapered section in the middle to suit some engines characteristics.

 

Naturally, the best way is adjustment on an engine dyno (or stand with club prop in place), but, who has access to those?

 

The RPM of the engine will be determined by the prop, providing the engine is reaching designed power. The idea is to get the rpm in the best torque band for the engine. Think of the prop as a sort of crude engine dyno.

 

Two strokes have their own quirks at idle, the main being, reversion of air flow thanks to port position and design. This kills vacuum at idle, so just have to settle for some sort of rather rough idle.

 

Roll on injection, don't need to concern about vacuum flucuations effect on fuel delivery.

 

Micgrace

 

 

[Guest Teenie2]

Guys its dead easy ( sometimes you guys scare me)(go see a level 2 if you need to ) .Lets look at a 582 ,on the side of the carbys is a vacuum port (I think it has a rubber blanks) you need to attach a manometer (spelling) to both of these ports at the same time (simple to make , a piece of clear tube about 10 feet long with a dia that will slide on the ports .Make a "U" shape with the tube and fill with a quantity of liquid that is easily visible, I use Penatrene (red) ,fill so the liquid fills the lower 2 feet of tube).Now run the engine at idle and watch the liquid ,the difference in vacuum (layman term ) will cause the liquid to sit at different level adjust the idle stops on the individual carby until the levels are as close as you can get .Thats the idle section done .Now increase throttle to mid opening ,watching the fluid level ,adjust the cable adjuster at the top of the carby to get the fluid level .And thats it, takes all of about 5min after you have access. NOTE If on a two stroke the levels are jumping around at idle,try lengthening the tube (acts like a pneumatic dampener).

 

Or maybe I really have no idea ,so maybe you should forget you read this.

 

Goodluck

 

 

[Guest micgrace]

Hi

 

The use of vacuum for tuning purposes is quite a legitamate way to tune. I have done so myself with a sensitive vacuum guage with a restrictor fitted in the line. This gets you in the ballpark with final adjustments by ear.

 

However, there is a slight variation in vacuum between cylinders due to slightly different sealing by rings, vavles etc. The length of intake introduces a slight reversion wave due to reflected pulses when vavles/ports close. This effect can be used to increase the effective power output of an engine.

 

Since most (4 strokes) prefer an air fuel mix w/w of around 14.7:1 and the air proportion is by far the largest, the most accurate way is by volume of air. This greatly reduces the error on fuel requirements compared to a vacuum reading without going into the calculations involved in that.

 

It may be noticed that volume of air going into an engine is the exact method used by the majority of efi systems to determine the volume of fuel injected. The MAP sensor is nowhere near as effective and has been basically discontinued in production in favour of reflected ultrasonic pulse type of sensor.

 

The airflow device used to do this on a carby unfortunately has been discontinued to my knowledge, such a pity is was superior to vacuum use in every way esp on extremely complex induction setups. (Heavily modified V-8 with 4 s/d webers, try tuning that!)

 

Micgrace

 

 

[Guest brentc]

Balancing carbies has never been a problem on Jabiru's to my knowledge!