Dynojet A/F Meter

[jensen]

Hi Brewsky,

This is a run from an old post, it's done with moderate temp and "normal" pressure.
It shows that I never get the A/F ratio above the 13, and lowering to 12.7 at the redline.
The bump untill 5000 rpm is introduced because the throttle opens to quickly, it isn't a constant vacuum carb. The second run is from my bomber, and you see that the bump is much smaller now.

You see that the max rpm with the dream is higher then the bomber.

This run did cost me 50 euro plus working time (of myself) to change needle hight and / or jetting. The point is, when you found a good setting, and it runs well, you don't have to touch it anymore, unless there are problems, and that for 50 euro and no hassle.

Notice the small gaps on the CB450 run at the high rev's (7000 to 9000 rpm), these were introduced by a not so good condenser. Without noticing in this run, if would have destroyed my points (burned). Also notice that the bump still exists, but less then on the dream. Later, Honda did a modification on the carbs by putting in a spring above the valve, mine doesn't have that spring, but you see clearly why Honda did the mod.

Jensen

[brewsky]

Jensen,
Do your results show single gear (top gear) readings at wide open throttle, or was the throttle opened slowly so you could correlate different carb circuits?

To determine the A/F ratio of the different circuits, it seems like someone would have to mark and note the throttle position corresponding with the RPM on the graph.

Maybe not so necessary on the CV carbs?

[jensen]

Hi,

All results are taken at the 3th gear. The throttle is opened proportional with the rpm, so the A/F ratio you see is from all circuits after each other, and the transition area between them.

Only the mixture at idling isn't shown correctly, but that is done with the gas analyzing at idling (and just above). The pick-up however is shown (from idling to the needle, and from needle to main jet.

The last 2000 a 3000 rpm is main jet only, before that area you see the needle (and transition area).

Jensen

[brewsky]

Jensen,
Are you able to equate throttle positions of 1/4, 1/2, 3/4, & WOT with specific RPM readings?
If so, what are they for the C78?
Thanks

[jensen]

Hi,

I'll try to explain were what works on the CB450 A/F curve first, because it's a CV carb, and gives a better understanding what happens.

The process is as follows. The bike is on the dyno and warm. The run starts as low as technical possible, in this case around 2000 rpm, lower isn't possible, remember, the run is done in third gear.

During the run the throttle is opened controlled, from stationary to WOT, remember, it's a CV carb, you can do that, the carb is regulating itself more or less. The engine is on a dyno, and has to deliver, so it has to work, work hard.

You can see here and here how it works and sounds

To keep it simple, in a carb there are four different sub systems working, the stationary system (stationary jet), the slide, the needle, and the main jet.



Looking at the partial AF curve 1 :

Systems working : stationary jet, slide angle and straight part of the needle.

Throttle opening : 0 to 1/4

When the engine is idling, the stationary system, together with the slide are in control, this is the area from 1000 rpm to 1500 rpm. When opening the throttle the air is sucked in, and because air is sucked in first (the gas is slower), the engine will run lean (see the peak around 2500 rpm). The needle is straight in this area, so is only metering a constant flow of gas while the slide is moving upwards.



Looking at the partial AF curve 2 :

Systems working : straight and angeled part of the needle

Throttle opening : 1/4 to 3/4

Around 3000 rpm, the needle starts to get smaller and letting in more and more patrol until the engine is running on the needle only (from 4000 rpm in this case).

Between 3500 rpm and 7000 rpm the needle is metering the gas, and around 6000 rpm the main jet is coming in.



Looking at the partial AF curve 3 :

Systems working : angeled part of the needle and main jet

Throttle opening : 3/4 to WOT

At 7500 rpm until 9000 rpm, the main jet is mainly in control, starting to take over completely after 8000 rpm.



I explained something about what you can see in the curves. The word power, and max torque are not used by me until now, and it is a misunderstanding that a dyno is only to measure power.

btw, I excluded the influence of the float hight, actually I excluded more, because I like to keep it simple.

Now, when you look at the complete picture, you will notice that the torque and power curve are reacting on the A/F curve. Please notice that there is a small time gap between the A/F curve and the power curve, this is due to the fact that gas needs to get to the sensor, which is a few meters (and thus time) away from the combustion chamber.

You can distillate so much info from a simple curve, that's why I like a dyno run every once in a while, it shows if everything is still working as supposed to.

Jensen

[brewsky]

Thanks for the detailed reply, Jensen.

I have a basic understanding of the different carb circuit's operation.

What I'm trying to better understand is the correlation of the dyno information with the actual carb circuits influence.

Most dyno runs I have observed have been done by the operator getting the bike in next to top gear under light throttle, then whacking the throttle open quickly. To me this suggests the readings are mainly resulting from the influence of the main jet, even at lower RPM's, and are intended basically to show max power.

I noticed the dyno operator on your 450 link also appeared to have the throttle full open through the progression of the test.

Since it has CV carbs, this probably isn't as significant as it would be for non-CV models, since the diaphrams are not supposed to allow more air in than the engine can take. But for a non-CV carb, I don't believe it accurately resembles most street riding conditions.

My main goal is to tune for best rideability on the street, and that would mean smoothing the transitions in power between circuit changes. That would mean looking at steady state and slowly increasing throttle results especially at and around the circuit transition points.

Being able to have a dyno run that can maintain a steady rpm at a given throttle opening long enough to get an accurate reading at that point I think would be helpful.

That is one of the reasons I got the A/F meter, to see the variations as they happen on the road, and to be able to feel the differences at the same time.

One question..... the gas analyzer probe used in your tests......how far was it inserted into the exhaust?....to the first bafflle, or was it pierced and extended further?

Thanks!

[jensen]

Hi Brewsky,

On the C78 it was done slower, proportional to the rpm, because of the direct slide.

I don't understand your point, when doing a run trough the whole spectrum you have all the data you need. Before the run the carbs are synchronized, the aircrew's are set, float hight is checked etc. On the dyno, the engine is under load, just as on the road, and the A/F ratio is measured with all possible slide positions.

When on the road you use the throttle to overcome the forces working against the speed of the bike, and those are in balance at a certain speed, so your throttle will differ between riding against the wind and with the wind, so there is not a constant relation between throttle position and engine rpm.

Are you suggesting that the engine is having another A/F ratio on the road ? Well, not much from what is measured on the dyno. What do you suggest? changing the air screw, needle hight, and main jet for every kind a ride ? I think the weather, altitude changes and moisture in the air have more influence on the A/F ratio then the difference between a road and a dyno on a tourer bike like the C78.

I rather use the dyno and gas analyser to set the carbs and ignition in a controlled and known environment, because I want to keep the engine in one piece, under load.

The gas analyzer is 20 to 30 cm from the tip of the muffler, as deep as possible.

I'm looking forward to see if the chart and curves of my C78 comes close to your measurements.
I noticed a logging function on your device, so please share the data when extracting.

Jensen