Topic: How Do You Use A Dynomometer To Jet A SOHC/4? (or any bike)

[rickman750]

My boss has given me the opportunity to use his Dynajet bike dyno at any A/H time I want.
He gave me a one-time, briefest instruction on the fans, and the "Run" button.
Several months later I gave my K2 a couple of full throttle power runs. On the first I fed throttle such that it was fully open at ~3k. The chart showed waaayyy rich through the mid revs (3k to 6k), then come back to reasonable air/fuel 6k to 9k.
On the second run I snapped the throttle open at 2k and it died away. I closed the throttle to half, and it picked up. Once it got to ~3k it could run at WOT, but again way rich.

I don't want to "just patch the hole", but I do want to learn how to dyno tune carbs PROPERLY. The boss doesn't want to do anything but (Woolich, Bazzaz, Power Commander, etc) injected bikes, and I'm loath to push him in case he says "If you can't figure it out for your self, stop using my dyno."

Who wants to help me learn? I will ask dumb questions, and display my misguided knowledge.

[Jerry Rxman Griffin aka MuthaF'er]

Whatever range it runs rich you simply want to lean it out. Let's say if it is rich as you say in mid-range and considering you haven't given any numbers I'm gonna to speculate your A/F is 12:1 or less?? If this is the case then you should begin by adjusting your needle position. Lower the needle to lean it out. That is you lower it to plug up the passage in the needle jet to allow less fuel passage. This will help actually patch the hole by lessening the size of the hole.

[scottly]

The chart showed waaayyy rich through the mid revs (3k to 6k), then come back to reasonable air/fuel 6k to 9k.

This is common for WFO, due mostly to the cam timing. The over-lap allows un-burnt fuel to pass into the exhaust, when operating the motor at RPMs below the torque peak. The needle has very little effect at WFO, regardless of RPM, Jerry. I usually start runs above 3.5-4k. Please post one of your graphs.

[rickman750]

I usually start runs above 3.5-4k. Please post one of your graphs.
[/quote]
I'm on holidays at the moment, but I should be able to sneak in to work tomorrow and burrow (there will be several bikes to impede progress) my way into the dyno room computer and grab the run files. I'm hoping that will let me open them on my Winpep7 viewer at home.

[scottly]

For what it's worth, I print a hard copy of the run off the dyno PC, then scan it at home for posting. I've yet to try saving the raw run data onto portable media such as a thumb drive, so let me know how that works.

[faux fiddy]

If this is the case then you should begin by adjusting your needle position. Lower the needle to lean it out. That is you lower it to plug up the passage in the needle jet to allow less fuel passage.

I shouldn't know anything about this, but my friend has gone through his rz to make it perfect as could be.  The needles themselves can be somewhat generic from the factory and his numerous modifications included using a surface grinder to shave a few .0001's here and there to change the shapes of his needles.  He eventually got rid of the hesitation between the first crack of the throttle through midrange and WOT so it was all smooth and responsive.

Of course he's pretty special at figuring this stuff out, and using a surface grinder. If you don't have one you pretty much run what you got.

 

[Jerry Rxman Griffin aka MuthaF'er]

The chart showed waaayyy rich through the mid revs (3k to 6k), then come back to reasonable air/fuel 6k to 9k.

This is common for WFO, due mostly to the cam timing. The over-lap allows un-burnt fuel to pass into the exhaust, when operating the motor at RPMs below the torque peak. The needle has very little effect at WFO, regardless of RPM, Jerry. I usually start runs above 3.5-4k. Please post one of your graphs.

Not sure where you are coming from but you do have a dyno  I wasn't suggesting affecting the top end which he said seemed to be OK?? I was suggesting leaning the mid-range. Please explain your thoughts.

[rickman750]

The chart showed waaayyy rich through the mid revs (3k to 6k), then come back to reasonable air/fuel 6k to 9k.

This is common for WFO, due mostly to the cam timing. The over-lap allows un-burnt fuel to pass into the exhaust, when operating the motor at RPMs below the torque peak. The needle has very little effect at WFO, regardless of RPM, Jerry. I usually start runs above 3.5-4k. Please post one of your graphs.

Not sure where you are coming from but you do have a dyno  I wasn't suggesting affecting the top end which he said seemed to be OK?? I was suggesting leaning the mid-range. Please explain your thoughts.

As I understand it, the RPM is (almost) irrelevant for tuning purposes. The carburettor opening is the overwhelmingly what matters. 0-1/8 = air jet; 0-1/4r = pilot jet; 1/4-3/4 = needle jet; 3/4-WOT = main jet.     Yes???

[rickman750]

This is my first attempt at an attachment, so....
Oh... and my first dyno run!!!
There are two "pulls". Both look the same. I can't figure out how to expand the A/F graph in WinPEP7. Sorry!

[flybox1]

Good thread.  I love this stuff.
Based the chart, and IIWM, id make only one change.....lower the needle(raise the clip position) to lean mid range out....and then retest.
You might see enough of a cumulative mixture change to also lean out the upper range.

[rickman750]

For what it's worth, I print a hard copy of the run off the dyno PC, then scan it at home for posting. I've yet to try saving the raw run data onto portable media such as a thumb drive, so let me know how that works.

I've installed WinPEP7 on my PC at home. My home PC is not connected to a dyno. I grabbed the ".drf" (Dyno Run File??) files from the dyno PC at work. The .drf loaded straight into my PC at home. Doing this lets me fiddle with the software and figure out how the UI is set up.

Works a treat!

[scottly]

Yes, WFO is controlled mostly by the main jet, but due to dynamics, RPM does play a big role. As I said before, these engines tend to run rich below the torque peak, then level out in the higher efficiency RPM range. Your bike is still rich from 6k up; the dashed red line at 13:1 is where you should be for max power (12.8-13.2:1).
Also, you might change the scaling to show the same scale for both torque and RPM.