Datsun Roadster Forum-311s.org

Daryl Smith wrote: Fri Oct 30, 2020 1:26 pm Was wondering about the variations in the A/F ratios, but, it's not that big of a range so really of little concern, and likely very little to be gained for the expense/time to smooth it out, if it was possible.
Great job, both of you.

Thanks
I was wondering myself about the variations in A/F so I sent the sheet to Dave Primo and his response was to not touch anything and that what I have there is almost exactly what you want to see with well tuned Mikunis carbs

What he explained is that The dips where its richens out in the beginning is totally normal as you transition to WOT and the accelerator pump kicks in and the subsequent dips come in as fuel is pumped to prevent the carbs from leaning out too much as you give more throttle .

he's only suggestion is an adjustment to make transition from pilot to main the little smoother when you mash the throttle

JT68 wrote: Thu Oct 29, 2020 3:26 pm
Suggest you limit the final timing to 34/35. You can't hear detonation over 5k, and the risk is not worth the minimal increase in power with pump gas. If you buy AV gas, no prob. The 123 makes these adjustments so simple.

The engine was always intended to make power in the 3-6k range. It really rips through the gears.

Yup I adjusted it down after the test

Agreed. It looked like the accelerator pump might be over richening for a bit. Mikuni was aware of the need to adjust that some and so they had the three holes drilled into the pump rod to change the volume of fuel pumped. As I recall, the pump jet and rod work in concert, where the jet primarily determines the duration of the squirt, and the rod determines the volume of fuel available to squirt. A fine tuning issue for sure, and one you might not even be aware of as an area to potentially adjust if not for seeing it on the dyno. On the street, a wide band probably would have caught it.
My issue has been the exact opposite, as I can't get enough response from the accelerator system unless I hit it harder than I want to (then it responds) so it goes lean with mild throttle changes enough to make it stumble. I had taken a video of the wide band showing me this but it has gone missing.
Truly a great result man, I'm very happy for you!

Thanks Greg
Very happy indeed

Interesting and excellent results.

I've dyno'd the 69 U20 40PHH stock cam unit a couple of times, more to check the results of jetting and found similar torque pull between 3000-4000 RPM.

Numbers were lower, 116 HP & 120 TQ, but the engine hasn't been cracked since I've owned it. May look at the cam upgrade.

For some reason, images aren't posting for me. Dyno chart here: August 2020 Dyno