Ailingwing wrote: ↑Thu Jan 17, 2019 4:24 pm
I always keep it on the center stand. Synced carbs again to be sure, was a wee bit off but solid now. Still bogs down a bit when I snap throttle. While still connected, my vacuum gauges plummet when I floor it, then sky rocket. I would think they'd ramp up first when I hit throttle. Running better, but still the odd backfire. Tested timing and timing isn't EXACTLY on F each time but damn close. Still smoking. The stutter (backfire) and hesitation when I gas it from idle is the most frustrating. I can deal with a little smoke! May try taking carb bank off again and re-testing everything. Tried vacuum leaks by spraying starting fluid at various carb joints. No change in rpms......
That's exactly the behavior you would expect when rapidly opening the throttle.
The vacuum is measured in the intake manifold between the throttle and the cylinder intake valves. When the engine is idling and the throttle is mostly closed, the engine is not drawing much air, and the throttle is at the maximum amount of restriction, so vacuum is relatively high.
When you rapidly open the throttle, all of a sudden there is little to no restriction to incoming air, AND at the same time, the engine still not requiring much air - so the vacuum plummets almost instantly.
As the engine speeds up, it starts drawing more and more air, more than can easily get past the throttle, so the vacuum drops once again.
Incidentally, this is the reason for accelerator pumps. When you rapidly open the throttle and the vacuum plummets, the fuel being drawn into the engine also almost completely stops - because the amount of fuel being sucked into the engine is dependent on the amount of vacuum present in the plenum. At low speeds it relies more on manifold vacuum to draw fuel (through the idle jet), at higher RPM it depends more on the vacuum generated by the volume of air flowing through the plenum (which is accelerated, causing a drop in pressure, i.e. vacuum).
Because the fuel almost completely stops, in order to keep the engine running AND make it start to accelerate, the accelerator pump squirts a little bit of extra fuel into the plenum. This gets the engine going, which speeds up RPM, which then raises vacuum, and things run normally.
This is why a malfunctioning accelerator pump or jet will cause the engine to bog down when the throttle is opened rapidly.
Check to see if you can see the accelerator pump squirting fuel when you rapidly open the throttle.