Flatslides and lurching deceleration

Here's a can of worms... hard to search for this very specific topic.

I'm using a TM34 on my latest XS build ( see "Hodaka tank") in combination with a custom 1 into 2 manifold. Absolutely rips from idle to WOT, but in certain coast down closed throttle decel situations under 30mph with little engine braking- things get a bit lurchy. I have a '92 Ducati 750ss with CV flatslides and 5" runners that has similar behavior.

My guess is this is due to lack of velocity in intake air stream and fuel drops out of suspension causing pooling of droplets and sporadic burn.

An interesting related note is that with this custom intake the first 2" closest to the carb is actually chilled in operation... Thermodynamics.... I have a couple ideas for dealing with that as it may be contributing to this minor issue.

Any expert tuners want to chime in? Not sure there's any easy fix...

You could try Googling:
"Carb heat"
"VW heat riser tubes"

I recall a small valve on my 1960 GMC straight-six, mounted in the big log intake manifold, that would admit raw air when the manifold vacuum went high enuff. Called an "AIR", some sort in air-injection for the inlet side, solved some sort of fueling issues. Pre-emissions era. Short-lived gadget...

Hey TwoM- you're probably on to something there. A vacuum that may also set up an unwanted harmonic pulse between the slide and intake valve... in my experience, tuning for the throttle position just off idle under load is the trickiest. It's asking a lot of a carb to perform smoothly at that instant. So the reverse is also true when the slide closes but usually not as pronounced... maybe a reed valve of some sort would work but difficult to tune for... was mostly wondering if this phenomenon is a "thing". With the Ducati i chalked it up to low rpm pulses from the external fuel pump i put on it (replacement for bad oem part in gas tank).

Need more test miles on the new XS setup to fully assess but it's not a big deal. Will try further searches as you suggest.

Got me a pair of lectron 34s flatslides that i did 2 immediate things...

1) polished slides to remove any remaining maching marks....Could possibly have rough action when slide is dropping causing lurch?
2) dont know if this an option, but lectron has different tension return springs. Initially it felt as throttle would hang too long. Went to a heavier spring and that solved it. Maybe return spring could be swapped for different tension to battle lurching.

Very good suggestions. I don't think it's mechanical but wouldn't hurt to double check slide and swap in a stiffer spring as an A/B comparison.

May take a longer spin tomorrow to zero in on exactly what it's doing.

Old memories resurfacing of my long-ago gas mileage experiments.
A few salient points to consider.

Induction fuel/air mix is a combination of vapor (individual hydrocarbon molecules), and droplets of various sizes (fog, mist, drizzle, ...etc.).

During initial combustion, it's the vapor that ignites. Then, as heat builds, the droplets vaporize, expanding (producing pressure like a steam engine), and join in the combustion fun. More heat, more vapor, more expansion, more combustion, repeat.

The distribution makeup of vapor/droplet ratios influence the rate of initial combustion.

Vapor pressure of fuel goes up with heat.
Vaporization of fuel goes up with lower pressures.
Fuel will vaporize until equilibrium, which takes time.

Air/fuel mix at low manifold vacuum, will have low vapor/droplet ratio.
Air/fuel mix at high manifold vacuum, will have a higher vapor/droplet ratio.

The initial combustion rates of these 2 conditions will be different...

barncat said:

... in my experience, tuning for the throttle position just off idle under load is the trickiest...

Click to expand...

Yes, given the above, you should see 2 scenarios that have max influence.
1- Dropping throttle at high rpm, producing high manifold vacuum.
2- Cracking throttle at idle rpm, producing rapid drop in manifold vacuum.

The pilot circuit has to handle both those extremes.

Some auto carbs have acceleration pumps, squirters that augment fuel delivery during throttle opening. Without them, the carb is too lean. But, the mixture is correct for idle, cruise, and dropping to idle (throttle closure).

This would imply that a 'non-squirter' carb must have its pilot circuits setup abnormally rich to be able to accomodate those scenarios...

A lot to consider there TwoM...

"Vacuum" in this case does not mean evacuated of "air", just the amount of suction or force drawing on a given instantaneous volume of air/fuel correct?

Does high manifold vacuum = high pressure?

Yes, your cases 1 and 2 above place mutually exclusive demands on the pilot system. I've got the acceleration side nailed. It's interesting in the course of tuning to realize most riding occurs at 1/8 throttle or less, save for bursts of acceleration, so still a bit of tweaking on closed throttle situations... may need to heat the intake near carb.

Yes, I'm using "vacuum" as anything less than atmospheric pressure. And, high manifold vacuum is very low pressure, low manifold vacuum is just below atmospheric. Look at manifold vacuum gauges, 0 = atmospheric, 30 = absolute vacuum.

More old memories bubbling up. Early Goldwings have mid-length intake manifolds, and use an air cutoff valve.

Google "Honda Goldwing air valve" to start you down the rabbit hole...

Digesting........

Obviously i don't own a vacuum gauge. This experiment is forcing me to consider the science not just tuning by rote. Will report back after more data in hand.