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Carb / mixer design questions for hit and miss engine

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Misterg:
Hi folks - I hope this is the right place to ask...

As you might have seen, I've just completed an open crank hit and miss engine of my own design.

I've noticed that it has to be adjusted to run quite rich in order to fire reliably when the governor is active (i.e. firing then coasting for a number of cycles). I'm guessing that this is normal behaviour, as I'm sure I've seen comments to this effect previously. Mostly out of curiosity, I'd like to understand why this is, and if there's anything in my design that's exaggerating this effect.

When running without the governor, it is very, very difficult to get the engine to fire every cycle (when it does, it takes off like a chainsaw!)- it is happiest firing every two cycles (4 crank rotations).

I though it might be struggling to clear the exhaust gasses, but I've taken some slow motion video which shows the (atmospheric) inlet valve being opened every cycle, so presumable the cylinder pressure was cleared before the inlet stroke.

From about 1 minute in:


(Compare the sound of firing with the cam/rocker movement).

I *believe* the ignition is OK (it is the Hobby King module with static timing at TDC - firing every crank rotation).

Which brings me back to fuel and the carb (mixer)...

It is very conventional, and while not a direct copy of any particular design, it has a single 1mm / 0.039" hole in a spray bar directed down stream which is common to a lot of the designs in circulation. It also has a check valve in the fuel supply in the form of a 2mm dia S/S ball in the elbow at the end of the spray bar - the 1.7 dia hole is actually 2.5 (other details as below).

The needle has a smooth taper and needs to be ~2/3 of a turn open for the engine to run - there's a range of about 1/4 of a turn where the engine will run in some fashion or other, which doesn't seem unreasonable, but obviously a finer adjustment is better.

So: Does this behaviour sound normal for this style of engine?

Any hints or tips to reduce the need to run rich?

Are there any changes I could/should make to the carb/mixer to improve it?

Carb working drawing:



Thanks in advance for any thoughts or advice :)

Andy

Roger B:
How are you judging that it is running rich, black smoke, sooty plug?

The mixture you will get with a relatively steady flow through the carb will be different to with an intermittent flow.

Misterg:

--- Quote from: Roger B on October 09, 2020, 03:31:24 PM ---How are you judging that it is running rich, black smoke, sooty plug?

The mixture you will get with a relatively steady flow through the carb will be different to with an intermittent flow.

--- End quote ---

Hmmm... I decided it was running rich because I had to open the needle valve more for hit and miss operation that I did for free running. If I let it free run at the same needle setting, it does indeed run very sooty.

I took the firing every other cycle (8 stroking?? - I would call it 4 stroking if it was a 2 stroke!) as a symptom of rich running too.

However your second point would probably explain all of that.

If it's all "normal", I'm happy  :Lol:



Roger B:
With an intermittent flow the mixture can become leaner as the inertia of the liquid fuel is more than that of air. This could explain your need to open the needle valve further when in hit and miss operation.

coulsea:
If the intake valve spring is too strong you will not get use of the whole intake stroke as the valve will only open when the suction is great enough to overcome the spring, if this only occurs near the end if the stroke you will not get enough charge to fire every time.

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