Weak Spark, Hard Starting

[GNEPIG]

"Yes, you magnificent blowhard,

.. i think i'm in love

[pggg]

Quote " lower the idle and you've reduced the vacuum"

Vacuum gauges read a higher vacuum at lower revs - lower vacuum at higher revs. Prom, any chance of explaining this to my dim brain? My theory is high vacuum at low revs stalls the pistons otherwise it would idle nicely at 20 revs. I know my theory won't hold water, so feel free to put me in my place

[prometheus578]

"We must kill the non-believers..."

"I've got an answer. Oh yes, I've got an answer. Have I got an answer." (Have I got an answer?)
(((An hour later...)))
Yes I do! As Scotty on Star Trek often remarked,"Aye, Captain. I'm givin' her all she's got... with those wonderful Constant Velocity Carbs..."
PGGG... You sir, are correct. Lower vacuum at higher revs. But, you are talking about an engine that's running. The engine here isn't.
On a CV carb, there are three vacuum zones; the inlet side, the venturi area, and the outlet side,(where we measure vacuum and where the pilot/idle circuit is) When at low speed, we have a certain value of measured vacuum (X) at either end, but a higher value of vacuum (X+) in the middle, at the venturi area. Let's accererate, shall we?
We twist the throttle, which opens up the butterfly plate at one end. This allows more air to pass, thus reducing the vacuum in that area, as well as our vacuum reading in the outlet area. But this increased airflow, when it passes the restriction of the venturi area, has to speed up, increasing the vacuum there. This vacuum is transferred through a small hole in the bottom of the slide, to the top of the diaphram area, where the vacuum acts on the diaphram, causing the slide to move upward, which in turn raises the jet needle which allows more fuel to flow, increasing the speed. "Und auf ve go!"
So yes, at higher RPM we read less vacuum... but that's a vacuum reading taken in the area of the pilot jet/idle circuit, which is not being used anymore to keep the engine running.
For starting... we need a certain value of vacuum (X) across the pilot hole to draw the fuel up from the bowl. Slow cranking... we get (-X). Faster cranking, we get (X+).
To provide a further example: This is the reason, when starting a bike, that one is supposed to NOT twist the throttle. Twisting the throttle opens the butterfly throttle plate, allowing more air to pass (at a slower speed) at this non-running/starting low RPM, which lowers the vacuum at the pilot hole area (and at the enrichener circuit), decreasing the fuel acquisition, resulting in hard starting.
There... took me two hours to research, figure this out and formulate my thoughts. Provided a better insight into the inner workings for me, it did, so was time well spent.
Of course, that's only if my demonstration was correct. I welcome all comments and rebuttals.

[LoserShoes]

But doesn't your whole theory get blown out of the water when you shoot a stream of starting ether down the carb?

Steve

[prometheus578]

"Ether I'm right, or I'm wrong"

Nope, reinforces it, actually.
A slow crankin' bike will, all things being equal, start with ether when it other-wise wouldn't. Normal cranking draws fuel from the pilot hole... the air flow past it has to be strong enough first, to lift fuel from the bowl to the carb throat, and secondly, then to atomize the fuel coming out the hole... suspending the fuel droplets into a mist, which is then pulled into the cylinder.
Spraying ether into the carb throat accomplishes both things: provides a source of burnable fuel, and this fuel comes already atomized (mixed into a fine mist) to be fired in the cylinder.
Never done it, and probably shouldn't want to try it, but I suppose that, I could spray ether into the carb while cranking the engine over with a wrench on the crank bolt. Slow moving piston would easily draw in this volatile mixture, it'd compress, receive spark and fire, whereupon the wrench would suddenly spin and snap my wrist bones.
Would any of you viewers out in TV land care to try this to prove me wrong? Thought not... I don't even care to try this to prove myself right. Just a mental exercise.

[prometheus578]

Poor man's diagnostic technique.

And I know I'm not the only guy, when doing the "Is it spark or fuel related" troubleshooting, to pull a spark plug, spray ether or carb cleaner down the hole (coming up on the compression stroke), replace the plug and try to start it. Great for dirtbikes after you've worn yourself out trying to kickstart the silly thing.

[BLUEJEEPLAB]

I've used the ether in the Spark Plug hole trick on my XT 500. I am usually loathe to do that because of the cleansing effect of ether on Cylinder walls, but desperate times call for desperate measures and that single 500 CC jug can kick your A_ _ after about twenty jumps (not to mention bust your ankle if you miscalculate the stroke)

[TopCatGr58]

Hey Prom,

Great Thesis work there!! It all makes sense to me! As for your ether wrench starting trick, try using a ratchet, so that when the engine catches, the ratcheting mechanism will just spin, but NOT the handle!! Good luck on finding a Square socket to fit the timing plate cover! Or you could use the alternator NUT/BOLT HEX HEAD, does it tighten CW or CCW? The engine is spinning CCW on that side, so I would think it would tighten CW. Should have enough torque pressure on it to not come loose from just turning the engine over under compression!?
T.C.

[prometheus578]

No Pain, No Gain

T.C., I initially was going to write about using a ratchet, but that wouldn't have satisfied my masochistic tendancies.