Hot start problem in the Extra - solved

gibbons

gibbons

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Those of you at Gaston's know I had a hell of a time with hot starts in the Extra. Twice I simply gave up and as soon as I let go of the ignition key the airplane fired up. <----- clue (or so I was told)

So I figured I had a bad ignition switch and you had to jiggle it just right to unground the mag. The problem with that theory is that it never happened when the engine was cold. <---- clue (I suppose)

This is a new problem for this airplane. Last summer it would be a bit hard to start when hot, but after several seconds it would fire and hold power without any real trouble. This summer has been different. In fact, it hasn't wanted to hot start even when the air temp was relatively cool. It's been that way ever since the annual. <----- clue (for sure)

So I called my old mechanic (he's out of the airplane business because of insurance and liability and sells real estate now - what a waste) and he told me to start with the easy stuff first. Pull the plugs and check the gap. We did, and what do you know.... the gap was much too wide on several (most) of the plugs. We cleaned and gapped them at annual and somehow really missed the gap.

I flew it today for about 45 minutes at full power then came back and let it sit for 10 minutes. It started up and ran fine after that.

Here's my question. How does the gap of the plug effect the hot start in this way? Why did it hit when I released the ignition switch? Why hot and not cold?

BTW, it idles a lot better now too.
 
Hot (relative term) fuel (vapor?) = leaner mix = lower combustion + wider gap = weak spark???

EDIT: Something I forgot; plug gap is like setting the timing. A wide gap is like an ill timed engine. The spark is supposed to be at TDC or just a bit BTDC. A wide gap would make it like way after TDC.

That you don't have this problem during cold starts I guess you have the correct plugs.
 
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gibbons said:
Those of you at Gaston's know I had a hell of a time with hot starts in the Extra. Twice I simply gave up and as soon as I let go of the ignition key the airplane fired up. <----- clue (or so I was told)

So I figured I had a bad ignition switch and you had to jiggle it just right to unground the mag. The problem with that theory is that it never happened when the engine was cold. <---- clue (I suppose)

This is a new problem for this airplane. Last summer it would be a bit hard to start when hot, but after several seconds it would fire and hold power without any real trouble. This summer has been different. In fact, it hasn't wanted to hot start even when the air temp was relatively cool. It's been that way ever since the annual. <----- clue (for sure)

So I called my old mechanic (he's out of the airplane business because of insurance and liability and sells real estate now - what a waste) and he told me to start with the easy stuff first. Pull the plugs and check the gap. We did, and what do you know.... the gap was much too wide on several (most) of the plugs. We cleaned and gapped them at annual and somehow really missed the gap.

I flew it today for about 45 minutes at full power then came back and let it sit for 10 minutes. It started up and ran fine after that.

Here's my question. How does the gap of the plug effect the hot start in this way? Why did it hit when I released the ignition switch? Why hot and not cold?

BTW, it idles a lot better now too.
Click to expand...

First, a wide plug gap produces quite a bit less heat during the spark and the spark is already pretty weak during starting. Second with reduced ignition temperature there is a correspondingly smaller range of mixtures that will fire and AFaIK the center of that range shifts towards rich. It's more difficult to achieve a specific mixture when the engine is hot, especially if you need all the cylinders at the same mixture. In summary a weak spark is likely to make starting more difficult when hot.

WRT starting when you let go of the switch, do you by any chance have a "shower of sparks" type of magneto?
 
lancefisher said:
First, a wide plug gap produces quite a bit less heat during the spark and the spark is already pretty weak during starting. Second with reduced ignition temperature there is a correspondingly smaller range of mixtures that will fire and AFaIK the center of that range shifts towards rich. It's more difficult to achieve a specific mixture when the engine is hot, especially if you need all the cylinders at the same mixture. In summary a weak spark is likely to make starting more difficult when hot.

WRT starting when you let go of the switch, do you by any chance have a "shower of sparks" type of magneto?
Click to expand...

I think there too many pieces of that trouble shooting puzzle missing. to give a complete answer.
 
gibbons said:
Impulse.
Click to expand...

Then (and this is just a guess) I suspect that either it was just coincidence that you got the mixture right at the same time as you released the switch, the timing was better when not cranking as fast. One other (very remote) possibility is that your mag switch was grounding one or both mags in the start position. If you only have one impulse coupler, the switch may be configured to ground one mag when cranking and if the p-leads got swapped the wrong one could be disabled.
 
Richard said:
EDIT: Something I forgot; plug gap is like setting the timing. A wide gap is like an ill timed engine. The spark is supposed to be at TDC or just a bit BTDC. A wide gap would make it like way after TDC.
Click to expand...

Excuse me???? The spark jumps gaps infinitely faster than the piston moves so I'm having trouble figuring out how gap would have an effect related to timing. If I figure this right, a pistons average speed would be around .0058 miles per second on an engine with a 5" stroke turning 2500 rpm, with the pisons speed slowing to 0 at the ends of the stroke so that the speed at 8 degrees BTDC, the piston is pretty slow. Light on the otherhand travels at 125,000 miles per second, so subtract a bit for the atmosphere in the combustion chamber... see my dilema?

I could see the difference in compression pressure between a hot and cold engine having an effect on whether the spark can get across or not.
 
lancefisher said:
Then (and this is just a guess) I suspect that either it was just coincidence that you got the mixture right at the same time as you released the switch, the timing was better when not cranking as fast. One other (very remote) possibility is that your mag switch was grounding one or both mags in the start position. If you only have one impulse coupler, the switch may be configured to ground one mag when cranking and if the p-leads got swapped the wrong one could be disabled.
Click to expand...

That's a good thing to check..., easy and free.
 
Chip I had a lycoming with one Impulse coupling that did what yours did (very poor hot starts and would catch after releasing the starter switch).

We tracked it down to a weak impulse coupling spring and when I changed that it was like new.

It happened twice in this engine, I have no idea why.

Apparently the spring, when hot would not have enough torque to produce the hot, retarded spark needed to get a start but would when cold.

iirc we put a few extra winds on it as a test before buying a new one (although the new one failed this way too)
 
Good feedback all. Gapping the plugs seems to have solved the problem. It still doesn't like to start hot, but at least it will.
 
Henning said:
Excuse me???? The spark jumps gaps infinitely faster than the piston moves so I'm having trouble figuring out how gap would have an effect related to timing. If I figure this right, a pistons average speed would be around .0058 miles per second on an engine with a 5" stroke turning 2500 rpm, with the pisons speed slowing to 0 at the ends of the stroke so that the speed at 8 degrees BTDC, the piston is pretty slow. Light on the otherhand travels at 125,000 miles per second, so subtract a bit for the atmosphere in the combustion chamber... see my dilema?

I could see the difference in compression pressure between a hot and cold engine having an effect on whether the spark can get across or not.
Click to expand...

Yep, I suspect Richard was thinking about the points gap not the plugs. Plug gaps can certainly affect starting, but not timing.
 
Steve said:
186,000 miles/second...it's not just a good idea...it's the law!

:goofy:
Click to expand...

FWIW, it's "the law" WRT light in a vacuum, but light goes even slower than Henning's 125,000 miles/sec in some media. Electrical current, which is what crosses the gap, flies through the compressed fuel air mix at something like 80% of the speed of light (in a vacuum) IIRC.
 
Steve said:
True, true.

You can even make charged particles go faster than the speed of light in water. Sort of a "sonic boom" of light.
fullpower.png
Click to expand...

Ah yes, Cerenkov radiation. And for you folks wondering if this violates the theory that nothing can travel faster than the speed of light, notice that Steve said the particles are going faster than light would travel through the medium, not faster than light would travel in a vacuum.
 
gibbons said:
Did I mention that the hot start problem was solved? :)
Click to expand...

Didn't all that stuff about the speed of light help???:D
 
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