Stuck Motor.....

Marshall Mauney · Posted: Mon Mar 03, 2003 8:12 pm Post subject: Stuck Motor.....

Well, my weekend was not the best. During practice for the first race of the season at Pat's Acres, I stuck my engine. Apparently, the spark plug worked its way loose from the head just enough to suck in some air. Plain, cold, dry, extremely lean air. End result - a seized piston at the end of the front straight.

Has anyone else had this happen?

MM
Crying or Very sad

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

VERY unlikely.
How much air can you blow past a loose plug thread using 15psi (1 Atm) in 3 milli sec's - (0.003 secs = 10,000rpm) ?
I would look for a more viable cause. like a leaking crank seal or carb boot etc.
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Chad
"Those of you who think you know everything are annoying to those of us who do !!"

Marshall Mauney · Posted: Mon Mar 03, 2003 8:42 pm Post subject:

David Killion · Posted: Wed Mar 05, 2003 11:00 am Post subject: Yes, heard of it

Yes, I witnessed a similar incident to a friend of mine @ Infineon last year, don't know the exact cause but it was claimed a loose spark plug.

Just as a note it's not just atmospheric pressure if the plug is loose because the piston is going to cause a vacuum on the down-stoke causing air to be pulled in from any opening - including the thin area around the spark plug threads. I think the theory has merit.
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David P. Killion - #71 Rocket RK-100 80cc Shifter
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Charley Schimmels · Posted: Wed Mar 05, 2003 11:53 am Post subject:

I'm in with David. I have had this happen on a twin cylinder two stroke before. In that case I could hear a change in the motor sound---burbling-ticking sound preceding the failure.

Marshall:

What does Mike Gent have to say? Has he torn it down yet?

Inquiring minds want to know!

Cheers!

Charley

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

Marshall Mauney · Posted: Wed Mar 05, 2003 5:21 pm Post subject:

Rob Martin · Posted: Wed Mar 05, 2003 5:45 pm Post subject:

Marshall,
I know you have ruled out temp.

When you transferred you carburation to another motor did you also transfer the fuel pump and airbox?

If not, I would closely examine your fuel delivery system and ensure that you dont have any pinched or kinked fuel lines. If the fuel pump fails to deliver an adequate supply of fuel you could end up with a lean condition even if the carb is jetted correctly. If your air filter or intake boot becomes damaged you could also increase airflow compared to your normal conditions.

Both of these items can contribute to a seizure under the right (or wrong) circumstances.

If the plug was loose prior the seizing, was there any noticable drop in power? If the plug was loose enough to allow air to be drawn into the cylinder, your compression loss should have been very noticable. Compression ~120psi, atmosphere 15psi

Was most of your fuel mixture being pushed out the plug hole?

Is it possible that you lifted-off the gas for a little bit longer than normal at the end of the straight. If so, the motor was still turning at a high rpm without receiving any lubrication or cooling from the incoming fuel mixture plus the possibility of cold air entering directly through the plug hole would be a recipe for a stick.

The loose plug was probably a contributing factor but likely not the only factor.

Donald Reese · Posted: Sat Mar 08, 2003 11:41 am Post subject:

HEY MM
In fact you will create a lean condition when the plug (becomes)? loose! the air/fuel mixture is drawn in from the pistons movement on tthe down stroke. if air was drawn in on that fairly cool afternoon it would in fact lean out, sorrry to hear about it. cheaper than a ford motor though! Wink

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Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

Marshall Mauney · Posted: Tue Mar 11, 2003 10:16 pm Post subject:

Interesting theory.... I hope I don't have the opportunity to learn more about this type of catastrophic failure!

Well, apparently, the new crank hasn't arrived yet, so I'm still without kart - or at least engine.

MM
(Keeping my fingers crossed that I'll get it together in time to get some practice in before the next RMC race at Pat's....

David Killion · Posted: Thu Mar 13, 2003 8:14 pm Post subject: Good thread

I'm sorry that this thread had to develop from the catastrophe of your engine, but I'm sure getting a lot out of this.

Chad - I definitely see your point and I had my 4-stroke hat on when making that above comment. I'm not entirely convinced that even under power there is never a vacuum in the cylinder - but I do like Rob's point that is made about letting off the throttle at the end of the straight - during a condition where little or no ignition is happening I could see a vacuum being created in the cylinder for sure. I'm new to the 2-stroke arena and I'm operating on mainly theory - there are probably many people with practical experience (like Chad) that are more qualified to make these assumptions.

I would like to hear from an engine builder on their theory on whether or not air can or will be drawn in through a loose spark plug or not. Under power or not.

If anybody has any interested in a nice description of the 2 stroke engine and a nice animated .gif file check out this site: http://w3.one.net/~jschust/animations.html I especially like the description on how the exhaust assists in forcing the fuel air mixture back into the combustion chamber reducing the common short-circuit problem with the 2-stroke design.
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David P. Killion - #71 Rocket RK-100 80cc Shifter
http://www.teammmi.com/html/team/dkillion/dkillion.htm

Chad Stapleton · Joined: 19 Jul 2001 Posts: 4403

Even when you are "off the throttle", the piston still compresses the gas in the cylinder (ie, +ve pressure) on the upstroke, and decompresses it on the down stroke untill the Exh port is opened. For thr rest of the stroke the Cylinder is open to the atmosphere via the exhaust until the piston covers the exh port on the upstroke again and the cycle is repeated.
Now, it is possible that even with the exh port open, the pressure in the cylinder may be less than atmospheric due to the residual pressure pulses in the exhaust, but it is unlikely to be significant in terms of the pressure differential required to force air past the plug threads in the few milliseconds that are available, and remember , this is when we are not burning the mixture so its not going to create a "lean burn" effect.

I slill favour the "hot plug" explanation for these failures rather than the weak mixture theory. Wink

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Chad
"Those of you who think you know everything are annoying to those of us who do !!"

Rob Martin · Posted: Fri Mar 14, 2003 12:15 am Post subject:

Chad,
My "off-the-throttle" comment was really related to the lubrication being provided to the cylinder at end-of-straight RPM's.

I can't imagine any 2 stroke would be capable of sustained high RPM in the event of no fuel/oil delivery.

Once engaged the drive train performs like a direct drive. As long as you are using the motor for braking the cylinder is in danger of running dry as fuel/oil delivery is minimal or absent.

I just realized no one has mentioned fuel/oil ratios as a possible contributor to the stick. I guess everyone just follows the Rotax recommendations. Wink

Roel Peters · Joined: 21 Jun 2002 Posts: 120 Location: Netherlands,

The cilinder pressure of a 2-stroke can be under atmospheric press., but only very little. As hot gas escaped trough the exhaust port the air gets a velocity/inertia in that direction, if the pressure has dropped to atmospheric level gas still can escape trough the exhaust valve because it already has a velocity and cant stop at once (thats why valves in a 4-stroke are closed beyond the bottom/top dead center). But this pressure drop, if any, would be very small.
And if air could be sucked in......imagine how much mixture would be pressed out as the piston goes up: pressure difference are 100 times greater, so there would be so little mixture left that the extra fresh air sucked in doesn't matter any more. (And there would be power loss)

The explanation of a hot spark plug sounds very logical: