wierd....i bet i can stump you....

[spanko123]

ok my buddy was runnnig his xtm x-cellerator and it was running good right? then he shuht it of give a break. then started back up and gav it a lil throttle and it jumps backwards. so he though it just rolled down a hill or sumthing. then he gave it a lil more gave and it was full blown going in reverse! its unbleviable. i can't belive it. anyone ever heard of this

has it ever happend before? its pretty crazy cuz it was running forward before it. then he shuut it off and started back up and vuoalla! it wwent foward again so anybody got a reason for this??? crazy huh :nope:

 

[TINY_MAXX]

I'm guessing that the crank started to spin in the opposite direction at start up..Other then that i dont know.

 

[spanko123]

i dont no almost positive that it didnt RUN backwards as in crank shaft goin backwards(ps. pistons go up and down so they can't go backwards lol i always used to say that) but i dont no must have been sumthing with the gear box or sumthing freaky any ideas people?

 

[cramey88]

The motor did in fact start in the opposite direction this is actually pretty common on a 2 stroke engine usually happens if combustion fires while piston is at bottom dead center.I have seen it before its tragic!

 

[Error401]

I've had that happen at the track. Luckily a friend of mine video taped it.

Engine running backwards at WOT.

 

[nitromaniac]

Now that was funny!!!

and it hopped right on that ramp too! good one!

Now see if you can get a vid of it running sideways!:hypno:

 

[lillightingboy21]

Haha that would be a site to see. But i have no clue on how that has happened.

 

[Waste Land]

He could have knocked his servo reversing switch backwards

 

[SlingItX]

Twisted

Error, it's a bit difficult to type this I'm laughing so hard - it actually took me a second to translate the image (a bit grainy) but then I started rollin'.

It must be pretty hard to stick those landings - obviously you're a pro.

spanko-
Never seen that myself, but I've heard it happening once or twice (crank spinning opposite direction). However it was directly related to the starter box wired wrong if I remember correctly.

Of course with a pull start if the piston came around approaching TDC and the nitro pre-ignited it could force the piston back down - I don't know, I'm just speculating. Possible, just not probable.

And if you'd be a little more clear in your post regarding all the specifics it might be a little easier to diagnose.

-SlingItX

 

[StrechM]

Now come on gentlemen, you know that this sort of thing usually only starts to happen around early May when the weather starts to warm up.

 

[Error401]

I find that it's the proximity of the moon that causes it mostly. Next time it happen see if it's nearly a full moon.

 

[SlingItX]

Great, and I thought it only affected my wife (I swear she runs backwards sometimes) now I have to worry about my buggies. Super. Makes sense, I have to go to the store to get plugs for my buggies too.

-SlingItX

 

[StaticSag]

Cramey88 has the idea here folks.....when I was younger my dad took my 83 CR60 MX bike and put it in 1st gear.....on the stand and spun the rear wheel backwards and it would crank and run backwards.....it is nuts but its true.....but I do remember him explaining it as something to do with it holding compression ( caused by too much backpressure) at BDC and the engine would build too much compression on the next upstroke and cause a backlashing effect that would spin the engine backwards. since its timed on the crank position to fire at TDC ...it really doesn't matter which way the crankshaft is turning....needless to say.....you can really scatter an engine...especially if ya try to shift gears....lol

-Sag

 

[Error401]

Did the engine on your bike have the intake timed by the crank? That would be tru if the carb and FA induction had nothing to do with the rotary port on the crank like model 2 strokes do.

Here's a decent website thaat explains the model 2 stroke, ports and valve timing (page 3).

http://www.rcnitro.com/rn/articles/engine_timing.asp

 

[SlingItX]

Where would the excess back pressure be coming from? I understand there is resonance from the tuned pipe, but would this be significant enough?

For a crank to fire backwards it would have to be combusting prior to TDC (given it started off spinning the correct direction), I'm just having a hard time wrapping my head around the reason. It takes X amount of compression for combustion to take place. What could cause the excess pressure to build up? Was his knee blocking the stinger slightly when he yanked the recoil? Flooding?

Aargh, I'm glad mine run the way they're supposed to.

-SlingItX

 

[Error401]

to my understanding, at BDC the intake port under the carb is closed (and has been for a while), the pressure in the crank case had been built up by the decending piston and the trnsfer ports on the cylinder have been open for a bit with the FA mix entering the cylinder and expelling the exhaust. IMO, unless something really freaky happens, the FA mix won't combust in this state.

If it fires before TDC you will get pre-det, which will cause pitting and other engine damage, but only at a low RPM will it suddenly reverse the direction of the crank and run backwards. Most engines do this when they die. Ever notice the loudish "Ping" or "clunk" when your engine dies on it's own? That's what's happening. The ping is the pull start and 1 way getting hammered by the reverse turning crank.

As stated before, 2 stroke model engines with rotary induction (crank timed FA intake) will run backwards, but not well, not long, and not over an idle. It's due to the port on the crank being 90 degrees out of normal for running backwards. If the port was moved 90 degrees clock wise when viewed from the front (clutch bell side), it would be ported to run clockwise. For some reason, ported as they are, some FA does get to the cylinder when they're running backwards, but not enough to facilitate proper running. Essentially, the way it's ported, it's trying to go bang, then get the fuel for that bang.

Reed valve engines are a completely different story. They are not rotary valve and don't care which way they run. This applies to any reed valve engine.

 

[SlingItX]

Many Thanks

Much appreciated Error. I spent some time reading the chart from the link in your post, now it's clear.

You missed your calling as a technical writer (maybe... not sure what you do).

-SlingItX

 

[YZ400Man]

I did have the engine in my buggy start to run backwards while I was driving it. I was driving it up and down the street. As I brought it back it sputtered a bit, I let off the throttle. When it returned to idle I started to press the gas and it went backward. Now after reading what error401 wrote I think I understand a little better what happened.

 

[Maxx Trooper]

I dont know if u guys have heard of this, but when i got my first nitro r/c.. a TRAXXAS Nitro Hawk. i started it up one time and it went in reverse, but not for long, the engine cut after going a few feet. weird though. i have also had it happen to my TRAXXAS Nitro stampede with a OS engine.

later

 

[StaticSag]

Originally posted by Error401
Did the engine on your bike have the intake timed by the crank? That would be tru if the carb and FA induction had nothing to do with the rotary port on the crank like model 2 strokes do.

Been 20 years since he done this but let me see if I got ya on this. That engine was not case reed inducted but was cylinder port inducted above the piston and the vacuum of the downstroke pulled the fuel in. The fuel entered directly on top of the piston. The backpressure was caused by the almost closed exhaust port (this was closed by an ATAC valve which closed the exhaust port at low RPM to increase low rpm power) The fireing was timed by a crank positioned sensor.

I am very new to the R/C scene (because my body can't take the punishment of MX after 25 years) so you kinda confused me with the last sentence there....after a little thought and reading that article I realize what you mean that the fuel air mixture is timed by the crankshaft due to the rotary valve configuration. Realizing there were more differences than I originally thought between the innerworkings of the 2 styles of 2 Stroke engines I decided to tear down my engine and peak in for myself. After that its hard to see how its possible for a nitro engine to run backwards. Thanks for giving me something to do this morning while the kids were napping....lol

-Sag