dcn10

I just got through shimming my diffs and now the engine fire up backwards. I don't see why working on the diffs would do this.
Thanks in advance for any thoughts.

firebird78

noway can shimming diffs cause the engine to crank backwards....the only ways it could is if you have a bump start back plate, or if you put the one way bearing on backwards and use a roto start..

dcn10

Its a pull start and I did not take it apart just shimmed the diffs. My diffs where sloppy. It just keeps starting in reverse. I forgot to say which one of my toys was doing this. Its a thunderfire.
I noticed that you have a Revo. My friend keeps braking his motor mounts. Where can he get a better motor mount?
Thanks

dcn10

Ya'll goin to love this. I put both front and rear diff's in backwards causing me to think the engine was running backward. I hope you laugh at me cause I shure did when I found out what I did.

tolofrhr

That was what I was gonna say. that's the only thing that it could be. I would laugh at you but I done stuff equally dumb.

chevyderek72

Haha havent we all

rjsdad

My engine started in reverse one time. What makes this interesting is that I didn't change anything off my buggy...no repairs, no switching of a part, nothing. the location of the piston must have been in the right spot...so when i yanked on the pull cord...it was running in reverse! It is possible since it is a two stroke engine. I didn't let it run long so I cannot tell you if it would have cut off eventually.

jpilkinton

I have always wondered this myself. Is it just the crankshaft air/fuel port window the only thing stopping it from running backwards?
I was wondering if your could cut the window out a little more on the opposite side and build the other side up. Basically reverse the port window for reverse rotation.
It would be interesting to see this. Then you could mount two engines crank to crank.....

rjsdad

Sounds like a project! LOL

SAVAGEJIM

By doing so, you will more or less ruin the engine. It will never run like it did before.
Advancing the crank induction port duration is one thing; that is if you are after power. But increasing that timing profile by cutting on the wrong side does nothing good for the engine.

It will still run, and yes, it will fire up in reverse, even without any port mods, but the power will be horrible and the engine will totally run like crap.

jpilkinton

I know that, that is why you would have to somehow reverse the crank timing.
I know that cutting in the wrong place would lead to disaster.... duh!
I was actually referring to reversing the crank timing(port window...hole in crankshaft where the fuel/air enters the crank....whatever you call it).
Of course it would take alot of time, work and reverse engineering.
I need to get me a degree wheel for these little engines.
I believe it could be done.

SAVAGEJIM

It might be,able to work, but the problem I can see is that the the induction port (the crank port we are talking about) is purposefully staggared from off-center so that the crank induction part of the cycle occurs at the proper time when the lateral ports, boost port, and exhaust port open and close at the appropriate time.

And since the crank induction port does not have its mid-duration coincident with the other ports' duration mid durations, cutting on the trailing edge of this port will not correct for this staggered effect.

See the attached timing profile map I made.

Also, here is the thread in which we originally discussed a bit of mapping the port durations of a timing profile (see post #8 oin this thread)
http://www.rcuniverse.com/forum/m_48...tm.htm#4852273

SAVAGEJIM

I totally forgot to mention what the words are on this port timing profile map.

*Schneurle is the duration of the lateral sleeve intake ports; the primary ports that take in fuel-air into the cylinder.
*Boost is the forward boost port that helps helps give an extra shot of fuel-air into the cylinder. Some VX/CXP engines have a sleeve boost port, but unfortunately, they are useless because the block does not have a boost passageway to feed that port
*Exhaust of course is the duration of the exhaust port.

*Induction is the crank induction port we are talking about cutting. Notice how the other ports are all nicely lined up on the bottom of this map. The crank induction port is really off set from the others to take advantage f when the other ports open and close so the crank case can induct a fresh fuel-air charge while all the other ports are closed.
There is not much overlap in which all four type of ports are open simultaneously. By cutting the trailing edge of the induction port, you also increase the time that all four port typs remain open. And unless you know what you are doing and understand how fluid flow utilizes these duration overlaps, you will only hurt the engine's power output.

jpilkinton

Thanks for that great information Jim! That is very interesting stuff!
Check this out Jim, this is what I was thinking(very hard to do sometimes!!!), of course everything would have to be bolted down securley, install degree wheel, take a dial indicator and find the degree when the port starts to open and see how many thousands(of an inch) the piston is from top dead center, then slowly turn in engine over untill the port is in closed position, get the degree on that, also and the measurement again on the piston from top dead center. And just reverse the whole thing. Wouldn't that work?
Kinda like dialing in a cam on a 1:1 engine....
What ya think bro?
I think the hardest part would be to "fill in" the port window to reshape it........
Of course I would never even attempt to try this on any of my so called good engines!!!
I have numerous small(.05 Infinity) engines I could tinker with..