I have heard of some old chainsaws running backwards.
I was thinking the compression would be the same. But, the port and transfer positions on these non reed valve saws would make them impossible to run.
Thought I would check .
Here is a question for you
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I was thinking the compression would be the same. But, the port and transfer positions on these non reed valve saws would make them impossible to run.
Thought I would check .
I would think the port and Reed valve would function just exactly the same. The problem is the spark is happening before top dead center in normal operation in reverse it would be after top dead center, I do believe.
huskydave
Addicted to ArboristSite
Yes but coil polarity needs to be switched or the engine will not start in that direction. You can start most engines in reverse if you just flip the coil. Mercury snowmobiles used to have 2 coils and you could kill the forward coil and start the engine in reverse, to start in the other direction you would kill the reverse coil with a single pole double throw switch wired to ground and the 2 coils. Modern ignitions can be programmed to do this but you still need a switch to tell the ecu which direction to go and what direction to run the starter if it has e start.
I was working on a friends rancher. He said it would start but run like crap. I started it and saw somehow it was running backwards. Put a new coil and n it and fixed all. Must be the coil was sending spark at the wrong time and when it fired it would kick the motor backwards. The flow through the engine is identical either way so I would imagine the compression should be identical backwards.
Running a 2 stroke backwards isn't anything new. The merc "dock crashers" del refers to changed timing based on the direction the engine spun over. Ez go, club cart etc basically used straight up timing on their 2 stroke golf carts no adjustments to the timing were done and all the switch did was change the starter motor direction. The piston doesn't know or care which way the crankshaft is spinning.
kshansen
ArboristSite Operative
A bit off topic other than the running backwards part.
I know a Detroit Diesel(actually a two stroke so close to on topic) will run backwards if you stall it going up a hill and let it roll backwards in gear. Biggest problem with that is even though it is a two stroke it does have an oil pump for lubrication. So if it runs for more than a few seconds there could be damage from lack of lubrication.
Now if you have need for a reverse running motor they did make them for either clockwise or counter clockwise operation.
I know a Detroit Diesel(actually a two stroke so close to on topic) will run backwards if you stall it going up a hill and let it roll backwards in gear. Biggest problem with that is even though it is a two stroke it does have an oil pump for lubrication. So if it runs for more than a few seconds there could be damage from lack of lubrication.
Now if you have need for a reverse running motor they did make them for either clockwise or counter clockwise operation.
Uniflow design with a wet sump, yes detroits were notorious for running backwards even though they were not designed to do so. Reverse rotation engines had a host of different parts to account for it.
huskydave
Addicted to ArboristSite
I have a 2003 MXZ 800 Rev with the x package and it has electronic reverse. It works slick but if the engine is not running perfectly it will have trouble restarting in the opposite direction. I had to re-jet this machine to 20 pilots and shim the needle 20 thou because it was setup too lean in the midrange from the factory.
In this video at 1:30 you can see the distributor rotate to give the correct timing for reverse. The distributor also rotates to advance timing as the throttle is advanced in forward and reverse. There was limited throttle in reverse mode.
If the system was fine tuned it worked well. Carl K. believed a shifting dog on the prop shaft was much weaker but his #1 man and genius Charlie Strang convinced him to change to the modern shifting system that we have today.
At 1:30.
Here's a great read:
https://www.amazon.com/Iron-Fist-Lives-Carl-Kiekhaefer/dp/0945903049
1950 Feather Craft Fire Fly / 1969 25 hp Johnson motor.
If the system was fine tuned it worked well. Carl K. believed a shifting dog on the prop shaft was much weaker but his #1 man and genius Charlie Strang convinced him to change to the modern shifting system that we have today.
At 1:30.
Here's a great read:
Iron Fist: The Lives of Carl Kiekhaefer
https://www.amazon.com/Iron-Fist-Lives-Carl-Kiekhaefer/dp/0945903049
1950 Feather Craft Fire Fly / 1969 25 hp Johnson motor.
huskydave
Addicted to ArboristSite
I restored one of the 6 cylinder Mercury tower of power engines last year, thing was a torque monster when I was done.
I restored a 1960 Glasspar G3 and put a 15 inch shaft Mercury 850XS on it.
Neighbor had a Mercury 150 stack 6 with that 15 inch shaft. I was afraid of that.
Should have built the transom back 20, just wasn't thinking.
huskydave
Addicted to ArboristSite
they haul the mail with a speedmaster lower unit and a cleaver prop.
FS60 (curved shaft} could be converted to an FS61 (straight shaft} by changing the ignition and recoil to run in the opposite direction. 020AV with points Could backfire and run backwards when the timing was off.
I had a China saw here that would start,run rough,backfire then run in reverse.
New coil fixed the problem.
New coil fixed the problem.
MPF
ArboristSite Lurker
Off topic backwards running part 2:
All of the hybrids ( cars, SUVs ) do not not have a reverse gear. The PCM directs the electric motor to go in reverse, keeps things simpler & less costly since it's a limited use affair.
Just don't go down steep boat ramps in drive!
Flywheel is going to be in same position running clockwise or counterclockwise.
For instance, if it was a dumb coil would it not be ******** 25 degrees or so instead of advanced ?
HansFranz
Addicted to ArboristSite
LOL "Incoming!" (Or would that be "Outgoing!")
Last time I took my bike for annual safety inspection and I was going through the brake light, turn signals, hi beam, lo beam routine, the inspector was like "Reverse...?" Stopped me cold for a second.
Uhhhh... 
Not an electrical engineer, but I suspect the coil just detects a trigger point to fire.
Coil also needs the magnets to charge. Not sure if they are symmetrical about the trigger point?
Would be interesting to put some timing marks on the flywheel, then spin the saw with a drill (spark plug out), and see where it fires with a timing light.
