spinning engine backwards....

averagebrunette

Fun Fly Superstar!!
I've heard and seen a few people spin the engine backwards to "clear things out". This is where not enough knoweledge gets kinda funny.

I understand timing in the 2cyl engine and that every time the piston comes to top dead center the plug fires, big fire ball, prop goes round and we have a good time. Intake happens just before compression and exhaust is the first thing out after the "bang".

How does spinning it backwards help for starting?? I see the piston hitting TDC and on the way down the exhaust port is still the first port it sees to relieve compressed gasses...

I'd really like to understand the "how's and why's" of this magical thing I'm seeing that helps with starting.
 

Jedijody

Proud father of Soldiers
It doesn't, it makes no difference. The engine doesn't know which direction it's turning, it pumps air and fuel equally well either direction.

Your knowledge is fine, whoever told you that is the one lacking it in this case.
 

Tired Old Man

Staff member
Lot's of glow people flip props against normal rotation to sort of "bump start" their engines. I never did quite get behind that concept.

As Jody said, a 2 stroke will run either direction, but the spin direction with the timing correctly established for that particular direction will run a lot better. They'll even run backwards with the prop on backwards too;);)
 

Tired Old Man

Staff member
Better to ask Jody:) Every so often everyone does something that drives them nuts trying to figure it out. With what we do that particular one is rather common with people when they are new. Can't tell you why here, perhaps Terry will. I can't be smug about it though, I did the same thing a few years earlier.
 

WrongWay

Well-Known Member
Lot's of glow people flip props against normal rotation to sort of "bump start" their engines. I never did quite get behind that concept.

As Jody said, a 2 stroke will run either direction, but the spin direction with the timing correctly established for that particular direction will run a lot better. They'll even run backwards with the prop on backwards too;);)
Starting procedure for Saito 100.

With glow ignitor off open throttle to half, put finger over carb inlet or exhaust inlet (when not hot) and flip it until it squishes (usually 3-4 flips)... this would be a condition most would call flooded.

Take finger off intake and again, with no glow ignitor flip it forward 6 times hard.

Reduce throttle to one click, hook up glow ignitor and briskly flip the prop backwards against the point of compression... You can do this all day and it will start on 1/2 of a back-flip.:willynilly:

The reason for back flipping is at a certain size a glow engine has enough compression that forward flipping may not make it through TDC before flame front propagation and the result is an engine that will start and run backwards. By back flipping it actually kicks backwards only to start forward...

Watch it on the Moki 1.8 and up... just moving the prop backwards slowly will gain a hard kick or run if your hand is not in the way to stop it... I used to just grab the spinner nut with two fingers and gently flip it backwards...

I did see a DA85 go from running forward to backward once. The idle was too low, the engine was almost stopped and the pilot stabbed the throttle only to have the engine hit and run backwards as approaching the ground for a pop to hover...he wound up in the mud with nothing broken and flying again in two mintues....100cc bird too..:biggrin:
 

Jedijody

Proud father of Soldiers
Lol!!!!!

I thought nothing got past Jody...........
Whatever gave you that idea? If I write with conviction it's because I don't write at all unless I know what I'm talking about. I'll also defend my right to express an opinion, or joke around.

I am very capable of mistakes, in our line of work it's part of the process at times. I'm also capable of faking them to make Pat feel better, lol. :biggrin:
 

JeffinTD

Well-Known Member
I have a 1.00 nitro that if you hand prop it forward, more often than not it will start and run backwards. Oddly it likes to run backwards if you flip it backwards.
 

Bunky f. knuckle

Cover shot, MA 10/09!!!
Whatever gave you that idea? If I write with conviction it's because I don't write at all unless I know what I'm talking about. I'll also defend my right to express an opinion, or joke around.

I am very capable of mistakes, in our line of work it's part of the process at times. I'm also capable of faking them to make Pat feel better, lol. :biggrin:
:) I got ya, Jody!! Just givin ya guff! ;)

We all make mistakes, I'm certainly not perfect. Noone is. And if there is one, show me, and I'll make a liar of them! LOL!!! :)
 

gkamysz

Member
I saw this thread yesterday and the only thing that came to mind was it may be a glow thing. Timed rotary intake when cranked backward would tend to blow a sloppy wet carb out. Obviously piston port or reed this doesn't matter.

Greg
 

Tired Old Man

Staff member
On a gas two stroke set up as most of ours are, most will run in either direction. Timing is what establishes how well. Those that fly multi-engine models find it very easy to eliminate "critical engine " issues by simply re-timing one or two engines (2 or 4 engine arrangements) and use a reverse rotation propeller. There are some pusher conditions that permit just mounting the prop upside down. Only one number you have to remember and that's 28. It will be used either before or after TDC, depending on which way you want it to run.

In either direction if the timing is not relatively optimized an engine will run like crap. With a normal rotation engine running backwards the timing is so far off they run like hammered crap.
 

averagebrunette

Fun Fly Superstar!!
Thanks soo much guys!! It feels good to know that what was in my head was right.

FYI- Jody... don't ever fake it!! ...especially for Pat ;) LOL
 

Mithrandir

Circus Staff
Lifetime Supporter
I start most slimers by flipping the prop backwards against compression because the leading edge of the prop won't cut my fingers......

if it don't start on the first flip.. I usually resort to an electric starter....
 

WrongWay

Well-Known Member
On a gas two stroke set up as most of ours are, most will run in either direction. Timing is what establishes how well. Those that fly multi-engine models find it very easy to eliminate "critical engine " issues by simply re-timing one or two engines (2 or 4 engine arrangements) and use a reverse rotation propeller. There are some pusher conditions that permit just mounting the prop upside down. Only one number you have to remember and that's 28. It will be used either before or after TDC, depending on which way you want it to run.

In either direction if the timing is not relatively optimized an engine will run like crap. With a normal rotation engine running backwards the timing is so far off they run like hammered crap.
The DA85 that ran backwards in flight for my buddy sounded like a diesel... ran like hammerd crap.

I start most slimers by flipping the prop backwards against compression because the leading edge of the prop won't cut my fingers......

if it don't start on the first flip.. I usually resort to an electric starter....
Yep, against the compression and not through the compression when flipping backwards... If the fuel air mixture is right it is a great way to get consistent starts..
 

JohnBer

Well-Known Member
i have bump started my 4 stroke glows for years, never had the guts to try it with the 450 r3 tho. that thing was a pita to start cold. electric starter every time. 2 stroke glows......our club banned em so i never messed with em much. funny thing, they said it was for noise, but allow gas 2 strokes of any size or over 1 ci 2 stroke glows? never did figger the sense that one out.
not sure i have any glow engines left anymore...... 30cc and over i run gas, under is electric.
running em backward to clear em out? sounds like a idea that would happen after taste testing a new batch.
 
Top