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Engine Backfiring Problem

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Ben Kelly27/07/2008 22:56:00
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I have an SC 91 Four Stroke that is installed in my Ultimate Bipe, the problem im having is that every time i try to start the engine it will backfire and throw the prop. I have been in contact with a couple of people and have adjusted the tappets so that they are in the correct positions as the gap was a bit too big and the 0.10mm feeler guage passed through too easily and im opening the needle valve about 2 - 2 1/2 turns and it still backfires every time. I dont think its the fuel because i am using Four Stroke fuel and glow plug. i dont know what else to try.

Hope someone can help me out with this.

Cheers.

Ultymate28/07/2008 08:35:00
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Ben in your post you don't say if your using a starter or a chicken stick, personally I've found this rarely happens when using a starter, if you're using a chicken stick then flicking or more correctly bouncing the prop backwards i.e. clockwise against compression. This way the engine bounces against the comprerssion and fires itself in the correct direction. One final question does your motor have a locking nut on the prop if so make sure it is really well tightened, hope some of this helps Cheers Brian

               

Ben Kelly28/07/2008 08:44:00
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I have got a locking nut on the front, when I said its throwing the prop what I meant its undoing the lock nut and the prop is freely spinning and I am using an electric starter to turn it over which is another reason I can't understand why it keeps backfiring.
Ultymate28/07/2008 08:51:00
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Another question then are you using a spinner with a relatively smooth backplate sometimes these can cause slippage. Try it without the spinner or make and fit some double sided washers from wet and dry sanding paper (so you have the grit side to both sides of the washers) and fit one either side of your spinner backplate. Cheers Brian
Bruce Richards28/07/2008 08:58:00
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1849 forum posts
You could also try making washers from cereal packet cardboard and putting these between the prop/spinner and the prop driver. (works for me) If it still trows the prop I think you may not be tightening the prop nut sufficiently.
Ben Kelly28/07/2008 10:48:00
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the spinner I'm using doesnt have a back plate that goes behind the prop. The prop is pushed on first against the front of the engine then the part of the spinner with the cutouts is put on, followed by the washer, prop nut and locking nut and then the front of the spinner which screws to the back piece. I have also had a couple of other people check that the prop and locking nuts are tightened sufficiently and still the problem persists. Could I send it off to somewhere like Just Engines and have them service it or somewhere that will do it. Thanks. Ben
Ultymate28/07/2008 11:45:00
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If your spinner is a plastic one that could be your problem in as much as however much you tighten your prop nut(s) all thats happening is you're crushing the plastic of the spinner instead of really nipping down tight on the prop. Before sending the motor off for servicing I'd try either a different type spinner or none at all and see if the prop loosens then
Eric Bray28/07/2008 11:58:00
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I have very little experience of four-strokes, as they were just coming in when I gave up using greasy engines, but for what it is worth, try a bit less nitro in the mix, or a different plug. What you are getting is pre-ignition - ie the mix is firing too soon, before the piston is reaching tdc, thus stopping it!
Nobby Clarke 228/07/2008 19:30:00
26 forum posts

Sounds like hydraullic lock to me

make sure you havn't got excessive fuel in the engine 21/2 turns out seems a bit on the rich side.

Is the engine inverted.? if so, try starting it with the engine is in the upright position.

Ben Kelly28/07/2008 22:03:00
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well i have tried the method of using sanding paper to grip the prop and this seems to have stopped it backfiring everytime i start it the only problem i have now it the engine dies out when i take off the glow plug at idle even when the trim is maxxed out to open it, but i guess this will need the idle mixture screw adjusting.

Thanks for all the help.

Ultymate29/07/2008 07:58:00
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1690 forum posts
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Ben thanks for reporting back with your outcome, make sure you're using a 4stroke plug and good luck with the idle adjustment.

                        Cheers Brian

Tim Mackey29/07/2008 08:30:00
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Ben. I have the exact same engine and have to say I never have any issues with it at all. One thing which does differ however is the mixture setting - I have my main needle out around only 1.5 turns compared to your 2.5.

I do however run an onboard glow system as my engine is inverted. 

Use an OS "F" type plug, and no more than 10% nitro in synthetic oil based fuel.

I take it the prop driver is in good condition and had not become smooth over time ?

Clive Moore29/07/2008 09:03:00
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150 forum posts
28 photos

Ben,

I have had a very similar problem on a Saito fs. I eventually traced the problem to the slow run needle being set to rich at the factory (it had been set 4 turns to rich and just flooded the engine), once leaned out it started perfectly every time without any backfire or prop throwing. May be an idea to check the setting.

Ben Kelly31/07/2008 18:51:00
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167 forum posts
35 photos

The engine is new its only been run a few times im using a 5% nitro fuel, i have checked everything for air leaks and looseness and i had to tighten up the exhaust port, the prop drive is as it should be ie. not smooth, i have also got the needle valve set at about 1.5 turns now but im sure its still the idle screw in the wrong place becaust as soon as i take off the glow plug it will cut out and i have checked the plug and thats fine.

Brian Parker31/07/2008 19:09:00
538 forum posts

It is either too rich, is there excess visable exhaust smoke when running with glow energised? If so weaken the mixture setting slowly then disconnect the glow. Hopefully all will be well.

 Or the wrong plug is fitted, try an OS 'F' type plug as advised by Timbo.

Good luck!

Ben Kelly31/07/2008 19:13:00
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167 forum posts
35 photos

i am using an OS F type plug but there is, not excessive amounts but more smoke than there proberbly should be so i will lean it out a bit more and keep trying

Cheers.

John Laird01/08/2008 21:23:00
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425 forum posts
42 photos

there is no mention of how far  you have the throttle open when trying to start the engine.

it should only be open a small amount .

I have found the hard way that open throttle starting on 4 strokes can cause backfiring  - shocked the bearings and pulled a saito 90 off the bulkhead - all because after sucking thru fuel to get it wet, I forgot to throttle back before starting.

Engine Doctor02/08/2008 11:22:00
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2249 forum posts
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Hi Ben .All fourstrokes try to back fire when primed and flicked like a two stroke.  First of all discard your plastic spinner ( never use plastic on a f/stroke ) and fit an ally spinner. You will have to addapt the the spinner adaptor nut to a lock-nut to enable the cap screw ,that pulls the spinner cone onto its back plate to be fitted . Starting a fourstroke is slightly different to a two stroke . First prime the engine until nice and wet . Dont connect your glow start yet . now flick the motor over as usual to make sure its not locked with fuel . When happy that it spins ok connect the glow. Now use a chiken stick ,or as I do and flick the motor backwards agianst compression using the spinner the motor will (should ) backfire and spin in the correct direction and usually carry on running. If you wish  to use a starter motor then prime the motor flick over WITOUT the glow connected to ensure it isn't locked now still without the glowconnected spinit over with the stater for a couple of seconds  to empty out excessive fuel . now connect the glow and sin with starter again . you should get an instant start. Once a four stroke is warm then a starter will get it going without all the priming etc . All the forgoing  assumes that the fuel mixture has been set up .  Good luck and let us know how you get on

E.D.

Martin Harris19/08/2008 08:55:00
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8662 forum posts
214 photos

There's a lot of excellent advice in the thread above but if none of it helps you, get someone competent to check your cam timing -  especially if the engine's history is unknown

Don't be frightened to lean on your spanners when tightening the nuts - correct torque is when you feel your eyes starting to bulge.  You'd need to have the strength of King Kong to strip the threads using standard spanner lengths.  I'm assuming you're not trying to tighten the nuts with a multi-way plug spanner which will not have sufficient leverage.

Also, be sure to tighten the main nut fully before tightening the lock nut against it with a separate spanner.

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