Rotax 618 back fire/hard starting

[Big Mac]

Hi
My thunderbird fitted with a Rotax 618 has suddenly developed a sucking
sound and backfire before starting this has also led to power loss
hence no flying.This has also created hard starting
This engine has been serviced regularly,every 25 hours clean air and fuel
filters,replace spark plugs
TBO including crank replacement was done 150 hours ago
Using 95 unleaded and Shell nautical oil ( 25 l petrol 500ml oil) premix
Regular flying twice per week
Checked valves they are free
Checked spark plugs colour of burn seems good (brown) 13 Hrs done since last service
Checked carburettors float levels correct and petrol supply OK.
Compression good
Dekoke done 50 hrs ago
Can anybody give me some further advice and possible cause
Allso looking for engine manaul with fault finding
Thanks
Big Mac

[zoobie]

Hi Bigmack.

I had similar problem a while back, but on 582. Engine would suddenly lose power and every now and then she would backfire quite badly. I eventually found that the exhaust was the problem. Have you made any mods to it?..if not, then check that the elbow that is connected to the Y piece is sealing properly. Mine was leaking there and I assume was causing a loss of back pressure. These 2 strokes rely on exact back pressure pules to function correctly.

Hope this helps. cheers Zoobie.

[Wargames]

Hi Bigmack.

I had similar problem a while back, but on 582. Engine would suddenly lose power and every now and then she would backfire quite badly. I eventually found that the exhaust was the problem. Have you made any mods to it?..if not, then check that the elbow that is connected to the Y piece is sealing properly. Mine was leaking there and I assume was causing a loss of back pressure. These 2 strokes rely on exact back pressure pules to function correctly.

Hope this helps. cheers Zoobie.

Agree with zoombie,

Backfire is normally the result of presure loss/gain of the exhaust. For engine to run normally, you need some kind of presure in exhaust. No to much, because then you are "chocking" the engine to death. Just make sure that there is no holes in exhaust, if not, check silencer for inner mufflers. Just take it off, and shake it. If there is a rattle, then you know where your engine is loosing its power.

If nothing, try someone else to lend you an exhaust as control to experiment.

Hope you get it right.

[t-bird]

The 618 have adjustable exhaust valves. These valves tend to get stuck in a position. This could lead to a back fire.

[Rudix]

The 618 have adjustable exhaust valves. These valves tend to get stuck in a position. This could lead to a back fire.

That was my 1st thought as well but we checked that, they are not stuck and even if you over-ride the normal operation it still does it.

It seemed slightly better with the air filter removed, once started it seemed to run nice and smooth at lower throttle settings but would not develop full power.

Fly safe,
Rudi
PS, I think we need to check the exhaust next.

[Big Mac]

Thanks gents
I intend removing the exhaust system first because it makes sence
that if faulty the sudden demise of back pressure will have a major
effect on a two stroke engine performance
Will give feedback once the problem has been solved
Thanks
Big Mac

[Big Mac]

Hi
Found the problem,after removing the exhaust system found No 1
piston seriously damaged and the top ring is missing.
Appears damage caused by detonation.
Will be removing engine this weekend and take same to Trevor Davies
for repairs.
Thanks again for advice
Big Mac

[Rudix]

Hi
Found the problem,after removing the exhaust system found No 1
piston seriously damaged and the top ring is missing.
Appears damage caused by detonation.
Will be removing engine this weekend and take same to Trevor Davies
for repairs.
Thanks again for advice
Big Mac

Oops !

I have seen that before on a 618, in that case the locating pin on the top ring came out allowing the ring to rotate till the open end got stuck in the exhaust port, exit one piston

Regards,
Rudi

[MADDOG]

Oops, story of my life - Piston Broke

[Big Mac]

Trevor Davies overhauled the engine. Fitted back to a/c yesterday and
test flight done. Everything sounds good and flying great.
The cause of the damage to the pistons appears to be the stator ,one of
the electrical wires worn through the insulation causing short possible
loss of current to spark plugs hence over detonation.
Once again thanks for advice
Big Mac

[Broker]

Hi BM...glad you are sorted out. May I please have the contact detail for Trevor Davies? Where doed he operate from, I also have a 618 in need of some TLC.

[Morph]

+27-11-952-1743

Plot 8 Reydal
Magaliesberg Rd
Tarlton
Gauteng

I think he uses the Crosswinds Airfield

http://www.tde.co.za

[Tailspin]

Hi There

Morph Beat me to it - i also have his cell number is you need it.

[Big Mac]

Hi Broker
Morph gave the address
The land line is not working as a result of cable theft.
You may contact Trevor on his cell 082 442 3925
Big Mac

[skidmark]

I have not gone through all the posts. But,. Paul Van Ass and I owened a Kitfox taildragger which had a 618 motor. We served our apprenticeship on that one! Rave valve's etc. But once you got it right its a great motor. He could give you a lot of help and info ....I think! pm me and I will give you his cell number. I think he's pretty well in touch with that motor as he made it his mission to get it right.