smoky 2 stroke

[pedrobedro]

Four stroking when it fires every other stroke. Get it running warm, open the throttle then give it a bit of choke and listen to the difference. The carb probably has three screws marked T, H and L and the H screw needs tweaking out a bit to let it rev but a lot of places richen the top end up a bit to stop it burning pistons. 2 strokes blow oil out the exhaust when they are not burning it all they don't have a dry crankcase, it is oiled by the oil in the fuel which is why you put it in there and why they run roller bearings not shells.

 

[norsa]

Amazing what you learn, thanks guys. Ill try a fresh mix but its the same mix as i used in the strimmer just before i did the hedge. I only need the choke to fire it and then knock it off or it wont run. Once fired it revs and runs smoothe but on full it has this knock/vibration and blue smoke. It is, as ive said unknown about its previous owner apart from the fact it fell out of his van and smashed the gearbox up. Maybe he used the wrong mix and it is burning it out. By the way, my Puch Maxi ran 25:1 but my fisie had auto feed i think.

 

[normspanners]

fuel can change its fuel/oil ratio if stood with an air leak in the container coz the petrol evaporates and leaves the oil, two stokes suffer greatly with this, my mate works in the forestry and says you can tell the difference on the saws when the fuel is a few weeks old they dont produce the same power, and fuel/ oil the plugs much easier some of this is poss down to the fuel losing octane rating as it ages

 

[normspanners]

Fuel
Octane rating

We recommend using a "premium" type automotive fuel with an octane rating of 91, a minimum of impurities and little or no alcohols (maximum 5%).

Some may have noticed that the Rotax documentation specifies an octane rating of 90, based on the RON standards used in Europe. This is the equivalent of an 87 rating under the Canadian AKI standards. A rating of 87 is indeed considered "regular" fuel, but we still recommend a "premium" fuel for two reasons:

First, when fuel is premixed with 2-stroke oil, the octane rating is reduced by about 2 points. An 87 octane fuel would therefore become 85 octane.
Second, fuel evaporates and loses its octane rating when it lays in your aircraft's fuel tank or in a plastic jug. A "premium", 91 octane fuel will see its octane rating reduced to unusable levels after as little as three weeks. Fuel with a lower octane rating would obviously have an even shorter usable life.
Too low an octane rating will create detonation and pre-ignition which can damage the piston crown and even melt a hole through it.

just found this and thought it interesting from a microlight aircraft site re rotax strokers obviously this is worse for an aircraft as as it gets higher tyhe air is less dense therefore the mixing becomes more critical!!
this could be the rattle ie detonation you are experiencing on full thottle
I know fuel deteriates as it ages but I didnt know it was as critical as that, and I have been involved with racing strokers for years but i know fuel has changed over the years and isnt as 'quality' as it used to be.
We dont like using v power that has stood for along time in a tank, in the rally cars, for any hard testing. just use it to warm up or any steady road milage.
this is why some supermarkets can sell fuel cheap we had some at our local SM that was old fuel in stock for the ministry, that they had bought and put somekind of octain booster in that clouded up and was blocking the in tank filters on the local cars, I found the problem and was interviewed on the telly re the literal hundreds of cars we repaired at the time.

 

[PTvor]

fuel can change its fuel/oil ratio if stood with an air leak in the container coz the petrol evaporates and leaves the oil, two stokes suffer greatly with this, my mate works in the forestry and says you can tell the difference on the saws when the fuel is a few weeks old they dont produce the same power, and fuel/ oil the plugs much easier some of this is poss down to the fuel losing octane rating as it ages

I've seen that happen. A friend was cornered into sorting out a toy motorbike which had been fiddled with. We put a cupful of fresh two stroke mixture in the tank and set it up, easier with two. After that it started from cold with no problems, ticked over etc.

A month later, in hot weather, it hadn't been collected. I thought I'd try starting it. No dice. Drained off the fuel and it was thick and goopy. The petrol had evaporated leaving the oil. A cupful of fresh 2 stroke mixture and it was fine.

Petrol also goes stale, I think because the volatile bits which make it light easily, oxidise. I don't know whether that affects the octane rating, but it makes it hard to start.

 

[norsa]

Didnt consider that for 2 stroke but was aware about the octane drop and only use fresh that day fuel in the rally car and any left over after the event goes in the service barge. Q: the adjusters, are they adjusting the amount of fuel or air that goes in? i.e in= lean, or in =rich, and is there a half decent base setting to work from??
Nick

 

[shenion]

The adjusters control the amount of fuel.

Usually L=1.5 turns, H=1.75 turns out from closed.

Close them lightly as to not damage the seat. Best to just go from where they are.

I run full throttle and adjust until it just stops hitting 4-stroke. Another method is to adjust for highest RPM then out 1/8 turn.

 

[norsa]

thanks all.

 

[shenion]

I forgot to mention: most modern 2S engines have caps on the adjustment screws that limit them to about 1/2 a turn.

 

[PTvor]

Out=rich usually.

They adjust the strength of the fuel/air mixture.

The way to set a carburetor up from scratch is to follow the maker's instructions.

If you haven't got them, screw the idle and main screws in as far as they will go, gently, no force. Then screw them out 1 and half turns.

Start the engine and have it running for three minutes to reach working temperature. it may just need a bit of choke.

Turn off the choke, if you've had to use it. With the engine running at speed screw the main mixture screw in and out. There should be a spot where it starts to drop off as the mixture becomes too lean, then as you screw it out, it reaches a plateau, then as you screw it out more, it starts to drop off as the mixture becomes too rich. Set the screw so that it's halfway between the two dropping off positions and with a tweak towards the rich side.

Same for the idle, twiddling the idle mixture and the tickover stop. The things to watch there is that the tickover is not so fast that it partly engages the clutch, and the engine has to pick up as you open the throttle. The slower the better, but with some engines, especially with a lot of hours on, it can be very hard to get a slow tickover.

If you haven't got a good reason to fiddle with the carburetor settings, (run it for a bit to burn off any oil which shouldn't be there, tried a fresh petrol mix) don't.