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M/C Mechanics

The Art of Pre-Mix

Mixing two stroke oil with the petrol is one of those 'chores' that the Japanese manufacturers very quickly decided to alleviate us from, when they started to make high performance two strokes with oil injection systems.

Which is all well and good, except that we have got so used to not having to mix two stoke 'Pre-mix' it can be a bit daunting, and a bit confusing. What the heck is thirty to one ratio? Do I bung the oil in the tank after I've filled it with petrol, or first? What if I've put in too much? How do I know I've put in enough? How the heck do you work the scale on the side of this bottle of oil?

If I am teaching you to suck eggs, go look at something interesting, but getting my pre-mix 'Right' is a chore that still perplexes me at times, and I've been doing it nearly thirty years!

If you look at the chart below, you'll see that I have created a table giving the number of cc's of oil you need to add to how ever many litres of fuel, to get a particular ratio.

Highlighted is the row for the quantities of oil needed to be added to 5L of petrol - for the simple reason my petrol cans; the generic polypropelene fuel cans you get a filling stations, are conveniently 5l, and I guess most people's are!

If you still work in gallons, use the row above. One Imperial Gallon = four and a half Metric Litres, as near as makes no odds. If you're working to US gallons, sorry, you'll have to figure that one out for yourself.

Fuel : Oil Ratio

Gallon

(imp)

Litres

Rich Mix (Leads to plug Fouling)

Usual Mixtures

Weak Mix (leads to seizure)

20.0%

10.0%

6.7%

4.0%

3.3%

2.9%

2.5%

2.0%

1.7%

1.3%

1.0%

5:1

10:1

15:1

25:1

30:1

35:1

40:1

50:1

60:1

80:1

100:1

0.0

0.0

0

0

0

0

0

0

0

0

0

0

0

0.1

0.5

100

50

33

20

17

14

13

10

8

6

5

0.2

1.0

200

100

67

40

33

29

25

20

17

13

10

0.3

1.5

300

150

100

60

50

43

38

30

25

19

15

0.4

2.0

400

200

133

80

67

57

50

40

33

25

20

0.5

2.5

500

250

167

100

83

71

63

50

42

31

25

0.7

3.0

600

300

200

120

100

86

75

60

50

38

30

0.8

3.5

700

350

233

140

117

100

88

70

58

44

35

0.9

4.0

800

400

267

160

133

114

100

80

67

50

40

1.0

4.5

900

450

300

180

150

129

113

90

75

56

45

1.1

5.0

1000

500

333

200

167

143

125

100

83

63

50

1.2

5.5

1100

550

367

220

183

157

138

110

92

69

55

1.3

6.0

1200

600

400

240

200

171

150

120

100

75

60

1.4

6.5

1300

650

433

260

217

186

163

130

108

81

65

1.5

7.0

1400

700

467

280

233

200

175

140

117

88

70

1.6

7.5

1500

750

500

300

250

214

188

150

125

94

75

1.8

8.0

1600

800

533

320

267

229

200

160

133

100

80

1.9

8.5

1700

850

567

340

283

243

213

170

142

106

85

2.0

9.0

1800

900

600

360

300

257

225

180

150

113

90

2.1

9.5

1900

950

633

380

317

271

238

190

158

119

95

2.2

10.0

2000

1000

667

400

333

286

250

200

167

125

100

The reason I've highlighted that row is simple - itís the one I'll use most often. So here's how I mix my fuel:-

The Accurate way:-

1/ Fill one five litre can with petrol at your favourite filling station, it will usually take a bit over five litres, make sure you get the extra bit to allow for spillage & inaccuracy.

2/ At home, in the comfort of your shed / back yard / on the kitchen drainer:-

- Using either a large graduated cylinder or a graduated measuring jug, measure 5 litres of petrol accurately into a second 5litre petrol can.

- Using another graduated measure, measure the appropriate quantity of oil, and add to the petrol you have just measured.

- Now, the 5 litre can should have a good gap left in the top, to allow for expansion - so screw the top securely on, and do a Tom Cruise Impression, like when he was in the film "Cocktail". Its not necessary, but it can make light of an otherwise uneventful experience, and give small children cause to look at you sideways and ask silly questions.

3/ Your fuel is now mixed: you can remove the cap and pour it into the tank of your machine, confident that it is the right mix.

 

The Usual Way:-

1/ Fill a five litre can with petrol at your favourite filling station. Carefully add the last half a litre, trying to catch it on EXACTLY five litres. You rarely will, but this is the rough & ready method, so donít worry too much about it.

2/ I need a mixture of about 3%, which is between 30:1 and 35:1. Looking at my chart, I need to add between 140 & 170cc of oil to 5 litres of petrol to get the right ratio. Hmmmm. Look at the table to work out roughly how much oil you'll need to add.

3/ Decide how you are going to judge the quantity of oil you're going to add.

My brand new bottle of twos stroke oil contains 500cc of oil. There is a scale on the side; Sometimes it is easy to understand. Sometimes its just marked in divisions. Sometimes it tells you how much oil the cap holds.

If I need about 150cc, then that's three tenths of the oil in the bottle - if there are ten divisions on the side, and I add enough oil to drop the level just past the third one down, thatís probably about right - dribble a bit more in, just to be on the safe side - then replace cap on petrol can and do the 'Shake it up Bay-Bee' dance.

Alternatively, If I know that the cap holds 5cc of oil, I could measure in thirty two cap fulls, instead, then wonder if I lost count and add a couple more, just for good measure, and perhaps, just another, because I spilled a bit down the side of the petrol can.

Either way, it'll be 'Close enough'.

4/ Add fuel to bike.

What do you mean you donít know what ratio of oil to use?

OK - I assume that you donít have a Montesa Cota 248, but something similar, and donít have a hand book. Well, if you look at the table, it gives ratios marked as 'Rich Mixture' and 'Weak Mixture' at either end of the 'Usual Mixtures'. Generally, the richer mixtures, 25:1 & 30:1, are used by slower speed engines, like lawn mowers, strimmers, chainsaws and Trial Bikes. The weaker mixtures 40:1 & 50:1 will usually be used by higher speed engines, like road racers, and may be motor-cross bikes. 35:1 is typical for 'normal bikes' (That use pre-mix) like old mopeds or commuter bikes.

The reason 'slow speed engines' use richer pre-mix, is that they wont use as much fuel, so they need proportionally more oil to see the same amount of lubrication. High speed engines, though they are turning faster, donít actually need much more lubrication, so they can use a weaker pre-mix to avoid the plug fouling with carbon deposits from the burned oil.

Now, heed the warning:- Rich = fouled plugs, Weak = seized engine.

It may be a lot easier to add a bit more oil to the petrol to richen it up, than it is to try and take some oil out, but itís a heck of a lot easier to remove and clean a fouled plug, than it is to potentially have to re-bore the cylinder and replace the piston & bearings.

If you donít know what ratio you should be using; do some trials;

Buy a gallon of petrol, and using empty, clean dry, pop bottles, mix up some fuel samples. About 200cc should be enough just to see how she runs. (You'll probably need the fuel tap on 'Reserve', with that little fuel though)

Start with a 15:1 mix, and see how long it takes until the plug fouls. If it doesn't - you might have something else to worry about.

Next try a 20:1 mix - but remember to drain the last of the 15:1 from both the tank and the carburettor float bowl first, then try it at 25:1, and so on, until she is running cleanly, without the plug fouling.

After the first few mixes in the 'rich' range, you'll probably find she runs fairly clearly, at something like 30:1 or 35:1. Which is probably about right, erring a bit on the side of caution.

But she may run better on a mixture a bit weaker. Remember the warning about replacing pistons and barrels, but you may like to try leaning it out a bit more.

By now the differences are going to be difficult to tell, so you probably ought to just knock back the mix a bit at a time each time you refuel, keeping a careful eye on the colour of the spark plug, and how she responds.

 

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