Compression and fuel economy

[at106]

In the last couple of months i've noticed my scooter seems to be using more fuel. When i had got used to riding (august ish) i would always get to 100kms on £5 before the fuel gauge read 0. But now im lucky if i get to 80kms. Im puting a similar amount of fuel in so i dont think fuel prices are the problem.

Last week i bought a compression tester because it was cheap And i tested my scooter and it produces just under 50PSI with the throttle wide open and the engine was cold.

The bike is a 2001 50cc 2 stroke peugeot vivacity with about 17000kms on the clock. And judging by the condition when i first got it it hasnt been looked after that well.

Do you think it's time for a rebuild?

[Kickstart]

Hi

At just under 50psi I am quite surprised it runs. I would expect the compression to be quite a bit higher than that.

If it is an accurate reading then I would agree with you that a new set of rings (or piston and rings) would be worthwhile at least.

All the best

Keith
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[at106]

[Vincent]

If it's a new tester, I'd be surprised if it's wrong. You might just get away with new rings if you're lucky. If it's on standard bore, remove the cylinder and push a new top ring up the bore about 3/4 of the way, (using the piston to keep it straight), and measure the gap in the ring with a decent set of feeler guages. Don't ask me what it should be cos I don't know but it'll be in the workshop manual, probably under "Piston Ring End Gap".

HTH, V.
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Space Is Deep

[at106]

[at106]

Just thought id update this, i tested the compression tester and it appears correct. So ive decided im going to rebuild the top end this weekend, as my dad is away so i can do it in the kitchen, where it'll be warm

Im also going to upjet it slightly as when i checked the plug it was getting on the lighter side of brown. Also when i fit the new piston into the barrel can i use 2 stroke oil to lubricate it?

And when i run the engine again am i right in thinking i should let the engine warm up, ride it varying the throttle for about 30 minutes and then let it cool down?

Thanks

[pepperami]

[MaybeGuy]

Don't upjet it.
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[at106]

[MaybeGuy]

Because you sound like a bit of a lemon, it's probably jetted properly, just in need of a rebuild, and if you jetted it, you'd probably get it wrong.
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[at106]

[Teflon-Mike]

I would ignore the compression test results.
Do you know what you are measuring?

Compression pressure is the pressure in the cyclinder, from squashing , the engine's 'displaced volume' into the combustion chamber.

A 50cc moped, displaces 50cc's. if it has a 10:1 compression ratio, the maths says that the combustion chamber volume is 5cc.

This is NOT a lot of volume.

Spark plug thread is what, 10mm? area of the spark plug hole is then 2/3 cm2, screw it out one turn, 1mm, and you have increased combustion chamber volume by 0,3cc.... which as a % of just 5cc is quite a bit.

If you had a 500cc cylinder, like on a 2.0l car, with 10:1 compression ratio, combustion chamber volume would be 50cc (size of your engine!) and 0.3cc would make bog all difference to readings on a compression tester.

Now, two types of fitting for a compression tester; first screws in like a spark plug, BUT usually NOT as far as a spark plug... we are now talking perhaps five or six threads or getting closer to a full cc of extra 'space' in the combustion chamber.

This will reduce recorded reading, and significantly so, on such a small combustion chamber volume.

Next kind of fitting is the 'bung' fitting, a tapered rubber cone, pushed & held in the spark plug hole.

This will only make any error bigger, before any question of leaking.

SO, compression testers on smaller engine's are NOT accurate, and will tend to read 'low' often VERY low, becouse of the volume of the spark-plug hole in relation to the combustion chamber.

NEXT:

Its a single, and I presume you were turning it over on a kick-starter or e-start.

This means it was turned over slowly, meaning more time for any 'leakage' past the piston rings to let pressure out of the pot.

Again, smaller the engine, more singnificant slow cranking will make this.

NEXT:

Its a two stroke.

Two strokes have ruddy great holes in the cylinder walls, called 'ports' that let the charge in, and the exhaust gasses out. And they extend nearly half way up the cylinder.

This means that while the pistoin displaces a nominal 50cc, there are holes letting any pressure out until the piston is half way up the stroke, so you are likely to only ever squash about 'half' the cc of the engine into teh combustion chamber.

So you are NOT squashing 50cc of air into 5cc, you are lucky to squash 25cc into it.

Two stroke compression ratio's are anomolouse.

The 'Calculated' Compression, is the swept volume divided by the combustion chamber volume, hence 50cc engine, 5cc combustion chamber = 10:1 compression.

The 'Effective' compression ratio, is worked out, measuring the displaced volume after the ports have closed, and dividing that by the combustion chamber volume, so if ports close half way up, = 25cc into 5cc combustion chamber, you get 5:1 compression.

The 'Actual' compression ratio, though likely to be something quite different; when the engine is spinning faster, and the harmonic tuning comes into play, the exhaust expansion chamber 'shock wave' 'supercharging' the engine blowing lost charge back into the cylinder as the ports shut.

This gives a compression ratio, with the engine running, something higher than the effective may be, but possibly not as high as a calculated.

SO: comparing measured compression readings on a small displacement two stroke engine, you have a LOT of variables to consider.

1/ small volume difference accuracy
2/ windage lossed spinning engine slowly
3/ What are you comparing readings to? A calculated C/R? An 'Effective' C/R, or an 'Actual' C/R and if an actual, at what cranking speed?

As Keith says, 50psi is barely 3bar, or 3:1 compression, and an engine with so low a compression is unlikely to even RUN, certainly at small throttle openings when choked so you dont have a full cylinder fill, and the actual C/R is dropped by whatever % throttle is closed!

So I would completely disregard that reading. It is obviousely erroniouse, which is NOT really to be unexpected, given accuracy errors possible on such a small two stroke engine.

This does NOT necesserily mean that your rings and bore are 'OK'... just means that they are possibly not as DIRE as such a low reading suggests.

BUT, from initial comments; I would not be leaping in to tear the top off the engine. You have marked a small loss of ecconomy, you have not noted a loss of speed or performance.

My starting point would be basic service and a new clean spark-plug; checking and cleaning the air-filter, and checking ignition and carburettor settings.

I would be checking tyre pressures, as THOSE are far more likley to cause poor fuel consumption.

I would be checking wheel bearings and brakes.

BUT.... biggest ecconomy changes are found in riding style.....

VF1000, has 35mpg book figure. 'Touring' on it, riding slow, sight seeing, I could double that to 70. Get a bit spirited, and start hooning it, I could halve it and be down into the teens!

I would not be surprised, that since you got the bike, you're confidence has increased, and you are now riding the thing rather more confidfently..... OK faster! This will dent the ecconomy.

Its also winter. Its cold out. How long do you warm it up? How long do you leave it on choke? This will dent ecconomy. More so more short runs you do, where on choke you could be bunging twice the fuel in for half the journey.

Would also tend to 'foul' the spark plug. The 'Choke' is an enritchening device; and that means sooty deposits, which agfter you have switched off choke, will take longer to clear..... so if you only ride 5miles or so.... quite possible that by the time you have switched off teh choke (or engine has done so automatically from an automatic choke, working on the engine temperature......) you are switching off at the end, before plugs clear.... making running less efficient, starter harding, and using more fuel, all the time, to get the same power.

ALWAYS start with the simple & easy, and look for obviouse answers, before the complicated and difficult.

Check the spark-plug, look at the choke, THINK about how you are using the bike.....

MIGHT save you a WHOLE load of wasted effort, and expensive and unnecessary repairs.

Compression testers are a great tool; BUT you have to know what you are doing with them; how they work, and what you are looking for, and they are not telling you that your rings are fucked or not, they are MERELY telling you what pressure you get in the cylinder when cold cranked.... and there may be any number of reasons for any particular reading.
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Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[at106]

Thanks for the good advice Mike
The ignition isnt adjustable so i'll change the plug and clean the air filter. The brakes dont bind but i'll definitely check the tyre pressures.

Im only using the choke to get it started, and let it warm up but then i switch it off, and i normally make atleast 10 mile journeys on it at a time so it gets a good run.

But you are also right about my confidence increasing so that probably adds to it.

[Teflon-Mike]

[at106]