2 stroke bike really bad fuel consumption

chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

Hello my 2 stroke cagiva w8 is using way to much fuel. I filled it with, about £7 worth of fuel the other day. The tank was bone dry at the time. Anyway i got at most 15 miles out of it, and she ran out. any suggestions on what it can be ? i really don't want a 4 stroke again. this consumption can't at all be right. i will add the exhaust has a little leak.
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1997 CB500

Fisty · Super Spammer Joined: 11 Apr 2007 Karma :

Id suggest a leak. thats about 15mpg......
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dextersaurus · World Chat Champion Joined: 26 Mar 2009 Karma :

Sounds like the exhaust has a leak

temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

the thing must reek of petrol!

Check the overflow hoses from the carby, see if its dribbling any fuel out. Check the tank for leaks, follow the fuel line down as well.

It's probably a good idea to check the float height and replace the float needle whilst you are at it.
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bladerunner · World Chat Champion Joined: 09 Sep 2006 Karma :

As said before....have a look if the carb is overflowing and draining your fuel. Would expect it to four stroke like hell if your jetting was allowing that much fuel to the engine. Was it on a run on national roads or start stop in town?
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chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

Some great reply here, thanks.

Anyone know the best mileage achievable i can get ? i am willing to sacrifice a little performance. if possible i would like to get around 55/60 MPG
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1997 CB500

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Suspect if set up reasonably and ridden reasonably that you could get that mpg.

All the best

Keith
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Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Does it smoke a fair bit? If not the fuel is not going through the combustion chamber.
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chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Nope. 15mpg is hideous (unless you are thrashing an RG500). 30mpg might be possible with a highly tuned 125 2 squeak being thrashed, but 55~60mpg is probably not a major issue. My RS125 averaged 55mpg over a few thousand miles when I kept a record (with a best of 74.5mpg when taking things very easy during the fuel strikes).

To the OP. Check the carb carefully. Is it the standard carb? Check the main jet size, that the carb needle valve is sealing, etc.

All the best

Keith
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chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

Just out of interest will lowering the needle help ? from what i know it leans out out a little.
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1997 CB500

bikersupermot... · Could Be A Chat Bot Joined: 26 Jul 2008 Karma :

my rs125fp / 3 gives about 35mpg. so u shud be getting at least double what you are.

what colours the smoke?
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

- · Super Spammer Joined: 22 Oct 2013 Karma :

chrisjose1913 · Nova Slayer Joined: 27 Dec 2012 Karma :

ok i think Teflon-Mike may be right, i put a fiver in today, so i can test and find the leak. Luckily it's also raining, this made it easier to locate a leak, with the rainbow affect on the ground. it appears to leaking out from the exhaust where the gasket should be ;/ it's leaking quit a bit too. i am thinking all this must be down to the exhaust and running way to rich ? i really hope the piston does not need replacing it only had around 500 miles on it i am glad i recovered it from the teen who owned it last. any suggestions ?
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1997 CB500

cagiva gezzer · World Chat Champion Joined: 17 Mar 2003 Karma :

My similar bike would do 60 mpg when running it in / taking easy. 40 when thrashing.

It may have had some "tuning" in the past and this may have made it too rich and its wasting fuel.

The exhaust gaskets on these bikes are o-rings and they can break up. A pair of o-rings and some sealant / gasket will tidy it up.
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

You put petrol in the tank.. and it starts dripping out of the exhaust?
Without kicking the engine over or anything?
It just runs right through?

Well... doesn't sound possible, TBH!

Obeying gravity; Fuel goes in tank filler, comes out tap, through pipe, into carb float bowl, via needle valve controlled by float; from float bowl, up through jets... into carb venturi under throttle slide.

Can then go two ways, depending which way is lower.. towards air filter or towards crank case. Crabk case is probably lower... so path would follow the inlet tract, down the inlet manifold, past the reed block into the crank case. At that point, I would expect it to stop.

Crank case is sealed, its a two stroke; it would probably take about 50cc or so to fill bottom of the crank case to the level of the crank seals of bottom of the reed block... would litterally be swimming in neat petrol.

To get up into the cylinder, from which it might start dribbling down the exhaust port and into the pipe to escape at the manifold gasket; it would have to completely fill the crank case, flood up above the level of the cylinder and transfer ports, which would usually be higher than the inlet tract, which means that long before the crank case was that full, it would have been going both ways out the carb and dripping out the air-filter, and well, probably every fucking where!

Dont make sense; to get to the exhaust flange, excess fuel would have to be pumped back up against gravity by the motor being turned over.

Are you sure its leaking from the exhaust flange, or is fuel dripping from there, having dribbled out of a split or pin hole in the tank, or a spl;it hose, onto the outside of the exhaust pipe, and run down to lowest point of pipe, where you see drip as if coming through the joint?

OR, have you kicked the motor over and then its dripped from manifold?

Follow the logic.

Investigate leak more carefully; lift the tank and check it for leaks off the bike, if you have to.

After that; then back to list; remove and clean carburettor; if float valve sticking, then could be flooding the engine, and if turned over to pump excess into the cylinder, leaking from pipe flange; but only if you have turned the engine over.
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