South West Bikers | Motorcycle Forum

Hi all,

To cut to the chase I sold my bike to my brother and it is having some problems.

The bike barely starts. When it does there is no acceleration at all. I rode it down a hill trying to roll start it and it would fire up for about 1 second and then die immediately.

Apparantly he has taken it to GT and had a spark plug changed and a K&N air filter. He informed me that the bike cutting out and not starting is because of an air filter. I don't believe this. The bike is under warranty though and has been serviced last year.

Any ideas or anybody had a bike keep cutting out and does not rev at all.

Check the fuel lines are not kinked...........

To state the obvious, and allow you to eliminate the one changed issue ..... the first thing to do is to put the original air-filter back in. This will return it back to it's original state when it was last working acceptably.

If the problem goes away, then the filter/lack of suitable jetting will be the culprit. If the problem remains, you need to start looking elsewhere (fuel, spark, displaced wiring connections, etc.,).

If it is still under warranty, it's up to them to sort. Note, that changing the air filter without appropriate jetting will allow the engine to run lean, possibly to the extent of detonating the motor (been there .... ), and certainly gives the manufacturer the opportunity to refute any warranty claim.

Good Luck

One of the main drawbacks of Websites is you need to be 'hands-on' with bike problems... might have to run through a series of possible solutions but I'd start with fuel blockage.

Check the fuel level !!!
Check the tap function, is it diaphram?
Blocked fuel line filters?
Fuel pipe to the carb OK?
Is ther any chance you've got water in fuel? Did you wash the bike just before the problem started?
Muck or water in float bowl?
Jets clean?
Mixture screw setting - usually 1 to 1.5 turns out from screwed in.

After the engine dies - kick it over a few more times and check the spark plug's wet. Check plug gap and try another plug if you have a spare.

Thank-you moose and you hit the nail on the head. It was the air filter after all!

The odd thing my brother noticed for the last 3 days is the bike would only work during the day not at night which seemed odd.

However after talking to the mechanic we found that the mixture was too lean causing the bike to cut out when the air was colder. My brother was stunned at how restricive the stock air filter was physically.

The mechanic at GT said either you keep to your stock filter or "rejet" the engine. I assume this is to add more fuel into the air/fuel mixture as there is more air getting into the engine.

Thanks for help.

Rejetting will add more fuel in to make up for the extra air yes. So it should effectively make the bike more powerful. How noticable that will be on a 125 i dont know but i doubt it would be amazing.

Banditmax wrote:Rejetting will add more fuel in to make up for the extra air yes. So it should effectively make the bike more powerful. How noticable that will be on a 125 i dont know but i doubt it would be amazing.

You are probably right, since 13bhp will probably go up to 15bhp which is insignificant compared to a 600 or my 650!

Anyways the problem is back now with the stock air filter. The odd thing is that the problem only happens at night. Back down to GT tomorrow to get them to sort the problem out before warranty expires in November!

It could also be dodgy fuel as my brother decided to stick super unleaded in and the problem started also, I used to use sainsburys cheap stuff as i am tight but could this really be the problem?

Very unlikely super unleaded will have made a difference although i filled my bike up once and got water in the fuel tank from it and that made it not run at all. If anything super unleaded shoudlve helped the jetting problem with the other air filter but as youve said the fault is back.

youve probably knackered the fuel jet with messing around with the filter. best to get it booked into gt but as youve played with the filter and mixture that will probably invalidate the stock warranty. take it to another honda dealer who doesnt know the history

It probably happens at night as the air is cooler and therefore more condensed meaning you can put more air in to the cylinder per square inch, which would again make it run a bit leaner.

spark plug

Funky wrote:It probably happens at night as the air is cooler and therefore more condensed meaning you can put more air in to the cylinder per square inch, which would again make it run a bit leaner.

This is what we thought but here is the odd thing.

We found that by just opening the petrol cap and shutting it again solves the problem. It wasn't the K&N filter after all!

When the petrol cap is opened there is a sucking noise as if there is a lower pressure in the petrol tank. I am going to get GT to look at the fuel lines on Monday with the full service.

It wasn't the spark plug as it just has been changed about a month ago.

the k and n probably highlighted the problem, get it sorted out and it should be okay to refit the k and n

Sounds like a blocked fuel cap breather there should be a small hole in the fuel cap which lets air into the tank have a look for it.

or a vent pipe on the tank?