Carburettor needles not staying where they should

[nobrob]

Hi all

I posted this on an alternative forum and didn't get much of a response, so I'm trying my luck here. My apologies if it's not the done thing.

I have two engines (two carburettors) and the needles (high speed and low speed) are both very easy to move. So much so that the high speed moves itself while the engine is running due to the vibration, annoyingly it moves itself inwards (leaner) until it's as lean as possible!

The only thing I think might be the reason is that I used to put after run oil in the carburettor, before being told it could cause damage, at which point I stopped.

Any thoughts on (a) why this might have happened and (b) how to fix it without replacing the seals?

Thanks

[ryanlownie]

Check the o-rings that sit on the needles. Simply take the needle all the way out and check that they are in good condition, if they are worn away then that will help them rattle out.

[nobrob]

I've had the needles out and checked the o-rings, they're not damaged in an obvious way. I'll be fair, I don't have much to compare them to, but they don't feel as tight as they should be, so could be worn.

One of the engines (and its carb) is less than six months old and I find it weird that it's happened to the other (which I bought second hand) at the same time.

I'm keen to find out what it is that might have caused this (after run?) and obviously avoid doing it again, or is it that the o-rings just need replacing every few months?

[carlin]

Hi Mate,

TBH i dont think the Afterun oil would of caused this issue.
I've used afterun oil for years never had problem doing seals in.

Sounds to me that the o-rings needs replacing.
If the rings have slight wear on them then they will need replacing.
Most needles have two little black o-rings on them, if you look closely on some show signs of wear like grazing against them or little bits of rubber threads.

This will be why your needles are moving, might help using some gasket sealant when you replace the o-rings also round the carb it will stop any air leaks also hold you needles little bit firmer too.

Hope this helps

[RCAshers]

Hi Nobrob your needle problem is not a unknown thing sometimes the O rings are not quite thick enought to hold the needle in the carb, the cure is to use some PTFE tape (white plumbers tape) take a small length around 2-3cm fold it in half along the length and wind it around the needle over the O ring smooth it down and replace the needle it should screw in and be tighter once you have the mixture right it should stay set. Hope all this helps. Regards Ashley

[triplesix]

You could replace the o-rings with a small "slice" of fuel line, as well!

[nobrob]

Good to know that this problem isn't new! And of course that After Run is unlikely to cause it

Still concerned by how quickly the O-rings have deteriorated, but from the homebrew solutions (PTFE, fuel tube) it sounds like a quick fix might be better than replacing them, at least for my wallet!

Thank you all for your help

[Neil Skull]

I have to spoil the party,

It could be after run or any chemical cleaner that reacts with the o rings and makes them soft. i think wd40 could cause that too.

Just wipe them dry and they normally go back to normal. but for a few pence it is safer to just replace them.

Another possible reason is vibration. if your engine bearings are on shagged out it could make things worse.

I have heard of this problem before and all the tips given so far are worth trying. but it would be good to know the cause!!!!

[mark christopher]

Originally Posted by RCAshers

Hi Nobrob your needle problem is not a unknown thing sometimes the O rings are not quite thick enought to hold the needle in the carb, the cure is to use some PTFE tape (white plumbers tape) take a small length around 2-3cm fold it in half along the length and wind it around the needle over the O ring smooth it down and replace the needle it should screw in and be tighter once you have the mixture right it should stay set. Hope all this helps. Regards Ashley

Originally Posted by triplesix

You could replace the o-rings with a small "slice" of fuel line, as well!

id highly NOT recomend the above, the primary function of the o rings is not to stop the needle moving but to prevent air being drawn past them, causing erratic tunning, but the correct seals, job done!!