Fuel pipe Diameter?

[Suntan Sid]

If you haven't seen it in "Show and Tell", my project bike, (Honda XL125RC), was finished, up and running and on the road last year, it runs ok but it has always been difficult to start, (no electric foot).

Yesterday I changed the fuel pipes from 5mm to 6mm ID.

I have just been out to start the bike up and, for the first time ever, it started up on the first kick.
After cutting the choke then letting it warm up for a few minutes, the idle speed seems to have increased by 2 - 300rpm.

According to my calculations, increasing the fuel pipe diameter by 1mm equates to a 44% increase in pipe volume!

There is no fuel pump or vacuum fuel sytem, it's just a gravity feed, I fitted an inline fuel filter, when I built the bike.

So, is it really possible that increasing the fuel pipe diameter has changed the flow of fuel enough to cure a difficult to start bike, or is this all just coincidence?
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[MarkJ]

Could it be the warmer weather we've been having? (If you're still in blighty) My carb'd bikes always seemed a lot easier to start once the weather started getting warmer. Not sure on the increase in RPM though

[Meatybeaty]

Hi had you already changed the jets or is this just with the bigger dia pipes , am new to this forum so had`nt seen your build

[Suntan Sid]

[Meatybeaty]

Cheers Suntan , will remember that one for future reference , could be of help to a lot of poor running bikes on here

[BTTD]

I wouldn't have thought it would make much difference, starvation at tickover revs would mean a real problem once running. I'd expect the bike to just not rev.
Maybe you knocked / righted something when you were changing the hose?

[spottedtango]

The old pipe may have had a slight blockage but the diameter of the fuel pipe would only really affect how fast the carb bowl fills up. Which would only really affect when the bike is accelerating as its using up the fuel in the carb bowl at a faster rate.

Also this is assuming the fuel pipe diameter is less than the inlet diameter of the carburettor (usually a brass piece you push the fuel pipe onto). If the fuel pipe diameter is bigger than the inlet then there will be no difference.

In reality that's the limiting factor as you can't get any more fuel into the carb without forcing the fuel in using a pump or boring the fuel inlet of the carburettor wider.

[Suntan Sid]

[Fizzer Thou]

There was a post on BCF a while ago where someone had a fuel flow problem and it was narrowed down to the in-line filter.From what I remember,until the filter became saturated,it effectively caused a vacuum and only a trickle of fuel flowed to the float bowl.

Maybe,now that the new in-line filter has been on the bike for some time,the flow has increased
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[Pete.]

[Ariel Badger]

My bike has 6mm hose feeding two heavily modified SU float chambers on a single AMAL GP 32mm carb. The float chambers pass over 3L per min and the bike manages about 4MPG on full bore. I doubt your 125 is going to notice the difference between 5 and 6mm tube.
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[Suntan Sid]

I'm happy with "coincidence"!
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[Suntan Sid]

Right, just been out to start it up, started first kick again, hoorah!
I'm happy to accept this is all just coincidence, but I've never been able to start this bike easily, it would alway require multiple attempts, even when I was using it daily last autumn, to run it in.

In the past I would kick and kick it, it would, reluctantly, catch then die straight away, doing this, at least, half a dozen times, at five minute intervals, it would eventually tickover on choke, but giving it any throttle would kill it. I would then have to leave it ticking over for at least 5 minutes before I could touch the throttle!

Now, first kick, it fires and idles on the choke, I can knock the choke off after 10 seconds and it responds to the throttle without coughing and conking out!

It just seems very strange, all I've done is change the fuel lines, even if there'd been dirt in the carb, I don't see how I could have dislodged it.

Coincidence or not, I'm still happy!
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[The Artist]

Yeah, it definitely sounds like something has cleared itself up in the carb area. Always nice when that happens without you having to do anything.