So the PO apparently replumbed with 1/4" vacuum line. And today it split.
Yeah.
Luckily I was doing maint on the bike and tore when I shifted the fuel tank, so I wasn't riding at the time and the bike was cool.
Anyways, in looking at replacements there's the age-old question of 1/4" vs 7mm vs 5/16" ... but then there's the fuel-starvation question.
Since 7mm has a 21.5% LARGER cross-section than 1/4", are we restricting our fuel-flow too much, and then having to compensate with either higher-flowing petcocks, or drilling out the existing petcock restrictor to increase vacuum to get more fuel flowing ...
1/4 (6.35mm) cross section: ~31.67
7mm cross section: 38.48
38.48 / 31.67 = 1.215
I wonder how many of the petcock-fuel-starvation issues are we actually doing to ourselves ...
Thoughts?
-mox
Still think the fuel starvation issue is NOT related to the lines nor the petcock/s flow ability. The 500 has huge fuel lines and chambers for a medium flow motor. The DR650 petcock has a 5mm ID brass connections and 4mm fuel switch holes. I bought one thinking it would be a good replacement. My CBR250RR 20000rpm racing bike had quarter inch ID fuel lines.
The big restrictions are the tiny holes inside the fuel switches and the vacuum switch. The fuel switch holes are only 3-4mm and right angles all through them ( fluid flow doesen't like right angle corners). Because of the way they are made you can't open them much without losing the sealing ridges.
The vacuum tap I haven't even looked at properly, but I strongly suspect it is the major culprit in high flow restriction. May NOT be oriface size, but vacuum dropoff at high rpm partially closing the switch. Tried using a fuel pump and tossing the vacuum switch but the leaky carbs won't take any pressure at all, and these carbs HAVE to have a turned off fuel when not running to stop them leaking.
They do make electric fuel cutoff valves that will close when the ignition is off. This seems like an ideal solution for a GS, other than losing reserve.
Unfortunately they're quite expensive. But they are around. I can dig up a link if someone really cares.
I'm cheap and went with 1/4" automotive fuel lines. It fits fine and the OEM clamps work on it. Not pretty, but a little lacquer thinner will remove the printing on it.
I recently replaced the fuel lines so I'll share my experience.
Researching this was a nightmare, so I'll be as comprehensive as possible. However, it's important to note that there's lots of variations between the different model-years, namely the sizes of the tank petcock outlets and the inlets/outlets of the frame petcocks. I assume these differences are the cause of the differing opinions seen here.
For my 2006, I replaced all the fuel hose from a 2m length of 7.2mm ID unreinforced nitrile rubber fuel hose (from Advanced Fluid Solutions if you're UK based).
The carb fuel T had an OD of 8.98mm, the frame petcock had ODs of 10.95mm, and the tank petcock had ODs of 8.13mm (9.31mm at the single barb).
I had to soften the hose in hot water for 60secs or so to fit it on the frame petcock connections, but it slid on easily after this. The other connections all fitted snugly.
I secured all the connections using spring clamps (you can get an assortment of sizes cheaply on eBay), and haven't had any leaks so far.
Fuel starvation was also something I was concerned about as I also fitted a fuel filter between the carbs and frame petcock, and 2 quick release couplers on the tank to frame petcock hoses (for easier tank removal).
These were all designed to fit 8mm ID fuel hoses, so the 7.2mm ID hose fitted well, but meant the fuel flow through each is constricted to ~6.35mm (1/4") diameter.
However, the ID of the carb fuel T is actually also 6.35mm, so even if all the lines were uninterrupted there would still be an unavoidable constriction.
Admittedly I don't do much motorway riding, but after a ~150mile trip last week at a sustained minimum of 70mph (usually more), I never experienced any forms of fuel starvation even with all these extra flow constrictions.
So I agree with Greg's opinion that fuel starvation is not fuel line size related (down to 1/4" at least).
Great data, thanks for sharing everyone.
@max
I've also got a 2006 and picked up some 1/4" Tygon LP1200 (here in California they're constantly pushing more and more ethanol)... but now after reading your post I'm worried that 1/4" ID will be too small to fit on the fittings!
-mox
Quote from: moxford on June 20, 2018, 05:25:14 PM
...
@max
I've also got a 2006 and picked up some 1/4" Tygon LP1200 (here in California they're constantly pushing more and more ethanol)... but now after reading your post I'm worried that 1/4" ID will be too small to fit on the fittings!
-mox
I had the same thoughts when looking at the 6.35mm or 8mm Tygon lines, worrying that the 6.35mm would be too tight for the frame petcock, and that the 8mm would be too loose on the tank petcock!
Afraid that I can't offer any actual advice on it though, as I went for the nitrile lines instead. Hopefully someone else will be able to share some experience.