Modern Vespa

I'm in the final stages of the rebuild of my LML, part of which included replacing the 10 year old hardened fuel pipe.

The new pipe went on fine, taking the same route as the old one (under the oil tank clip thing). The tank seemed to drop back into the frame with the minimum of force, so I don't think that anything got pinched. However, when I add 1 litre of fuel to the tank and switch the tap from off to reserve, the fuel barely reaches the banjo. Is this normal? The fuel fills the pipe to within a couple inches of the banjo, but not enough to make its way into the carb.

I made a simple test of removing the pipe from the banjo and dropping it straight down into a container. The litre of fuel took 1 minute 10 seconds to drain completely, is this within specs? It's a fast-flow tap and was working fine with the old pipe, so I don't think it's at fault.

I cut the new pipe as short as I could without putting any tension on it. Maybe I could cut another inch off, but that would be all. As it stands, it would seem that the last litre of fuel in the tank isn't going to be of any use. Or with the motor running, is there a kind of siphon effect that will draw the fuel through the pipe?

that's a good fuel flow rate, your tap is checking out fine

Thanks - so the fact that the pressure of 1 litre of fuel isn't making it through the final 11 inches of uphill travel isn't a cause for concern? Maybe I'm just overthinking things. Just that I don't want to get this thing all back together and the issue only becomes apparent later on in a WOT, reserve tank situation.

Your tap is cool. The hoses and routing may not be.

The routing inside the frame is the same whether the pipe is connected to the carb or dropped straight down into the container. If the flow rate is good into the container, it would suggest that there are no kinks or pinches inside the frame. When connected to the carb, it goes directly from where it pops out of the frame to the banjo, pretty much a straight (but slightly uphill) run of around 11 inches. Pipe is Motion Pro btw.

oh wow I didn't see your flow test was in-frame.

i'm wondering about line length, maybe it's too long (24" is the consensus). hopefully others will chime in. your tap is good, that's a good start but not the finish line.

I neglected to measure the pipe before installation, but assuming the 36 inch length printed on the packet is correct, I've got a 13.5 inch offcut, so the pipe as fitted should be around 22.5 inches long. I could probably cut another inch off but I don't want it to put under tension in case it pops off going through a pothole etc.

Did you install any sort of aftermarket fuel filter in the mix?

Which fast flow fuel tap did you buy? Because some fast flow taps are faster flowing than others.

Do you have a link to the Motion Pro fuel line?

No filter, BGM tap (working fine it seems). Here's the pipe I have:
Motion Pro 12-0054 Tygon Clear 1/4" x 3' Premium Fuel Line https://a.co/d/6Cbqfy4

Tygon is an excellent hose for fuel, but i've found that it can get a kinky crimp in it fairly easily. Kinks are no bueno. 1/4" line sounds a little small to me. I'd step up to a 5/16".

Trying to get my head around this. Wouldn't the good flow rate into the container indicate that no kinks are present? The problem seems to be a lack of pressure to make it uphill from the frame exit to the carb.

Don't cut it any shorter. Should around 25" with autolube.
Check your issue by adding more than 1 liter and see if it rises(?).

There were mystery flow issues with the earlier iterations of the BGM tap. Never solved mine (cut out downhill) and went SIP no issues. Supposedly BGM solved it on v2, idk.

8mm rubber anti-kink hose is better here, especially if this is a kitted bike.
You won't be able to see fuel flow, but if you loosen the banjo bolt at the carb you can verify.

I think it's gravity.
But you could be releasing a kink with it disconnected from the carb(?).

Im no qualified chemical engineer , but I remember from working with pipelines that flow through pipes is reduced at corners and bends, it doesn't need to be a kink to slow the flow rate. When you say you have added one litre , does this completely flood the fuel tap inlet.

Clearly as you increase the fuel in the tank you increase the " head " on the fuel line, eg like a u tubed manometer. Is it worth trying your tank with a similar level of fuel to what you will run the bike at?

Good luck with the fault finding and rebuild

Thanks guys, I've got some 8mm rubber hose on order, hopefully that will cure the issue. Not looking forward to removing the tank again.

I bought the Motion Pro branded hose on a recent trip to the US. It seemed to get good reviews regarding its ethanol resistance. You guys in the US are ahead of the game in this, in Europe it's only been added for the last few years. But I have to say it seems a bit flimsy and liable to kink, so the thick rubber type should be a lot better.

The hose I replaced was the Toyox clear reinforced type from Scooter Center, but it went completely hard over the years. It hadn't actually cracked but I just thought it a good idea to replace it when the scoot was apart anyway.

I had the same.
After a year with ethanol fuel it also became completely hard, and shrank
That was the recommended hose at the time

Two year old rubber hose (internally braided, ethanol resistant) no issues.
Just pulled a tank. Still supple at the connections and easier to re-connect.

The BGM faster-flow would cut out with low fuel downhill here.
There's an FMP video out there somewhere that addresses the issue with the early BGM taps cutting out.

Btw it's easier to replace by feeding the hose down the carb first, lubricated with diluted dish soap or silicone grease.

Although the scooter has been running fine for years with the BGM tap, my OCD will not allow it to stay! New SIP tap on order, together with a length of old school black rubber hose. Just booked a trip touring Corsica in less than a month, that should focus the mind on getting this thing running again ...

Just to close off this issue, I replaced the 10 year old BGM "faster flow" tap (ref. BGM3030) with a new SIP version (ref. 15878100) and the same 1 litre of fuel flowed perfectly up to the carb.

So in conclusion, the BGM tap has a design fault whereby the last 1 litre (or more) in the tank is of no use. I see that the version they currently sell has an N on the end (BGM3030N), presumably for "new" - maybe they've fixed the problem on the new version.

I have been reading this with interest.

I have kitted 1976 VBC Super and I have been having fuel flow issues too.

I had no idea that the length or the fuel line was so critical. The last time I replaced the line I left it a few inches longer than it needed to be to facilitate installing the tank. I guess I need to pull the tank and cut the extra off.

It also has the original, funky plastic tap and I suppose I should change that too. What is recommend?

After using the toyox branded ones for 6 months each they both shrunk by 2-3" and were rock hard. Switched to a Bridgestone rubber one and cut it at 25" and have had zero issues since. I came to the 25" by measuring the banjos on both ends which are 1/2" long. 24" of line from banjo outlet to banjo outlet with a 25" hose. Perfection.

Length isn't as critical with an 8mm rubber hose.

Does your '76 Super still have the metal tank?

The tank has been replaced with another aftermarket metal tank that seems to be of very high quality.

I like the gas cap even better than the original. It doesn't have the attached flip over lever but rather a cap that unscrews like an automotive cap.

Is 8mm the equivalent of 5/16?

Sounds like you got a PX style tank, PK has this improved cap too.

Pretty much, 7.95mm is close enough to 5/16.

Sorry, meant to say metal oil tank.
The reason I asked is with the LF autolube tank the lever is nearly impossible to deal with, compared to the P series tank.
Some things have to be done to the lever stem to keep from going mad on those older tanks, basically allowing you to install the lever after the tank is installed.

I replaced a broken USA market plastic lever with this one. Don't get the plastic/cheaper version.

https://www.scooterwest.com/chrome-euro-fuel-lever-kit-for-oil-injected-large-frame-vespa-eflever-c.html