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Discussion Starter #1 (Edited)
I think I've finally diagnosed a little problem that I've been grappling with for a long time which would account for a loss of power under load.

I don't know why it's taken me so long to notice but after changing my fuel filter today after a particularly poor run I noticed that there's an awful lot of air getting into the line somehow.

Naturally when I changed the filter I ensure that all the air was pushed out by the weight of fuel and also that the hose was tight and clamped where appropriate. Nonetheless, at numerous stages during the journey I could look down and observe an almost empty fuel filter. This was especially evident when it coincided with occasions of poor pickup.

So I'm wondering where I should be looking? Tonight I shall blow compressed air back up through the petcocks to see if either one of those is blocked but I honestly doubt that as there's a reasonable strong flow towards the carb. Is there any way it could be coming back from the carb itself? Should I be checking the float or could a slightly loose slide be the culprit?

Here's the setup as it stands. Any tips or ideas appreciated.

 

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I bang on about this but its worth a try in your case as well.

See if the fuel filler cap is sealing so well and so thoroughly that the breather is somehow not breathing. On my 350B I had this problem, the breather wasn't a straight hole from the outside of the cap into the tank, it must breath somehow near where the filler cap seal is, despite there being a hole externally in the middle of the filler cap.

When I first put my bike together I found this to be a problem
 

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Discussion Starter #3
Cheers, I have another cap which I will also try to see if it improves things. I'll also check the breather hole anyway.... Good idea.
 

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Make sure you have the filter right way round. It looks reversed in the photo. They sometimes have an arrow mark to show direction. A little air in the filter is normal but as you know not a lot of air. Lastly check your float needle. It may be hanging up at the seat.

Cap is easily checked... Loosen it when the starvation occurs and see if it goes away.
 

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I go with the breather in the cap, too. That little hole leads to a labyrinth to prevent raw gas escaping easily, and then to open air around the perimeter of the cap beyond the rubber seal.
 

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Discussion Starter #6
Nice tips fellas, muchas graçias...

@hulagun The flow on the filter's defo correct - the arrow going away from the fuel source and to the carb. But good lateral thinking.

I'll try to loosen the 'non-standard' fuel cap I have on the Old Gal. Might just be the source of the woes.
Incidentally, I bought a double banjo for the carb a month or two back but I just couldn't for the life o' me figure out a decent way to plumb the sucker in! I wanted to use fuel filters like the one above 'cos they is small and there ain't much space to play with but bit just turned into a mess. Petrol don't like to flow uphill too much.

If anyone has figured out a better bit of plumbing than that shown above, I'll buy 'em a drink! Yes, I'm talking to you @oepie...
 

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I run a duel nipple inlet banjo on my Motobi 125 Dellorto UB22. I dislike inline filters. The UB22 has a perfectly adequate Dellorto internal filter that fits around the banjo bolt. Maybe your carb can use one of those.

Have you seen the L-shaped inline filters? I use them on my Triumph because it's Amal carbs have no internal filter, and under tank clearance is tight. They fit nicely. Might be helpful in eliminating that high spot you have in your fuel line, which may encourage a trap bubble that might inhibit flow.

If you are running a Dellorto ME18, the original spec should be:
main jet 80, pilot 35, slide 40 or 50, needle G3 or G4, atomizer 258A.
Some adjustment may be needed for wear, and modern fuels.

Hope this helps!
Ivan
 

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Petrol don't like to flow uphill too much.
Not strictly true, Sheriff! So long as the carb is below the level of fuel in the tank, and there's no tank-vacuum or blockage in the taps/lines, the fuel will flow to the float chamber (..even uphill!) Air in the fuel filter is of no consequence - in fact it's a necessary part of its 'breathing' - lest the system is pressurised (..fuel injected).

If you check what's already been suggested and you find the fuel is flowing freely from the taps, a sticking float needle is the most likely culprit. Remember to shake the float whilst you have it in hand, and confirm it's still airtight. Floats (..especially the old soldered-seam brass type) can become porous over time, and any fuel ingress will interfere with its correct operation.

Best of luck ;)
 

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I with Steve on this - obviously hard to say from pics but if it were mine, I would start at the carby & work back up the line to the tank & filler cap. Whilst this may be the long way, a really good clean out & re-set is certainly not going to harm anything.

Your fuel hoses look new/newish in the pic so they shouldn't be problematic insofar as leaks.

Lets us know how you get on, it's always great info for the memory bank in future. ;)
 

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Discussion Starter #10
Thanks peeps for the tips, I've definitely made some progress and learned something - always a good combination!

I've adopted the belt & braces approach:

- Thoroughly checked tank cap breather and slightly enlarged the hole and ensured it has NOT been obstructed by the seal;
- Entirely removed the in-line filter. I'm now firmly of the opinion that The Flying Ant DOES NOT LIKE in-line filters and the small banjo filter will be sufficient (that's a drink for @hulagun right there!);
- Swapped out the 6,5 gm float in the chamber for a spare I happened to have. (drink for @750four...)

So this morning I reserved judgment until I was an hour in to my ride even though I could feel a difference immediately. After that I felt pretty confident that the problem has been remedied. Of course, my fuel/air settings will need to be adjusted to account for the improvement in fuel flow but I'll work that out quickly enough after a ride or two. Hell, I'll even treat the Old Gal to a new plug.

Let me know whenever your in my 'Hood guys and I'll show you the best craft beers you could wish for....
 

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Awesome! Glad to help. :f4:

FWIW, if the float takes on gas it usually sinks, and won't cause a lean condition, but the opposite. It runs rich (assuming you can even get it started).
 
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