fuel flow & low gas

[beechfront]

lately i have noticed when i get about 70 miles on a tank it starts to sputter at low rpms like it's running out of gas - but when it reaches about 35-4000 rpm it takes off. If I fill up the problem goes away. I think maybe I need to re-work my fuel lines/filter placement. Wierd how it's only at low rpms.

[oldnortonrider]

MINE HAS DONE THE SAME THING; for 10 years of riding since I've had this beast.

I FOUND THE CURE ; AND if you think about this >> it'll be "a sometimes cure" for a lot of other problems.

Turn it onto "PRIME".

I have rebuilt my "octopus" , to what I feel is near perfection. And it does include the "vacuum piston diaphragm". My theory why I don't need to use "prime", when the tank has a good bit of gas in it> is because the gravity flow helps get the gas through this maze, when there's more of it "pushin' ".......??

I always run it on "prime" , in tight traffic >> no matter, if I'm running on a full tank. When dropping down to low idle conditions with a hot engine, it sometimes will not want to put the right kind of vacuum to the diaphragm. And sometimes, might "stutter", if I don't got it on "prime".

My float needles don't leak. But no matter, I don't shut the engine off, with it on prime.

45 years of riding experience, has shown me> you never can tell when one might decide to leak.

I've got the 3.9 gallon tank; 70 miles would put me with 1 & half to 2 gallons left "pushing down" on the octopus maze and those 4 buggerbear carburetors. Sometimes, I think the vacuum piston diaphragm, just gets a little rheumatism in it.

Of course, if you are running without the vacuum piston diaphragm and original octopus setup > yer runnin' on "prime" all the time, when the petcock is on "ON" . But I'm guessing, yer running with the diaphragm.

I could write a book on this subject.

[Mike Hart]

Next time it is doing that, instead of speeding up the revs, switch your petcocks to Prime and see if it makes any difference.

Had the petcocks out to inspect? Maybe lower portion of filter towers (on the petcocks) is not clear, so, good flow when tank full, but when fuel level gets lower, bad flow....

JAT

[beechfront]

I'll try the prime next time to see what happens.

I have a brand spankin' new octopus in there, so i hope there's not a problem with that.

my first thought was also that less gas means less 'force' to push the gas through.

the tower filter idea also makes sense.

but the thing i don't understand is why does this only happen at lower rpms? wouldn't it require even more fuel when pulling at higher speeds?

[oldnortonrider]

THE PISTON in the diaphragm is not always all the way open at low rpm IMHO, depending on how strong of vacuum pull, you've got going, to get it all the way open. I think, when we've got a full load of fuel, it doesn't have to be all the way open> to GET ENOUGH THROUGH TO RUN THE ENGINE PROPERLY.

AND, you've just got the one cylinder pulling (sucking), through the vacuum hose, to the diaphragm. Everything has to be, just about perfect. Cannot have any "leaks" from that one intake manifold, through the hose to the diaphragm. No leak in the diaphragm (outside gasket) either.

A whole lot of things can affect that vacuum "pull", all the way from the combustion chamber, back to the butterfly>at the low RPM.

Air bubble(s) in the fuel line(s)..........., fuel's moving faster with the high rpm. (air pockets/bubbles in the lines). The octopus invites air bubbles; and I use inline filters which makes it more aggravating. It's kind of like trying to get a siphon hose going, by the time everything gets from the tower filter(s), to the carburetors.

When it works, you can't beat it. Sometimes you gotta fudge a little bit to make it work.

[skids]

Re: fuel flow & low gas

When my standard did this, I pulled the filter towers out of the tank (actually the affected one) and I noticed that it was full of gas all the way to the top. There was no visable crud plugging the screen. I blasted the screen with carb cleaner spray and gas freely ran through it again. It *might* have something to do with the tank sealer (caswell's) partially disolving and changing surface tension.

Originally posted by beechfront
lately i have noticed when i get about 70 miles on a tank it starts to sputter at low rpms like it's running out of gas - but when it reaches about 35-4000 rpm it takes off. If I fill up the problem goes away. I think maybe I need to re-work my fuel lines/filter placement. Wierd how it's only at low rpms.

[fusionking]

Try seafoam

[skids]

I must try that stuff.

Originally posted by fusionking
Try seafoam

[beechfront]

oldnortonrider

thanks a bunch - now that makes sense. I never noticed this problem until:


A - I hooked up the new diaphragm, and
B - Installed in-line fuel filters


Got to looking last night, and the vaccuum line to the octopus is looking pretty old, and also takes a pretty aggressive loop. I'm wondering if it isn't getting the vaccuum it needs to open the valve fully.

Also, there is almost always some air in the fuel lines. is this going to cause me problems?

[oldnortonrider]

When you've had all those goodies apart>
>and put it back together> it'll have air pockets/bubbles everywhere, in the octopus, the diaphragm, and the in-line filters.

It's a job to get the engine to running right. One of the ways, that helps, is> when you've at least got it going on 2 or 3 cylinders (you've started out with the petcock on "prime"), switch
the petcock(s) again for a bit, back to "ON", while it's running>gets the fuel to moving down those other 2 lines, and through different parts of the diaphragm >i.e.> sucking air pockets and bubbles out from where they don't need to be.

Every time I ride it > I'll run it some , for a while on "Prime", and some on "ON". The more fuel you're using, like in top gear, pulling a bit, the more of an "air clean-out", you're going to get.

I keep a roll of that small vacuum line; I generally keep a fresh piece on my machine, maybe not more than 2 years old.

[nashville_bill]

I had problems with air in the fuel lines when I first put fuel filters on my bike. I figured out that all fuel filters are not created equal and that the filters that I had bought had a resistance to fuel flow. I bought some new ones, at Walmart, and haven't had any fuel flow problems in about a year now. They were cheap, made out of metal, and fit old Fords. This is how I have them installed:



And this is how they look when installed on bike.



I have since replaced the rubber hose with stainless braided fuel line from Summit Racing to prevent kinks by the petcocks.

[oldnortonrider]

Resistance to fuel flow >

........ >> definitely the key words. I long ago developed a "distrust" for those tower filters above the petcocks. They're bad to clabber up. If there's any trash in my tank, I want to suck it on through. Some day, I'll get brave, and try 'em again............maybe..........

I use the glass "see through" filters, with the plastic mesh. Not much of a filter resistance; stops the chunks. Clear lines (the blue ones).

At times, I just have to be able to see "it all moving", to be able to understand it. Especially if I'm running/flushing fuel through one of the carburetors with the drain plug out. I do a lot of "messing around" with these fine Mikunis while they're still on the machine. I got sick and tired of pullin' 'em a long time ago.
Although, much of the things you're going to do with them>they'll have to come off.
And, I got paranoid, about stressing out my intake manifolds, everytime, I, either, pulled the carburetors off, or put 'em back on.