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Old 08-08-2020, 04:57 PM   #1
rhysmort
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Default 41FL Starving for fuel - Advice please

I've spoken previously about this but now have deterimed it down to the carb. replaced everything else.

So long story short, now I'm able to get to higher speeds the knuck starves for fuel, loses power and dies, engine still idles but won't have power for a minute or two. 74" motor m35.

I spoke with a friend who has a similar aged knuck, same problem. He's tried 3 linkerts on it with the same detrimental effects. 35, 74 and 74b with bombsite.

The bike runs spot on, 1 kicker until it gets to higher RPMS. The only way he solved it was going to a Super E, now I've heard of links causing issue in the past especially on dual setups, but in your guys opinions, is there anything glaring I'm missing as to why?

I'd prefer to keep the link but it's going to get me killed if I am for example on the highway. I have an M74 I was going to try as I thought the fixed HS jet may solve the issue. But as my friend said, it made no difference. As far as I know my engine is run of the mill, no hot cams, low comp pistons etc.

Any thoughts ideas would be welcome. I've been through the carb etc, everything has been freshened. As mentioned it's only at 70ish MPH I'd hazard a guess,

thanks!
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Old 08-08-2020, 07:20 PM   #2
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Default Re: 41FL Starving for fuel - Advice please

Tell us about the whole system that the gas must pass threw. Starting at the gas cap vent.
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Old 08-08-2020, 09:52 PM   #3
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Default Re: 41FL Starving for fuel - Advice please

This happened to me years ago. It was a gas cap vent just like Joe mentioned. Pingel petcock, s&s super-e and two chiwanese ebay gas caps...
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Old 08-09-2020, 03:05 AM   #4
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Default Re: 41FL Starving for fuel - Advice please

what's the fuel line size, fuel tap size. Fuel cap has unblocked breather hole, Float not sticking or incorrectly set. Fuel line not routed with any high bends causing air lock.
I can feed two 34mm Mikunis through a single 1/4" pipe so would think your issue must be fairly easy sort.
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Old 08-09-2020, 03:24 AM   #5
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Default Re: 41FL Starving for fuel - Advice please

Good morning gents!

I should have added fuel cap is vented, infact drilled it out a little as well incase. No difference with cap on or off.

Running a 1/4 inch fuel line with a 1/4 NPT petcock. Petcock is new, checked the flow from tank without petcock and with, exactly the same. Fuel goes directly into the bowl, removed the strainer off the bottom as I know they can cause issue.

Carb is an m35, fully rebuilt, replaced everything, new bushings as well.

I ran the same petcock and fuel line on my flathead, without any issue. Which leads me to believe it's the carb. I've set bowl height, polished internals to make sure there's no stiction with the needle.

It doesn't do it sitting either, revs freely, it's only under load. Again pointing to the carb. Need to suss it so I can have confidence in riding it further distances!

cheers!
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Old 08-09-2020, 07:22 AM   #6
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Default Re: 41FL Starving for fuel - Advice please

again way too little info

petcock is what - usa or some china crap that the rubber in the petcock made from used condoms is swelling up and blocking the fuel from flowing

2 - knuckles run hotter then any other 74 inch even a flat head so the 1/4 inch line vapers and stops flowing - 5 /16 fuel line and a fuel hose cover is the first place to change - then the float valve its self is it stainless steel or again china rubber tipped < more total garbage -

today with out shop experience your left to the advertisement from company's who are profit driven so you get to buy the junk we dont in the field - it shows up to be installed when the bike comes in - only to hit the garbage pail
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Old 08-09-2020, 07:51 AM   #7
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Default Re: 41FL Starving for fuel - Advice please

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Originally Posted by johnjzjz View Post
again way too little info

petcock is what - usa or some china crap that the rubber in the petcock made from used condoms is swelling up and blocking the fuel from flowing

2 - knuckles run hotter then any other 74 inch even a flat head so the 1/4 inch line vapers and stops flowing - 5 /16 fuel line and a fuel hose cover is the first place to change - then the float valve its self is it stainless steel or again china rubber tipped < more total garbage -

today with out shop experience your left to the advertisement from company's who are profit driven so you get to buy the junk we dont in the field - it shows up to be installed when the bike comes in - only to hit the garbage pail

John,

That's really good info, I didn't know that about the fuel line. I'll try there also! I've always used 1/4.

My petcock was about 65 dollars, american made, I forget the brand. My float valve is stainless, the rubber tibbed shite is dreadful. I always buy good replacement stuff where possible if OEM isn't available. I mean - finally found a decent pig snout so I can scrap my jammer, taken me 3 years!!

I'll try with fuel line first! thanks again!
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Old 08-09-2020, 09:42 AM   #8
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Default Re: 41FL Starving for fuel - Advice please

Quote:
Originally Posted by joe49 View Post
Tell us about the whole system that the gas must pass threw. Starting at the gas cap vent.
Quote:
Originally Posted by johnjzjz View Post
again way too little info
EVERYTHING in system.
Pictures of the whole system as it is. That includes routing of fuel line.
As you've been told above. We don't live down the street from you. We would be more able to help with more info.

''you jump in and start changing things and your quick to lay blame and ignore other possibilities''.

Trouble Shooting is the process of elimination. 1) proving what is working correctly 2) proving what is not working correctly 3) though changing for known good (not new) parts is used, also.
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Old 08-09-2020, 12:35 PM   #9
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Default Re: 41FL Starving for fuel - Advice please

the german 8mm hose we use that on all race stuff - its nitrile rubber resists heat better then other type rubber hoses - up to 50 Psi todays gas and diesel - its not the same as neoprene rubber that is fuel hose but its not as good
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Old 08-09-2020, 12:39 PM   #10
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Default Re: 41FL Starving for fuel - Advice please

Quote:
Originally Posted by joe49 View Post
EVERYTHING in system.
Pictures of the whole system as it is. That includes routing of fuel line.
As you've been told above. We don't live down the street from you. We would be more able to help with more info.

''you jump in and start changing things and your quick to lay blame and ignore other possibilities''.

Trouble Shooting is the process of elimination. 1) proving what is working correctly 2) proving what is not working correctly 3) though changing for known good (not new) parts is used, also.

Hap jones side fill, with petcock routed from the left hand side, between the jugs, straight into the bowl of m35.

I was using 1/4 but as John mentioned will try 5/16 filter is on petcock so that's literally it Joe.

Bike runs and rides perfectly until high rev/speeds in doesn't do it unladen. Only when ridden.

I've checked flow from the tank without a petcock, really good, checked with petcock installed, same flow. Now changed the fuel line to 5/16 so that's next, if no change, I'll look at carb next.

Cap on/off makes no difference, when the bike loses power, I have to pull over and kill engine, and then it acts like a stuck float, which says fuel lock of some kind, theres no stiction I can see in the carb/float etc.
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Old 08-09-2020, 01:22 PM   #11
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Default Re: 41FL Starving for fuel - Advice please

Ditto the 5/16 fuel line. No filter on carb fuel inlet? No tank strainer on petcock? Is the needle a rubber tipped one? They can swell up. Did you save the original needle and seat? TECH tip; OEM needle and seat can be lapped. TEST; with carb or bowl inverted suck on the inlet if it leaks lap it. 400/600 grit Clover.
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Old 08-09-2020, 02:54 PM   #12
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Default Re: 41FL Starving for fuel - Advice please

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Originally Posted by joe49 View Post
Ditto the 5/16 fuel line. No filter on carb fuel inlet? No tank strainer on petcock? Is the needle a rubber tipped one? They can swell up. Did you save the original needle and seat? TECH tip; OEM needle and seat can be lapped. TEST; with carb or bowl inverted suck on the inlet if it leaks lap it. 400/600 grit Clover.
I've got a strainer on the petcock, never ran a a filter, fuel here isn't bad so never ran a filter and strainer. I mean my tank was as clean as a whistle so was the petcock strainer!

I have a good quality needle and seat, the original fitting snapped, the 'rebuilt' carb I had was trashed, so have had to go through it all myself. Hose/carb it seems. I'll try 5/16 and go from there. If it's something that simple I'll kick myself.

Why does it only happen in higher revs/4th gear. Is it due to the engine running at it's warmest?
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Old 08-09-2020, 04:14 PM   #13
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Default Re: 41FL Starving for fuel - Advice please

yes could be vapor lock. ....or restriction lets you run the float bowl out of gas. You still didn't say does it seem to be running out of gas? How much fuel in the tank is it better with more or less fuel. On another branch of the why it quits, What plugs are you running a hot plug can cut out on you. ...and have you pulled and read a plug when it happened?
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Old 08-09-2020, 04:25 PM   #14
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Default Re: 41FL Starving for fuel - Advice please

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yes could be vapor lock. ....or restriction lets you run the float bowl out of gas. You still didn't say does it seem to be running out of gas? How much fuel in the tank is it better with more or less fuel. On another branch of the why it quits, What plugs are you running a hot plug can cut out on you. ...and have you pulled and read a plug when it happened?
I had clear fuel line on it, and when I pulled over, minus no power, it would still just about idle, after a few seconds you'd be able to rev it a little but it seems fuel starved from what I've seen experince wise. I've also seen the fuel 'refill' in the hose, like it's empty and boom it refills like the float has accepted fuel back in..

I forget what plugs they are but there's very few early knuck plugs available on the market, short reach ones.. Sorry I can't remember what the code is but they're correct for the bike.

It acts the same with a full tank or less.. It feels like when it's out of fuel, same symptoms. I've got it running slightly rich on purpose, plugs are a biscuit brown colour.
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Old 08-09-2020, 04:38 PM   #15
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Default Re: 41FL Starving for fuel - Advice please

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I have a good quality needle and seat,
That isn't a answer to the question. Is it a rubber ended needle? Have you pulled it and looked to see if it is swollen or plugged with crap from the good fuel over there?
You have no hills in the fuel line right? You can have the fuel line go down and make a U and back up to the carb but the other way it will vapor lock easier. ...and protect the line from heat. As john said hose will deal with heat better.
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Old 08-09-2020, 05:08 PM   #16
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Default Re: 41FL Starving for fuel - Advice please

here we go... another 47 page post.... just get a better carb & quit flogging a dead horse
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Old 08-10-2020, 03:53 AM   #17
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Default Re: 41FL Starving for fuel - Advice please

chuck a big inline filter in the line see if that makes a difference(there cheap)
It will act like a reservoir.
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Old 08-10-2020, 03:30 PM   #18
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Default Re: 41FL Starving for fuel - Advice please

if you have a tank that supports a petcock on both sides like a brit bike y it to the single line to carb

pull the bowl and cork the bottom - fill it with water even and see if the float is side sticking to the bowls side wall - as they tell you to off set the float some actually off set it the wrong direction - go figure

also make sure the floats tang over the top of the needle is not hanging up - if the tang has lots of side play they will stick - and even seen some that only let the float travel 1/2 the way down

your looking for a needle in a hay stack but the engine can and will burn up if it keeps leaning out
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Old 08-10-2020, 05:29 PM   #19
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Default Re: 41FL Starving for fuel - Advice please

Evening all.

I've got some 5/16 fuel line, Will start there.

John, I'll try that trick, I'm 99% sure it's not hanging up. I've even polished the needle. I have an M74 here off a running pan which I know 110% works, so if it's still doing it with the 5/16 that's my next move!
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Old 08-11-2020, 09:38 PM   #20
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I didn't see any mention of air cleaner (or lack there of). In my experience, any of those bird catcher-y type deals (or nothing at all) look awesome but suck at speed for real riding. That half globe screen is fine around town but will restrict air at hi speed, and if u put it under one of those air scoops, you'll absolutely feel it. Even any of the mini filters, I'd give pause. If any of these items are in use, I'd put a stock style air cleaner on and see what happens.
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