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Discussion Starter · #1 ·
Bloody hell, I did the usual mod to the tubes on my 300 EXC, ran two breathers up near the headstock (but extended them and pointed them downwards because water was getting into the float bowl). I looped the overflow over the top of the inlet boot and ran it down to past the swingarm along with the other two breather tubes.

It's been OK for ages but this morning I found that my engine and exhaust was full of fuel (being a dimwit I forgot to turn off the fuel tap yesterday). There must have been two pints in total in the exhaust and engine!

Luckily I spotted that something was wrong this morning as the garage stunk of fuel so I did not juts try and kick the engine over.

It has taken at least four hours to get all the fuel out of the engine, I had to remove the carb and reed block, stand the bike up vertically and then blast the crankcase with an airline. (I cleaned the carb out too!)

So here's a tip....DON'T run the overflow tube up over the back of the carb! :mad::mad::mad:


(And turn your fuel tap off when the engine is not running!)
 

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Noob mistake. I'd have expected more from you... :ROFLMAO: :LOL::ROFLMAO:

The safest way is to tee-off one or two of the breather pipes and put additional tubes higher, and leave the originals how they are. The overflow, as you've found out, needs to stay where it is.
 

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Top pipes need to go to the air box. They are atmospheric pressure balance tubes, they do not suck. They need to be in air to allow the carb to work. If blocked or in water your motor will stall. If you are getting water in your carb it’ll be coming through elsewhere. Most likely the air filter.
 

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Discussion Starter · #4 ·
Top pipes need to go to the air box. They are atmospheric pressure balance tubes, they do not suck. They need to be in air to allow the carb to work. If blocked or in water your motor will stall. If you are getting water in your carb it’ll be coming through elsewhere. Most likely the air filter.
The water was getting down the tubes when I was jet washing the bike! yes the tubes need to be at atmospheric pressure which is why I personally don't think that its a good idea to put them in the air box which may well be slightly below atmospheric pressure. (I doubt that it is much lower because its not exactly an 'air box' in the true sense of the word, in my humble!)

I think that water in the float bowl is more likely have got in there via the tubes rather than the air cleaner side of things, any water getting past the air cleaner is more likely to go straight into the engine. (also not great). Since extending the top tubes and running them down the front frame rail a few inches no water has got into the float bowl!
 

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On my 13 i put two up towards the headstock just above the coil on the left side, put a full loop in, then had about 3 to 4 inches hanging down.

the other 2 just left them as std
 
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Discussion Starter · #6 ·
On my 13 i put two up towards the headstock just above the coil on the left side, put a full loop in, then had about 3 to 4 inches hanging down.

the other 2 just left them as std
Yep, I've done the same sort of thing, it was the overflow pipe that fucked me up due to me being an idiot!
 
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Discussion Starter · #8 ·
I always run them into the air box, never had a problem




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Fair enough but 'technically' the airbox is not going to be at atmospheric pressure, this means that in theory there is less air pressure in the float bowl which means less pressure differential between the float bowl and the carbs venturi, the result of all of this is that less fuel is delivered into the air stream. Its all probably a very marginal difference because our bikes hardly have a true airbox anyway.
 

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I think the top area of a modern “air box” will just have air flow, like the headstock area, I would be massively surprised if there is any negative pressure there at all


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Discussion Starter · #10 ·
I think the top area of a modern “air box” will just have air flow, like the headstock area, I would be massively surprised if there is any negative pressure there at all


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Yep, fair enough.
 

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So here's a tip....DON'T run the overflow tube up over the back of the carb! :mad::mad::mad:
I wonder why this should lead to a flooded engine. A leaking float needle valve is what caused the problem. I reckon...

My upper vent hoses are routed under the seat and lead into the right side cover. The overflow hose is looped as JD recommended. Never had a problem in ten years...🤷‍♂️ And yours worked for ages too...
 

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Discussion Starter · #12 ·
I wonder why this should lead to a flooded engine. A leaking float needle valve is what caused the problem. I reckon...

My upper vent hoses are routed under the seat and lead into the right side cover. The overflow hose is looped as JD recommended. Never had a problem in ten years...🤷‍♂️ And yours worked for ages too...
Yes it was a leaking float valve that was the start of the problem (well the start was me leaving the fuel tap on). Normally if the carb floods the fuel will just run out of the overflow a bit like how the overflow in a bath works but because I was/am an idiot I looped the overflow tube over the back of the carb, this meant that when the float bowl flooded the fuel just started to piss out of the needle jet and straight into the engine, it filled the crankcase up then started to fill up the lower part of the exhaust pipe! If I had not noticed something 'odd' I could have tried to start the bike and I would have hyro-locked it up which may well have damaged it.

I was surprised that even when I thought I got all of the fuel out of the crankcase the bike still wouldn't start, I had to stand the bike up vertically on the back wheel with the reeds and carb removed and then crank the engine over on the starter (spark plug removed and kill switch held down to stop the whole thing catching fire!). Only after all of this farting about did the engine fire up! Posting all of this up obviously alerts everyone to my 'dickheadedness' but it might stop some other muppet doing what I did! I had run this setup for at least 3 years prior to all of this.

The breather mod is a good idea, it defo stops the engine stopping stone dead in deep water, looping the overflow is a really bad idea, JD should not recommend doing it! I think the idea is that it stops fuel being wasted when it splashes over the top of the overflow pipe inside the float bowl, well in the end I wasted more fuel because of the mod!
 

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Discussion Starter · #14 ·
Stics fucked it.
Well.....I was fucking about with the STIC block and I reckon that the little spring clip thingy that is part of the float valve got caught up on the tang that operates the float valve, that's why the carb flooded.
 

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The breather mod is a good idea, it defo stops the engine stopping stone dead in deep water, looping the overflow is a really bad idea, JD should not recommend doing it! I think the idea is that it stops fuel being wasted when it splashes over the top of the overflow pipe inside the float bowl, well in the end I wasted more fuel because of the mod!
Thank you Pete for taking the time to answer copiously, much appreciated. Yeah well, no doubts about the breather mod, I guess many of us have tried it with success. As you say JD's intention was to stop fuel pissing out when the bike is leaned over or on steep downhills. I have done his mod but I only made one loop leading the overflow hose back through one of this little hose holder thingies (found no term for it) that are bolted to the float bowl. It was free as the hose which normally goes through it was re-routed as mentioned above.

However, I am now in doubt if I should revert this mod. Not sure.... At least two things have to happen (tap open, defective/sticking float valve) that the engine is flooded.... 🤔

Cheers Steve
 

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Discussion Starter · #17 ·
Thank you Pete for taking the time to answer copiously, much appreciated. Yeah well, no doubts about the breather mod, I guess many of us have tried it with success. As you say JD's intention was to stop fuel pissing out when the bike is leaned over or on steep downhills. I have done his mod but I only made one loop leading the overflow hose back through one of this little hose holder thingies (found no term for it) that are bolted to the float bowl. It was free as the hose which normally goes through it was re-routed as mentioned above.

However, I am now in doubt if I should revert this mod. Not sure.... At least two things have to happen (tap open, defective/sticking float valve) that the engine is flooded.... 🤔

Cheers Steve

Steve you are correct that two things have to occur, I was a fuckwit and did not fit the float valve properly to the float 'tang'. Having said that I have heard that the floats can stick on these carbs and of course you could always be unlucky and end up with a small bit of crap in the float valve which would cause it to leak. My overflow tube was setup so that it went UP through one of the tube brackets, it then went over the back of the 'bell mouth' of the carb and then back DOWN through the other tube bracket. Really no part of the tube can be higher than the top of the float bowl (well really the top of the needle jet).

If I had jumped down on the kickstarter without clearing all of the fuel out I'm sure I would have done some expensive damage to my engine and as this engine has cost me enough already I don't need to give it another excuse to rape my wallet!

Personally I would not loop the overflow and wanted other people to be aware of the 'potential' problem that it can cause.
 

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Steve you are correct that two things have to occur, I was a fuckwit and did not fit the float valve properly to the float 'tang'. Having said that I have heard that the floats can stick on these carbs and of course you could always be unlucky and end up with a small bit of crap in the float valve which would cause it to leak. My overflow tube was setup so that it went UP through one of the tube brackets, it then went over the back of the 'bell mouth' of the carb and then back DOWN through the other tube bracket. Really no part of the tube can be higher than the top of the float bowl (well really the top of the needle jet).

If I had jumped down on the kickstarter without clearing all of the fuel out I'm sure I would have done some expensive damage to my engine and as this engine has cost me enough already I don't need to give it another excuse to rape my wallet!

Personally I would not loop the overflow and wanted other people to be aware of the 'potential' problem that it can cause.
Well shit happens! At least you didn't kill the engine using the kicker. Last time I was working on the carb I screwed the main jet in but forgot to tighten it. Don't ask how the bike felt once the jet fell out :-( Me friggin' idiot was out in the nowhere and had no idea what was going on. However, I made it back to my van.

I think it makes no difference if the overflow hose is routed over the bell mouth or if it makes a smaller loop - like mine - from one tube bracket to the other. I see no way how fuel should find its way through it once the needle valve is stuck open. I admit that makes me nervous now. Never thought that this could cause a problem and I wonder if others had a problem.
Thanks again!
 
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I think it makes no difference if the overflow hose is routed over the bell mouth or if it makes a smaller loop - like mine - from one tube bracket to the other. I see no way how fuel should find its way through it once the needle valve is stuck open. I admit that makes me nervous now. Never thought that this could cause a problem and I wonder if others had a problem.
Thanks again!
It's just about the levels - if the top of the looped overflow is above the top of the float bowl, then the fuel will overflow through the jets and into the intake rubber, in preference to the overflow. I can't see any reason to do anything with the overflow - just leave it alone.

My mate had exactly the same issue but he's dangerous with spanners, and he literally just grabbed all the tubes and stuck them pointing upwards in the air box.

It's a case of a little knowledge is dangerous.
 

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Discussion Starter · #20 ·
It's just about the levels - if the top of the looped overflow is above the top of the float bowl, then the fuel will overflow through the jets and into the intake rubber, in preference to the overflow. I can't see any reason to do anything with the overflow - just leave it alone.
Yep, that's it exactly!
 
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