Running bike on prime?

[Srengam]

Sorry if this gets a little long (i'll give the short version):

I've had a local mechanic give my bike a once over and get it through it's MOT. He's worked on it before and remembered it wasn't running perfectly, said he's "done the valves" for me whilst he's at it.

Anyway, it passes, no problems, but when I pick it up he says "right, I've sorted it out but it needs to run on prime now to give it a bit more air so it runs better now your carbs are all clean".

So I don't seem rude quizzing him; I ride it home on prime then switch it back to on when I got home. The bike won't run now unless it's in the prime position now. It cuts out after about 300 yards in the on position.

I've lifted the tank up and the vacuum hose is where it should be on the tap.

But I thought the rule was: Never leave it in prime, always leave it in the on position.

The bike seems to accelerate faster now too, so he's definitely done something good to it, but it's developed a "rasp" in the lower rev range <4.5k ish. (If this is helpful info).

Now I really want to like this guy, he comes across as really nice, and I hope you'll all say, "yeah the mechanic is right, it's a common fix", but I just can't see why it won't run in the on position?

Any advice would be great, should I take it back, thank him again because he's done his job better than he needed to, or look for problems myself?
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Suzuki GSX600F Katana (1997)

[CaNsA]

The air:fuel mix is off.
With it being set to prime, the extra force of the petrol in the tank pushes more petrol out of the bottom of the tank and into the carbs.

If that makes sense.

I'm sure someone will say that i am chatting shit, but science doesn't lie.

[Casper]

Is the vacume on at the carb end? He may have blocked it off or he did this as your tap is fucked. Try it on Reserve as the on may be blocked.

Either way he should have told you this and not gave you bull shit. Prime is not good long term. If you have a carb problem you now have a chance of your petrol pouring out all over the floor.

[Srengam]

I had the tank off two weeks ago to clean it, also cleaned fuel tap and filter whilst I was at it, but the tap was fine then. I dropped the bike off with him running fine in the "on" position.

What part of the bike does the other end of the vacuum hose go to? It seems to go down the middle of the bike so can't follow it properly without taking the tank off fully (I could take it off tomorrow to see if it's an easy enough job to get to the other end of the hose.)
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Suzuki GSX600F Katana (1997)

[_Iain_]

Vac hose goes from tap to one of the inlet manifolds - usually connects to one of the ports you'd use to balance the carbs with.
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Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[Casper]

See the top of the carbs you have a black pipe sticking out the top of each one. Vacume pipe will attach to one of these. The others need to be covered. Some bikes may have more than one attached but one will do.

Why a mechanic is fucking about with your bike when you did not ask i dont know.

https://i.ebayimg.com/t/carburetors-carbs-Suzuki-GSX600F-Katana-600-90-97-/00/s/MTIwMFgxNjAw/$T2eC16FHJHQE9nzEyHGcBQQGhGRZ2g~~60_3.JPG

[Srengam]

I've just noticed the thick hose that should be connected to the back of the air filter housing was actually folded in half, not connected to anything, and hissing as it's trying to pull air in through the reduced diameter, I've put it back on.

Also, a clip that holds the fuel hoses in place is missing. The hoses are also routed in a mish-mash way where as before I done them all tidily.

These obviously aren't causing the problem, but they are suggesting a bit of a bodge job maybe.

I'm beginning to suspect that the vacuum hose has also been left out (at the other end) when it's been put back together.

I'll take the tank off once I've got less fuel in it so it's lighter cos it's full now.

I suppose I am OK to use the bike then in prime position for now? And should I return it to the "on" position when parked up?
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Suzuki GSX600F Katana (1997)

[_Iain_]

Either way, if you leave it on prime and the float sticks you'll wake up to the entire contents of your tank in the crankcase, the engine locked solid with fuel, and the petrol will fuck up things such as the rubber seals at the end of the crankshaft.

I have pictures to this effect.
____________________
Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[Casper]

Yes turn it to on or res when not in use as the fuel should not flow if you do get a carb problem.

[Srengam]

[Srengam]

Am I right here;

The main fuel supply is from the hose at the top of the picture, which connects to the "on" outlet of the fuel tap. When the bike is running, it creates a vacuum inside the carbs, this then pulls the fuel tap open so fuel can get down?

Also, where does the other hose go to from the fuel tap? I had assumed one hose was for on position, and the other is for priming, I thought that the priming hose doesn't need the vacuum to let fuel flow through.

I'd really like to learn a bit more about what's going on here.
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Suzuki GSX600F Katana (1997)

[Casper]

Fat hose is fuel, whether on Pri, On or Res. The thinner hose is vacume what operates the diaphgram in the tap.

Manual download

https://www.manualedereparatie.info/en/download/Suzuki-GSX-600-F-750F-1100F-87-93-Manual.html

Or the later model

https://www.manualedereparatie.info/en/download/suzuki-gsx-600-f-gsx-750-f-gsx-750-1998-2002-Service-manual.html

[Srengam]

I've got three hoses in total on the fuel tap, 2 fat ones and 1 thin one.

Any idea what the extra fat one is for, or where it goes?

Also, I think i'm starting to understand now. Am I right in thinking that the vacuum operation on the tap is entirely a safety mechanism that keeps the fuel in the tank when the bike is off. If the bikes running then it can be in any position and operate exactly the same (except it'll go further in Res) obviously.

Am I also right to assume then, that since the bike is running fine in Prime position, then there has to be vacuum in the carbs (or air movement which creates vacuum in the line), but it's not getting to the fuel tap, in which case this HAS to be a problem somewhere from the top of the carbs where the hose connects, or the hose itself, most likely as suggested that the hose isn't on where it should be.

Thanks again for your help.
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Suzuki GSX600F Katana (1997)

[Casper]

All honesty i dont know. I have never seen a tap with three pipes. All the vacume one does is suck the diaphgram open in the tap to let the fuel flow. It sucks from the carbs. No suck no fuel. Prime is a diaphgram bypass to allow you to fill the carbs after they have been drained for whatever reason.

My carbs only has one fat fuel pipe feeding the four carbs. Yours could be one pipe feeding carb 1+2 and the other feeding 3+4. Have a good read of the manual as it should tell you for sure.

[Srengam]

I've checked the manual and my bike's carbs do have two fuel line fittings, one on each side as you thought. Number 4 on the diagram.

Also on the diagram, above the second carb on the left, there is a hose which looks like it's on one of the "nipples" that you mentioned and shown on the photo earlier, so I presume that this is the vacuum hose.

As suggested earlier, I suspect all 4 four "nipples" will now be capped, or more likely; three will be capped and one will have a hose near it but not attached.

Thanks again.
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Suzuki GSX600F Katana (1997)

[Srengam]

How strong is the vacuum that pulls the diaphram on the fuel tap, Is it strong enough to feel or hear if I pull the vacuum hose off the fuel tap and feel with my thumb, or try to hear it (although the engine sound would probably mask it)
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Suzuki GSX600F Katana (1997)