Hello all,

Its been a while since I visited, but the old GS has been in semi-retirement for some months, just doing little journeys and tooling about town.

Anyhow, she came into regular service again recently as I have been travelling to a customers site and as the company pay me 40p per mile, I figured I would use the little GS and maximise my profit.

But, she has issues, doing a lot of motorway work, she starts to stutter and stumble after a time on the motorway, about 15 minutes at 85+ mph and she starts acting up. Yesterday, I was quite late and the motorway was empty, so it would have been rude not to give it some beans, after a while she coughed, spluttered and came to a complete-jerky stop. So there I was on the hard shoulder, wondering why I didn't take up the breakdown cover with my insurance...... But I pressed the start button and away she went, same thing happened a few miles on. Once I got off the motorway and was back to nurdling along at 60-65 she was fine (as fine as one can reasonably expect now that the mileometer is seeing numbers its already seen once before).

I figured from this that I have some kind of fuel starvation issues. I had a quick look when I got home and firstly noticed that the fuel lines were past their best, probably would have crumbled if  had touched them, So I got new fuel lines, after fitting the new fuel lines, (bloody tight squeeze trying to get the two fuel lines on the tap under the tank) I turned on the tank tap and heard the fuel gurgling down to the frame tap. I opened one of the drain screws in the carb bowls and drained out the fuel, then I turned the tap to "PRI" expecting fuel but got nothing. I took the pipe off the frame tap that goes to the carbs, still no fuel, I took each of the pipes from the tank off the frame tap and got fuel everywhere.

Do I have a shafted fuel tap? am I correct in thinking that fuel should flow freely with the tap in the "Pri" position and will only flow in the "on" or "reserve" position when the bike is running?

Now, when I started riding, bikes all had an ON / OFF / RES on the fuel tap and no vacuum. I never used to turn them off otherwise I would forget to turn them back on, get half a mile down the road and come to an embarrasing stop. Given that these old taps were always on, and I never had any issues, can I just run a pipe from the tank to the carbs and let the float valves deal with it as that is essentially what happened when I used to leave fuel taps on?

OK, now I know I could spend the cash on a new fuel tap and do a propper job, but I figure the bike is worth about £75.00 and thats only because it has good tyres, so I dont want to spend money on it, its old, its very high mileage, it rattles (possibly an understatement given that I once had someone ask me if it was a deisel while it was idling) and the paitwork is, well, to be honest their aint much of it left, and it smokes more than me and I'm a 20 a day man.

Hooking the fuel lines directly up to the carbs should work theoretically as long as the float height is set correctly so that will stop the flow of gas.  By doing so you will lose the reserve function of the tank and when you run out of gas you are out of gas (assuming you take from the reserve nipple on the bottom of the tank).

If you wanted a safety net, even though it would be some work, you could cap the reserve hole and run the line off the main fuel nozzle, keep a screwdriver, as well as a wrench to loosen up the tank, and if you ever ran out of gas you'd just have to lift the tank, turn the petcock to the off position, switch the hose, turn it back on, and bolt the tank back on. Like I said, it'd be a bit of work, but you'd just have to get used to filling your bike up at a set distance each time before the reserve hit, and that would just be your emergency option, and it's definitely better than walking to the a gas station.

You are running into a known problem: The vacuum petcock (the frame mounted petcock) does not flow enough fuel for sustained high-speed (70+ mph) running. Pretty soon you use up the fuel in the float bowls and the carb jets are sucking air.  Sputter...  Die...  By the time you pull to the side of the road and hit the starter, more fuel has flowed into the bowls and off you go like nothing's wrong.

Fix:   You can modify your stock petcock to operate without vacuum (becomes three position:  ON / OFF / RESERVE) or you can replace the petcock and use a simple on/off petcock like one sourced from a honda CRF or Pingle brand. Generally, you will not have a reserve position with this type so be warned.

Do a search on this forum for user 'jenya' and words like "modify petcock" etc. You will find a ton of info.

Cheers!

Thanks for the info guys.

I take it this issue is cured by the fuel tap mod then? I will do a search and give it a go, but to be honest, I'm not at all bothered about loosing the reserve function, I've never run out of fuel in 15 years so hopefully, I wont anytime soon.

I have a fuel petcock from an 05 on mine, and havent experienced this problem.  they dont attatch the same, you would have to make a bracket to mount it, but it will work as the factory intended

I replaced the oem petcock with a fuel valve intended for lawnmowers - cost about 5 USD.

Does a Lawnmower tap give enough fuel flow ?

Need it to cope with 30 minutes full whack on the motorway. (or 30 minutes 70mph if any officers of the law happen to be here)

I've had the same experience on motorways.  It's great to know why it happens.  It never bothers me though as I have a simple two-stage plan...

As soon as I detect a splutter, I flick to Reserve for a couple of seconds, then back to On.  That does enough to clear the issue.  Hope it helps.

... Or just run it in the "Prime" setting all the time.  No reserve, but also not dependent on vacuum to open the flow.

Is one method more / less economical than the other?

No, fuel mixture is determined by the carburettors, not the fuel tap.

You should be able to ride on PRI with no fuel starvation issue as the vacuum-operated valve is by-passed in this mode.  If it's starving in PRI that means your fuel flow is restricted, most likely by the fuel filter in the tank or the vent in the gas cap.  No vent = poor flow.  Check your vent first.  When the bike sputters and dies, open the tank.  If you hear air rush in your vent is plugged.  Then check the filter if the vent is good.  If both are clean you should be able to ride fine on prime.  Best of luck with your bike and getting to your job!

Thanks for all the info guys.

I did the non-vacuum mod and all seems well with it now. 

Sorry its taken so long to post the results, but I been away for a few days so yesterday was the first proper test. Early morning, motorway empty, 95mph for 30 miles and no problems.