80 GS550 loses power on highway

[stecz]

I had an interesting thing occur... I drove my my bike a short distance on the highway (less than 10 miles). It acted like it was running out of gas (losing power, surging). I stopped and got gas, but was only able to cram 1.2 gallons in or so.

It ran fine, I drove another 10 miles on the highway and could feel it losing power, surging, and then couldn't maintain speed. I pulled over and removed the gas cap, replaced it, started, ran fine for another 10 miles or so... then the same thing.

So, on the way home, I loosened the cap and drove it all the way home, ran perfect, no surging, no loss in power.

So, I'm obviously drawing a vaccuum in the tank and gas isn't coming out as fast as it should.

The question is, how is the tank normally vented? Should the gas cap have a vent hole (mine does not). I was thinking of drilling a tiny hole in the gas cap and be done with it, but I figure I want to learn what should be happening first.

Thanks

[DimitriT]

My 82 550L has an entire valve assembly in the cap. I have no idea how it works but there are two little metal balls in there with a vent hole. I guess equalizes the pressure somehow. Are you saying your cap doesn't have any of this? Have you taken it apart?

[stecz]

nope, it doesn't have any of that....

I learned something else... The fuel valve doesn't turn off on primary (but it does on reserve, and if I draw a vaccuum, it starts to flow... on primary, it flows all the time).

So, I left the bike parked and on primary and it dumped a bunch of fuel on the ground through the airbox overflow hose...

I have a fuel valve on order at the suzuki dealer, but it sounds like I may need to get a gas cap.

[Hotblack]

The three positions on your fuel petcock are reserve (res), on, and prime (pri). The prime position is only to use if your carbs have been sitting a long time and are empty, or if you've been silly enough to run out of gas (I have). It flows fuel directly to your carbs by gravity. You should never ever ever leave it on prime after you fill the carbs though, you run the danger of getting raw fuel into your cylinders and worse, having it leak past your rings and into your engine oil. Nobody wants that. You ought to check your oil for gas smell, or better yet just change it to be safe, if you've done that. This doesn't always happen, but if the fuel leaks past the inlet AND one of the overflow hoses is blocked, you'd be in trouble.

You've discovered that drawing a vacum on the fuel cock does pass fuel in reserve and on, so you should just make sure the vacuum hose is properly hooked up and drawing vacuum. It's also a good idea to pull the assembly out of your tank and make sure the screen is clean.

As far as the cap, it certainly could be blocked, and that has certainly caused these symptoms for others. As far as I know all gas caps are vented, it's just not necessarily easy to see how. On the L models with the lock in the cap, I've had the whole thing apart and cleaned up the lock and venting. On the other type, where it's just a plain cap under the locking door, I've never looked at it, but I'm sure that it is vented somehow. You probably will discover how if you look carefully and thoroughly enough, of course a new gas cap is relatively cheap too. Good luck!

[stecz]

Crap... Prime.... not primary (it just says PRI)... Ok, I guess I learned something new.

I wondered why it said "on"...

I'm pretty sure the fuel valve isn't the right one (althought it is vaccuum actuated) because it interferes with the carb, so I have a new one on order, but at least I know this one works properly...

[Hotblack]

I think that 'pri' setting gets just about everyone at first. :shock: Not a bad idea to have one that doesn't knock agin' your carb though. Some people like to order the Pingel petcocks, they are stone simple and reliable, they only have and 'off' and an 'on' (equivalent to the 'pri' setting on the OE units) and no vaccum hookup, but you HAVE to REMEMBER to turn them off when you turn the bike off, or you know what can happen. With my memory, I'd have a very short working relationship with Mr. Pingel.

[skreemer]

Nerobro has the loss of power issue if he has the bike open full throttle for more than 3 or 4 minutes... It starts to chug and cough and generally act like it's out of gas... He doesn't have the snap down lif over his gas cap so he just reaches down and pops it for a little bit and it runs just fine...

Nerobro has the plain cap type and there is a vent in it we know his is blocked up (PO put kreem or whatever it is on it)... I'm betting a blocked/clogged vent is probably the issue.

[Hotblack]

I just looked at my 750 cap, which is plain under the locking door, and there is a teeny little hole under one of the locking wings, which I'm sure leads up above the rubber gasket. It looks like kreem would do a number on that, too, and no easy way to take it apart for thorough cleaning.

[AOD]

Sometimes a clog in the carb bodies can cause you to run out of fuel when you ride at a sustained highway speed. It's impossible to see unless you remove and disassemble the carb bodies.

For starters, ensure your petcock is flowing properly and there's no clogs in the fuel line to the carb, before you start tearing into it.

~Adam

[rckrzy1]

Why would you be at full throttle for 3-4 minutes ?

Nerobro has the loss of power issue if he has the bike open full throttle for more than 3 or 4 minutes... It starts to chug and cough and generally act like it's out of gas... He doesn't have the snap down lif over his gas cap so he just reaches down and pops it for a little bit and it runs just fine...

Nerobro has the plain cap type and there is a vent in it we know his is blocked up (PO put kreem or whatever it is on it)... I'm betting a blocked/clogged vent is probably the issue.

[AOD]

Interesting question...

[8trackmind]

Why would you be at full throttle for 3-4 minutes ?

To pass a car.[-o<

Mine does it as well but only two up running 85 or so. Never had it happen solo.

[skreemer]

Coming out of the speed inhibitors AKA Toll Booths it's a straight shot of about 30 miles right to Downtown Chicago. Nerobro and I are speeders... we know it's bad... we do it anyway... most of the time we're travelling at 10:00pm at night and it's dead on the highway... We just open em up and do a "highway run"...

[Martin_K]

I think that 'pri' setting gets just about everyone at first. :shock: Not a bad idea to have one that doesn't knock agin' your carb though. Some people like to order the Pingel petcocks, they are stone simple and reliable, they only have and 'off' and an 'on' (equivalent to the 'pri' setting on the OE units) and no vaccum hookup, but you HAVE to REMEMBER to turn them off when you turn the bike off, or you know what can happen. With my memory, I'd have a very short working relationship with Mr. Pingel.

Oh man I wish I read this yesterday. I just bought my first bike two days ago (an '81 GS400L) and on my first day the engine completely flooded with gas (I had to change to oil and about 4L of gas came out, ouch). How did this happen, well I was told there were problems with the fuel petcock and if left in the ON position there will be some fuel leaking. I had the same three options, RES, ON, PRI, I assumed RES is for reserve and I had no idea what PRI was, but assumed that it was to turn it off. Haha how wrong was I. After the hard lesson, I removed the fuel line to see what all the options really did on the bike. It tuns out RES cuts off the fuel, PRI gives it constant fuel and ON is suppose to give it fuel but this morning no fuel was coming out when it was in the ON position, so I had to ride with the valve on the prime position (is that ok to do so?), I put it on RES when it's parked (but I notice there's a bit of gas leaking from the vavle). I was told there's a problem with the vacuum diaphragm as well (I have not idea how all of that works, but I think right now the vacuum tube is blocked and plugged into the petcock).

Now I'm shopping around for a new fuel petcock. Hopefully it shouldn't be too much trouble to fix.

[GS850_Coug]

Martin

The RES is for reserve, allowing you to use the remaining fuel in the tank (unfiltered) and requires vacuum (engine must be turning), PRI is for prime (allows fuel to flow without the engne running), and ON is the run position (fuel only flows when vacuum is applied to it).