81 Gs550T No vacuum

[jrock141]

Had some problems getting my bike to run, so I switched it to prime and everything worked great. Ended up replacing the petcock which now works perfectly (I suck on the line and it opens, but doesn't keep leaking air). I road with this new petcock at 70mph for about 50 miles. Once I got to my exit, I pulled in the clutch to coast. I couldn't get the RPMs back up afterward though. The throttle wouldn't respond right away, but after holding it wide open, the petcock would finally open and then she'd go real fast. I basically kept having this problem whenever I'd have to stop and go. Once the RPMs go down, the petcock seems to stay shut as if it's not getting vacuum pressure. Then I hold the throttle open and nothing happens until the vacuum pressure builds up. Once it builds up enough to overcome the valve, it pops wide open and I'm going faster than I wanna be. Once i got home, i switched it to prime and it worked perfectly. Switched it back to on and it wanted to die unless I kept the throttle way up. Though I could just run it on prime, I like having the feature of a reserve so I don't have to keep guessing when I'll get stranded. Am I correct in my assumption that I'm receiving little vacuum pressure? Or is my petcock spring a little stiff? What could I do to get more vacuum or to get the valve to open easier? Any ideas? I'm not open to the idea of an On/Off switch as I like the security of a reserve function to let me know when I need fuel.

[tom203]

It sure sounds like petcock to me. Where did you get this petcock? Is it brand new? Might be a clog in petcock or a defective unit that only flows well in prime. You don't need much vacuum to get it to work - if your motor is running well, it's producing enough vacuum. If it was struggling to increase revs, holding the throttle plates open would likely decrease the vacuum. Don't be tempted to blow backwards into petcock- it will likely destroy the diaphram valve.

[AJ]

It also could be the vacuum line from the carbs is old and leaking. That would also affect performance in addition to the petcock problem.

As a test, switch the petcock to the Prime position while riding and see your problem goes away. Probably good for you to know that just for safety reasons. In the old days, there were no vacuum petcocks. They were added in the 1970s for convenience and because too many people were forgetting to turn the petcock off when parked, or to turn it back on for riding.

[jrock141]

Bought the petcock on ebay from K&L supply Co. Here's a link of an updated auction. http://www.ebay.com/itm/1981-SUZUKI-...item4ab20cb619 .

It seems to work. After removing the tank, no fuel came out on the "on" or "reserve" settings, but it flowed freely when switched to prime. I sucked lightly on the vacuum line to simulate a vacuum effect and the valve opened, but let no excess air out after it opened to it's full extent (I wasn't trying to go blue in the face, just seeing if it worked).

So the vacuum pressure is decreased with more RPM's. Shouldn't it just be a constant rate?

Also, the vacuum line works fine as I'm able to only get air through it as the valve is opening, once opened it is air tight. The bike runs fine in the prime position and remembering to turn it on/off isn't a big deal to me, but by doing this I lose my reserve function. I don't like being afraid and filling up with gas every 80 miles out of fear of running out.

My father's loaning me his vacuum gauge this evening so I'll be able to test things. I just have a hard time believing that the petcock is bad.

Edit: Also when I had the tank off, I forgot to mention not only did gas flow in the prime position, but it also flowed in the other positions when i sucked on the vacuum hose to open the valve. Again, I find it hard to believe that diaphram is bad. It seems very positive in it's opening and closing. It's a definite On/Off Open/Closed.

[Nessism]

Sounds like the petcock is fine. Most likely the problem lies somewhere else in the fuel system. Do you have a fuel filter in-line? If so, make sure it's a high flow type intended for a gravity fed fuel system. Another thing to check is the fuel tank vent (in the cap).

You might want to check the Newbie Mistakes thread to see if this list triggers anything you might have overlooked.


Good luck

[AJ]

jrock, I'm not sure you understood my post (or I perhaps I misunderstood your response). I believe you that the petcock works when you pull a vacuum by sucking on the hose.

Correct me here if this is wrong:
1) Bike runs fine at high speed / high RPM
2) Bike does not run well at low speed / low RPM.

What I think is happening is that the carbs aren't supplying enough vacuum at low rpm to keep the petcock flowing. This is most likely because the hose is bad (the rubber gets hard and small cracks develop, especially down at the carbs where it's hot from the engine). At high RPM, your engine is drawing so much air that it can still pull enough vacuum to keep the petcock open. At low RPM, the leak is large enough that either the petcock won't open or the mixture leans out and the engine can't keep running.

You can sometimes fix this by pulling the hose off and cutting half an inch or so off both ends, then reinstalling, but the hose is dirt cheap, so I usually replace it. There may also be seals if the vacuum hose connects to a T fitting between carbs, but you'd have to pull the carbs off to test that..

Checking with a vacuum gauge sounds like a good idea, but I'd check that hose before I do anything else.

[tom203]

If you got a rebuild kit (and not an entire petcock), I'd be very wary. The rebuild kits might be sorta different 30 years later- companies change manufacturing processes and sometimes the new stuff isn't completely compatible, even if it appears to fit fine.

[jrock141]

I doubt it would be anything involving a fuel filter as everything is stock and it runs PERFECT on the prime setting. Also, I did the gas vent test (waited for it to die, took off gas tank opening, no air was sucked in). If the the tank had a vacuum, this thing wouldn't be running on prime either.

Again the bike will run at high RPMs. At first there's a delay, while I suspect it is building up vacuum pressure. Then the pressure finally overcomes the valve to open it and I start moving fast. As long as I hold that high RPM (ex, stay on the highway and avoid red lights) it'll run perfectly. Once I slow down, it likes to die on me.

I guess I'll test the vacuum's suction with the stock hose as well as a new hose and see if there's any difference. Hopefully a hose switch is all that's needed, and the vacuum itself is working fine. If that doesn't solve it, I can't even imagine what the next step would be. Thank you all for the advice. I really appreciate hearing from all of you. This is a heck of a lot cheaper than taking it to the shop.

[Big T]

Are you sure you have the vacuum line hooked up correctly?

To the #2 carb?

[jrock141]

It's hooked up to the second carb in. Not the first carb you see from the left side, but the next one (which is what I assume you mean by #2). Sorry I didn't have a chance to test things last night. I'll head out there now and see what it says for a suction rating. My dad said it registers 30 on his vacuum line for milking cows, but that most cars will have around 15. I'm assuming a motorcycle will be less, somewhere around the 10 area but we'll see.

[tom203]

Before the petcock milking, I'd still like to know if it was a rebuild petcock kit.Don't forget that most cars have a common intake manifold, while #2 carb on a suzuki is all by its lonesome making vacuum - intake pulse every other revolution, so ain't lot of vac power available.

[Steve]

..., so ain't lot of vac power available.

Yet there is no problem lifting a column of mercury several inches, because that little pulse of vacuum happens rather frequently.

.

[jrock141]

Put the vacuum meter on it, and the needle kept fluttering all over the place. I'm guessing with each cycle of the motor, the needle would move to a high number then drop back down. Long stories short, I can't give an exact reading on the vacuum pressure as it's something that changes more than a few times per second. That being said, I was at least confident that I had vacuum pressure.

Though I doubted that the hose was the problem, it seems to be what needed fixing. I removed it, could not for the life of me find any cracks or holes. I then cut about 3/4" of off of each end and reinstalled it. Took the bike for a ride and it worked perfectly. Went and bought a new line and installed that. Went for another ride and it worked fine.

I apologize for doubting anyone who said replace the vacuum hose, but I guess you guys were right. I still can't find any blockages, holes, or cracks on the old one, but for whatever reason it was the problem. Now I get to ride again. Thanks

[AJ]

Congrats! You learned something. Better yet, go ride your bike to celebrate.

[Steve]

I then cut about 3/4" of off of each end and reinstalled it. ... I still can't find any blockages, holes, or cracks on the old one, but for whatever reason it was the problem. Now I get to ride again. Thanks

Could be that it was just a bit loose on the fittings.

Doesn't take much of a leak to cause problems.

.