Petcock?

[Spyder]

I have a 83, GS 750t. 4into1, K&N pods, Jetted? Runs good, LOTSA POWER ! ! ! Cruise to 70mph= fine, Rippit to 70mph= falls on its face like the float bowls are running dry. Is this a common symptom of an old petcock? Does it sound like something else? Any feedback is greatly appreciated.
Spyder...

[Dave8338]

Could be the petcock, or the inline filter (if equipped), or carbs in need of a GOOD cleaning, or air leaks causing a lean-out condition, or too small on the main jets, or any combination there of...

[jmanz6]

Mine would actually run out of gas after a few minutes unless I had it on Prime. You could be having the same type of issue just not as bad as mine. I bought a new OEM petcock just in time for winter to come so I can't say for sure if it fixed the problem.
But I do know mine was bad.

[Spyder]

No inline filter. D&D pipe (if it helps). Doesn't matter position the valve is in, no change. could this be a float problem?

[80GS1000]

Sounds fuel starvation from your petcock and possibly jetting. A Pingel fuel valve plus a Dynojet stage 3 kit should sort you right out. Will it go over 70 MPH? When I first put my Dynojet kit on the bike with pod air filters and a V&H 4-1 pipe, the bike would sputter on the freeway like it was running out of gas. It was, really - the stock petcock couldn't keep up with the thirsty float bowls. Smooth sailing since the Pingel was installed.

[robertbarr]

A healthy O.E.M. petcock switched to Prime with a hose attached (leading to a fuel container) can drain a 5-gallon tank in less than 2 minutes. Not even a fully laden semi-trailer truck at 70 m.p.h. uses fuel at that rate.

There are valid safety concerns with a manual fuel tap that I think make the O.E.M. petcock the only reasonable choice. That's my opinion; there are others. Pingel obviously has its share of advocates, and there might be a few good reasons for using their petcock, but fuel delivery rate isn't one of them.

The original units hold up for several decades before showing their age. I'd say that's another reason to opt for O.E.M.

[80GS1000]

A healthy O.E.M. petcock switched to Prime with a hose attached (leading to a fuel container) can drain a 5-gallon tank in less than 2 minutes. Not even a fully laden semi-trailer truck at 70 m.p.h. uses fuel at that rate.

There are valid safety concerns with a manual fuel tap that I think make the O.E.M. petcock the only reasonable choice. That's my opinion; there are others. Pingel obviously has its share of advocates, and there might be a few good reasons for using their petcock, but fuel delivery rate isn't one of them.

The original units hold up for several decades before showing their age. I'd say that's another reason to opt for O.E.M.

Did the 83 750 come with a manual or vacuum operated fuel tap? The vacuum ones hardly pass enough gas to keep a stock motor running. And you have to crank the motor for the vacuum type to flow at all when it's on "prime". Garbage stuff.

[Spyder]

I just replaced the old vacuum operated petcock with OEM new out of box. Haven't had chance to test yet, as it's in the 30's outside right now. Maybe this weekend. Draining the tank was very, very slow on prime. I have high hopes that this is (or was) a (if not the) big problem.

[Steve]

The vacuum ones hardly pass enough gas to keep a stock motor running. And you have to crank the motor for the vacuum type to flow at all when it's on "prime". Garbage stuff.

As robertbarr mentioned, a properly-operating stock petcock will empty a 5-gallon tank in about 2 minutes. From personal experience, I can tell you that semi trucks get over 5 miles per gallon, so it would take over 25 miles to go through 5 gallons. For most trucks, this would take about half an hour. You trying to tell us that your bike can burn gas at a rate exceeding 2.5 gallons per minute? You either have a severe jetting problem or are going incredibly fast.

Oh, and all this time I (and everybody else around here) have been operating on the (evidently mis-informed) idea that the purpose of the PRIME position was to bypass the need for vacuum for fuel flow. I have always understood that by placing the petcock in the PRIME position, you could fill the float bowls first, then hit the button to start the bike.

Draining the tank was very, very slow on prime.

Couple of things to check:
Pull the petcock off the tank, inspect the inlet screen. That screen is your primary fuel filter, so expect there to be some crud on it. Clean it carefully.

If that does not fix it, examine the petcock valve assembly. You can remove the diaphragm to get to the back side of the valve, some times you can also remove the valve itself from the body. Make sure there is nothing stuck in any of the passages.

Check your hose from the petcock to the carbs. It is not unheard of for them to deteriorate and collapse internally, blocking flow.

.

[Nessism]

I think you guys are exaggerating with this 5 gallons in 2 minuets flow business. My petcock(s) won't flow anywhere near that rate, but it flows plenty of fuel for the bike to run properly.

[jmanz6]

The prime position does allow fuel to flow WITHOUT the engine vacuum to operate the diaphram. When I drained my tank to install the new OEM petcock it took about 10 minutes on the prime position. I would suspect your problem will be fixed with the new petcock, if it isn't then your jetting is off and you need to install larger jets to compensate for the extra air you are getting into the engine.

[Spyder]

Yeah Jman, I agree with the flow theory. I put new petcock in, results are pending due to COLD weather. As soon as the temp goes up a few degrees I'm on it. And for the record it took about 25 minutes to drain 2 gallons from tank. Fuel line and vacuum line are new.
I also installed a bracket, mounting the back end of the carb assembly trying to prevent frothing in float bowls. Maybe a good idea?

[Dave8338]

I also installed a bracket, mounting the back end of the carb assembly trying to prevent frothing in float bowls. Maybe a good idea?

Never heard of "frothing" as being a problem. What are you running in that bike, anyway?

[robertbarr]

Never heard of "frothing" as being a problem.

Perhaps he means 'sloshing'? Just a guess...

[woodman]

Id try it with the gas cap open or removed to see if its a venting problem