Ugh dang petcock

[mrbill5491]

Went on a nice ride yesterday and the petcock looses its vacuum at a round 85 mph. Felt the first stumble and knew what exactly what it was. Pulled in the clutch and coasted to a stop, engine dead. Sat there for a couple minutes and cranked it over a couple times until the carb bowls refilled, then took off. Didn't happen again the rest of the day.
Kind of spooked the guy behind me when all of a sudden he was on me lol.

This happens every now and then, once it does, it won't do it again for the rest of the ride. Not sure why it looses the vacuum like that. The # 2 carb has new rubber boots front (all the carbs have new boots and clamps) and back and clamps and is sealed up good, no air leaks anywhere. Even the vacuum hose is new. Gas cap is fine too. Just looses the vacuum long enough to starve the fuel bowls dry. Looks like a another new petcock coming up. It's the only thing I can see. Any other thoughts?

[Steve]

Does this happen after a llooonnnngggg pull at full throttle?

You obviously know that it takes engine vacuum to pull against the diaphragm to open the fuel valve. When the carbs are held wide open, there is very little, if any, vacuum in the intake tracts to pull on the diaphragm.

The first attempt at diagnosis will be to back off the throttle just a bit to see if that helps.

I know there are some long hills in your area, I am guessing that you are hustling up one of them at wide-open throttle when this happens.

.

[GSX1000E]

If you are an enthusiastic rider, try turning the petcock to prime while riding and see if that works.

[mrbill5491]

Does this happen after a llooonnnngggg pull at full throttle?

You obviously know that it takes engine vacuum to pull against the diaphragm to open the fuel valve. When the carbs are held wide open, there is very little, if any, vacuum in the intake tracts to pull on the diaphragm.

The first attempt at diagnosis will be to back off the throttle just a bit to see if that helps.

I know there are some long hills in your area, I am guessing that you are hustling up one of them at wide-open throttle when this happens.

.

It happened on a nice long flat part of the hiway and was doing around 85mph. I had just gotten into 5th gear and a few minutes later it hit. As of a matter of fact, that when it tends to do it, not pulling a hill. Backing off the throttle does nothing nor pulling out the choke. Sometimes it will do it once and not do it again during the rest of the ride.

[mrbill5491]

If you are an enthusiastic rider, try turning the petcock to prime while riding and see if that works.

I was thinking about that. Been thinking about just gutting the petcock, seal off the vacuum ports and installing a inline manual shut off valve on the fuel line.

[Steve]

I was thinking about that. Been thinking about just gutting the petcock, seal off the vacuum ports and installing a inline manual shut off valve on the fuel line.

Before you do anything drastic, please try something.

I know you said in your first post that "the gas cap is fine", but that might still be a problem. See if you can reproduce the problem. When it happens, open the gas cap, see if you hear a rush of air as you open it. If you do, your vent is blocked. It might be open enough for slow equalization due to lower fuel draw at more-modest speeds, but the fuel flow at 85 mph might be drawing fuel quicker than it can equalize.

Does this happen rather soon after filling the tank? If so, I would definitely go with the tank vent suggestion.

.

[tkent02]

The tank vent seems more likely that the lack of vacuum. I ride in mountains where long full throttle pulls are several minutes long, especially on the smaller engines. Sometimes WOT for several thousand feet vertically without backing off. Fuel keeps flowing just fine.

If the tank vent is partially clogged as Steve suggests, it would do exactly as you describe.

[mrbill5491]

Before you do anything drastic, please try something.

I know you said in your first post that "the gas cap is fine", but that might still be a problem. See if you can reproduce the problem. When it happens, open the gas cap, see if you hear a rush of air as you open it. If you do, your vent is blocked. It might be open enough for slow equalization due to lower fuel draw at more-modest speeds, but the fuel flow at 85 mph might be drawing fuel quicker than it can equalize.

Does this happen rather soon after filling the tank? If so, I would definitely go with the tank vent suggestion.



.

Yeah thought of that too at one time, gas cap is fine. No whooshing sounds ever. And no doesn't happen soon after filling the tank. It could set a week, go out and it would do it. Like I said it does it once every great while. And it was at 4,000ft elevation. It is just one of those annoying hiccups. Gonna clean the gas cap vent for sh*ts and giggles anyways. Did order a new petcock today.

Oh yeah, this is what the guy said that was behind me when it happened "Bill-- that Suzuki of yours really slows down quickly when the fuel is cut off! Engine must have great compression!" Went from 85 mph to 0 in almost nothing flat. When I felt the motor starting to stumble I pulled in the clutch before the motor died and let it coast to a stop.

[cowboyup3371]

I just thought of something as it sounds similar to an issue I faced in Colorado before moving out here. I would be riding along our two-lane highway between the base and the house at 65mph when I would suddenly lose power for about two or three minutes. I typically wouldn't get it back until I'd dropped to 2nd or 3rd gear and 45mph-ish. After several days of experiencing this, I put my side covers back on the bike (hadn't finished painting them at that time, and never had another issue. The best I could figure was somehow the air flow was severely disturbed once I got to higher speeds.

I don't know if that's the case for you but if you don't have your side covers on, put them back on and give it a shot.

[mrbill5491]

I just thought of something as it sounds similar to an issue I faced in Colorado before moving out here. I would be riding along our two-lane highway between the base and the house at 65mph when I would suddenly lose power for about two or three minutes. I typically wouldn't get it back until I'd dropped to 2nd or 3rd gear and 45mph-ish. After several days of experiencing this, I put my side covers back on the bike (hadn't finished painting them at that time, and never had another issue. The best I could figure was somehow the air flow was severely disturbed once I got to higher speeds.

I don't know if that's the case for you but if you don't have your side covers on, put them back on and give it a shot.

Sides covers are always on. And yes the vent tubes are in their proper routes.

[Steve]

Based on cowboy's comment, I am going to ask another question (or three).

Do you have vent hoses on your carbs?

If so, where do you have them routed?

Those vent hoses are the atmospheric reference for the float bowls. If the ends are sitting in a stream of moving air, the pressure will be a bit lower, preventing the proper "push" to get the gas through the jets, into the air stream. If the hoses are long enough, route them behind the airbox or up, under the seat, to a pocket of relatively still air and go for another ride.

.

[mrbill5491]

Based on cowboy's comment, I am going to ask another question (or three).

Do you have vent hoses on your carbs?

If so, where do you have them routed?

Those vent hoses are the atmospheric reference for the float bowls. If the ends are sitting in a stream of moving air, the pressure will be a bit lower, preventing the proper "push" to get the gas through the jets, into the air stream. If the hoses are long enough, route them behind the airbox or up, under the seat, to a pocket of relatively still air and go for another ride.

.

Vent hoses are on, they are of the proper length and routed the correct way (under the seat). They are not exposed to streaming air.

[mrbill5491]

Yea! Got my new OEM petcock in yesterday. Gonna install it this weekend WITH new OEM sealing washers. I was inspecting the new petcock and noticed it was set on prime for some reason.