Engine dies due to no gas because vacuum isn't strong enough

MCPhotographer · May 9th, 2023, 06:37 PM

Hey everybody. Thanks for taking a look at this post. I have a question because I can't seem to figure out the reason why this is happening. I've rebuilt the carburetor, it's been dipped in berryman's and ultrasonically cleaned. This carburetor was rebuilt not that long ago wit a rebuild kit and is only been sitting for a few months. I decided to thoroughly clean it before trying to get it running again just to make sure that I didn't have any idling issues or throttle issues. Anyways I've got it in and installed and the bike would not start. I decided to check to see if there was fluid flow because it did not seem like the engine was getting gas. Sure enough there was no gas running into the carburetor. So I attached a brake bleeder to the vacuum side of the petcock and applied pressure to it. The gas started to flow freely and the engine runs fine as long as the brake bleeder is connected with pressure. Once I remove that and connect it back to the carburetor with the hose the bike will run for a while and eventually just shut off because it's not getting any gas. So the problem is there's not enough suction to cause the venturi to allow the gas to flow. Does anybody have any suggestions or has experiqenced this problem before? Has the petcock gone bad? Or is it the carburetor that's got the issue? I don't know if the carburetor is not producing enough suction or if the petcocks faulty and requires more suction than it should.

Guaire · May 12th, 2023, 07:01 AM

Is there compression on the cylinders?

ducatiman · May 12th, 2023, 01:57 PM

Quote:

Originally Posted by MCPhotographer

So I attached a brake bleeder to the vacuum side of the petcock and applied pressure to it. The gas started to flow freely and the engine runs fine as long as the brake bleeder is connected with pressure.

please post a pic of this setup with brake bleeder installed

MCPhotographer · May 15th, 2023, 08:53 AM

Ducatiman, the carburetor is currently assembled and situated in the bike. I literally took a Mighty Vac and connected a hose to the petcock vacuum tube and gave it a couple of squeezes and let it dangle. The bike ran fine when this was done.

Guaire - Not sure I understand the question. I assume there's compression. Bike runs fine as long as there's sufficient suction on the petcock.

Triple Jim · May 15th, 2023, 09:09 AM

Vacuum petcocks take very little vacuum to open when they're working properly. They have to be designed that way since if you hold the throttle wide open for a while, causing the vaccum to be only slight, you can't have the fuel stop flowing. If you put the vacuum hose in your mouth, it should take only a little sucking to make it open. A Mighty Vac is way overkill.

DannoXYZ · May 15th, 2023, 09:24 AM

Hey good job figuring petcock wasn't getting enough vacuum. This is out of Honda manual, but same idea.

Check your hoses on carbs. Make sure they match this layout.

Note petcock should have dedicated vacuum hose that's not shared with anything else. Make sure there's no cracks in hose and both ends securely clamped.

Measure how much vacuum it actually takes to open petcock. It might have leaky diaphragm and needs rebuild.

MCPhotographer · May 15th, 2023, 09:25 AM

I appreciate the input. So then what do you think might be causing the issue then? What would cause the carburetor to not generate enough suction as it ran?

DannoXYZ · May 15th, 2023, 02:22 PM

It may not be carbs. Petcock hasn't been confirmed fully operational. Hook up vacuum gauge again and measure what vacuum-level is required to open up petcock to full flow. And leave it for while and see if vacuum drops. Sure sign of leaking diaphragm.

Also want to verify that there's zero flow when petcock is at rest with no vacuum.

MCPhotographer · May 15th, 2023, 03:06 PM

Hey Danno. I don't know why I didn't see the picture and diagram you listed earlier, but I didn't. That is definitely helpful. I will spend some time on it tomorrow. I'm thinking the vacuum hose is loose/leaking on one of the ends. Need new hose and new clamps to be sure.

May 9th, 2023, 06:37 PM	#1
MCPhotographer ninjette.org member Name: Sarge Location: Los Angeles Join Date: Jan 2014 Motorcycle(s): EX 250, 06 CBR 1000, Duc 848, 100yr Road King, VTX 1300 Posts: 35	Engine dies due to no gas because vacuum isn't strong enough Hey everybody. Thanks for taking a look at this post. I have a question because I can't seem to figure out the reason why this is happening. I've rebuilt the carburetor, it's been dipped in berryman's and ultrasonically cleaned. This carburetor was rebuilt not that long ago wit a rebuild kit and is only been sitting for a few months. I decided to thoroughly clean it before trying to get it running again just to make sure that I didn't have any idling issues or throttle issues. Anyways I've got it in and installed and the bike would not start. I decided to check to see if there was fluid flow because it did not seem like the engine was getting gas. Sure enough there was no gas running into the carburetor. So I attached a brake bleeder to the vacuum side of the petcock and applied pressure to it. The gas started to flow freely and the engine runs fine as long as the brake bleeder is connected with pressure. Once I remove that and connect it back to the carburetor with the hose the bike will run for a while and eventually just shut off because it's not getting any gas. So the problem is there's not enough suction to cause the venturi to allow the gas to flow. Does anybody have any suggestions or has experiqenced this problem before? Has the petcock gone bad? Or is it the carburetor that's got the issue? I don't know if the carburetor is not producing enough suction or if the petcocks faulty and requires more suction than it should.

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May 12th, 2023, 07:01 AM	#2
Guaire ninjette.org member Name: Bill Location: Arlington, VA Join Date: Apr 2011 Motorcycle(s): 2005 EX250, 2014 Royal Enfield Continental GT Posts: 88	Is there compression on the cylinders?

May 15th, 2023, 08:53 AM	#4
MCPhotographer ninjette.org member Name: Sarge Location: Los Angeles Join Date: Jan 2014 Motorcycle(s): EX 250, 06 CBR 1000, Duc 848, 100yr Road King, VTX 1300 Posts: 35	Ducatiman, the carburetor is currently assembled and situated in the bike. I literally took a Mighty Vac and connected a hose to the petcock vacuum tube and gave it a couple of squeezes and let it dangle. The bike ran fine when this was done. Guaire - Not sure I understand the question. I assume there's compression. Bike runs fine as long as there's sufficient suction on the petcock.

May 15th, 2023, 09:09 AM	#5
Triple Jim Guy Who Enjoys Riding Name: Jim Location: North Carolina Join Date: Jul 2016 Motorcycle(s): Ninja 250 Posts: A lot. MOTM - Oct '18, Aug '17, Aug '16	Vacuum petcocks take very little vacuum to open when they're working properly. They have to be designed that way since if you hold the throttle wide open for a while, causing the vaccum to be only slight, you can't have the fuel stop flowing. If you put the vacuum hose in your mouth, it should take only a little sucking to make it open. A Mighty Vac is way overkill.

May 15th, 2023, 09:24 AM	#6
DannoXYZ ninjette.org certified postwhore Name: AKA JacRyann Location: Mesa, AZ Join Date: Dec 2011 Motorcycle(s): CB125T CBR250R-MC19 CBR250RR-MC22 NSR350R-MC21 VF500F CBR600RR SFV650 VFR750F R1M ST1300PA Valkyrie-F6C Posts: A lot. MOTY - 2018, MOTM - Nov '17	Hey good job figuring petcock wasn't getting enough vacuum. This is out of Honda manual, but same idea. Check your hoses on carbs. Make sure they match this layout. Note petcock should have dedicated vacuum hose that's not shared with anything else. Make sure there's no cracks in hose and both ends securely clamped. Measure how much vacuum it actually takes to open petcock. It might have leaky diaphragm and needs rebuild.

May 15th, 2023, 09:25 AM	#7
MCPhotographer ninjette.org member Name: Sarge Location: Los Angeles Join Date: Jan 2014 Motorcycle(s): EX 250, 06 CBR 1000, Duc 848, 100yr Road King, VTX 1300 Posts: 35	I appreciate the input. So then what do you think might be causing the issue then? What would cause the carburetor to not generate enough suction as it ran?

May 15th, 2023, 02:22 PM	#8
DannoXYZ ninjette.org certified postwhore Name: AKA JacRyann Location: Mesa, AZ Join Date: Dec 2011 Motorcycle(s): CB125T CBR250R-MC19 CBR250RR-MC22 NSR350R-MC21 VF500F CBR600RR SFV650 VFR750F R1M ST1300PA Valkyrie-F6C Posts: A lot. MOTY - 2018, MOTM - Nov '17	It may not be carbs. Petcock hasn't been confirmed fully operational. Hook up vacuum gauge again and measure what vacuum-level is required to open up petcock to full flow. And leave it for while and see if vacuum drops. Sure sign of leaking diaphragm. Also want to verify that there's zero flow when petcock is at rest with no vacuum.

May 15th, 2023, 03:06 PM	#9
MCPhotographer ninjette.org member Name: Sarge Location: Los Angeles Join Date: Jan 2014 Motorcycle(s): EX 250, 06 CBR 1000, Duc 848, 100yr Road King, VTX 1300 Posts: 35	Hey Danno. I don't know why I didn't see the picture and diagram you listed earlier, but I didn't. That is definitely helpful. I will spend some time on it tomorrow. I'm thinking the vacuum hose is loose/leaking on one of the ends. Need new hose and new clamps to be sure.