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Post by GrumpyUnk on Jan 14, 2018 14:15:04 GMT -5
EdwarT:When I removed the carb and placed my hand over the intake manifold and tried to start on electric start the engine was sucking air in, but even with a full float bowl the engine doesnt suck fuel through the carb.
It seems impossible that you can feel the vacuum provided by the engine with your hand, yet the same vacuum will not pull fuel from the floatbowl. Not. Gonna. Happen. Unless the passages in the carburetor are plugged. The vacuum in the intake should pull fuel from the floatbowl unless there is internal blockage. As I think, the only other thing that could prevent or lessen fuel flow is if the float bowl itself is under vacuum that opposes that from the intake. If the float bowl is sealed, intentionally or not, it would not have atmospheric pressure pushing on the fuel, and forcing it up into the carb passages. It is that pressure differential that allows fuel to flow. Without a vented floatbowl, fuel will not flow (readily. It may marginally over time) Have you compared your vacuum and other connected tubes on your carburetor to another one that has not been worked on? Is it possible that someone has connected something to some place it should not be? I have not closely inspected for venting, but I think the 'elbow' that is on most carbs that points towards the ground is the vent for the float bowl. It comes out on the left(seated on the bike) side of the carburetor and immediately points downward. Some machines have more than one vacuum connection to the intake, some have them plugged, and some just 'fold' the vacuum line, and some are connected to a canister, apparently valved, that controls evaporative emissions from the gas tank. Do you have a picture showing all the connections to yours? I second the idea of the non-vacuum fuel supply valve. Run at WOT, there should be about ZERO vacuum to keep the fuel valve open, leading to leaner mixture, increased temperature, and possible slowdown and/or damage. tom
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Post by edwardtennant on Jan 14, 2018 17:48:44 GMT -5
Hi Grumpy, I will double check the jets in the carb and make sure they are clear next time i am with my bike 9after all its 10 miles away). I will also make sure the float bowl vent isnt blocked or anything. I know the tubing is correct as it has been running fine since March when i got the scoot second hand. When i get back to the bike i will take a photo of the carb setup. Here is a crappy MS paint drawing of the carb stuff imgur.com/a/Iz9dy
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Post by FrankenMech on Jan 14, 2018 18:59:00 GMT -5
If the fuel filter picture representation is correct it is backwards. Fuel flows from the outside of the filter element to the interior. It is a small point to check and should not be a real problem.
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Post by lostforawhile on Jan 14, 2018 21:38:33 GMT -5
EdwarT:When I removed the carb and placed my hand over the intake manifold and tried to start on electric start the engine was sucking air in, but even with a full float bowl the engine doesnt suck fuel through the carb. It seems impossible that you can feel the vacuum provided by the engine with your hand, yet the same vacuum will not pull fuel from the floatbowl. Not. Gonna. Happen. Unless the passages in the carburetor are plugged. The vacuum in the intake should pull fuel from the floatbowl unless there is internal blockage. As I think, the only other thing that could prevent or lessen fuel flow is if the float bowl itself is under vacuum that opposes that from the intake. If the float bowl is sealed, intentionally or not, it would not have atmospheric pressure pushing on the fuel, and forcing it up into the carb passages. It is that pressure differential that allows fuel to flow. Without a vented floatbowl, fuel will not flow (readily. It may marginally over time) Have you compared your vacuum and other connected tubes on your carburetor to another one that has not been worked on? Is it possible that someone has connected something to some place it should not be? I have not closely inspected for venting, but I think the 'elbow' that is on most carbs that points towards the ground is the vent for the float bowl. It comes out on the left(seated on the bike) side of the carburetor and immediately points downward. Some machines have more than one vacuum connection to the intake, some have them plugged, and some just 'fold' the vacuum line, and some are connected to a canister, apparently valved, that controls evaporative emissions from the gas tank. Do you have a picture showing all the connections to yours? I second the idea of the non-vacuum fuel supply valve. Run at WOT, there should be about ZERO vacuum to keep the fuel valve open, leading to leaner mixture, increased temperature, and possible slowdown and/or damage. tom The vacuum for the petcock comes from behind the butterfly valve, there is always some vacuum or fuel couldn't be pulled through the carb, it will fluctuate as the throttle is opened or closed, but there is enough fuel in the bowl, for this to not have an effect. if you hook a vacuum gauge to one of the ports, then blip the throttle, you'll see what I mean
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Post by GrumpyUnk on Jan 15, 2018 11:36:50 GMT -5
lost: there is always some vacuum or fuel couldn't be pulled through the carb,
Yabbut. He is reporting that there is no fuel being pulled into the engine. Or did I get it wrong?
I never held the throttle wide open on any engine I owned while I had a vacuum gauge connected. I know, or have seen, the measured vacuum after the throttle plate dip way close to zero. Likely not zero, but, there has to be enough to operate the vacuum fuel valve. Older carburetors had a built-in valve called the "Power Valve" which was vacuum operated. When manifold vacuum dropped low, the valve opened, and dumped fuel into the venturi, where there is vacuum pulling the fuel from the float bowl. The vacuum in the venturi is NOT the same as manifold vacuum. You develop vacuum relative to atmospheric pressure. With a zero reading, atmospheric pressure will still push air into the carb, and thus the intake. I cannot state yes or no, absolutely, that running with WOT will keep the vacuum enough to operate the fuel valve. I do not know. Given these small engines were developed to be used more in urban settings, where traffic flow has multiple stop and go situations, consistent long term running at WOT is not likely part of the criteria used to set parameters for fuel delivery/control. They don't run them at WOT long enough to run out of gas in the float bowl. I think that here in USA, WOT is more common. May be wrong with that assumption also. tom
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Post by edwardtennant on Jan 15, 2018 13:02:23 GMT -5
Hi just to clear some things up. The float bowl has fuel in it. The engine turns but doesnt pull the fuel from the float bowl into the engine. If i remove the carb and place my hand over the intake i feel suction when on the electric start.There is not enough vacuum on the starter motor to open the fuel petcock. I am going down to my bike tonight and i am going to install new vac lines, clean the carb more thoroughly, check to make sure the timing chain hasnt slipped, and also check to make sure there are no air leaks near the manifold.
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Post by edwardtennant on Jan 15, 2018 13:02:54 GMT -5
Also the fuel filter is in correctly, i just drew it the wrong way around for some reason, sorry for the confusion
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Post by edwardtennant on Jan 15, 2018 16:26:50 GMT -5
Another update, got back to the bike, and the bike is running again after swapping vac lines, cleaning carb again, and resetting valves. Thanks for your help guys
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Post by lostforawhile on Jan 15, 2018 17:55:11 GMT -5
lost: there is always some vacuum or fuel couldn't be pulled through the carb, Yabbut. He is reporting that there is no fuel being pulled into the engine. Or did I get it wrong? I never held the throttle wide open on any engine I owned while I had a vacuum gauge connected. I know, or have seen, the measured vacuum after the throttle plate dip way close to zero. Likely not zero, but, there has to be enough to operate the vacuum fuel valve. Older carburetors had a built-in valve called the "Power Valve" which was vacuum operated. When manifold vacuum dropped low, the valve opened, and dumped fuel into the venturi, where there is vacuum pulling the fuel from the float bowl. The vacuum in the venturi is NOT the same as manifold vacuum. You develop vacuum relative to atmospheric pressure. With a zero reading, atmospheric pressure will still push air into the carb, and thus the intake. I cannot state yes or no, absolutely, that running with WOT will keep the vacuum enough to operate the fuel valve. I do not know. Given these small engines were developed to be used more in urban settings, where traffic flow has multiple stop and go situations, consistent long term running at WOT is not likely part of the criteria used to set parameters for fuel delivery/control. They don't run them at WOT long enough to run out of gas in the float bowl. I think that here in USA, WOT is more common. May be wrong with that assumption also. tom there is always vacuum on the manifold as long as the engine is running, it doesn't take much to open that little petcock, remember the vacuum acts on the entire surface of the diaphragm in there, multiplying it's force over the surface area. It will dip when you open the throttle momentarily, before the engine RPMs catch back. A properly operating petcock works just fine at WOT, plenty of people run them full out for long distances, including myself.
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Post by GrumpyUnk on Jan 16, 2018 10:05:17 GMT -5
Okay, now I know. Thanks. I wondered if they(designers) had tested them. I watched 90 doing his MD runs to the beach, which was a lot of WOT, but I thought he had replaced the vacuum valve with a manual shutoff. Yep, the area of the diaphragm multiplies the force of the vacuum, essentially, so I guess it doesn't take a lot to move the valve. One I had must have had a punctured diaphragm for as soon as you cranked it up, got it started, it would die, and fuel would flow out the air cleaner. Ugh. No fuel to dribble until the valve opened, and then it was the deluge. tom
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Post by lostforawhile on Jan 16, 2018 11:25:41 GMT -5
It could be that sucking gas through the vaccume line,but that would go in the intake directly , not in the filter, it sounds more like a stuck float or dirt in the float needle valve, gas would flow into the bowl until it overflowed
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Post by edwardtennant on Jan 16, 2018 11:42:53 GMT -5
I got the no start sorted Lost, the float needle was stuck, the so it was pulling a vacuum inside the float not letting fuel out of the bowl. It starts and runs now
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Post by lostforawhile on Jan 16, 2018 21:16:24 GMT -5
might be a good time to source a rebuild kit and do a through cleaning of your carb, some time in the future, if dirt has made it's way in, I had a piece of fuel filter mesh stuck in a jet
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