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Post by Deleted on Sept 20, 2013 19:22:04 GMT -5
Deni carbs dont appear to have an accelerator pump...how does it work?  |
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Post by millsc on Sept 20, 2013 20:17:04 GMT -5
Most carbs for 150s don't have a pump
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Post by rockynv on Sept 21, 2013 5:27:17 GMT -5
The slide dropping down and multiple jets works good enough to eliminate the need for the accelerator pump.
Even at that most small engines do not have or really need them
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Post by Deleted on Sept 21, 2013 5:52:29 GMT -5
So what adds fuel when the throttle gets turned? If I ever have to replace it can I use one with the pump? Even the rochesters for cars back in the day had a pump.
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Post by pmatulew on Sept 21, 2013 7:27:16 GMT -5
"Back in the day", cars with Rochesters got about 11 miles to the gallon on a good day. They also had cavernous and poorly designed intake manifolds. Extra shots of gas were a spit in the wind hoping that some of it would actually reach the intake valve.
Flushing more fuel down the drain is not what scooters are about. Our small displacement engines respond quickly enough to rpm changes that the added complexity of an accelerator pump is generally unnecessary.
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Post by pmatulew on Sept 21, 2013 7:32:36 GMT -5
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Post by rockynv on Sept 21, 2013 8:32:20 GMT -5
"Back in the day", cars with Rochesters got about 11 miles to the gallon on a good day. They also had cavernous and poorly designed intake manifolds. Extra shots of gas were a spit in the wind hoping that some of it would actually reach the intake valve. I still have my exhaust gas analyzer and such that I used to remeter the Quadrajets which when set up for economy could get up to 21 mpg highway in vehicles such as a 1969 Sedan DeVille 4 door with the 472 cu engine. They used Stellite valves and seats in those engines which would stand up to running lean a bit better than what was the norm for that era. Put in a B&M Transpac to shorten the lag in the shift points of the transmission and lean out the metering rods setting the secondaries to cut in at a higher throttle setting and you could get very good fuel economy as the 2 primary barrels were quite tiny. In the 1980s they used similar technology bulding staged 2 barrel carbs for 4 cylinder engines such as the Holley 5280 to increase drivability while upping fuel economy. |
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Post by phssthpok on Sept 21, 2013 9:14:45 GMT -5
So what adds fuel when the throttle gets turned? If I ever have to replace it can I use one with the pump? Even the rochesters for cars back in the day had a pump. The runner length (distance from venturi to valve face) on a car is much, much greater than that found on a scooter. The reason carbs on cars had accelerator pumps was because flopping the butterflies to WOT from idle would kill the vacuum present in the manifold required to pull the fuel from the bowl, through the venturi. By adding in a mechanical 'shot' of raw gas, the engine was supplied with the fuel necessary to continue operation (read: NOT die for lack of fuel) long enough to bring the RPM up far enough to create the needed airflow (vacuum/air pressure differential/however you want to think of it) to pull the fuel through the normal metering channels of the carburetor. On a properly tuned scooter (correct A/F mixture, proper idle speed, etc.), it's all but impossible to work the throttle 'fast enough' to kill enough vacuum between the valve face and the butterfly in the carb to 'kill' the engine. The manifold lacks sufficient volume for this to occur. By the time the vacuum has dropped even marginally, some fuel has reached the cylinder and the RPM's have begun to climb. Additionally the plunger (with the long needle in it) acts as a vacuum actuated airflow restrictor upstream from the throttle blade. As shown in pmatulew's linked video, the plunger is drawn upward (overcoming the spring that normally holds it down) by vacuum applied to the diaphragm in the dash-pot *. This 'delay' also helps to minimize the possibility of a rapid enough change in vacuum to 'kill' the engine. Think of it as a constantly variable, RPM sensitive 'choke', used to ensure proper 'pull' on the fuel supply through the main jet. Here is another short video of our (typical GY-6) carb in action. While they never really let it get down to a true idle, you'll note that no matter how fast they work the throttle, the plunger moves only as fast as the vacuum conditions dictate/allow. * If I'm not mistaken, on the typical GY-6 carb this vacuum passage is actually a small hole in the bottom of the plunger right next to where the long needle sits, and is oriented between the needle and the throttle blade. |
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Post by rockynv on Sept 22, 2013 6:29:15 GMT -5
On a slide carb the slide will not open unless there is enough vacuum differential between the inlet and the back side of the slide to pull the diaphragm against the spring so it is basically impossible to get to a low vacuum sitiation that would require a mechanical pump. Slide carbs are considered by many to be the next best alterative to fuel injection due to this and the fact that the system does not really require a battery in order for it to work making it better suited for off road and remote locations where a dead battery would leave an electronicly fuel injected bike stranded.
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