Yellow Bullet Forums banner

21 - 29 of 29 Posts

·
Registered
Joined
·
29,019 Posts
To clarify, you are not saying the secondaries drain when the engine is not running? If a bowl empties when the engine is shut off it is likely the idle air bleeds are plugged or the metering block or plate is leaking, as mentioned in posts above.

The engine has to be running for this to occur, right? If it runs well, and you like the way it runs, don't mess with it. It is doing this because the secondary is feeding more idle fuel than the primary, but so what?, if the engine likes it, it likes it. Better to listen to the engine than the internet. One of the most important rules of carburetors is don't fix them when they are already running good.
 

·
Registered
Joined
·
180 Posts
Discussion Starter #22
To clarify, you are not saying the secondaries drain when the engine is not running? If a bowl empties when the engine is shut off it is likely the idle air bleeds are plugged or the metering block or plate is leaking, as mentioned in posts above.

The engine has to be running for this to occur, right? If it runs well, and you like the way it runs, don't mess with it. It is doing this because the secondary is feeding more idle fuel than the primary, but so what?, if the engine likes it, it likes it. Better to listen to the engine than the internet. One of the most important rules of carburetors is don't fix them when they are already running good.
LOL, thanks Yeti... I havent checked it yet to see if it drains if I just shut the motor down instead of just shutting the pump down first.. I just happen to shut the pump off first and noticed it draining.. Your right, I think im going to leave it alone now since it runs and idles perfect now..

Thank you everyone for the help!!
 

·
Registered
Joined
·
65,219 Posts
It is pulling fuel from the primary, but the secondary is feeding so much more fuel it runs out more quickly. Because the secondary idle air bleed is small (should be ~.026" or so in a vac. secondary Holley or clone carb) and the piss hole is (looks to be) too large the idling engine is getting the majority of idle fuel, and possibly air if the secondary is open too far, from the secondary, so when the secondary bowl runs out the engine dies.

Fuel is still in the primary bowl because the primary isn't discharging as much fuel as the secondary.

Didn't OP say he originally had to turn the primary mixture screws all the way closed to get it to idle. This is because it is getting way too much from the secondary side. Should only be a small portion, not the majority of idle fuel.

The secondary idle air bleed needs to be small ~.026 or so ~ to make the vacuum secondary opening transition smoothly. With that small a bleed, if there is a lot of discharge orifice area, like the mixture screws in the primary or the too-large piss hole, the fuel discharge will be much more sensitive to intake vacuum changes and A/F will be erratic when throttle position and load, therefor intake vacuum, is changed.

The piss hole is just a tiny fixed discharge orifice and the source of fuel is the same as the T-slot. The piss hole is right below the T-slot in the same passage, so fed from the same source, from the idle jet in the metering plate and idle air bleed in the body above the main venturi. The piss hole feeds when the throttle is closed and the slot is not discharging. This serves to keep the circuit wet "primed" so when the secondary opens the fuel is there and ready to flow with no lag.

The area of T-slot above throttle blade is air bleed in, T-slot below the upper edge of throttle blade is fuel feed out. It is commonly said that the intake vacuum below the slot pulls the fuel into the bore, but actually it is the depression resulting from the air velocity passing the edge of the throttle blade which causes a low pressure that draws fuel through the idle circuit and through the slot. At part-throttle, and particularly when the edge is near the slot, the flow area between the edge of the butterfly and the bore wall acts as a venturi. At or just very slightly below the edge (in the vena contracta) there is lower pressure than further below in the bore or manifold. (same as the "pressure recovery" in another thread today) As the blade opens and the area of slot changes from air bleed in to discharge out and the fuel flow increases proportionally to the ratio of slot in to slot out.
Thanks Yeti. Had to read that twice but I have a better understanding now on what's going on with his modified carbs. Still though, the engine dying when the fuel level in the secondary bowl only just disappears from the site window --- still has me scratching my head -- since there is still fuel in the bowl.
 

·
Registered
Joined
·
29,019 Posts
I don't see why this would make you scratch your head. Low float level makes a carb lean for the same reason high float level make a carb rich.
 

·
Registered
Joined
·
65,219 Posts
I don't see why this would make you scratch your head. Low float level makes a carb lean for the same reason high float level make a carb rich.
Jets still covered with fuel.

Primary bowls still full.
 

·
Registered
Joined
·
29,019 Posts
Jets still covered with fuel.
Gravity head. Higher and lower float level cause more or less head pressure. You never noticed high or low float level causes a carb to be rich or lean, really?

Primary bowls still full.
Secondary idle/transfer circuit is discharging fuel at much higher rate than the primary, so much more the engine is idling almost entirely on the secondary, ergo, secondary runs out first with only small change in primary float level.
 

·
Registered
Joined
·
65,219 Posts
In a properly modified carb, yes ... I have witnessed that happening.
 

·
Registered
Joined
·
29,019 Posts
I don't see what "a properly modified carb" has to do with gravity. Gravity is the same everywhere for everything. Idle has the least fuel flow. The less fuel flow there is, the more significant the effect of the gravity head.

The OP's combo is outside the norm of what most people consider correct calibration for "a properly modified carb" but if it runs good so be it.

The big secondary pissholes would be detrimental in a circle track or road race carb because they would over-fuel on deceleration and load the manifold with too much fuel, so on throttle pickup to get off the corner it would be blubbering rich, a stinky poo flat spot that a lot of folks usually think is too lean because it misses when they touch the throttle, so then when they richen it up it gets worse and they get frustrated. If there is a WBO2 it reads lean because of the misfires, further compounding the frustration.
 

·
Registered
Joined
·
180 Posts
Discussion Starter #29
I ran the motor again and shut both the ignition and fuel pump off at the same time and the fuel never drained from any of the bowls.. Im going to leave it the way it is, I just thought something was wrong when I shut the pump off and only the rear bowls continued to drain while it was ideling
 
21 - 29 of 29 Posts
Top