How to rebuild a Rochester Quadrajet 4MV carburetor
(→Float/fuel bowl: add image, text) |
(→Idle problems: Add text, images) |
||
| Line 366: | Line 366: | ||
[http://vetteworks.tripod.com/qjet.htm Rochester 4MC, 4MV, M4MC, and E4ME Carburetors] from vetteworks.tripod.com | [http://vetteworks.tripod.com/qjet.htm Rochester 4MC, 4MV, M4MC, and E4ME Carburetors] from vetteworks.tripod.com | ||
| − | ===Idle | + | ===Idle circuit=== |
*[http://www.73-87.com/7387garage/drivetrain/qjetidle.htm Q-jet Idle Problems] from 73-87.com | *[http://www.73-87.com/7387garage/drivetrain/qjetidle.htm Q-jet Idle Problems] from 73-87.com | ||
| Line 373: | Line 373: | ||
This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air). | This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air). | ||
| + | |||
| + | ====Idle bypass air==== | ||
| + | Similar to drilling the primary throttle blades, the idle air bypass orifices introduce air below the primary throttle blades. These orifices can be enlarged/reduced to help some engines idle better. | ||
| + | |||
| + | The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller. | ||
| + | |||
| + | There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit. | ||
| + | |||
| + | Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting. This can be fairly easily removed if it's encountered. They block them in various places, sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets. | ||
| + | |||
| + | The older 'tunnel' type (circles): | ||
| + | |||
| + | [[File:Bypass 1.jpg]] | ||
| + | |||
| + | |||
| + | The later 'tab' type, most look like the one here: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is): | ||
| + | |||
| + | [[File:Bypass2.jpg]] | ||
===Power piston=== | ===Power piston=== | ||
Your Privacy Choices