How to rebuild a Rochester Quadrajet 4MV carburetor
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Some basic [http://www.73-87.com/7387garage/drivetrain/myqjet.htm tips and links], from from 73-87.com. | Some basic [http://www.73-87.com/7387garage/drivetrain/myqjet.htm tips and links], from from 73-87.com. | ||
| − | ===Basic adjustments | + | ===Basic adjustments=== |
[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 problems=== |
*[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 | ||
| − | ===Idle discharge ports=== | + | ====Idle discharge ports==== |
The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws. | The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws. | ||
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#[http://www.73-87.com/7387garage/drivetrain/damon.htm Performance Tune Your Q-jet Secondaries, Part 1] from 73-87.com | #[http://www.73-87.com/7387garage/drivetrain/damon.htm Performance Tune Your Q-jet Secondaries, Part 1] from 73-87.com | ||
#[http://www.73-87.com/7387garage/drivetrain/damon2.htm Performance Tune Your Q-jet Secondaries, Part 2] from 73-87.com | #[http://www.73-87.com/7387garage/drivetrain/damon2.htm Performance Tune Your Q-jet Secondaries, Part 2] from 73-87.com | ||
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| + | ====Secondary air valve==== | ||
| + | [[File:17080213 3519 CPX 1980 Chev, rear po, hot air choke.jpg|thumb|480px|left|Arrow indicates air valve limiter]] | ||
| + | If the engine exhibits a flattening out at midrange rpm and everything seems to be in order, check to see that the secondary air valve spring tension isn't too tight, causing the air valve to not open fully. | ||
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| + | The next thing is to check to see that you're getting full opening of the secondary air valve. Some carbs had the amount of the air valve opening set to a lot less than it's capable of. The fix is easy, just look where the red arrow (left) is pointing, that's the stop. It can be carefully bent to set the opening amount. | ||
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| + | I will use a Dremel to slot the linkage to make bending it easier- I prefer that over grinding the stop in case it needs further adjustment to lessen the opening amount, because the idea is to increase the opening until you either reach the max it will open or to where it doesn't help performance any more- in which case you'd decrease the opening back to where it was best. The linkage can be bent several times w/o breaking as long as the cut isn't made so there's not enough metal, and it's not bent too many times. | ||
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| + | ====Pulloff orifice==== | ||
| + | Once that's squared away you can go about quickening the opening rate by enlarging the pull off orifice. One way to do this to go very large on the factory-installed orifice, then solder the opening shut. From there, enlarging the orifice is as easy as using a small numbered bit in a pin vise. | ||
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| + | If you go too large, a touch w/a soldering iron will close it back up. And plan on going too large during the tuning of the orifice size, because you will keep enlarging it until you go too far and get a bog, then you back up to where it was best (just like the opening amount of the air valve). | ||
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| + | Some pulloff diaphragms have a 90 degree bend in the vacuum tube. If there is enough length before the bend, the 90 degree can be cut off in order to reach the orifice. If not, the tube can be carefully ground down at the 90 degree bend enough to access the orifice with a drill bit, then enlarge the orifice oversize as explained above. Reseal the tube and tune it by changing the orifice size at the soldered over tip of the vacuum tube. | ||
==Casting smoothing== | ==Casting smoothing== | ||
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