How to rebuild a Rochester Quadrajet 4MV carburetor
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====Secondary air valve==== | ====Secondary air valve==== | ||
[[File:17080213 3519 CPX 1980 Chev, rear po, hot air choke.jpg|thumb|480px|left|Arrow indicates air valve limiter]] | [[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. | + | 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. The secondary air valve only opens when the engine is under a load and the primary throttle blades are opened about half way or more. If the AV is adjusted so the spring pressure is too light, it can open too much, too soon- and the result is a bog. |
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| + | The AV shouldn't open when you jazz the throttle in PARK/neutral. Only when under a load, and at sufficient RPM do you want the AV to open. Also, there shouldn't be a noticeable transition into the secondaries. Many guys think the carb should "bang" into the secondaries, but what's happening is a bog when the secondaries open, followed by the engine 'catching up' to the carb- this gives the impression of acceleration. A timing slip will show otherwise. | ||
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| + | You can get a 'feel' for how it's adjusted by pushing the AV open (engine OFF) w/a finger. It should shut w/o hesitation when you let off pressure. After you've been working w/the Q-jet for a while, you will get a feel for how the AV is adjusted just by doing this. | ||
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| + | There is an adjustment for the AV spring. Should you want to try different adjustments on the air valve, use a 3/32" allen wrench to loosen the lock screw under the AV spring adjustment. 3/4 of a turn from first contact of the spring is the base setting, changing the setting 1/8 to 1/4 turn at a time will get it dialed in. | ||
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. | 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. | 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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| + | Conversely, if the carb is going into the secondaries smoothly, yet the engine seems to lay down even though everything is as it should be, the air valve may be opening too far; that is, the engine doesn't need all the flow the Q-jet can supply, at WOT. If that is the case, carefully adjust the stop to provide less AV opening until it accelerates cleanly and strongly to redline. | ||
====Pulloff orifice==== | ====Pulloff orifice==== | ||
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