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| Vacuum leaks https://slantsix.org/forum/viewtopic.php?t=13407 |
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| Author: | R. Wood [ Sun Jun 19, 2005 6:19 pm ] |
| Post subject: | Vacuum leaks |
I am getting around 13 psi on the vacuum gauge, when tee it into the line from the distributor. When I spray carb cleaner to check for leaks, I get higher at revs at virtually every vacuum hose joint - - even the new ones that seem snug. Also getting higher revs when I spray into the base of the carb, around the EGR valve...... seems like it's leaking everywhere! The carb base gasket (and the EGR for that matter) is nearly new. Here's something else - when I remove the Purge line at the cannister, and hold my thumb over it, the vacuum immediately increases to 14.5 psi. Could the cannister itself be leaking? Or is doing that just relieving some of the vacuum load? I am having to set the curb idle at around 950 rpm to keep the car from stalling, and I suspect the vacuum problem is having at least something to do with this. Any ideas on how I can fix it? Thanks. |
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| Author: | SlantSixDan [ Sun Jun 19, 2005 6:44 pm ] |
| Post subject: | |
Well, you're working on a vehicle that's between 18 and 33 years old, so it is possible that it's "leaking everywhere"! There are lots of leak points, gasketed and otherwise. Throttle shaft bushings wear and create big vacuum leaks. EGR valves stick partially open or burn up their gaskets. Charcoal cannisters and their purge valves degrade and break. Carburetor castings warp and create internal or external vacuum leaks. Diaphragms crack or develop holes in distributor vacuum advances, EGR valves, choke pull-offs, heat-A/C mode door actuators, control valves and selector switches. Vacuum hoses crack or become porous. PCV valves deteriorate. Intake manifolds leak at their junctions to the head. Some intake manifolds even develop leaks right through the casting, and cracks aren't unknown in the intake floor right below the carb, directly into the exhaust hot spot. And previous owners hook stuff up wrong and do other mayhem. What kind of vehicle are you working on? What mileage? Which carb? Iron or aluminum intake? A few notes: 1) If you're seeing 13 psi on the vacuum gauge, you've got a really strong turbocharger. I think you're really seeing 13 in. Hg. ("inches of Mercury"). 2) The line from the distributor isn't a good place to check your vacuum. That's not manifold vacuum, it's ported. Use the choke pull-off hose instead; it's got constant manifold vacuum. |
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| Author: | R. Wood [ Mon Jun 20, 2005 8:33 am ] |
| Post subject: | |
It's a '75 Valiant, cast iron intake, Carter BBS. The car has 85k miles on it but the engine has 155k. The carb is a recent re-man; it's the one I had choke issues with (still do, actually - the electric choke from CarbsOnly did not link up at all with this Carter). But getting back to the vacuum leaks, the items that have not been recently replaced or attended to are the charcoal cannister / purge valve and the diaphragm in the distributor vacuum advance. Also I don't think the vacuum amplifier has been touched. I will recheck the vacuum at the choke pull-off (thanks for the tip) and I'll report back with the number of inches, not pounds, lol |
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| Author: | SlantSixDan [ Mon Jun 20, 2005 9:58 am ] |
| Post subject: | |
Quote: It's a '75 Valiant, cast iron intake, Carter BBS. The car has 85k miles on it but the engine has 155k. The carb is a recent re-man; it's the one I had choke issues with (still do, actually - the electric choke from CarbsOnly did not link up at all with this Carter).
Interesting. Take pictures and show us how it doesn't link up.Quote: But getting back to the vacuum leaks, the items that have not been recently replaced or attended to are the charcoal cannister / purge valve and the diaphragm in the distributor vacuum advance. Also I don't think the vacuum amplifier has been touched.
Well, the vacuum amplifier is easy enough to test. Pull off its large diameter supply hose and cap it. If that increases your manifold vacuum at idle, the amplifier's bad. The distributor vacuum advance can be tested by applying mouth suction to the hose. Assuming the hose is good, if you can suck air (can't pull a vacuum), the diaphragm's ruptured.The charcoal canister's trickier. It's been a long time since I had a carbureted car with a canister, so I can't advise you reliably. |
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