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| Hissing sound at idle when hot? Why? https://slantsix.org/forum/viewtopic.php?t=47127 |
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| Author: | 1966DartConvertible [ Mon Nov 28, 2011 4:13 am ] |
| Post subject: | Hissing sound at idle when hot? Why? |
During the run yesterday, I noticed a distinct hissing sound seem to be coming from the carb/manifold area. This only happens once the engine is fully warmed-up. I cant pin point exactly where, just wondering what are the possibilities? Vacuum hoses are all new. Carb gasket and o-rings are new. Air cleaner is tight and does not wobble/move. The engine drives fine, but idles a bit erratic once the hissing sound started. The hiss goes away as soon as I hit the gas... Any idea where i should start? Thanks. Syed |
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| Author: | Slanted Opinion [ Mon Nov 28, 2011 6:09 am ] |
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The hissing is most likely a vacuum leak. Check to see that the nuts securing the carb to the manifold are tight. Beyond that you can find the leak with a little bit of carb cleaner spray. Spray suspect areas while the engine is idling. The engine with smooth out and speed up when you've spray the area that is leaking. - Mac |
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| Author: | DonPal [ Mon Nov 28, 2011 8:43 am ] |
| Post subject: | Hissing Sound When Hot |
I just fixed a hissing problem when hot that turned out to be coolant leaking past an exhaust manifold stud. Coolant pressure buildup when heated was enough to push the coolant past the stud threaded connection. A new stud, resealed. stopped the hiss (and unseen coolant lost). I verified the problem by monitoring coolant level.....it showed up fairly fast (several hun dred miles) |
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| Author: | SlantSixDan [ Mon Nov 28, 2011 10:16 am ] |
| Post subject: | |
I'm voting against it being an external vacuum leak. You can get a hiss like this from the carburetor if the throttle plate position is "just so" relative to the various ports and passages, or if the throttle shaft/bushing junction is worn. Try a really careful adjustment of idle mixture and speed and basic ignition timing. |
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| Author: | Aggressive Ted [ Mon Nov 28, 2011 1:06 pm ] |
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Quote: I noticed a distinct hissing sound seem to be coming from the carb/manifold area.
Last weekend my engine started doing the same thing. I started to adjust the idle mixture but that really didn't help solve the problem. It just compensated for the apparent vacuum leak.I found that the nuts holding the carb to the intake manifold were not very tight. I went ahead and tightened them which helped a little, but I went ahead a replaced the carb to manifold gasket and that did the trick. My old FelPro "thick" gasket had started to delaminate and was leaking between the layers. The new FelPro "thick gasket is not laminated........but made from a solid piece of material. |
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| Author: | 1966DartConvertible [ Mon Nov 28, 2011 4:17 pm ] |
| Post subject: | |
Quote: I'm voting against it being an external vacuum leak. You can get a hiss like this from the carburetor if the throttle plate position is "just so" relative to the various ports and passages, or if the throttle shaft/bushing junction is worn. Try a really careful adjustment of idle mixture and speed and basic ignition timing.
Thanks Dan. Will try to adjust further.The throttle shalf/bushing was worn when I bought the carb but I sent it for re-bush and was nice and tight prior to installation. Also, dont know if this might be related. I cannot get the idle to go below 750rpm even with both the curb idle and fast idle screw not touching the cam. Syed |
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| Author: | Doc [ Mon Nov 28, 2011 4:21 pm ] |
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Quote: ... I cannot get the idle to go below 750rpm even with both the curb idle and fast idle screw not touching the cam.
As a test, disconnect the throttle pressure (kickdown) linkage to see if that is keeping the carb from closing completly. If so, you may need to re-adjust the TP linkage in a way that allows a lower idle speed adjustment.DD |
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| Author: | SlantSixDan [ Mon Nov 28, 2011 4:49 pm ] |
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If the car passes Doc's good test, it is quite probable whoever rebushed your carburetor didn't get the throttle plate back in its exact correct orientation. |
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| Author: | 1966DartConvertible [ Mon Nov 28, 2011 9:33 pm ] |
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Quote: Quote: ... I cannot get the idle to go below 750rpm even with both the curb idle and fast idle screw not touching the cam.
As a test, disconnect the throttle pressure (kickdown) linkage to see if that is keeping the carb from closing completly. If so, you may need to re-adjust the TP linkage in a way that allows a lower idle speed adjustment.DD Failing that, maybe will remove the carb and see if something is amiss. Syed |
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| Author: | 1966DartConvertible [ Tue Dec 06, 2011 6:35 am ] |
| Post subject: | |
Quote: I'm voting against it being an external vacuum leak. You can get a hiss like this from the carburetor if the throttle plate position is "just so" relative to the various ports and passages, or if the throttle shaft/bushing junction is worn. Try a really careful adjustment of idle mixture and speed and basic ignition timing.
Found the "hiss" to be coming from the "bowl vent valve" on top of the carb float bOwl. Is this normal? Is it supposed to vent out air constantly like that when hot? It does not affect idling quality that much now, just the annoying hiss... syed |
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| Author: | SlantSixDan [ Tue Dec 06, 2011 12:07 pm ] |
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Quote: Found the "hiss" to be coming from the "bowl vent valve" on top of the carb float bOwl. Is this normal? Is it supposed to vent out air constantly like that when hot?
No, definitely not. It sounds as if your carburetor may have an internal fault, or perhaps a leak at the economizer diaphragm or its cover, which is right next to the bowl vent and a much more likely source of vacuum hiss. (Toldjya you should've let me send you that new carburetor…!  )
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