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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Cars: 1966 Dodge Dart 270 Convertible,
1990 Alfa 164 Twin Spark
1980 BMW 735i 5-speed

http://www.weddingcar4u.blogspot.com/

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

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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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

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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一期一会
Too many people who were born on third base actually believe they've hit a triple.

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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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

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

_________________
Cars: 1966 Dodge Dart 270 Convertible,
1990 Alfa 164 Twin Spark
1980 BMW 735i 5-speed

http://www.weddingcar4u.blogspot.com/

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

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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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Will disconnect the kickdown linkage and if that doesnt work, will get the garden hose and try to pin point the leak...

Failing that, maybe will remove the carb and see if something is amiss.

Syed

_________________
Cars: 1966 Dodge Dart 270 Convertible,
1990 Alfa 164 Twin Spark
1980 BMW 735i 5-speed

http://www.weddingcar4u.blogspot.com/

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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Cars: 1966 Dodge Dart 270 Convertible,
1990 Alfa 164 Twin Spark
1980 BMW 735i 5-speed

http://www.weddingcar4u.blogspot.com/

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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一期一会
Too many people who were born on third base actually believe they've hit a triple.