Hissing Carb

[jbuening]

Quick question and let me know what you think. With the hood open, engine idling, and standing by the drivers wheel i can hear the carb making a hissing/sucking sound. It has been this way since the rebuild 4 months ago and cannot figure it out. Is this normal or abnormal?

Background: 1970 351w standard bore, stock heads (new valve job, guides, and hardened seats), Edelbrock 600cfm manual choke carb, Typhoon polished aluminum dual plane intake, stock flow mechanical fuel pump, and the following CompCams items: XE262H cam w/lifters, true dual roller timing chain, standard length pushrods, stamped rockers, slightly stiffer than stock springs per their recommendation, retainers w/10° keepers, and the adjustable valvetrain kit they sell for non-adjustable heads (basically a spacer and a tighter fitting nut).

The carb was completely rebuilt by me when it was found that the motor would not idle under 1500rpms. It turned out to be an internal vac leak in the carb.

I've sprayed carb cleaner everywhere till i'm blue in the face and have found no apparent leaks. Even replaced the intake gasket and used Grey RTV silicone instead of the end corks. Engine is rough idling for the first minute or so and i constantly have to tap the gas to keep it from dying. Vacuum guage bounces everywhere when cold, but once it is able to idle fine i am getting a steady 15 in Hg but still have the hissing sound.

If i were to have a vac leak or improper valvetrain adjustment, wouldn't this show up with the vac guage? Why is it bouncing like crazy for the first few minutes? Finally, could it be my air/fuel mixture? I don't have a wideband air/fuel meter, but have tried everything from 1.5 turns to 3 and i still get the hissing sound. I've even disconnected and put a pipe plug in the vacuum port in the intake to eliminate any external sources. Could my carb be too small? It doesn't backfire at all and the timing is set at 16° initial w/20° centrifugal for 36° total.

So, any ideas besides pulling my hair out? Probably something elementary that i'm overlooking. Guess it wasn't a quick question after all Thanks in advance!!!!


The best i can describe the sound is someone sucking through a straw.

 

[DJCarbine]

I have a basically stock 289, and the Holley 4bbl on it does the same thing. Sounds like someone sucking water through a straw, but trying to suck up the last few bits and getting air too.


Not sure with your cam combo how much vacuum you are supposed to pull, but I pull a steady 18 inches and I feel that the carb sound is fine. I can only REALLY hear it with the air cleaner off.

 

[10secgoal]

The vacuum guage points at a vacuum leak. If it goes away when it warms up I would think that things are expanding and creating a seal once it's warm.
Look for a gasket, or something that can expand and contract with heat, like and intake. Have you tried retorquing the intake and carb ? Maybe look for a crack in the hose that may run to the pcv or the brake booster.

 

[MitchGT]

pcv valve? maybe hooked up to the wrong vacuum port?
block off all the vacuum ports (manifold and carb). if you rev the motor a bit, then suddenly close the throttle, does the noise get louder? I know that the heads just had valve guides done, but that can leak, especially when cold, and under higher vacuum(with the choke on), which would go away after it warmed up( and the choke open). you would definately hear that if you reved up the engine and suddenly closed the throttle, plus you might get a little smoke out the tail pipe. I don't know if this helps at all, but at least it gives you somewhere to look. does it make any ticking sounds? Maybe a sticky lifter could give you erratic vacuum.

 

[jbuening]

pcv valve? maybe hooked up to the wrong vacuum port?
block off all the vacuum ports (manifold and carb). if you rev the motor a bit, then suddenly close the throttle, does the noise get louder? I know that the heads just had valve guides done, but that can leak, especially when cold, and under higher vacuum(with the choke on), which would go away after it warmed up( and the choke open). you would definately hear that if you reved up the engine and suddenly closed the throttle, plus you might get a little smoke out the tail pipe. I don't know if this helps at all, but at least it gives you somewhere to look. does it make any ticking sounds? Maybe a sticky lifter could give you erratic vacuum.

I've unhooked the PCV valve hose and plugged it and it still hisses. I have the hose going to the back center port on the edelbrock, the one that usually has a pipe plug in it. I have the front port plugged. It has a slight hesitation when i open the throttle blades quickly. I'll check what you mentioned to see if the hiss gets louder when closing the throttle. So are you saying that if i rev the motor and then let off of it and it gets louder, then the guides are leaking? It did have a ticking sound right after the rebuild because i failed to check the lifter preload. I ended up having 0.15" preload with the max allowable being 0.06", thus the reason for adding the adjustable valvetrain conversion kit. I pretty much trashed the set of new pushrods due to that fact and was the cause for the ticking sound i did have. After i put that kit in, the ticking went away and i had tons more power. The only thing i notice now is a slight hesitation when you mash the pedal and that annoying sucking sound when you lift the hood. I can stick my ear next to the bores of the carb and hear it so i know that is where it's coming from.

 

[smallory]

I too have a bit of a sucking sound at low idle on my stock 289 w/ MC2100 carb, and the hesitation upon mashing the throttle down. The sucking sound doesn't bother me, but the hesitation and near stalling after a brief bit of gas followed by a coast (i.e., to get out into traffic) is troublesome and unsafe.

Please let me know if you figure out what's troubling your carb.

 

[glastang]

Sounds like a bad power valve to me.

 

[jbuening]

Sounds like a bad power valve to me.

Thank goodness it is an Edelbrock

 

[67coupestang]

mine did that when i installed a phenolic spacer... turns out i didnt tighten down the carb studs enough...

 

[Black1987Stang]

My friends 86 2.8L comanche does that...i always figured it was the air rushing through past the throttle plates or something with the fuel since it gets louder the more gas u give it.

 

[jbuening]

I think i have a vac leak though the throttle linkage. I have a very slow leak of black sludge that is coming from the drivers side of the carb around the back linkage. Thats the only thing i can think of. The carb has a fresh rebuild but doesn't come with anything to fix that amnit:

 

[JB66]

Is the base of the carb flat? The sound you describe I would say is a result of the throttleplates being partially open. Is the idle speed adjustment screw cranked down to the point of almost binding the spring? If it's a 4V,it may be the secondaries not being fully closed. Otherwise maybe a float level adjustment. I agree on the too little vacuum thought. Something's not right.

 

[1965-Mustang]

My 1965 Mustang also does this. It has a Speed Demon 575 CFM carburetor. I just spent my whole afternoon trying to tune it correctly. I watched the Demon DVD and followed the instructions but with no luck. I have the loud sucking noise that you speak of. The base of the carb is flat, it is new, and it sits flat on the intake. I torqued the bolts down to 7 ft lbs as per instruction, this seemed to be very little but I don't want to warp the base. I also torqued in an 'X' pattern. When I mash the pedal I also get the hesitation. I performed a test as the DVD told me to see if it was running lean, and the results told me yes, it is running lean. But no matter how rich I put the mixture screws the lean test will always show its lean. I sprayed carburetor cleaner and found the engine RPM drops when spraying the edges of the intake and at the base of the carburetor.

Also when driving normally down the road it will idle fine, but as soon as I mash the pedal to WOT it will continue idling at 2000 RPM after that.

Sorry don't mean to hijack, but I think I have a very similar problem.

 

[JB66]

At this point, I would pull the carb off, turn the idle speed screw all the way out, turn the carb upside down (after emptying the fuel bowls) and spray carb cleaner on top of the throttle plates to see if they are closed all the way.

 

[MitchGT]

When I bought my car the PO used the wrong carb bolts and just used extra washers to get rid of the unneeded length of the bolts. When I changed all the seals on the holley and went to remount the carb, I must have lost a few washers, because when I torqued down the bolts, there was a small leak still, and the car wouldn't run right. I figured I must have rebuilt the carb wrong, and just bought a new one to get the car running more quickly. When I went to install the new one I discovered the carb bolt problem. I bought carb studs and used them instead of the bolts, and the problem was solved. If your carb bolts are too long they bottom out in the manifold before they put enough pressure on the gaskets. Maybe an additional washer on each bolt might help? It's probably the cheapest fix to try, and it's quick.

 

[jbuening]

At this point, I would pull the carb off, turn the idle speed screw all the way out, turn the carb upside down (after emptying the fuel bowls) and spray carb cleaner on top of the throttle plates to see if they are closed all the way.

Do the throttle plates form a tight seal on the bores? I may have to try that and see. So with the carb upside down and the throttle plates both closed, i should be able to spray carb cleaner on the throttle plates and it should not leak through into the bores?

As far as hijacking this thread, have at it!!! I guess i don't feel so bad cause it seems like others have the same problem so it will help more than just me hopefully!

The base of the carb is flat, as far as i can tell. After doing the above test, i'm going to buy the thicker carb gasket to see if that helps. I do use bolts to attach the carb, but i do have studs and will try that trick. The bolts are pretty small but it's still worth a shot. The carb previously would not idle below 1500 rpms, and turned out to be an internal vacuum leak in the carb. The upper gasket had a tear in it between the bowl and the throttle bore. So i bought the big rebuild kit and rebuilt the carb. I then had problems with a lean condition and determined that a piece of dirt had clogged one of the idle jet tubes. So off went the carb and cleaned the entire carb again and put it back together. Now it responds well to both idle mixture screws, so i know it isn't plugged. As far as the vacuum, i know another individual with the same motor and cam and he is pulling 15 in Hg so i know the vacuum is balls on. Something is definitely screwy when the motor is cold and has new intake and valve cover gaskets, along with another new PCV valve and vacuum hoses.

As mentioned before, i have a very small puddle of black sludge in one of the pockets of the intake where the carb has leaked something through the secondary throttle blade shaft. This is the only thing i can come up with but i wouldn't think it would leak enough air to cause the hissing sound. I've sprayed carb cleaner everywhere until i'm blue in the face and have found nothing. I even tried the unlit handheld propane torch trick and still nothing. Any other ideas?

 

[JB66]

With the speed adjustment screws backed off, the plates should seat well enough to retain the fluid. I'm still thinking the secondaries are stuck.

 

[ashford]

the low speed air bleeds do make this soundwith a bigger cam. im not entirely shure where they are at on an edelbrock but the high speed bleeds are brass tubes sticking out of the boosters where they are attaced to the body. the low speeds should be near these. if you put your finger over them the sound should disappear and kill the engine.