The carburator is *hissing* at me..?

[Marck]

Hi guys..

I'm hoping you can help me sort this deal out, I'm just spinning my wheels at the moment.

Carb is a Holley 750 DP, and it's on a 440 stroked to a 512.

The problem is, when it goes from normal idle (no problems here at all) to any movement of the butterflies (slow is better, quick is worse), it makes a very noticeable short hissing sound, and hesitates. Sometimes, (only when in gear and not fully varm yet), it stalls.

The carb has very recently been disassembled and cleaned, I've dialed in the mixture screws using a vacuum gauge, and after seeing some youtube videoes from Holley, I put in a bigger injector nozzle, went from 28 to 37 I think.

So now I'm wondering if a bigger accelerator pump diaphragm might be the way, or am I missing something obvious?


Cheers.. 

-Marck

[XH29N0G]

Quick thoughts.  

• I don't think the accelerator nozzle size is the source of your effect.  That only controls the rate (maybe a second order effect on amount) of gas added when the blades are opened.  
  
• The stalling when not yet warm could be that the idle is optimized for idle with vacuum, but not quite rich enough when the blades are first opened.  I just swapped carbs (yesterday) and on the first run with my A/F, I could see that it was lean and my car stalled several times when first starting out.  I made the idle a little richer 1/8 turn and then later another 1/8 turn and it now is at A/F of 13 at idle and doesn't stall.  I suspect you have the same issue.  
  
• Make sure you don't have vacuum leaks because this can throw things off.
  
• The hissing is something I do not know about.  I would not be able to hear it because my engine is loud.  When mine was more tame, I could hear the carburetors hissing when I switched to an unsilenced air cleaner.  I wonder if that could be entirely normal.  If other's don't write in quickly with a solution, you may want to provide a video/recording of it.
  

Good luck with this.

[69wannabe]

Depending on how your 512 is built you may be undercarbed. I am running a 493 ci RB engine and have an 850 holley DP carb and it ran good out of the box but I have had to go up a few jet sizes in front and a couple in the back I think and I went from 31 squirters up to 35's. The 750 may be a bit lean on a good built stroker engine. I think several guys even run 950's on their 500 in stroker engines but the 850 works good for the way mine is built. Maybe a few more will chime in....

[BSB67]

I put in a bigger injector nozzle, went from 28 to 37 I think.

Why?

So now I'm wondering if a bigger accelerator pump diaphragm might be the way, or am I missing something obvious?

Not the problem

Depending on how your 512 is built you may be undercarbed.

Not the problem.



If it was not whistling before, it was something you did, or did not do on the carb rebuild.   Otherwise look for a vacuum leak somewhere....it will be small.

[John_Kunkel]

With the engine idling smoothly, grab the carb's throttle lever and wiggle it straight up/down and back n forth; if the idle changes noticeably or makes the hissing sound, the throttle shafts are worn and sucking air....fairly common.

Loose throttle shafts can be bushed or the throttle plate replaced.

[Marck]

With the engine idling smoothly, grab the carb's throttle lever and wiggle it straight up/down and back n forth; if the idle changes noticeably or makes the hissing sound, the throttle shafts are worn and sucking air....fairly common.

Loose throttle shafts can be bushed or the throttle plate replaced.

Just tried that, and the idle didn't change at all.. Thank you

I put in a bigger injector nozzle, went from 28 to 37 I think.

Why?

So now I'm wondering if a bigger accelerator pump diaphragm might be the way, or am I missing something obvious?

Not the problem

Depending on how your 512 is built you may be undercarbed.

Not the problem.



If it was not whistling before, it was something you did, or did not do on the carb rebuild.   Otherwise look for a vacuum leak somewhere....it will be small.

I put in bigger injector nozzles after seeing this video from HolleyPerformance on Youtube around 3 minutes in.
https://www.youtube.com/watch?v=k-Nx5HEzvlY
It was hissing before I did all of the above. All was done to remove the hissing end the hesitation, but with not much effect, other than it seems to start up more easily now.
Any tips for seeking out leaks?

Depending on how your 512 is built you may be undercarbed. I am running a 493 ci RB engine and have an 850 holley DP carb and it ran good out of the box but I have had to go up a few jet sizes in front and a couple in the back I think and I went from 31 squirters up to 35's. The 750 may be a bit lean on a good built stroker engine. I think several guys even run 950's on their 500 in stroker engines but the 850 works good for the way mine is built. Maybe a few more will chime in....

I know of two guys that run similar engine sizes, with 750's on them, with no problems. Sadly they are of little to no help for me, but I appreciate your thoughts.

Quick thoughts.  

• I don't think the accelerator nozzle size is the source of your effect.  That only controls the rate (maybe a second order effect on amount) of gas added when the blades are opened.  
  
• The stalling when not yet warm could be that the idle is optimized for idle with vacuum, but not quite rich enough when the blades are first opened.  I just swapped carbs (yesterday) and on the first run with my A/F, I could see that it was lean and my car stalled several times when first starting out.  I made the idle a little richer 1/8 turn and then later another 1/8 turn and it now is at A/F of 13 at idle and doesn't stall.  I suspect you have the same issue.  
  
• Make sure you don't have vacuum leaks because this can throw things off.
  
• The hissing is something I do not know about.  I would not be able to hear it because my engine is loud.  When mine was more tame, I could hear the carburetors hissing when I switched to an unsilenced air cleaner.  I wonder if that could be entirely normal.  If other's don't write in quickly with a solution, you may want to provide a video/recording of it.
  

Good luck with this.

I have actually tried getting this on video, but the engine drowns out the "hissing" on the video. It is easy to hear in person, but for some reason the mic on the phone/camera doesn't pick up on it.
I have adjusted the idle screw, and I'll see if this helps on the stalling. 

[HPP]

Nearly all carbs will hiss when you hit the throttle without an air filter and your head under the hood. You don't notice it when its covered by the filter and under the hood and you are in the cabin.

If it hesitates or stalls when you hit the throttle, you have tuning to do with the pump shot.

[Paul G]

I my opinion that 750 is just too small, but it should work well, just down on power. I run a 750 on warmed up 360.

How does the car cruise at steady speed? Does it run down the road smoothly, or a little rough/or little stumble while cruising? If the main jets are too small it can have lean stumble at cruise, and may go very lean on tip in. Going lean when out of the transition slot and the main jets are not flowing enough fuel yet. Which could be the hesitation on throttle opening.

Make sure your accelerator pump lever is adjusted correctly, and you get a squirt of fuel on the slightest opening of the throttle. That has to be right before you do anything else.

Make sure your floats are set correctly. Should have a slight amount of fuel dribble out of the bowl sight hole when engine is running. 

Ignition timing is a leading cause of carb stumbles, really. Are you running enough base timing?