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Username Post: vacuum leak through choke?        (Topic#32470)
LtFrankDrebin 
Super Senior Member
Posts: 5264
LtFrankDrebin
Loc: Coopersburg, PA
Reg: 01-25-01
04-25-03 03:07 PM - Post#218419    

I'm suspecting a vacuum leak on my '56 235 6cyl. I sprayed starting fluid in likely leak areas, but the only area that caused an increase in rpm was on the choke cover. So I took the choke cover off and found that the gasket was missing. I put one on but it occured to me that air could also be entering through the heat tube from the exhust manifold and from around the choke valve shaft. Looks like the choke system on the single barrel carb is pretty open to the atmosphere. The choke pull-off piston doesn't have any kind of seal, so the vacuum passage to the throttle body could easily pull in outside air. My car is beginning to be a real pain to drive. I cannot get a good idle from it and when I'm coasting, the car starts jerking back-and-forth. At low speeds, this jerking can get pretty violent if I don't release the clutch. Is it possible to have a vacuum leak here and if so, how can I remedy it? I thought about sealing off one end of the choke heat tube, but that would probably impede choke operation. Too bad these carburetors didn't have choke pull-offs like later cars did; with a vacuum cylinder similar to the vacuum advance at the distributor.

Wyatt

'56 210 Townsman wagon 6cyl/3spd OD | '58 Impala Sierra Gold 348


 


Gene_Schneider 
Ultra Senior Member
Posts: 12262

Loc: Wisconsin..USA
Reg: 09-27-02
Re: vacuum leak through choke?
04-25-03 03:23 PM - Post#218420    
    In response to LtFrankDrebin

The choke has a slight vaccum leak.It is necessary to pull the hot air from the manifold up to the choke cover...The cover should not leak around the outside.The choke piston is not a perfect seal and will have slight leakage but should cause no running problem.This is the vaccum sorce for the heat tube. Sounds as if your chokes is doing fine.What is the "low speed" that the bucking starts at.?

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


 
LtFrankDrebin 
Super Senior Member
Posts: 5264
LtFrankDrebin
Loc: Coopersburg, PA
Reg: 01-25-01
Re: vacuum leak through choke?
04-25-03 04:05 PM - Post#218421    
    In response to Gene_Schneider

I can feel a bucking at all speeds if I let off the accelerator, but the bucking becomes more pronounced as the car's speed drops. There are also fuel stains down the outside of the carburetor and a hint of wetness around the cast iron throttle body. Why is this carb leaking fuel? I rebuilt it a few months ago.

Wyatt

'56 210 Townsman wagon 6cyl/3spd OD | '58 Impala Sierra Gold 348


 
jim_h_erie 
Contributor
Posts: 220
jim_h_erie
Loc: erie pa.
Reg: 10-17-01
Re: vacuum leak through choke?
04-25-03 04:41 PM - Post#218422    
    In response to LtFrankDrebin

Sounds like the body of the carb might be worn where the throttle shaft goes thru. if it is it`ll suck air when the throttle is closed and spit gas around the shaft when it`s open. you can drill and bush the shaft openings if thats the problem. just a thought.
Jim 56 Delray



 
Anonymous 

Re: vacuum leak through choke?
04-26-03 01:23 AM - Post#218423    
    In response to LtFrankDrebin

Hi Wyatt,

If you see external fuel leakage on that Rochester, I would suspect the float level is too high, or the float is getting heavy.



 
LtFrankDrebin 
Super Senior Member
Posts: 5264
LtFrankDrebin
Loc: Coopersburg, PA
Reg: 01-25-01
Re: vacuum leak through choke?
04-26-03 01:51 AM - Post#218424    
    In response to jim_h_erie

The shaft doesn't seem loose. But, to see if there was a leak there, I placed some grease at both ends of the shaft where it passes through the throttle body. Didn't make a difference. The next time I rebuild one of these carbs (I have three here), I'm going to seal off the one end of the shaft by JB Welding a piece of steel onto the throttle body over the end of the shaft. That'll eliminate the possibility of a vacuum leak there. I could just have a bad carburetor with leaks inside??? I started the car yesterda and it absolutely would not idle. I had to hold the pedal a little passed idle to keep it running and if I let off the pedal totally, the engine would shut off as if I turned off the ignition.

Wyatt

'56 210 Townsman wagon 6cyl/3spd OD | '58 Impala Sierra Gold 348


 
57wagon 
Very Senior Member
Posts: 1271
57wagon
Loc: stony creek va.
Reg: 08-10-02
Re: vacuum leak through choke?
04-26-03 02:01 AM - Post#218425    
    In response to LtFrankDrebin

could there be something wrong with timing.and check those points.bad points can cause.bucking and missing also.coil wire inside dist.can be momentarily grounding out.this usually causes sputtering and popping back tru carb.


i get off on 57 chevys,i get off on screamin guitars.and a 60 chevy p/u. http://community.webshots.com/user/57210man


 
LtFrankDrebin 
Super Senior Member
Posts: 5264
LtFrankDrebin
Loc: Coopersburg, PA
Reg: 01-25-01
Re: vacuum leak through choke?
04-26-03 02:26 AM - Post#218426    
    In response to 57wagon

I replaced all the ignition components except for the coil. I would say the problem definitely lies in the fuel delivery system since the engine tends to want to die after coasting to a stop. After releasing the clutch, you have to keep one foot on the gas to keep it from dying. It kinda sucks working three pedals at once!

Wyatt

'56 210 Townsman wagon 6cyl/3spd OD | '58 Impala Sierra Gold 348


 
Gene_Schneider 
Ultra Senior Member
Posts: 12262

Loc: Wisconsin..USA
Reg: 09-27-02
Re: vacuum leak through choke?
04-26-03 06:40 AM - Post#218427    
    In response to LtFrankDrebin

It sounds as if it will be necessary to remove the bowl cover and do some checking.Would suggest to buy a can of choke cleaner and use the plastic "straw" that comes with it to blow uot the idle circuit passages in the bowl cover.Check shop manual as to their locations.Also check power piston to be sure that its moving freely.Check imprint on gasket to be sure that cover isn't warped.A little stain from gas may be found but should not be wet with gas.If gas level is too high due to flooding its normal for the gas to run out around the throttle shaft.

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


 
Robert_May 
Old as Dirt Member
Posts: 6939
Robert_May
Loc: San Marcos, CA USA
Reg: 01-03-02
Re: vacuum leak through choke?
04-26-03 01:06 PM - Post#218428    
    In response to LtFrankDrebin

Silly question, but do you know how to set the F/A idle screws? This sounds a lot like a bit of stuff in the screws blocking the flow of fuel.
You also don't indicate whether the plugs are indicating a rich or lean mixture which would tend to lead to what is going on.
Also, is the dwell set right and the timing somewhat right? A dwell that is way off can do exactly what you are complaining about as the timing will be either advanced or retarded, depending upon the way that the dwell is in error.
If you are not sure about dwell setting or timing, just ask! There's been a lot of discussion on the various forums which will tell you how to do it.

Robert May Fat? Eat less and you will get thinner!


 
Alegm 
Poster
Posts: 29
Alegm
Reg: 01-05-13
08-12-18 05:36 PM - Post#2742890    
    In response to Robert_May

Hi
I know it was centuries ago but did you solve the bucking problem on your 56 chevy?
Im having exactly the same problem on my 58 biscayne
Thanks



 
LtFrankDrebin 
Super Senior Member
Posts: 5264
LtFrankDrebin
Loc: Coopersburg, PA
Reg: 01-25-01
08-13-18 08:24 PM - Post#2742997    
    In response to Alegm

Yes, but not really. I believe I had some bad valves. I could get the car idling better by making the fuel mixture richer, but all I was doing was compensating for the poor sealing of the cylinder head valves.

'56 210 Townsman wagon 6cyl/3spd OD | '58 Impala Sierra Gold 348


 


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