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Right air/fuel ratio?

  
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Right air/fuel ratio?

 
chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/29/09
06:10 PM

I know the right a/f ratio should be 14.7 or so. But here is what i got.
71 350 sb
stock specs with exception to 1.5 roller rockers, 1 5/8 headers, RPM intake, demon jr 625(electric choke). The carb has 74 jets. The initial jet size was a 70. I took them out cause the car was really hard to start and all the plugs were white.

This is what the plugs look like w/74 jets. cylinders 1,7,4,6 are completely white. cylinders 2,8,3,5 are different shades of brown. All plugs are the same heat range and are gapped at .40(since im running a late model HEI with coil in cap).
Ive developed a hesitation under low acceleration. It first developed at cruise during part throttle operation. Then it started from idle to about 1500 rpm. As long as i keep accelerating im fine. Ive checked fuel press its at a steady 5.5 to 6 psi. Ive adjusted the idle metering screws with a vac guage to read the highest possible vacuum. Ive ordered a rebuild kit for the carb and hope this helps solve the issue.

I guess my question is this, Is the engine rich or lean? I have a a/f monitor and it sweeps to lean during part throttle operation. But at idle its always rich. It should be operating the opposite than it is. Right?  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 09/29/09
07:08 PM

First things first, and we need to get this straight is that the conventional procedures for reading spark plugs really only apply after you have ran the engine at full throttle for a good amount of time (usually 13-14 seconds or however long it takes to make a quarter mile) and shut the car off and pulled the plugs. You cannot read the plugs with the same views as car enthusiasts usually have with plugs that have been idling around for hours at a time.

Even though I doubt your A/F monitor is a wide-band O2 sensor, so it's accuracy is questionable, what you're seeing is not that out of the ordinary. Engines like these need to idle rich - you're setting the idle rich when you are setting to maximum vacuum. Vacuum increases when combustion quality is improved, and your engine is starved for fuel at such low speeds because it is highly inefficient and is relying on a weak signal to atomize the fuel - so you need to throw more fuel at it. Hopefully you have a fully functional vacuum advance operating on manifold vacuum as well.

A lean cruise speed is ideal for fuel economy, and a slightly rich air to fuel mixture is ideal for peak power.  
~Gibs

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/29/09
09:16 PM

Its not a wideband monitor, i didnt want to spend 350 bucks to get one. Would cruising on the highway/interstate for an hr somewhat qualify for a decent way to read plugs? I do this on a daily basis not much full throttle though.
It has a new adjustable vac advance on the distrib. the problem just seems to be in the carb. The car was down for a while and it didnt have this problem before the work was started.  

cbtlr77 cbtlr77
User | Posts: 95 | Joined: 08/08
Posted: 09/30/09
12:49 AM

Old Gibby can be harsh at times but he knows his ****.Period.  

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/30/09
06:56 AM

Ive been in the forum for a while and know of his knowledge and directness.  

BigBlockBuck BigBlockBuck
User | Posts: 56 | Joined: 09/09
Posted: 09/30/09
02:05 PM

If there's any question about the air/fuel ratio, go richer and work your way back leaner but sounds like your jetting might be ok. Under light accel and cruising the carb pretty much runs off the idle circuits. I would adjust that and see what happens. Also check the float level and maybe there's some crap floating around if you said it had been sitting.  

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/30/09
02:17 PM

im rebuilding the carb. There were some small particles in the carb when i disassembled it. The float levels are ok, the feul is just at the bottom of the sight glass window. Im going to make sure its not in the vac advance when its all back together though.  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 09/30/09
03:12 PM

We can forget about reading plugs if you have a air fuel monitor. Even a narrow-band O2 has a reasonable degree of accuracy at part-throttle operation near stoichiometric conditions.

I'm unsure if you're at the speed to engage the main circuit but you could try jetting up another size or two if you need some more fuel at part throttle. Most likely the engine is still running from the idle restriction rather than the main jet at the speeds you're experiencing hesitation but it's difficult to say - you will just have to try different things. Jet size also affects the amount of fuel in the idle well very slightly, so it may be worth a try.

How far out are your idle air needles to achieve the highest vacuum reading?  
~Gibs

mpayne mpayne
Guru | Posts: 1117 | Joined: 10/07
Posted: 09/30/09
03:49 PM

I agree he's harsh but fair. That's why he's my second in command.  

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/30/09
05:11 PM

My needles are out 1 1/2 turns.
upon taking the carb apart I noticed my throttle was sticking a little bit on the base plate and one of the blades was bent ever so slightly so the idle feed circuits were unbalanced. I tapped it with a rubber mallet and the bind in the linkage is fixed and i matched the blades the best i could. Maybe its time for a new carb?  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 09/30/09
05:46 PM

Blades bent?

Yeah, the carb has probably been damaged in the past by having a way too large return spring. That ruins the bushings in the throttle shafts and causes leak problems as well. This is an all too common problem/  
~Gibs

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 09/30/09
06:27 PM

Throttle plates/butterflies not blades  

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 10/02/09
02:41 PM

Got the carb back together and on the car. Everything seems to be ok now. I put 76 jets in it and it starts and doesnt have part throttle hesitation anymore, so far anyways.
Ok so the car runs better, however i think my manifold is warped or something. Cause now my throttle is sticking again. I have another manifold to use so i can use that. I guess Mad  

chevman24 chevman24
Enthusiast | Posts: 430 | Joined: 08/08
Posted: 10/04/09
05:07 PM

The only time my throttle is binding is when the engine is running. this doesnt make sense!!  

GibTG GibTG
Guru | Posts: 1985 | Joined: 10/03
Posted: 10/05/09
02:40 PM

How would a warped manifold make the throttle stick?

It's gotta be in your linkage or carb.  
~Gibs

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