carb question 4 bbl

bluebuick91 · 11-22-2006, 10:11 PM

have a 79 chevy truck 3/4 ton 350 turbo 400 4bbl quad carb wen i rev it up and let off gas it pops out the exhaust pipe runs great just wen i rev it up and let off gas pop pop pop but loud like a gun some times, if it is the carb i have 80's carb a new one sitting but it has 3 wire plug ins on it only difference,would it still work, looks the same exept the plug ins thanks
jenn

silicon212 · 11-23-2006, 01:55 AM

Does your car have a smog pump? If so, check the vacuum line to the diverter valve.

To answer your other question, the other carb you have is called a 'feedback' carb, and will not properly operate without an ECM (computer).

hotrod_chevyz · 11-24-2006, 02:28 AM

Quote:

Originally Posted by bluebuick91

have a 79 chevy truck 3/4 ton 350 turbo 400 4bbl quad carb wen i rev it up and let off gas it pops out the exhaust pipe runs great just wen i rev it up and let off gas pop pop pop but loud like a gun some times, if it is the carb i have 80's carb a new one sitting but it has 3 wire plug ins on it only difference,would it still work, looks the same exept the plug ins thanks
jenn

Look up the numbers on the carb. If its made for a 350, it will work. The electrical parts inside the carb will serve no function, as your 79 truck has no computer to hook it to. All the electric parts inside the carb does (for the most part) is send information back to a computer to adjust ignition timing advance. The three wire plug is called a "tps".

It will still mix fuel the same as one without any electrical parts. If its new and made for a 350 it will probably work better than what you have.

I wouldnt rule out exhaust problems. Take it to a muffler shop and have it checked out, get a second opinion.

If you stand behind the truck and smell gas, its not mixing fuel right, and a different carb could correct the problem you have.

I would check the distributor vaccume advance diaphram it could be bad. Or the wrong hose could be hooked to it. If you want to know how to check it just ask and i will give you info on it in greater detail.

Toke_k19 · 01-04-2007, 06:38 PM

To me it sounds like your firing order is off to creat that backfire or it the firing actually cummin out of the carb air intake value either way you should stick wit checking the order if its firing out the exaust and it its out the carb dont replace with the emc carb thats to much work heh...hit me up if this helps any

Blue Bowtie · 01-09-2007, 10:40 PM

The E4ME will work on a non-computer installation, but in a compromised state. There is no power valve in the E4ME, and power enrichment is through the rich stop adjustment of the main metering jets. In order to get it running right at high throttle angles, it's going to have to be excessively rich at low and moderate loads and throttle openings.

I haven't found anything on a 4MV/4MC that can't be repaired unless someone has taken a hammer to the die castings, or gone overboard in trying to drill air bleeds. Even those can be replaced if you are industrious.

ChaunceyShives · 01-10-2007, 12:30 PM

Silicon212 is on the right track. It doesn't sound like a carb problem. It sounds like an exhaust related problem. If the truck runs ok, leave the carb alone. You won't have any success with your feedback carb.

silicon212 · 01-11-2007, 02:27 AM

Quote:

Originally Posted by hotrod_chevyz

... The electrical parts inside the carb will serve no function, as your 79 truck has no computer to hook it to. All the electric parts inside the carb does (for the most part) is send information back to a computer to adjust ignition timing advance. The three wire plug is called a "tps".

It will still mix fuel the same as one without any electrical parts. If its new and made for a 350 it will probably work better than what you have.

...

The TPS is Throttle Position Sensor.

You forgot about the M/C solenoid - Mixture Control solenoid - this is what more or less regulates fuel flow in the carb and is duty-cycle controlled by the computer depending on input from the EGO, MAP, TPS, CTS and VSS along with reference pulses from the distributor. This mixture control solenoid, along with its companion idle air bleed circuit, replaces the older power valve and idle mixture circuit, and directly controls the metering rods that determine fuel flow through the primary jets.

As Blue Bowtie noted, this is a less-than-optimal setup if it's not controlled by the ECM. It's designed to be operated by the computer for optimum efficiency and power - but as with the CALPAK on an EFI engine, it is also designed to have some level of functionality in the event that ECM control goes south.

Through personal experience, the engine runs REAL rich if the ECM control is removed from a feedback carb. Knock out the distributor electronic spark control, and you have a recipe for an engine that has power equivalent to a Yugo GV while having a fuel appetite equivalent to an SD40-2 locomotive.

hotrod_chevyz · 01-11-2007, 11:37 AM

Good catch guys, i wasnt even thinking about that while i was answering..i was busy thinking about other problems it might have had besides the carb.

I know how they work. I've rebuilt so many q-jets i could build one in the dark.

Blue Bowtie · 01-14-2007, 04:56 PM

Quote:

Originally Posted by hotrod_chevyz

...I've rebuilt so many q-jets i could build one in the dark.

Ohhh! Maybe THAT explains it. Did you have the lights on for this one?

Sorry - Couldn't resist...)

Just for giggles, you might want to make sure the secondary throttle valve stop adjustment and secondary AV windup spring are adjusted correctly, and they are returning smoothly and fully.

mekaniclyinclined · 02-03-2008, 10:40 PM

change the fuel filter before you mess with the carb a lean misfire will do that to and the filter is cheap quick and easy even if it doesnt fix it it dont hurt

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