Help a carburetor noob out.

[natto fire]

Backstory:
Most of the cars I have owned did hard work on have been EFI, so the fuel systems have been fairly reliable and diagnosis was rather easy if something did arise.

I now have a carbed car and was hoping to get some pointers on diagnosis. When I first got the car it ran great, would always start on the first crank, and idle smoothly albeit a bit highly. I didn't drive the car regularly for almost a year but would still start it and drive it a couple miles every other month or so and still seemed to run well. I have been driving it more regularly now and have had problems with cold idle and sometimes idling in general as well as "twitchy" throttle response.

Technical stuff:
The car is an '80 Corolla with a feedback carburetor. Carb was rebuilt by a reputable shop a little over a year before I bought the car.

As mentioned above, I am having trouble with cold idling (stalling) and warm idling (too high ~1500 RPMs). I also get a weird problem that after getting the engine warm and then parking for a while it will start up fine but can all of a sudden stumble and not respond to the throttle at all and then die and will then be very difficult to restart. Fuel economy is also not near where it should be at ~22mpg (car weighs 2600lbs and has a 1.8 w/5 spd)

I have been doing some reading and it says to check the basics first, and I did replace vacuum lines, fuel filter, and a cap and rotor, as well as adjust the ignition timing. Things I have not done include checking the EGR thoroughly and a compression test. The compression test will be done tomorrow, but this seems unlikely to be causing the symptoms I am experiencing, however, I have noticed some blue smoke upon startup which makes me question the integrity of the valve guide seals.

So anyone have some tips for me, as far as the symptoms I am having? I know the obvious solution is to buy a new car, swap in a 4A-GE or to get dual Mikuni side draft carbs whilst nixing all the emission shit, but I am extremely poor and would much rather turn a screw/bolt here and there than lighten my wallet.

Thanks in advance and sorry for the long post...

Cliffs: So many people recommending books on the internet but anything longer than a couple paragraphs in a forum post is faux pas. Probably not irony, but pretty damn close.

 

[SSSnail]

The only carburetor I've ever worked on was for remote control cars, but I figure they work the same way. What you're describing seems like it's not getting enough fuel (running too lean).

 

[franksta]

How's the fuel pressure? Float level? Spark plugs and wires?

I'm not familiar with that specific carburetor. Does it have a "fast idle" cam? It's a little step-shaped gear that keeps the throttle plate open a little when the temperature is below a certain level. Then as it warms up it allows the throttle to close.

EDIT: It will look similar to this.

 

[Vetterin]

Definitely sounds like a choke adjustment is needed.

 

[alkemyst]

if a pro did it, chances are it could still be off.

Most of today's mechanics are morons...

 

[natto fire]

Originally posted by: franksta
How's the fuel pressure? Float level? Spark plugs and wires?

I'm not familiar with that specific carburetor. Does it have a "fast idle" cam? It's a little step-shaped gear that keeps the throttle plate open a little when the temperature is below a certain level. Then as it warms up it allows the throttle to close.

EDIT: It will look similar to this.

I forgot to mention I have done plugs as well. The fast idle is controlled by a slew of vacuum modulators and a thermo switch. I am going to test the thermo switch. Fuel pump is mechanical, and I don't have a gauge to check the pressure, float level looks good as there is a sight glass for that.

 

[MiataNC]

What do the plugs look like? (black/wet/sooty=rich....white/blistered/bubbled=lean)

With the air cleaner top off, do a cold start after sitting overnight. You should be able to see the "choke" open to allow more air and less fuel as the car warms up.

Some of your symptoms could be related to low fuel pressure (if lean). Mechanical pumps can partially fail (torn diaphram or worn arm/cam), and fuel deposits can gum up the idle-mid-high circuits of the carb.

Fuel in he sight glass does NOT eliminate a float that is out of spec or sticking.

 

[PottedMeat]

What engine is this? If it's a 4AC I can scan some pages about the carb.

 

[Turin39789]

sounds similar to a problem I had with an 87 tercel with a similarly horrific carb.

Mine was flooding, that was cousing the hard starts, would get going if you opened up the intake.

My solution - sell the beast, never did figure out what the real problem was.

 

[natto fire]

Originally posted by: PottedMeat
What engine is this? If it's a 4AC I can scan some pages about the carb.

It's the 3TC.

I will try the cold start and watch the choke. The climate here is notorious for destroying rubber, so I might replace the fuel pump anyways as a preventative measure.

 

[Iron Woode]

I also get a weird problem that after getting the engine warm and then parking for a while it will start up fine but can all of a sudden stumble and not respond to the throttle at all and then die and will then be very difficult to restart.

classic vapour lock.

make sure your pump is working right and your return line is clear.

check your engine's vacuum as well. carbs are notorious for vacuum leaks.

also is there debris in the tank? check your fuel filter as well. particles can get into the carb and wreak havoc with tuning and economy.

 

[drnickriviera]

I don't know about your car, but our mid 80's corolla had a million miles of vac lines. If one sprung a leak it would cause all kinds of problems. I just had to take them off one by one and test them with the mighty vac.