A Tale of Two Carburetors

[exister99]

Carb #1: When it is on the engine the car will start right away when cold and hold an idle. The exhaust fumes are cloudy, burn my eyes and reak of unburnt fumes. It will idle for a while, but as soon as I tap the accelerator pedal it dies. After that I can only start it if the accelerator pedal is completely depressed, and it will die unless I keep the pedal completely depressed, in which case it weakly idles. The fuel level in the carb's bowl sight is in the lower half of the little cloudy circle as it should be. When I pull the spark plugs they are black and a couple of them are wet. Two days ago I took apart the choke plate and reassembled it to spec.

 

Carb #2: When it is on the engine the car won't start at all. The fuel level in the bowl sight hole is above the cloudy little circle. I just rebuilt this carburetor and clearly I should have replaced the float. When I pull the plugs they are all wet, and there is fuel visible in the cylinders.

 

So based on the above situation here are my three theories. Please feel free to rate them from most to least likely.

 

Theory #1: Both carburetors have a flooding problem. The main problem with this theory is that I just set the choke on both carbs. Granted carb #2 needs a new float, but I can't find anything wrong with Carb #1 which was working fine before this rich situation came on suddenly. Is there anything in the carb besides the choke and the float that could make it run rich.

 

Theory #2: I am getting too much fuel pressure. Two holes in this theory. One, don't fuel pumps typically put out too little fuel when they fail; and two, if I have too much fuel pressure why is the fuel level in Carb #1 correct? I will know more once I get a chance to hook up a pressure gauge to the fuel lines. Has anyone ever had engine flooding due to a component outside of the carburetor?

 

Theory #3: My ignition coil is weak. This theory has two strength. First, a weak spark would cause incomplete combustion and consequently rich running and unburnt fuel in the exhaust. Second, if I hook up a timing light to the car while it is running the timing light only flashes intermittently. The distributor was just rebuilt and I also replaced the ignition control module. Any other causes for a weak or intermittent spark. Wires and spark plugs are all brand new BTW as are the cap and rotor.

 

I have a third, newly rebuilt carburetor on the way, and I am trying to rule out any other causes for this problem so I can be ready for action if I get the same behavior with the new carb as with Carb #1.

 

I hope you have found this tale entertaining. Please contribute any thoughts you may have, and Merry Christmas!

[NorthWet]

I would start with #3)... although you have sort of a "chicken and egg" problem. Faulty ignition can cause sooted plugs, but sooted plugs can cause faulty ignition. If the plugs aren't firing properly, your timing light's flashes will look erratic. try a new/clean set of plugs and immediately on startup check with timing light.

 

Distributor might have issues, or the coil/ignitor ground might be corroded/faulty.

[exister99]

Yeah, in this case the timing light will cut out alltogether for a few seconds at a stretch when connected to the #1 plug wire and will cut out in a much shorter cycle when connected directly to the main ignition wire. At first I though that the cutting out was due to the wet plugs not firing, but then I realized that wet plugs will still draw current. The current will leak to the block instead of generating a spark, but it should be coming down the wires nonetheless, right?

 

Thanks for your input.

 

I would start with #3)... although you have sort of a "chicken and egg" problem. Faulty ignition can cause sooted plugs, but sooted plugs can cause faulty ignition. If the plugs aren't firing properly, your timing light's flashes will look erratic. try a new/clean set of plugs and immediately on startup check with timing light.

 

Distributor might have issues, or the coil/ignitor ground might be corroded/faulty.

[Allouez]

I'm interested in what you find out...

I'm not a mechanic, but your problems sound somewhat similar to mine, as I'm now on carb. #2, a carb. that came from my former driver and worked very well though my problems persist. I only wish I could offer some advice...

[exister99]

Perhaps you could elaborate on the problems you have been having. Are you belching unburnt fuel fumes from your exhaust? Be warned that this rich running is bad business for the catalytic converter and the engine as a whole, so I am keeping it to minimum.

 

I will definitely post the resolution. There always is one... eventually.

 

I'm interested in what you find out...

I'm not a mechanic, but your problems sound somewhat similar to mine, as I'm now on carb. #2, a carb. that came from my former driver and worked very well though my problems persist. I only wish I could offer some advice...

[NorthWet]

...At first I though that the cutting out was due to the wet plugs not firing, but then I realized that wet plugs will still draw current. The current will leak to the block instead of generating a spark, but it should be coming down the wires nonetheless, right?

 

Thanks for your input.

The timing light senses a high-voltage pulse. If the plugs are sooted, the voltage will bleed to ground along the insulator, possibly never developing the threshold voltage/rise-time needed to trigger the timing light. If the plug is so wet it takes more voltage to fire than the coil/wire/plug can supply, then, again, you will not get enough voltage pulse to fire the light.

[exister99]

You make a good point. I will probably go ahead and change out the coil anyway since it is several years old and I can get one for cheap.

 

Any thoughts on the high fuel pressure theory? Could high pressure be making Carb #1 run rich without driving the level in the bowl sight above the correct level? Is that even possible.

 

The timing light senses a high-voltage pulse. If the plugs are sooted, the voltage will bleed to ground along the insulator, possibly never developing the threshold voltage/rise-time needed to trigger the timing light. If the plug is so wet it takes more voltage to fire than the coil/wire/plug can supply, then, again, you will not get enough voltage pulse to fire the light.

[NorthWet]

If the fuel pump was putting out too much pressure, it would force the float-valve's needle off of its seat and raise the level in the sight glass.

 

What does #1 look like (looking down the air horn) while the car is running? Choke plate closed all of the way? What happens if you prop the plate open a little with pen/scewdriver/dowel/baby's finger? Does the chokeplate ever open up? (My Datsun's electric choke fell apart some years ago, so I don't even use it... just pump the acc-pump a few times and crank... works better in above freezing weather.) If the chokeplate is open, can you see liquid fuel dripping out of the discharge nozzle or accelerator pump shooter?

[exister99]

That's what I've been thinking too is that there isn't really a way for the fuel pump to cram too much fuel into the engine without raising the fuel level in the bowl.

 

I've been thinking back to when Carb #1 failed and trying to figure out which component could fail that way. I was cruising right along, warmed up and all, when I started to hear like a low pitched hum. That went on for like 20 minutes on the freeway until I lost power and couldn't go faster than 35 mph. Then as I was limping into a parking lot the engine died completely and wouldn't start at all. I have rebuilt 2 of these carburetors so far and I can't think of any part inside the carburetor that would fail so quickly and catastrophically. Any ideas?

 

 

 

If the fuel pump was putting out too much pressure, it would force the float-valve's needle off of its seat and raise the level in the sight glass.

 

What does #1 look like (looking down the air horn) while the car is running? Choke plate closed all of the way? What happens if you prop the plate open a little with pen/scewdriver/dowel/baby's finger? Does the chokeplate ever open up? (My Datsun's electric choke fell apart some years ago, so I don't even use it... just pump the acc-pump a few times and crank... works better in above freezing weather.) If the chokeplate is open, can you see liquid fuel dripping out of the discharge nozzle or accelerator pump shooter?

[NorthWet]

My first thought is to eliminate cold-start from the picture. Prop-open the choke plate and see how it runs. Second thought is checking the idle-mixture screw, though I can't think of a failure mode that would cause it to go super rich. Third is possibly a main jet backed out inside of the float bowl; this seems very unlikely, but possible; it also wouldn't be presenting a problem at idle.

 

If all of this is at idle, you seem to have just a handful of causes: Stuck choke, stuck/misadjusted float, misadjusted idle mixture screw, clogged idle air jet, loose carb top screws (or failed gasket), of truly broken carb casting.

 

Or, again, it may not be fuel but rather ignition issues. Always worth ruling out ignition prior to touching fuel system. Anybody near by with a known-working carburetor?

[BoostedBalls]

Clogged air passages in the carb?

 

I would lock the choke in the open position and see what happens.

[Allouez]

Well, not to take time from your probs., but mine is an 83 wagon gl. Got it from a junkyard and ran well for the first couple of months, and then began to lose power and eventually choke and hesitate off-the-line. In time, it began blackening the carb/air filter and eventually quit all together. I had replaced wires and plugs prior to the probs., and was told it was anything from a clogged cat. to after-market fuel cap vacuum-lock and eventually the carb. So I replaced the carb w/one from an 81 hatch that ran great and for a time my issues seemed resolved, but now the hesitating is back (as is the blackening). Note the carb is NOT identical to the 83's; there is one fewer vac. line on the 81...I was told that it shouldn't be an issue in this case however. I plan to open the cat. converter this week, but your theory on weak coils caught my attention. Mine is original, as is the cat. and most everything else on the car. I've had a LOT of these older Subarus and have never had one with these issues, so I'm at a loss. No computer (thank God), and very simple, but I've not been able to figure it out.

I've had 3 mechanics look at it and nothings been solved...

 

Anyway, if you have any thoughts, I'm all ears. And good luck to you too.

 

P.S. I plan on replacing the coil regardless...do you have any recommendations along those lines? Factory, or a different coil?

Thanks!

[NorthWet]

Clogged air passages in the carb?...

I assume that this comment was in response to mine, so...

 

The air jets for the main and idle circuits are at the top of the air horn, just above where the venturi narrowing begins. The jets/orifices point upwards/outwards, and if they get plugged it will effect the strenght of the vacuum signal applied to the fuel jet, causing more fuel to get sucked in. (Analogous to putting your hand over the carb's airhorn while the engine is running.)

[exister99]

I swapped out my catalytic converter a couple years ago. This was after rebuilding the carburetor and chasing everything else down. If the cat is original you can almost bet it's toast. As a preliminary test run your engine with the air filter lid off. Then get down to where your eyes are level with the lip of the air filter housing. Rev the engine by tugging on the accelerator cable. If you see droplets of fuel and fumes belching out of top of the carb you can bet your exhaust is clogged, probably at the cat. Now to make extra sure remove the cat and Y pipe assembly and give it a shake. If there is junk rattling inside the cat it is probably the culprit. The cheapest one I could find sold for $215. It required some modifications to get it to fit, and still doesn't fit as well as the original. Plus I had to fashion my own heat shield because it kept trashing my passenger side DOJ boots and joints. I kinda wish I had spent an extra Franklin or two for a dealer one that would give me an exact fit.

 

As far as the blackening of the carb and filter, that sounds like misrouted exhaust due to a valve problem...

 

Well, not to take time from your probs., but mine is an 83 wagon gl. Got it from a junkyard and ran well for the first couple of months, and then began to lose power and eventually choke and hesitate off-the-line. In time, it began blackening the carb/air filter and eventually quit all together. I had replaced wires and plugs prior to the probs., and was told it was anything from a clogged cat. to after-market fuel cap vacuum-lock and eventually the carb. So I replaced the carb w/one from an 81 hatch that ran great and for a time my issues seemed resolved, but now the hesitating is back (as is the blackening). Note the carb is NOT identical to the 83's; there is one fewer vac. line on the 81...I was told that it shouldn't be an issue in this case however. I plan to open the cat. converter this week, but your theory on weak coils caught my attention. Mine is original, as is the cat. and most everything else on the car. I've had a LOT of these older Subarus and have never had one with these issues, so I'm at a loss. No computer (thank God), and very simple, but I've not been able to figure it out.

I've had 3 mechanics look at it and nothings been solved...

 

Anyway, if you have any thoughts, I'm all ears. And good luck to you too.

 

P.S. I plan on replacing the coil regardless...do you have any recommendations along those lines? Factory, or a different coil?

Thanks!

[exister99]

P.S. I plan on replacing the coil regardless...do you have any recommendations along those lines? Factory, or a different coil?

Thanks!

 

I would just go with the cheap one from Napa. I have three of those laying around and as far as I know they are all in perfect working order. I have never seen one go bad.

[exister99]

OK so here is what I did last night.

 

Replaced the coil to no effect.

 

Spliced a fuel pressure gauge into the fuel lines, propped open the choke plate and started her up. The fuel pressure reading was exactly 2 psi, perfect!

 

Then I peered past the choke plate into the main barrel, and there was literally a stream of fuel pouring into the main venturi and main barrel. I'm talking like there was a mini faucet in there just pouring fuel. When I pulled on the accelerator cable I got the expected spray in the venturi from the accelerator pump, but then it went back to the stream. The exhaust gas was rich and acrid as before so I shut her down and went inside to give the garage a chance to clear out.

 

Now I've peered down this carb many times before, but I have never seen fuel just gushing into the venturi like I did last night. That can't be normal. I was looking over the design of these carbs and trying to figure out what could be causing this. The main jet points down at a 45 degree angle to vertical so it is possible that it could have worked its way loose. I didn't take out or retighten the main jets when I rebuilt this carb so they probably haven't been tightened since they left the factory over 20 years ago. I do in fact do a lot of bumpy driving on some pretty rough tracks so it is possible they could have been jiggled loose. Another point of concern is the two bolts on the bottom of the float that are removed to access the main jets are leaking fuel, which is not a good thing either.

 

You mentioned that clogged air jets could be to blame as well. Would clogged air jets significantly increase the amount of fuel coming into the venturi. I did have a meltdown of the plastic air suction valve silencer, which deposited a few chunks of melted plastic in my air filter housing. That was a couple years ago, but it could have taken a few years for the plastic to work its way down the air jet.

 

In any case I have a rebuilt carb arriving on Tuesday. Once I get that in I will take apart the one on there now in hopes of finding the root cause of this stream of fuel in the main barrel.

 

Merry Christmas.

 

 

I assume that this comment was in response to mine, so...

 

The air jets for the main and idle circuits are at the top of the air horn, just above where the venturi narrowing begins. The jets/orifices point upwards/outwards, and if they get plugged it will effect the strenght of the vacuum signal applied to the fuel jet, causing more fuel to get sucked in. (Analogous to putting your hand over the carb's airhorn while the engine is running.)

[exister99]

So the brass float that I ordered came in yesterday. I installed it in carb #1 (the one that was flooding). Then I installed carb #1 and voila! It started right up. The problem now is that it idles insanely high, 2000-3000 RPM and that is with the idle speed screw all the way out and the timing retarded. When I rebuilt it I set everything to spec, screwed the idle air mixture screw in until lightly seated and back out 4 turns. Adjusted the main throttle plate according to spec using the fast idle screw. I did take out the idle speed adusting screw and put it back in.

 

So let me pose this question. If you had a carburetor on the bench and you knew it was idling too high on the car how would you adress the problem? Is there a procedure that you would follow to adjust the three screws involved in idle speed so that you can achieve a decent idle once the carb is on the engine?

 

BTW, I know that the high idle is due to the carb and not some vacuum leak because when I install the other carb it idles just fine, only rich.

 

Merry Christmas!

[NorthWet]

So the brass float that I ordered came in yesterday. I installed it in carb #1 (the one that was flooding). Then I installed carb #1 and voila! It started right up. The problem now is that it idles insanely high, 2000-3000 RPM and that is with the idle speed screw all the way out and the timing retarded. When I rebuilt it I set everything to spec, screwed the idle air mixture screw in until lightly seated and back out 4 turns. Adjusted the main throttle plate according to spec using the fast idle screw. I did take out the idle speed adusting screw and put it back in.

 

So let me pose this question. If you had a carburetor on the bench and you knew it was idling too high on the car how would you adress the problem? Is there a procedure that you would follow to adjust the three screws involved in idle speed so that you can achieve a decent idle once the carb is on the engine?

 

BTW, I know that the high idle is due to the carb and not some vacuum leak because when I install the other carb it idles just fine, only rich.

 

Merry Christmas!

High idle speed is probably: Misadjusted fast-idle screw (rides on fast-idle cam); too tight throttle cable or throttle-shaft obstruction; REALLY misadjusted idle-speed screw.

 

I would start by ensuring the choke/fast-idle cam are at their closed position, backing the fast-idle screw off of the cam, and seeing what the idle speed is.

[jacobs]

When an engine is idling too fast, too much fuel and air is getting into the engine. For this to happen, you must either have a vacuum leak outside the carburetor or the butterfly is somehow being held open and leaking. This can be caused by the fast idle cam, fast idle screw or misadjusted butterflies. To check the butterflies, remove the carburetor, close the butterflies and see if there are any gaps around the edges. It's very possible this is the problem if you loosened the screws that secure the butterflies to the throttle shaft. I don't remember if both barrels are the same size but you NEVER want to mix up the butterflies. Always reinstall them in their original location and facing the same direction as originally installed because the angles are different along their edges. If you don't, they will leak causing a fast idle condition.

[exister99]

Last week I bought a rebuilt carburetor and installed it. I am pretty happy with it except when the temp drops below 60 degrees it is really hard to start and won't hold an idle until it is warmed up. The choke plate is in the same closed position as it is on my other carbs that start up just fine.

 

I have been tweaking the idle air mixture screw and the idle speed screw to see if I can get a mixture that will help it cold start, but no luck yet. I will be checking the timing and readjusting the accelerator cable today.

 

Life would be peachy if I could just get consistent cold starts like I used to have. Can anyone help me out with some carb tuning suggestions?

 

Thanks in advance.

[Gloyale]

Get a MAf, a distributor, an ecu, and an intake manifold from an SPFI and go to town. Oh and get the y pipe with the o2 sensor as well. Way better gas milage too:)