General Fiat 500L - Engine not idling

[bellucci]

Hi everyone,

I have an idling problem with my Fiat, and I was wondering if anyone here can help guide me. I’m not a mechanic but based on everything I’ve read on this forum, I was able to get my engine running, at least.

I have a 1970 Fiat 500L with a 126A engine (595cc) and a 28 IMB carburetor, named Bellucci. According to the previous owner, this car used to run but I’m not sure how or for how long. The car sat for several years in his garage without it being started, so I thought it was a perfect idea to buy it from him.

I have taken the carburetor down and cleaned it out entirely. I soaked it, removed all debris, and made sure that both the choke and throttle levers snapped back into place without any hesitation. The float needle has been replaced along with the gaskets. The plastic seat that sits between the carb and the engine used to have a crack in it so that’s been replaced as well. I used an air gun to dry all the cavities and used a needle to make sure nothing was blocking any holes to the best of my abilities. I’ve confirmed that fuel is making it into the carb by taking the top off after running the engine for 10 minutes. The carb bowl has plenty of clean fuel in it.

The gas tank has been fully cleaned out. A new fuel sender has been installed. A new air filter has been installed. A new battery has been installed. The oil has been changed. Spark plugs have been replaced and a new T line has been installed where fuel can circulate from the carb back into the main fuel line. The fuel pressure is strong. I have not touched the distributor nor made any adjustments to it. At this point, I'm 30% an IT guy and 70% a wannabe mechanic and I'm starting to rethink my life's decisions.

I get in the car, turn the key to the on position, pull the choke lever up, the dashboard displays the “G” letter along with the word “press.” I push the gas pedal all the way down and pull the start lever and the engine starts to crank. Usually after the second attempt, the engine starts running. After the engine runs for a minute with my foot on the gas, I gradually push the choke lever down until it’s completely down. Then I get ready, take my foot off the gas, and immediately make it to the back of the car only to push the throttle down by hand to keep the engine running. If I’m late by a second, the engine dies. This happens whether the engine is hot or cold.

With the throttle down with one hand, I get around 1200 to 1400 RPMs give or take some and the engine continues to run steadily. I have a digital RPM gauge attached to one of the sparkplug wires. Please note, the RPMs are all relative and may not be 100% exact. There is no backfiring, but the engine does run rich.

While the engine is running as I’m holding the throttle down with my left hand, I screw in the mixture-screw on the carb all the way in with my right hand and it makes no difference with the idle. When I unscrew the mixture-screw out by 4 to 5 turns, then I get more black smoke from the exhaust and sometimes it back-fires into the air intake and the air filter. This tells me that more fuel is making it into the engine than it can burn in time. If I screw in the mixture-screw by 2 turns, then there won’t be any backfiring and the black smoke is under control. All of this is said and done with one hand on the throttle keeping it halfway down while adjusting these screws. One more thing, the idle speed screw is all the way in and I still have to push the throttle down.

I do know that the engine has to run on its own before the idle can be adjusted, so I shouldn't even attempt to adjust the carb, but I may be completely wrong.

What could be happening here? Why wouldn't the engine run on its own to allow me to adjust the idle settings?

Thanks for any help anyone can give.

 

[smart51]

Hello and welcome. I'm not sure I can get you running in one giant leap, but I may be able to start the process. In your second photo you have an arrow pointing to a screw. That's the idle jet holder; that needs to be tight. The idle mixture adjustment screw is below that in a small tube. Turn it all the way in, then turn it 2 full turns out. Try it from there. By 3 turns out, you've used all the adjustment it's got.

Why are you starting it with an open throttle? It should start and idle with your foot off the pedal.

 

[bellucci]

Hello and welcome. I'm not sure I can get you running in one giant leap, but I may be able to start the process. In your second photo you have an arrow pointing to a screw. That's the idle jet holder; that needs to be tight. The idle mixture adjustment screw is below that in a small tube. Turn it all the way in, then turn it 2 full turns out. Try it from there. By 3 turns out, you've used all the adjustment it's got.

Why are you starting it with an open throttle? It should start and idle with your foot off the pedal.

I will keep the idle jet holder screw tight and leave it. I wasn't sure if it needed adjustments or not but good to know. On the idle mixture screw, I'll set it to 2 turns and will give it a try and see if it helps.

As far as the open throttle goes, the engine would crank but wouldn't run if I don't keep the throttle down. At least it wouldn't run on the second or third try. Maybe I'm being impatient. I'll try and see if I can get the engine started without the throttle.

Thank you for the pointers. Appreciate it.

I'll report back.

 

[Bounding Bambino]

Something tells me its running extremely rich, and its just barely able to idle because the carb is dumping fuel into the engine. When you mentioned black smoke, this was my first red flag. I have a suspicion that there might be a missing jet in the carb or a float adjustment issue where fuel is just bypassing orifices and just dumping into the engine.

With the car in neutral and carb throttle at the ~50-60% position that you have it in photo one, it should be revving to the moon.

You have replaced the bottom carb spacer which is great, those are known for air leaks and erratic idle issues, so scratch that off the list.

If you haven't done so already, i would just do a quick once over and ensure all the parts are there.
some critical parts are 9,10, 4, 6, 8, 5, 3, 2, 35, 15, 12

8 is called the "starter valve" basically a choke plunger, that slides up/down when actuated by the choke lever 30. if this isnt fully seated, when off, it will continue to dump fuel into the carb.

Part 2 is the choke jet. 4 and 6 do the main fuel/air metering into the carb. With 35 being the main fuel jet.
9 is the idle jet and has super small passages. If this is missing, the car will absolute run rich and fuel will just dump in.

Then, just to eliminate other issues, i suggest a compression test. This will rule out low compression or a valve that is not fully closing.

 

[bellucci]

Hello and welcome. I'm not sure I can get you running in one giant leap, but I may be able to start the process. In your second photo you have an arrow pointing to a screw. That's the idle jet holder; that needs to be tight. The idle mixture adjustment screw is below that in a small tube. Turn it all the way in, then turn it 2 full turns out. Try it from there. By 3 turns out, you've used all the adjustment it's got.

Why are you starting it with an open throttle? It should start and idle with your foot off the pedal.

I tried to start the engine this morning without pressing the gas pedal and the engine would crank but would not start. I tried this process 7 times with 10 second intervals and couldn't get it started. I'm a little worried if I do this enough times I may wear down the starter. Then I started to simply tap on the gas pedal as the engine was cranking. It helped a little as it made the crank speed a bit faster but would not get the engine running. I did this for 4 or 5 times. Then I held the gas pedal down and started the cranking process again and after the 3rd time it finally started. Right now, this is the only way that I know of to get the engine running. I do, however, agree that I should not have to hold the gas pedal down like I'm doing.

 

[smart51]

I don't know enough to solve the problem, but I can make guesses. Opening the throttle adds more air, of course. It sounds like you might be running very rich. Have you checked the idle jet is the right size / even fitted?

I've had my engine idle just on the starter circuit by having the mixture screw turned all the way in. I wonder if your starter circuit is putting too much fuel in. Will it start with the choke lever all the way down, without having the throttle open?

 

[bellucci]

Something tells me its running extremely rich, and its just barely able to idle because the carb is dumping fuel into the engine. When you mentioned black smoke, this was my first red flag. I have a suspicion that there might be a missing jet in the carb or a float adjustment issue where fuel is just bypassing orifices and just dumping into the engine.

With the car in neutral and carb throttle at the ~50-60% position that you have it in photo one, it should be revving to the moon.

You have replaced the bottom carb spacer which is great, those are known for air leaks and erratic idle issues, so scratch that off the list.

If you haven't done so already, i would just do a quick once over and ensure all the parts are there.
some critical parts are 9,10, 4, 6, 8, 5, 3, 2, 35, 15, 12

View attachment 435504

8 is called the "starter valve" basically a choke plunger, that slides up/down when actuated by the choke lever 30. if this isnt fully seated, when off, it will continue to dump fuel into the carb.

Part 2 is the choke jet. 4 and 6 do the main fuel/air metering into the carb. With 35 being the main fuel jet.
9 is the idle jet and has super small passages. If this is missing, the car will absolute run rich and fuel will just dump in.

Then, just to eliminate other issues, i suggest a compression test. This will rule out low compression or a valve that is not fully closing.

Thank you for this and for pointing out the specific parts! The engine does runs super rich - you're definitely on point with that. If I stick around for 5 minutes in the garage while it's running, I end up with black gunk in my nose. I have to always have a fan running to air it all out. I don't know how much black smoke is considered okay for this car, if any.

I actually used this same diagram before taking the carb apart and cleaning it. I'm pretty anal with making sure that nothing is missed, and I even ordered a second identical carb (rebuilt) to replace some parts in the original one. I guarantee that no parts are missing from the carb based on this diagram.

I did replace #28, #27, #26, #12, #38 and #41. I didn't touch the float adjustment and kept the original float, but I noticed that the needle in #38 went up and down easily and loosely when the carb was dry as I was testing it. I don't know if the bottom portion in #38 needs to stay in place and the ball at the tip controls the fuel flow or not. I'll see if I can post a picture.

I will take the carb out and double check #8. I think you may be onto something with #38 and the float adjustment. The bowl shouldn't be full of fuel, right? Just about enough to cover the main fuel jet, right?

Thank you again for your help.

 

[Bounding Bambino]

I will go thru my documents to see if i have any specs for setting up the float. There are some general rule of thumbs for position.

the ball in the bottom of the needle should be spring loaded and push into the needle if the float is forced slightly.

 

[jackwhoo]

If I stick around for 5 minutes in the garage while it's running, I end up with black gunk in my nose. I have to always have a fan running to air it all out. I don't know how much black smoke is considered okay for this car, if any.

DO NOT RUN ANY ENGINE IN A CLOSED GARAGE YOU WILL KILL YOURSELF .

Engine exhaust gas contains carbon monoxide - a deadly gas.

The main garage doors MUST be open when starting/running an engine.

 

[Toshi 975]

I would check the float in your carb as the brass parts often develop virtually invisible hairline cracks. I remove the float and immerse it in a mug of boiled water, if you have a crack then the heat will force out expanding air causing bubbles.

 

[bellucci]

If I stick around for 5 minutes in the garage while it's running, I end up with black gunk in my nose. I have to always have a fan running to air it all out. I don't know how much black smoke is considered okay for this car, if any.

DO NOT RUN ANY ENGINE IN A CLOSED GARAGE YOU WILL KILL YOURSELF .

Engine exhaust gas contains carbon monoxide - a deadly gas.

The main garage doors MUST be open when starting/running an engine.

Thank you - I had a rookie moment initially but now I work on the car outside.

 

[bellucci]

I don't know enough to solve the problem, but I can make guesses. Opening the throttle adds more air, of course. It sounds like you might be running very rich. Have you checked the idle jet is the right size / even fitted?

I've had my engine idle just on the starter circuit by having the mixture screw turned all the way in. I wonder if your starter circuit is putting too much fuel in. Will it start with the choke lever all the way down, without having the throttle open?

Interesting. I've never tried turning the car on without the choke when it's cold. When the engine is hot, however, it fires up just fine with the choke lever all the way down. I'm beginning to suspect that my problem is too much fuel making it into the engine.

Here's one thing that I've observed. When the engine is cold and I get it running, with the gas pedal pressed all the way down, as I lower the choke lever, the RPM increases and the engine starts to rev up to a point where I have to press the gas pedal half way down instead of all the way. Not sure if this means anything but I wonder if this also points in the direction of having too much fuel in the engine.

I'm in the process of taking the carb apart and adjusting/testing the float. Just wanted to share this piece of info.

 

[smart51]

Interesting. I've never tried turning the car on without the choke when it's cold. When the engine is hot, however, it fires up just fine with the choke lever all the way down. I'm beginning to suspect that my problem is too much fuel making it into the engine.

Here's one thing that I've observed. When the engine is cold and I get it running, with the gas pedal pressed all the way down, as I lower the choke lever, the RPM increases and the engine starts to rev up to a point where I have to press the gas pedal half way down instead of all the way. Not sure if this means anything but I wonder if this also points in the direction of having too much fuel in the engine.

I'm in the process of taking the carb apart and adjusting/testing the float. Just wanted to share this piece of info.

In the summer, I never touch the choke. There's no need. The weather forecast tells me the temperature is about 15°C in Huston at the moment. My 500 would probably start without the choke, but wouldn't really like it. I might use half choke until I pulled out of the end of my street.

I had trouble with my carb when I got the car. The spring inside the float jet had lost its strength and was not pushing the ball bearing out. The result was my fuel level was too high. A new jet and a good clean later and all is well.

 

[smart51]

Interesting. I've never tried turning the car on without the choke when it's cold. When the engine is hot, however, it fires up just fine with the choke lever all the way down. I'm beginning to suspect that my problem is too much fuel making it into the engine.

Here's one thing that I've observed. When the engine is cold and I get it running, with the gas pedal pressed all the way down, as I lower the choke lever, the RPM increases and the engine starts to rev up to a point where I have to press the gas pedal half way down instead of all the way. Not sure if this means anything but I wonder if this also points in the direction of having too much fuel in the engine.

I'm in the process of taking the carb apart and adjusting/testing the float. Just wanted to share this piece of info.

In the summer, I never touch the choke. There's no need. The weather forecast tells me the temperature is about 15°C in Huston at the moment. My 500 would probably start without the choke, but wouldn't really like it. I might use half choke until I pulled out of the end of my street.

I had trouble with my carb when I got the car. The spring inside the float jet had lost its strength and was not pushing the ball bearing out. The result was my fuel level was too high.

 

[bellucci]

In the summer, I never touch the choke. There's no need. The weather forecast tells me the temperature is about 15°C in Huston at the moment. My 500 would probably start without the choke, but wouldn't really like it. I might use half choke until I pulled out of the end of my street.

I had trouble with my carb when I got the car. The spring inside the float jet had lost its strength and was not pushing the ball bearing out. The result was my fuel level was too high.

Oh this is great! I noticed that the ball inside the float jet is very loose. I'll send pictures today with what I'm seeing. And yes, the weather has been pretty warm here - interesting how that comes into play. I'm a noob with cars so all of this is very interesting. I'll try the choke halfway and see once I put the carb put back together.

 

[bellucci]

I managed to take a few pictures and noticed some things. Please note, these pictures were taken on a cold engine. The first and second image shows the needle moving in and out as the float moves up and down. I believe that this is the normal behavior and that's how fuel makes it into the bowl to start with. Meaning, it should be expected for this needle to freely move up and down. Please correct me if I'm wrong.

Image 3 shows the fuel level in the bowl which is just above the main jet. I noticed that in one other cavity, there's 3 times more fuel which tells me that the bowl must have been filled up by more than 50%.

The last picture is the one that makes me a little concerned. I have 2 pins and they're not 100% identical. The one that came from the repair kit has the ball bearing pushed in more than the original. Maybe that's my problem? I need to switch back to using the original?

 

[smart51]

I think the ball bearing to tapered point is the important dimension. I doubt the position of the shoulder makes a difference. When the float is floating on the fuel, it will displace some of the fuel so the fuel level will be higher, perhaps up to the level in the other cavity. In both cases, I don't think it matters. To set the float height, hold the carb top so that it is vertical. Let the float fall under its own weight and close the valve. There should be 8mm (from memory) between the float and the top gasket. That sets the fuel level when the valve closes. After that, everything is good.

 

[bellucci]

I think the ball bearing to tapered point is the important dimension. I doubt the position of the shoulder makes a difference. When the float is floating on the fuel, it will displace some of the fuel so the fuel level will be higher, perhaps up to the level in the other cavity. In both cases, I don't think it matters. To set the float height, hold the carb top so that it is vertical. Let the float fall under its own weight and close the valve. There should be 8mm (from memory) between the float and the top gasket. That sets the fuel level when the valve closes. After that, everything is good.

Got it. I will change out the pins and go with the original one and see if that helps. The distance that you mentioned between the float and the top gasket is 7mm for me if my eyes are not failing me.

I'm going to clean the carb again, test the float and reassemble and try it again. Will provide an update in a day or two. Thanks for the quick response.

 

[Bounding Bambino]

The taper on the needle from the kit is also concerning. Look at the massive difference between the 2. the angle might be the same, but the overall length of the taper (the new one being much shorter), might prevent the needle from fully sealing off fuel flow (needle body bottoming out in seat). One thing you can do/test... when you have the float attached to the top half of the carb, flip the top half over so the float is closed from gravity, and try and blow into the carb fuel inlet (block off the return barb with your hand), and see if its fully sealing and not letting any air thru.

On avg, fuel pressure is around 2-4psi, we can only blow about ~1psi, so it should easily seal shut against human breath.

 

[Toshi 975]

This thread has reminded me that last year an owner of a car fitted with a 650 engine and running a 28IMB carb had trouble trying to get it running properly after he rebuilt the carb. As a last straw he sent the carb to me to have a look at. He had got a rebuild kit from a well known UK supplier which I found to be pretty poor quality. The main gasket was about half the thickness of the originals but worse it had that shorter fuel inlet valve insert which made it impossible to adjust the float properly. I re-rebuilt the carb using parts I was happy with and all was well