Technical 500L (126A1 Engine) Low Speed

[bellucci]

Hi All,

My Fiat 500L has a 126A1 engine and uses the 28 Weber carb. I have cleaned it out completely along with the fuel tank and the fuel sender. I have a filter between the main fuel line and the fuel pump. The engine idles just fine, and I get a steady flow of fuel in the filter (1/2 way full) at idle.

I'm able to go from idle to 1st gear and then to 2nd gear and then to 3rd, consistently, without any issues. Everything operates as expected from 1st to 2nd to 3rd.

However, after I change into the 3rd gear, sometimes I'm able to maintain a steady speed (not accelerate any further but keep a steady speed) at 3rd but I don't have the power/RPMs necessary to accelerate enough to reach 4th. When I do have a steady speed at 3rd, I once changed into the 4th gear, and it shifted just fine but without additional power or speed, so I eventually went back to 3rd. I'm able to shift into all 4 gears.

At other times, when I accelerate and change into the 3rd gear the car slows/reduces power. It feels sluggish as if it was doing better in 2nd. When this happens, even if I push the gas pedal to the floor, nothing happens. In this scenario, I noticed the fuel filter was 1/3 full instead of 1/2 (at idle).

Do you think I'm having a fuel supply issue where not enough fuel is making it to the engine or do I have a gear problem? I'm thinking maybe the fuel pump is not able to keep up with the demand or maybe I need to adjust the float in the carb to allow for more fuel storage in there?

The car runs lean, no backfires, no black smoke. I would like to be able to hit 45/50 MPH if possible.

Thank you for taking the time to read all this. Any suggestions would be appreciated.

 

[the hobbler]

Hi All,

My Fiat 500L has a 126A1 engine and uses the 28 Weber carb. I have cleaned it out completely along with the fuel tank and the fuel sender. I have a filter between the main fuel line and the fuel pump. The engine idles just fine, and I get a steady flow of fuel in the filter (1/2 way full) at idle.

I'm able to go from idle to 1st gear and then to 2nd gear and then to 3rd, consistently, without any issues. Everything operates as expected from 1st to 2nd to 3rd.

However, after I change into the 3rd gear, sometimes I'm able to maintain a steady speed (not accelerate any further but keep a steady speed) at 3rd but I don't have the power/RPMs necessary to accelerate enough to reach 4th. When I do have a steady speed at 3rd, I once changed into the 4th gear, and it shifted just fine but without additional power or speed, so I eventually went back to 3rd. I'm able to shift into all 4 gears.

At other times, when I accelerate and change into the 3rd gear the car slows/reduces power. It feels sluggish as if it was doing better in 2nd. When this happens, even if I push the gas pedal to the floor, nothing happens. In this scenario, I noticed the fuel filter was 1/3 full instead of 1/2 (at idle).

Do you think I'm having a fuel supply issue where not enough fuel is making it to the engine or do I have a gear problem? I'm thinking maybe the fuel pump is not able to keep up with the demand or maybe I need to adjust the float in the carb to allow for more fuel storage in there?

The car runs lean, no backfires, no black smoke. I would like to be able to hit 45/50 MPH if possible.

Thank you for taking the time to read all this. Any suggestions would be appreciated.

Difficult to "definite-diagnose" from this side of "the big pond", but it does sound like a lack of fuel, especially at low speed. The first thing that I would check is your 'slow-running' (aka, incorrectly, the 'idle') jet---I have run into this problem before, and it turned out to be a bit of muck intermittantly blocking the jet. You say that you have put a filter between the "main fuel line and the pump"---you do NOT need this filter. You already have 3 filters in the system----1 on the 'tank-unit', another in the top of the fuel-pump and finally, 1 in the top of the carburettor. Which model of '28IMB' carb do you have? On the early models there was a filter under a big brass cap---on the later versions the filter is a very small one IN the pipe into the top of the carb (no brass cap). Check that this little filter (if that is the version that you have) isn't blocked---if it is, take it out and dispose of it---do without it!

 

[bellucci]

Oh that's a great point! I had forgotten about the filter inside the carb. The carb that I have is the 28 IMB with the brass screw on top and a tiny filter inside it. It has an inlet and an outlet which is supposed to recirculate the fuel, but the outlet has been closed off using a short rubber hose to help maintain better fuel pressure.

I am going to check the filter in the carb, remove it completely, then the idle jet and finally bypass the filter between the line and the pump altogether and do a straight connect. Let's see how that goes. After all this, I will end up with one fuel filter, and it will be the one inside the tank.

Thank you for your reply and suggestions!

 

[the hobbler]

Oh that's a great point! I had forgotten about the filter inside the carb. The carb that I have is the 28 IMB with the brass screw on top and a tiny filter inside it. It has an inlet and an outlet which is supposed to recirculate the fuel, but the outlet has been closed off using a short rubber hose to help maintain better fuel pressure.

I am going to check the filter in the carb, remove it completely, then the idle jet and finally bypass the filter between the line and the pump altogether and do a straight connect. Let's see how that goes. After all this, I will end up with one fuel filter, and it will be the one inside the tank.

Thank you for your reply and suggestions!

If you have the filter on top of the carb (under the brass cap) you won't have one IN the feed pipe. You can leave this one in place if it is clean. You should have one in the top of the fuel-pump (as well as the filter as part of the tank-unit). The idea that shutting off the 'return' from the carb "to maintain better fuel pressure" is a load of balony! On the '126'this return went back to the tank with a separate pipe. On MY '28 IMB' when I had such a carb fitted, I took a return back to a METAL 'T' piece, secured to the side of the engine bay, where the fuel pipe from the tank converts from 'fixed' to 'flexible'. I led the 'return' hose round the front of the engine bay, secured with plastic-sleeved "P" brackets---I also led the 'return' pipe through a length of small-bore water-hose to protect it where it was against the front bulk-head. Double-check that 'slow-running' jet to ensure that there is no muck in its chamber, also check the idle-mixture-screw orifice for any contamination.

 

[bellucci]

Sounds good. I'll keep the filter in place in the carb and will check the idle jet and the idle mixture screw. I'm going to bring the carb down and make sure everything is good with it. As far as the return pipe from the carb goes, thanks for the insight. The previous owner told me he always had it shut off but I know that that particular carb was made for fuel circulation so it baffled me. I'll look into that as well.

One last question for you. Are you using a 6mm I.D. for the fuel hose or anything larger in diameter?

 

[fiat500]

If you have the filter on top of the carb (under the brass cap) you won't have one IN the feed pipe. You can leave this one in place if it is clean. You should have one in the top of the fuel-pump (as well as the filter as part of the tank-unit). The idea that shutting off the 'return' from the carb "to maintain better fuel pressure" is a load of balony! On the '126'this return went back to the tank with a separate pipe. On MY '28 IMB' when I had such a carb fitted, I took a return back to a METAL 'T' piece, secured to the side of the engine bay, where the fuel pipe from the tank converts from 'fixed' to 'flexible'. I led the 'return' hose round the front of the engine bay, secured with plastic-sleeved "P" brackets---I also led the 'return' pipe through a length of small-bore water-hose to protect it where it was against the front bulk-head. Double-check that 'slow-running' jet to ensure that there is no muck in its chamber, also check the idle-mixture-screw orifice for any contamination.

I have never understood the advantage of re-routing the fuel return like that. Any surplus fuel being short-circuited into supply via the back of the engine bay must gain heat rather than lose it; that presumably being one of the design intentions or a return pipe?

Why not just block it off?

 

[the hobbler]

I have never understood the advantage of re-routing the fuel return like that. Any surplus fuel being short-circuited into supply via the back of the engine bay must gain heat rather than lose it; that presumably being one of the design intentions or a return pipe?

Why not just block it off?

The constant flow of the petrol can makes it run cooler---I have NEVER had a hot-start problem with my engine. My apprenticeship was on Mercedes-Benz cars in the days of the early, simple, petrol fuel-injection---2-plunger injection-pumps giving a carefully calibrated AMOUNT of fuel, as against the later 6-plunger injection-pumps (or in the case of the 600s, a straight-eight injection-pump!) which were not only calibrated the amount, but also timed the injection point. All these injection systems had (and HAD to) a "constant-flow" fuel-supply system, and I remember that the return-pipe, back to the tank was noticeably colder than the 'supply' pipes. I will concede that the "supply" pump supplying fuel to the injection-pump (which like a diesel pump, created the pressure as well as the injection timing) was a tad more powerful than the Fiat's fuel-pump----if I remember correctly, they supplied 1 gallon per minute!

 

[Goldnrust]

The workings of fuel injection, old fashioned or modern (where a ‘dead head’ non return design is common) is kind of irrelevant to a carb discussion.

On one level I agree with @fiat500 that circulating the fuel though a small loop in the engine bay might actually add heat overall as you are simply keeping a larger amount of fuel in the hot engine bay as opposed to in the cooler environment of the line through the cabin and the tank.

On the other hand if the real issue is not general engine bay heat, but the hot spot where the fuel line runs especially close to the exhaust manifold, then keeping the fuel circulating past this point, should help stop it from vapour locking.

… That said, there seem to be regular threads on here where the poor running of an engine is put down to a fuel flow issue. Pumps, lines, filters, etc are suspected, but I can’t remember the last time that by the end of the thread the concern about fuel flow had not turned out to be a red herring and in fact the issue had been ignition or carburettor related.

The ignition system in the 500 has a hard life, being blasted with hot air coming off the engine. Condenser and coil failures seem common and points will need comparatively regular maintenance (cleaning and checking gap) to perform at their best.

Air leaks at the base of the carb are common, as are blocked jets. A thorough cleaning is never a bad idea.

Back to the original post, if you do indeed have a fuel flow issue, then adjusting the float away from factory spec will only make things worse. But a dirty or sticky needle valve can cause fuel starvation, so back to the previous comment about a clean carb.

If your fuel lines are clean, the inside of the tank is clean and the filters clean then the only likely problem of poor fuel flow is the fuel pump itself failing

 

[Toshi 975]

Very short video showing the return fuel flow from a Weber 28Imb carb when I ran it on the bench. I was surprised by the flow rate.

 

[fiat500]

Very short video showing the return fuel flow from a Weber 28Imb carb when I ran it on the bench. I was surprised by the flow rate.View attachment 463472

I have the return plumbed back to the tank. When the engine is idling and the fuel tank level is low, I can hear the returned fuel splashing into the tank. I was prompted to do that after a particularly serious fuel lock issue 70 miles from home, after a long descent of steep hills in summer caused the fuel at the engine to be virtually static for about ten minutes,

 

[the hobbler]

I have the return plumbed back to the tank. When the engine is idling and the fuel tank level is low, I can hear the returned fuel splashing into the tank. I was prompted to do that after a particularly serious fuel lock issue 70 miles from home, after a long descent of steep hills in summer caused the fuel at the engine to be virtually static for about ten minutes,

Very interesting little video Peter----the rate of flow proves what I have always said---the standard fuel-pump om the 500 and 126 engines will provided far more fuel than will ever be required to run one of these engines---even at high speed. There is absolutely no need under normal (or even slightly abnormal) conditions to warrant the complication (and expense) of an electric fuel pump.

 

[bellucci]

Very interesting little video Peter----the rate of flow proves what I have always said---the standard fuel-pump om the 500 and 126 engines will provided far more fuel than will ever be required to run one of these engines---even at high speed. There is absolutely no need under normal (or even slightly abnormal) conditions to warrant the complication (and expense) of an electric fuel pump.

I'm glad you confirmed this! I was actually thinking that an electric pump would help so good to know the existing pump is plenty.