Technical How do I reduce Emissions??

Currently reading:
Technical How do I reduce Emissions??

Mark0474

Member
Joined
Jun 15, 2021
Messages
16
Points
54
Hi
I have a 1971 500f with a Weber 28 IMB carb fitted. I am trying to get it through the spanish ITV, MOT and its failed twice on emissions.

The max the emissions can be is 4.5 and my readings are over 7.

The engine was rebuilt, carb done at the same time, I have also put some additives in the fuel to run through the system. It seems to run well, sometimes does a little hiccup when accelerating but otherwise is fine.

When cold and I rev the engine, it looks like fuel is coming out of the exhaust, this clears once warm. I'm not familiar with these carbs so any help is always greatly appreciated.

Would replacing the carb for the stock Weber 26 make a difference?

Thanks
Mark
 
The 28 IMB is normally the carb for the 652cc '126' engine, so is a bit on the large size for a standard 500 (if that is what you have). Yes, a 26IMB might cure your problem, OR try fitting a slightly smaller main jet. The normal main jet for a '28' (when fitted to the 652cc engine) is a '115'---try a '110' and see how that goes.
 
Thank you, it was kinda what I thought but I haven't played with carbs too much, its a dark art!

I'll look at getting a smaller jet otherwise the engine builder has offered to swap carbs to a stock 26.

Thank you
 
One of the main suppliers of Fiat 500 parts in the UK said some years ago that it was pointless putting a 28IMB on a stock 499cc engine as all you would do is end up up using more fuel and you may be proving that point. There is nothing remotely mystical about the little Weber carbs and the only adjustments you can make are the tick over rpm rate and more importantly the idle mixture screw. To set the idle mixture screw you start by screwing the screw right in without forcing it then easing it back 1 1/2 turns. Get the engine warmed up then make minor adjustments to the idle mixture screw either way to achieve the smoothest running by ear and feel. Although there were several revisions of the 26IMB carb throughout production the jetting remained constant. Most revisions were about making the carb more robust with the 26IMB 10 being the last in the range. The 28IMB also had several revisions with the jet sizes changed but these generally went hand in hand with other engine revisions.
 
I had the same experience as Mark with my stock '71 Giardiniera this year, failing inspection for emissions, with my carb adjusted as Toshi suggests. The car never failed this test before, so I imagine it has become more stringent.
Long story short, I had to find a garage equiped with a gas analyzer (few are these days), and time was spent finding a setting that resulted in a very rough idle, but gave acceptable values.
Inevitably, I reverted to the previous setting after inspection, and may have to go through this rigmarole again once a year from now on.
 
Thank you for the invaluable information.

I’m torn as to whether or not put a stock carb on but in the meantime I have a local garage who will look over the carb and they also have a gas analyser, fingers crossed we can find a compromise.
 
In Greece at our local MOT the measurement is at idle, do they test yours at idle or at high rpm, just curious.
 
It’s measured at idle in Spain. The technician wasn’t impressed!
 
My Giardi was tested at idle, my petrol injection Peugeot at 2500 RPM a few days later.
Was told that's as per directives.
 
You mentioned in your last comments that the emmissions are tested at IDLE--have you tried a smaller slow-running (idle) jet? On the 28IMB (on a 652cc 126) the slow-running jet is a 50---you might be able to get a smaller one to try. Also, have you checked the float adjustment---with the carb top-cover lying 'on its back', the clearance between the float and the gasket should be 7mm. The easiest way to check the adjustment is with a drill bit of the appropriate size, slid between the float and the gasket.
 
The sizes smaller than a '50' are:---47+45+42+40+35. I would suggest that you try a 45 and see what emission result you get with that---at least to get you through the test!
 
Thank you so much for the information, I think I need to start ordering a few bits and see what can be done.
 
Just a thought...
In Germany there is a Catalytic Converter kit for Fiat 500s...
(I think) can't remember where I saw it...
 
Just a thought...
In Germany there is a Catalytic Converter kit for Fiat 500s...
(I think) can't remember where I saw it...
Hi, I did see them as well but with a 1000euro price tag. I am just waiting on some idle jets to arrive and will give those a go. I was also told to put a load of ethanol into the fuel to get it through inspection and then afterward to fill it up with normal fuel.
Thanks
 
Hi
I have a 1971 500f with a Weber 28 IMB carb fitted. I am trying to get it through the spanish ITV, MOT and its failed twice on emissions.

The max the emissions can be is 4.5 and my readings are over 7.

The engine was rebuilt, carb done at the same time, I have also put some additives in the fuel to run through the system. It seems to run well, sometimes does a little hiccup when accelerating but otherwise is fine.

When cold and I rev the engine, it looks like fuel is coming out of the exhaust, this clears once warm. I'm not familiar with these carbs so any help is always greatly appreciated.

Would replacing the carb for the stock Weber 26 make a difference?
?
Thanks
Mark
Mark, what does that 4.5 mean? Is it 4.5% CO, which would be quite generous, or is it something else? Hydrocarbons would be parts per million and would be quite a high number. It's maybe been mentioned already, but do you have a new air-cleaner fitted?
 
There are universal Catalytic converters for £50-100... I am sure an enterprising person could adapt one... even if only temp to reduce emissions....
 
Ok im going to chime in my 2cents.. Because I daily drive a classic Mercedes, that up until a few years ago i had to do the "sniff" tests to pass emissions and get my annual renewal here in Ontario, Canada. Modern cars with OBD2 are easy to pass the test, as long as your engine light is not on, it will pass. For older pre obd-2 cars from mid 80's to around 95-97, they actually do a emissions "sniff". And you literally have no ideal if you will pass or fail because the CO/NOX limits are very tight, and there is no real way to test for those values at home in the garage without a sophisticated exhaust sniffer.

The fail proof way to pass the test is using methyl hydrate as a additive to the fuel. Its normally marketed as "gas line antifreeze" in colder climates, but its basically a alcohol that burns clean and hotter. It helps weak catalytic converters to "warm up" and give its that little help to pass NOX/CO tests. I had to get my tests done in the winter, and that extra little bit of a exhaust temp bump was a bit of a help.

My car always passed with flying colours even with nearly 500,000km's on it. But i wanted a grantee i would not have to waste time and a hr out of my working days dealing with the whole "bureaucracy" that is/was emissions testing here in Canada. They have since removed any requirements, which is a sigh of relief for classic car owners.

Using methyl hydrate on a non cat car will help lower co values as it is a much cleaner burning fuel. The car will be a bit grumpy if you use too much, but it will not harm the fuel system at all. Most older cars are always tuned/carbed towards the richer end anyways, so it should help if it just need that little bit of "persuasion" to pass.

Nothing beats a proper tune with a wideband o2, but its a tip you can try.
 
Last edited:
Thanks for the info with regards to the Methyl hydrate, I was told to add a good bit of Ethanol to the fuel tank as this would achieve the same thing you describe, burns cleaner. I had cleaned out the carb, adjusted the float to 7mm and checked all gaskets and also the exhaust gaskets.

This morning I managed to get the car through emissions, everything was stock and I had the idle jet wound out 1.5 times and my emissions were still too high, I wound it in half a turn and the emissions dropped and it passed inspection.

I find that driving it feels like the engine loses power and then suddenly wakes up, normally in about 3rd gear but has happened in the other gears. Could this be that the carb is getting a bit too much fuel, is it worth me changing the jets as mentioned above as they have just arrived in the post?

Thanks all for you help and advice
 
Back
Top