Technical Backfire from carb at speed

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Technical Backfire from carb at speed

Stupid question following:D..Isn't the main jet the one on the side (spark plug side) that can be removed while carb is on? While trying to make my engine smoke and smell less i wanted to check the main jet, i thought it was the one on the side so i removed mine and it is 112, so i thought it is ok.
 
Stupid question following:D..Isn't the main jet the one on the side (spark plug side) that can be removed while carb is on? While trying to make my engine smoke and smell less i wanted to check the main jet, i thought it was the one on the side so i removed mine and it is 112, so i thought it is ok.

You are correct Thomas. Also the adjustment of the mixture screw WILL affect whether the engine runs rich or not and hence the colour of the plugs. The more you unscrew, the richer the mixture. Some people make the mistake of adjusting at too fast an idle which can badly affect the result. You want the throttle closed right up as far as possible whilst still maintaining a healthy tickover; then you do you adjustment for smooth running.(y)
 
The adjusting mixture screw has to do only with idle if I am not wrong. A reach micture depends on the main jet isn't it?
 
I stand corrected gentlemen---teach me to check my references before making an arse of myself! The MAIN jet (1.12mm) can be accessed from the outside of the carb, with it in-situ. The IDLE jet needs the carb to be removed to change---on a 500, the idle jet is a 0.45mm jet. if the carb is taken off to check the idle jet, it is worthwhile giving it a good clean and checking the float-level (7mm with the top of the carb held up vertically). Also check that the needle valve is not 'ridged' and that the little alloy washer that goes under the needle valve is in place. lastly,make sure that the fuel enrichment device (the 'choke') is going fully off and not causing any enrichment during normal running.(y):)
 
I stand corrected gentlemen



.....as I may be also corrected; but as far as I am simplistically aware, the idle circuit of the carb, is in operation over the lower range of engine RPM and the main jet kicks in properly at higher revs. So if you are chugging around a lot and not giving it much welly, the correct mixture adjustment becomes even more critical when we are looking at spark-plug colour as an indication of a correctly adjusted carb. But of course, I might be talking out of my exhaust. :spin:
 
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I stand corrected gentlemen---teach me to check my references before making an arse of myself! The MAIN jet (1.12mm) can be accessed from the outside of the carb, with it in-situ. The IDLE jet needs the carb to be removed to change---on a 500, the idle jet is a 0.45mm jet.

The idle mixture jet is located above the idle mixture screw on the outside of the carb and is easily accessible in situ :)
I noticed the other day that the original Fiat workshop manual has a sectional drawing of the carb showing the idle jet inside the carb top cover in a vertical position which is completely wrong. This same drawing has been reused in all the other off shoot manuals.
 
Hmmmm...I failed to mention that I purchased a new carb a few months ago due to the original being badly warped and no matter what I did to it, I could not stop fuel from leaking out of it.
Maybe I just swap the original idle jet from the old carb to the new?
 
If the back-firing wasn't apparent with your original carb, my advice would be to take the carb off, have the old carb and the new carb side by side on the (well protected!) kitchen table, and then check the jetting in your original carb vis-a-vie the new carb and then bring the new carb to the same jet spec as the old carb. Make a careful note of what was in old and new carbs prior to doing the actual swap, and have a number of small envelopes/zip-top bags to hand so that if you need to store any of the jets you can do so in a 'notated container'. If you haven't got a full list of original carb specs, let me know and I will pass them onto you. Do not assume that float setting etc are correct in the new carb--check everything.(y):)
 
If the back-firing wasn't apparent with your original carb, my advice would be to take the carb off, have the old carb and the new carb side by side on the (well protected!) kitchen table, and then check the jetting in your original carb vis-a-vie the new carb and then bring the new carb to the same jet spec as the old carb. Make a careful note of what was in old and new carbs prior to doing the actual swap, and have a number of small envelopes/zip-top bags to hand so that if you need to store any of the jets you can do so in a 'notated container'. If you haven't got a full list of original carb specs, let me know and I will pass them onto you. Do not assume that float setting etc are correct in the new carb--check everything.(y):)

I swapped out the idle jet and have to take it out for a spin now to test it. Visually they looked the same. If this doesn't do it then I will take the carb apart and start swapping parts. Please send over the carb specs and I will take a look. Maybe it is the float setting also.
Thanks!
 
According to my 'book of words' the following are the specifications for the "Weber 26IMB 6" which is the correct carb for your car (500L). The specifications are the same for the IMB 4, the '1' and the '10'. Only the '3', which was used on the '500 Sport' has (slightly) different specs. The '10' additionally, has "CO limiter for idling.


Throat diameter:----------------26mm
Venturi:--------------------------21mm
Main jet diameter:---------------1.12mm
Idle jet diameter:----------------0.45mm
Starting jet diameter:-----------0.90mm (F5)
Main air-jet diameter:-----------2.35mm
needle=valve seat diameter:---1.25mm


As per Peter's query, is the noise that you are hearing a light 'phut phut' or a bang, loud enough to shake the budgies off their perches? When my engine is hot, the backfire from mine is 'distinctive!'(y):)
 
The backfire is more or less a phut-phut. Not that loud but I can hear it from inside and someone behind me can hear it also. It happens during deceleration from a higher RPM usually. I think I'm just going to swap out all internal components of the old carb to the new carb. I can't imagine what else could be causing it.
 
The backfire is more or less a phut-phut. Not that loud but I can hear it from inside and someone behind me can hear it also. It happens during deceleration from a higher RPM usually. I think I'm just going to swap out all internal components of the old carb to the new carb. I can't imagine what else could be causing it.

I had the "phut-phut" on my previous engine in those circumstances, and no adjustments of timing, valves, carb made any significant difference. It did get better with an electronic distributor (the only benefit!). I get the same thing with my current engine on very steep downhills when I let the engine over-run using engine "braking". My theory is that the worst it can be is surplus fuel burning and then escaping past the closed valves under great pressure. I think a more likely explanation is that as we don't have fuel injection which can be turned off automatically by a computer-control, the engine is forced to accept a bit of petrol in circumstances where none is needed. As the throttle valve is closed with your foot off the pedal, there isn't enough air drawn into the engine to burn the fuel properly, so when the exhaust valves open and let the hot mixture out it burns up spontaneously giving a"pop".
Just my theory, based on (I hope) logic and the fact that it seems impossible to avoid this happening sometimes.:bang:
 
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I had the "phut-phut" on my previous engine in those circumstances, and no adjustments of timing, valves, carb made any significant difference. It did get better with an electronic distributor (the only benefit!). I get the same thing with my current engine on very steep downhills when I let the engine over-run using engine "braking". My theory is that the worst it can be is surplus fuel burning and then escaping past the closed valves under great pressure. I think a more likely explanation is that as we don't have fuel injection which can be turned off automatically by a computer-control, the engine is forced to accept a bit of petrol in circumstances where none is needed. As the throttle valve is closed with your foot off the pedal, there isn't enough air drawn into the engine to burn the fuel properly, so when the exhaust valves open and let the hot mixture out it burns up spontaneously giving a"pop".
Just my theory, based on (I hope) logic and the fact that it seems impossible to avoid this happening sometimes.:bang:



This makes total sense. And it’s exactly what I’m experiencing.
But it didn’t do it before:
A-installed the new carb
B-heads reconditioned with new valve seat and valves.
I’m going to swap the carbs and see if it happens with the original carb. If not then I will start swapping components. This is very time consuming!
 
Having listened to your description of the 'fault', I think Peter's description of the cause and chain of events is pretty well spot-on. I had come to the same conclusion, which is probably why it only really happens on my car when the engine is warm and needs less fuel. For what it is worth, do the carb jet swap, make sure everything is correct with regard to valve settings and timing , and then just go and enjoy the car. Do NOT try to compare the running characteristics of a 40+ year old car with a modern car---enjoy the simplicity of your classic and "go with the flow, man".(y):)
 
Paolo is correct - backfiring is a timing issue.

It's caused by the ignition being too far retarded.

Other causes include a too-rich mixture or a cracked distributor cap. If you've had the engine to bits I would suspect the timing first .. then the mixture.
 
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Re: Backfire at speed

Tried timing it. Advanced...same. Retarded...same. So I decided to swap the new Weber out with the old. Bingo! No more backfiring! So it is something related with the jetting on the new carb. It's just dumping too much fuel into the cylinders. At this point I'm inclined to just leave it now since it is purring like a kitten.
 
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