Technical Backfire/Bog on Acceleration When Cold

[fiatseattle]

Hello All! I've got an interesting one going on....
Before we get going here's the specs:
1976 124 1800 - running stock marelli distributor (new old stock) Dwell angle is 52deg, 10deg BTDC.

32/36DFEV Redline Weber - new.
Main/Secondary Jets: 137/145 Air Correctors: 160/165 Idle Jets: 55/60

The issue: When it is cold, up to fan activation, on acceleration it will bog down and backfire under heavier loads (when the main & secondary circuits are being used above 2500rpm) until throttle is released, left to idle 10-30seconds then it is fine for the most part. It will occasionally bog for a second and then recover afterwards until hot. I had initially jetted it using Midwest Bayless's settings of: 125/145 mains, 185/175 airs, 55/65 idle but it was running so lean it would backfire regardless, hence the current jetting.

Ive adjusted the electric choke and verified voltage. Ive checked for air leaks and found none. Checked fuel pressure found at 2.5-3psi. I dont know what to check next and am driving myself nuts. Any ideas/help would be much appreciated!

 

[bugsymike]

Hello All! I've got an interesting one going on....
Before we get going here's the specs:
1976 124 1800 - running stock marelli distributor (new old stock) Dwell angle is 52deg, 10deg BTDC.

32/36DFEV Redline Weber - new.
Main/Secondary Jets: 137/145 Air Correctors: 160/165 Idle Jets: 55/60

The issue: When it is cold, up to fan activation, on acceleration it will bog down and backfire under heavier loads (when the main & secondary circuits are being used above 2500rpm) until throttle is released, left to idle 10-30seconds then it is fine for the most part. It will occasionally bog for a second and then recover afterwards until hot. I had initially jetted it using Midwest Bayless's settings of: 125/145 mains, 185/175 airs, 55/65 idle but it was running so lean it would backfire regardless, hence the current jetting.

Ive adjusted the electric choke and verified voltage. Ive checked for air leaks and found none. Checked fuel pressure found at 2.5-3psi. I dont know what to check next and am driving myself nuts. Any ideas/help would be much appreciated!

If with engine switched off and air cleaner out the way, shine a torch down the inlet tracts of the car and progressively open the throttle, you should see a fine steady jet of fuel from the accelerator pump squirting down into the carb (note if both chokes/throttle valves open at the same time you will see a double jet squirting into both tracts.
The idea is as you floor the throttle it squirts extra fuel in at that point which stops it back firing as you make it temporarily weak before the normal jetting takes over. This is assuming your normal jet sizes are correct.
Often the accelerator pump jet blocks and give a poor fuel delivery or the accelerator pump diaphragm leaks so poor delivery of fuel to jets.
It may look like this, or a double if both tracts/throttle butterflies open at the same time depending on design.

 

[fiatseattle]

If with engine switched off and air cleaner out the way, shine a torch down the inlet tracts of the car and progressively open the throttle, you should see a fine steady jet of fuel from the accelerator pump squirting down into the carb (note if both chokes/throttle valves open at the same time you will see a double jet squirting into both tracts.
The idea is as you floor the throttle it squirts extra fuel in at that point which stops it back firing as you make it temporarily weak before the normal jetting takes over. This is assuming your normal jet sizes are correct.
Often the accelerator pump jet blocks and give a poor fuel delivery or the accelerator pump diaphragm leaks so poor delivery of fuel to jets.
It may look like this, or a double if both tracts/throttle butterflies open at the same time depending on design.

The accelerator pump is working - I will be testing the jet sizes later this week by setting idle to 1400ish when warm and fiddling with idle mixture to see if the sizes are good.

 

[bugsymike]

The accelerator pump is working - I will be testing the jet sizes later this week by setting idle to 1400ish when warm and fiddling with idle mixture to see if the sizes are good.

I thought the idle adjustment had little effect once throttle open a bit.

 

[fiatseattle]

I thought the idle adjustment had little effect once throttle open a bit.

It doesn't after 2500rpm but it will indicate if the idle jet size is correct and if performance is improved overall it will help confirm correct jetting size for the mains (I have been chatting with an old tech at my machine shop about it. I wouldn't have thought this either if I hadn't chatted him up). If only I have a 2 channel AFM and a dyno machine HA!

 

[bugsymike]

It doesn't after 2500rpm but it will indicate if the idle jet size is correct and if performance is improved overall it will help confirm correct jetting size for the mains (I have been chatting with an old tech at my machine shop about it. I wouldn't have thought this either if I hadn't chatted him up). If only I have a 2 channel AFM and a dyno machine HA!

Just found this in an old car manual 1975-82 Fiat 131 Mirafiori with some jetting details for the 1800 as a rough guide.
It also has some details on the twin contact point system, which was raised in another post.

 

[124BC1]

Your fuel pressure seems a little low for the 124 Spider, the factory pressure afaik is 0.24 - 0.30 bar = 3.6 - 4.35 psi.
If there is a fuel filter fitted, might it need to be changed?
Is the filter sock fitted around the fuel pick-up pipe in the tank possibly choked with crud? (this pick-up pipe is part of the fuel tank gauge sender unit).
If removing the tank sender unit for examination, clean any paint/dirt off and spray penetrating oil on the little nuts that secure the s/unit in the tank (7mm wrench size) - they're easily sheared otherwise.
If these 2 items are ok, then your pump may be starting to fail? But before replacing the fuel pump, if it's an original electric type in the trunk, check the internal filter screen is clear - the pump can be separated into 2 halves (3 screws) to check the filter screen.

I've never heard of "raising the idle speed to 1400 rpm and fiddling with the idle adjuster screw to check if the idle jets are good". Like @bugsymike I was of the opinion that at this engine speed, the fuel/air mixture was coming not just from the idle hole but also from the progression holes (both are fed from the idle jet), so I don't know if fiddling with the idle mixture screw will tell you anything worthwhile.

The old-fashioned way of determining if the idle jet fitted was correct was to set it approx. 2 turns out from lightly seated and then try turning it in/out and seeing how the engine idle changed - screwing it in by approx. 1 turn should weaken the mixture and cause the engine idle to become rough and start to miss before cutting out, screwing it out should cause the engine idle to also become rough but this time start to 'hunt' (engine seems to surge up and down in a sort of rhythm). Try listening to the exhaust gas exiting the tail-pipe - running weak sounds 'splashy' and you can feel the misfiring if you place your hand in the exhaust gas stream, running rich, you can hear the rhythm and feel the pulses, will also likely smell the rich mixture. On the 124's., iirc, the idle mixture screw usually gave the correct mixture when set at approx. 1.1/4 - 1.1/2 turns out from seated (verified using an exhaust gas analyser).

Have you check and verified that the float height has been set correctly? (some carbs require the height to be set with the top cover gasket in place)?
Have you checked that the throttle plate (throttle valve/butterfly valve) is the secondary choke is closed fully at idle speed. I've heard reports of this secondary throttle stop needing to be re-adjusted even on brand new replacement carbs.

 

[bugsymike]

Just a thought, how good are "Redline Weber carbs" compared with genuine Weber from Italy, I found this online https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-503977--.html
In the past I have had issues with the smaller chinese carbs and poor running/manufacture.
Perhaps others on Forum can comment?

[email protected] Sun Mar 18, 2012 7:44 pm​ the "Marketing Scam" is the "USA Made" webers that are made in China and assembled in the USA, those are horrible.

 

[s130]

Another possibility.

Spark plugs are known to sometimes break down (internal tracking?) under heavy loads, or no loads at all. Not sure why or what happens under heavy engine load but it does apparently happen.

With one or more missing sparks raw fuel and air mixture is just pumped into the exhaust and also delivers no power output causing hesitation etc. Later on when a correct air / fuel ratio has built up in the exhaust system the backfire will occur. So in practice you get a loss of power / hesitation and later on the backfire.

Used to be an old kids trick to turn the ignition off, coast in gear, and then turn the ignition back on. Bang!

 

[bugsymike]

Used to be an old kids trick to turn the ignition off, coast in gear, and then turn the ignition back on. Bang!

Did that as an apprentice in 1969 and blew a silencer apart, boss made me weld it up in my dinner hour, it worked best with carbs with an accelerator pump, so you revved the engine, turned off ignition, pumped the throttle and then turned it on again.
Nothing to do with that but I went to a breakdown on a old Ford Transit that the owner had been trying to start for ages filling the exhaust with unburnt fuel, I found the problem and it fired up with a hell of a bang and set off seagulls all around Torbay.

 

[fiatseattle]

Your fuel pressure seems a little low for the 124 Spider, the factory pressure afaik is 0.24 - 0.30 bar = 3.6 - 4.35 psi.
If there is a fuel filter fitted, might it need to be changed?
Is the filter sock fitted around the fuel pick-up pipe in the tank possibly choked with crud? (this pick-up pipe is part of the fuel tank gauge sender unit).
If removing the tank sender unit for examination, clean any paint/dirt off and spray penetrating oil on the little nuts that secure the s/unit in the tank (7mm wrench size) - they're easily sheared otherwise.
If these 2 items are ok, then your pump may be starting to fail? But before replacing the fuel pump, if it's an original electric type in the trunk, check the internal filter screen is clear - the pump can be separated into 2 halves (3 screws) to check the filter screen.

I've never heard of "raising the idle speed to 1400 rpm and fiddling with the idle adjuster screw to check if the idle jets are good". Like @bugsymike I was of the opinion that at this engine speed, the fuel/air mixture was coming not just from the idle hole but also from the progression holes (both are fed from the idle jet), so I don't know if fiddling with the idle mixture screw will tell you anything worthwhile.

The old-fashioned way of determining if the idle jet fitted was correct was to set it approx. 2 turns out from lightly seated and then try turning it in/out and seeing how the engine idle changed - screwing it in by approx. 1 turn should weaken the mixture and cause the engine idle to become rough and start to miss before cutting out, screwing it out should cause the engine idle to also become rough but this time start to 'hunt' (engine seems to surge up and down in a sort of rhythm). Try listening to the exhaust gas exiting the tail-pipe - running weak sounds 'splashy' and you can feel the misfiring if you place your hand in the exhaust gas stream, running rich, you can hear the rhythm and feel the pulses, will also likely smell the rich mixture. On the 124's., iirc, the idle mixture screw usually gave the correct mixture when set at approx. 1.1/4 - 1.1/2 turns out from seated (verified using an exhaust gas analyser).

Have you check and verified that the float height has been set correctly? (some carbs require the height to be set with the top cover gasket in place)?
Have you checked that the throttle plate (throttle valve/butterfly valve) is the secondary choke is closed fully at idle speed. I've heard reports of this secondary throttle stop needing to be re-adjusted even on brand new replacement carbs.

I was informed by Weber Redline that the Fuel Pressure on a weber should be 2.5-3psi, the filter sock is clean, tanks has no corrosion. The pump is an electric pump that was replaced last year. The one thing I have not checked is the float level, its on my list....

 

[fiatseattle]

Just a thought, how good are "Redline Weber carbs" compared with genuine Weber from Italy, I found this online https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-503977--.html
In the past I have had issues with the smaller chinese carbs and poor running/manufacture.
Perhaps others on Forum can comment?

[email protected] Sun Mar 18, 2012 7:44 pm​ the "Marketing Scam" is the "USA Made" webers that are made in China and assembled in the USA, those are horrible.

Weber Carbs are made in Spain now, the Italian ones stopped being produced the 90s. The on I have is Spanish.

 

[fiatseattle]

Another possibility.

Spark plugs are known to sometimes break down (internal tracking?) under heavy loads, or no loads at all. Not sure why or what happens under heavy engine load but it does apparently happen.

With one or more missing sparks raw fuel and air mixture is just pumped into the exhaust and also delivers no power output causing hesitation etc. Later on when a correct air / fuel ratio has built up in the exhaust system the backfire will occur. So in practice you get a loss of power / hesitation and later on the backfire.

Used to be an old kids trick to turn the ignition off, coast in gear, and then turn the ignition back on. Bang!

Spark plugs are brand new.

 

[fiatseattle]

I'm going to get a look into several things tonight. 1. check the float level. 2. recheck choke settings. 3. Remove plugs and check burn pattern for lean/rich condition. 4. Verify fuel pressure. 5. I do have access to an exhaust gas analyzer and can check that this weekend. Ill update when I get some more data.

 

[s130]

Spark plugs are brand new.

Funny you should say that because last year I had a duff plug so replace them all. Very very short while later one of the brand new NGK plugs failed. In 50+ years I had never experienced this so was baffled and then really mad!

Plugs, leads, distributor cap can be the cause of misfires.

 

[bugsymike]

Weber Carbs are made in Spain now, the Italian ones stopped being produced the 90s. The on I have is Spanish.

Did you check that website I gave you at #8 it mentioned that.

Funny you should say that because last year I had a duff plug so replace them all. Very very short while later one of the brand new NGK plugs failed. In 50+ years I had never experienced this so was baffled and then really mad!

Plugs, leads, distributor cap can be the cause of misfires.

I have trusted NGK since 1975 working on Mazdas

 

[fiatseattle]

Funny you should say that because last year I had a duff plug so replace them all. Very very short while later one of the brand new NGK plugs failed. In 50+ years I had never experienced this so was baffled and then really mad!

Plugs, leads, distributor cap can be the cause of misfires.

Did you check that website I gave you at #8 it mentioned that.

I have trusted NGK since 1975 working on Mazdas

I have also had bad plugs out of the box, which is why i want to check them. The carb i have is directly from weber redline, I have a whlsale acct with them so Im not too worried about that. Points, condensers, dist. cap & rot have been checks and are in good working order. I also use NGK plugs, they are very reliable but even then there is that possibility. The symptoms have been present before and after the new plugs so Im less inclined to think its them.

 

[s130]

This could prove to be an interesting hunt / quest.

Basically we are down to fuel, carburettion, mixture and of course electrical.

You quote:

32/36DFEV Redline Weber - new.
Main/Secondary Jets: 137/145 Air Correctors: 160/165 Idle Jets: 55/60

Are you sure that the jets, air correctors and emulsion tubes are correct for your car?

I may be able to find some stock / base values for the 32/36DFEV.

 

[fiatseattle]

This could prove to be an interesting hunt / quest.

Basically we are down to fuel, carburettion, mixture and of course electrical.

You quote:

32/36DFEV Redline Weber - new.
Main/Secondary Jets: 137/145 Air Correctors: 160/165 Idle Jets: 55/60

Are you sure that the jets, air correctors and emulsion tubes are correct for your car?

I may be able to find some stock / base values for the 32/36DFEV.

The jetting sizes may not be, I began with sizing from Midwest (125/145 mains, 185/175 airs, 55/65 idle) but it was way too lean. Emulsion Tubes are F11 and pump jet is a 55. I have the weber baseline sizes but am going to start over (after performing the checks previously mentioned if they turn up nothing) with:

• Primary Main Jet: 135
• Secondary Main Jet: 150
• Primary Air Corrector: 175
• Secondary Air Corrector: 175
• Primary Idle Jet: 60
• Secondary Idle Jet: 65
• Emulsion Tubes: F11
• Pump Jet: 55

I can then check the exhaust gas measurement and see..... I also failed to mention I have a 4:2:1 exhaust on it with no smog equipment.

 

[gengis]

See below the jetting I had in my Spider…European head & 4-2-1 exhaust. It ran without flat spots/backfires and pulled like a train.

Primary

Main…140

Air Corrector…170 increased to 180

Emulsion tube…F50

Secondary

Main…140

Air Corrector…160 increased to 170

Emulsion tube…F50.

Hope this helps.