Car stalls without warning

Ics19 said:
@jimrat Might be worth sticking back the stock 3 ohm coil as Alex says - it could be the module isn't meant to run with 0.8 ohm and is shutting down to protect itself. But then again, you say the module isn't getting hot? 0.8 is generally correct for the separate HEI modules like in the AEI200 packs on 124s and later Montecarlos. There's no ballast resistor on the euro/uk cars that had the Marelli factory 135/155/178 distributors.
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The Marelli coils had a primary resistance spec of 3.1 to 3.4 ohms per the manual. That implies an internal ballast for that coil. I don't think that there are many Kettering type ignition systems designed to drive a 0.8 ohm coil primary resistance.
 
dllubin said:
On page 10-87/88 it has a complete parts diagram of the carb with everything labeled. There is also a chart showing what the values of jets, etc. should be.
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That's usually useful, but those Weber part diagrams are not 100% specific when it comes idle and main layout. Even the ones that cover operation apply some side view artistic licence with respect to how the jets are physically located. The OP car is a euro 1500 model so will have as standard a 34 DATR 7/250 or 34 DATR 28/250 (if it has the heater disc on the auto choke) so not covered by the US specs. The stock jetting wiki page does have the euro 1500 listed though (albeit as 7/270 and there is a discussion comment that queries this).
 
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Hi

I measured the amount of fuel pumped to the carb - I'm only seeing 160ml per minute - it should be ten times that amount - the pump is rated at 100 litres per hour. So fuel starvation is the issue. I havent found the root cause - it's definitely not blocked filters, hoses or tank vent, so it must be the pump. Some pumps dont like running with no back pressure, so I may need a restrictor in the return line. Or it might be a dud pump. I'll report back

Cheers
Jim
 
Good find. Nice to narrow it down to fuel.

Which pump do you have? and where did you mount it?

You could try a bench test of the pump to check the flow rate in ideal conditions before you replace.
 
Cancel that! I bench tested the pump and it is pumping the correct amount. I had measured the flow in the return line to the fuel tank. I have just found the tiny (<1mm) restriction at the brass outlet nozzle, which presumably is there to ensure petrol goes into the carb, rather than down the return line. Hmmm back to the drawing board.
 
Summary so far:

Car idles fine but stalls repeatedly if taken for test drive. Restarts immediately after stalling.

Since my original post I have:

Swapped carbs (both 34 DATR 7/250) - no change
Rechecked fuel system - getting correct flow rate into carb, filters are clear, tank is venting.
Electronic ignition - have tried a 0.8 ohm Lucas coil and 3.2 ohm Magneti BK2A coil. Still stalls, but worse with Magneti coil (as expected?).
Plugs aren't sooty or wet.
 
When the car stalls, does it die immediately like an electrical issue or does it cough and sputter like it is running out of fuel?
 
Update:

The car still stalls occassionally (once or twice on a 10 mile run) but is running a lot better. When it stalls, it sputters and stops over a few seconds. After driving 10 miles, the plugs look like this:

1712559781543.png


Looks like they are getting too hot, which means running lean or too much advance?

I rechecked the timing: with vacuum connected to the Marelli SE100CX, I'm getting 20 degrees advance at idle, increasing to 40 degrees at 4000 rpm. I think this is quite a bit more advance that the stock Marelli S135 (which had no vac advance), but there's not much info out there on S135 (or S178) advance curves.

One thing I havent mentioned is the car is fitted with a K&N air filter (with new filter element). Could this be causing a lean condition? Do I need to go up a size on my main and secondary jet sizes?

And one final observation: I have noticed that the hot idle speed changes. I'll set it to 900 rpm one day, then the next day it will idle at 1200. It's not the linkage. I know this sounds like an air leak, but I used 1.5 bar compressed air to pressurise the engine, and apart from a small leak at the exhaust flange, it all held pressure.

1712561243580.png


Cheers
Jim
 
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It really does sound like an air leak. Some thoughts - Although that air pressure test is interesting, it test while cold and does not include the carby or its gasket or a bunch of carby related toobes.. :) Perhaps something is warping as it get hot and permitting an air leak. That high idle is as you say, classic air leak symptom
 
Good news'ish - what have you changed to only now have it stall occasionally?
jimrat said:
I'm getting 20 degrees advance at idle, increasing to 40 degrees at 4000 rpm
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That sounds a tad high - a few searches has a 7/250 carb'ed S135 1500 dizzy advance at only 23 degrees @ 4500 rpm + 5 degree static (I'll plot this when mine is out of hibernation!), while the 1300 dizzy is few degrees more at 28. I'd assumed you'd left the vacuum advance plugged. This is the 1500 dizzy mechanical curve, so 5 to 28 total advance across the rev range:
1712576260377.png
 
lcs19 - I'm not sure what I've changed to now have it only stall occassionally. It could have been float level - I reset the height to 7mm using a drill bit rather than vernier callipers, and made sure both floats were identical height. I also noticed the brass float valve housing wasnt screwed in really tight, something that was mentioned in my workshop manual.

I found this info for the SE100CX in the Fiat Uno workshop manual. It looks like the total centrifugal advance on the SE100CX is similar to the original Mareli S135.

1712593865888.png


So should I:
- leave the vacuum connected, and retard the timing to bring the total advance down to 30ish @ 4000rpm or;
- diconnect the vacuum and just run with centrifugal advance?

Cheers
Jim
 
I would try it without the vacuum advance. Also, you might want to go with weaker advance springs as the engine will be happier with full advance by 3,000 - 3,200 rpm.
 
Good spot on the carb - always nice to rule out the mechanical adjustments first :)

As above, start without the vacuum unit. There's a chance that S135 springs might be transferrable into the SE100 to optimise the advance.

Try to plot what the curve is for reference - set the timing to 5 BTDC at <1000 to keep it simple for now.
 
Many distributors have two centrifugal advance weights whose movement is locked together via a mechanical linkage. They often use two springs. I've had good sucess removing one spring to speed up the advance.

On my Ducelier distributor I ended up with one spring that was very much weaker than the original two stock springs. That was to bring max advance down from ~5K rpm to ~3K rpm. Also added a stop to drop centrifugal advance range from ~28 degrees to ~10 degrees.
 
Update:

Carburettor has been professionally cleaned and tested
Vacuum advance disconnected and timing set to 5 degrees at idle. The timing curve is approx. 6 deg @ 1000 rpm; 12 deg @ 2000 rpm; 19 deg @ 3000 rpm; 28 deg @ 4000 rpm.

Car idles and pulls well, but stalled twice on a 2 mile test drive. This is just town driving, so not highway speeds, just 20-40mph. Today it stalled just after changing gear (i.e. higher revs), and again when on part throttle. When it stalls, I feel it lose power, foot on the clutch and the engine stops. Coast to a stop, turn the key and it starts again.

I'm losing the will to live :(.
 
jimrat said:
Vacuum advance disconnected and timing set to 5 degrees at idle
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As you're just town driving then I'd of thought it should be fine, so yeah, not your issue. The curve isn't a great match to stock and it would be interesting to see what happens at higher revs, but that's another day - here's your values (5 degree static removed):
1712576260377_se100cx.png

I think I'd go back to the original distributor setup to rule out the module as that seems the last change you did before the problems started?
 
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