Alfa 105 running on 3 cylinders at idle

[poordecisions]

Hi All,

This Xmas allowed me to spend some time on the Alfa GT Junior 1600 1973 that I collected back in October. I drove it to Leatherhead from Durham which was... eye opening.

Having had time to delve into it, and whilst changing the starter motor it became clear that the car was running on 3 cylinders. I was surprised by this as there would be no way that I’d be able to drive that asthmatic engine on 3 cyclinders at 80mph all the way home.

The carbs are dual Weber DCOE 40’s. I have read about an idle “circuit”, but am by no means a carb maestro.

So, the symptoms:

Runs on 3 at idle (cylinder no 1 not firing)
Cylinder 1 has spark at idle, but the cylinder doesn’t combust
Runs on 4 when accelerator pressed
Covering the intake on cylinder 1 increases the revs, covering decreases revs (air starvation) on other cylinders.

Complications found by me:

Aux Venturi on cylinder 1 is size 60, not 45 as per the others and should be held in by a grub screw, not a spring as per the other 3. My carbs should use springs, though despite that the Venturi was orientated correctly (I have ordered the correct size)

Drops of fuel (not loads, but some) are pushed out at idle through the intake on cylinder 1, I had assumed this was something to do with the Venturi.

All jets and tubes are the same size across the board.

I am no carb expert. Id like to think I am a competent home mechanic, but have only worked on BMW twin carbs in the past.

I have not tested compression yet - am desperate for it not to be piston rings.

What I have done so far (to no avail):

New spark plugs
New HT leads
New Coil
New dist cap
(New rotor, but didn’t fit, so using old for now)
New fuel pump
New Venturi ordered but not fitted

Any help or pointers would be greatly appreciated.....

Will

[E-bmw]

I too am no carb expert and I am sure there will be more specific information along shortly.

For me when you have eliminated the obvious that you have found to be incorrect & cleaned/replaced all the jets etc I would be looking to get the carbs set up & if the spark is at the right time & strength, which from what you say it should be then that should sort the issue.

Could be an idle jet, idle screw or similar issue as a starting point as it sounds like it is OK off idle..

[poordecisions]

Sardonicus said:

Drops of fuel spitting out on idle indicates a sticking or damaged intake valve IMO this will be masked as the revs rise giving the impression your now on all 4 cylinders well more than likely 3 an a bit (comp leaking via intake valve) if my suspicions are correct as in the cylinder showing some compression if not full a compression test or preferably a leak down test would be my next move TBH before jumping to conclusions even if it appears I have done

Edited by Sardonicus on Thursday 4th January 15:44

Thanks, I’ve ordered a compression kit to check tomorrow.

Is there anything that I should look for on the carb for the idle? It seems a little strange as it is running on 4 on acceleration (assume due to fuel pumps squirting?) - so my inkling is that there is little / no fuel on idle.

Happy to be corrected!

[GreenV8S]

Didn't you say it had a different size venturi on the cylinder with the misfire? I'd have thought that would make a big difference to the fuel mixture and could easily throw the mixture out, which could well lead to the misfire you're seeing. A misfire could also account for the fuel spray you mentioned, so I'd start by fixing the problems you can see and then see what is left.

[kev b]

Is it on points or electronic ignition?

[poordecisions]

kev b said:

Is it on points or electronic ignition?

Points, which look relatively new

[poordecisions]

GreenV8S said:

Didn't you say it had a different size venturi on the cylinder with the misfire? I'd have thought that would make a big difference to the fuel mixture and could easily throw the mixture out, which could well lead to the misfire you're seeing. A misfire could also account for the fuel spray you mentioned, so I'd start by fixing the problems you can see and then see what is left.

Yes, though it’s the Aux Venturi (which I thought weren’t that important)

Probably wrong!

[poordecisions]

I’m sorry I don’t understand that - could you elaborate please?

Thanks,

Will

[kev b]

OK this is a long shot but I have seen it a few times, as it is on points then the distributor can wear such that the dwell (point gap)on each cylinder can vary.

If you set the points with a feeler on the lobe that gives the widest gap then one of the other lobes may give a narrower gap causing a misfire or vice versa. If you used a dwell meter then this averages out the dwell between all four lobes and can cause the same problem.

You can check for lack of spark with a strobe on each plug lead in turn.

This used to be common on Vivas and Victors but I have never seen it on an Alfa.

As others have said a leakdown test is where I would start. Also check the idle jet and circuit is clear on the misfiring cylinder first and that the seal between the carb and manifold is ok.

Old Webers can wear around the spindle seals letting air in, a blob of grease will stop this long enough to check.

[227bhp]

What made you come to the conclusion it's running on 3?

[GreenV8S]

If I understood correctly, you have two twin choke carbs. Is it possible to swap them over and see whether the misfire follows the carb?

[Peanut Gallery]

Just a random thought - I have had two anti dieseling solenoid valves fail on me - a little electric solenoid that shuts off the idle jet when the ignition is turned off. They were both a real PITA to find when they went, as it was not something I had even considered.

If your carbs have these, does pulling the wires off make any difference? - Also noting you have had electrical fun with the car in the past.

[tr7v8]

Common problem on 105s is the rubber carb mounts, they split and let air in. From memory there is a brace to stabilize the weight of the carbs which gets left off which makes it worse.

[GreenV8S]

Spraying cold start around the outside of the intake is a good way to detect air leaks.