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Hi Folks,

Finished putting the 2L back together for my Super resto - been in a world of pain with a surging / wandering idle on the car - any / all suggestions welcome - I think I've been staring at it too long - sorry this is a little long winded - but appreciate any help.

The story so far....

Rebuilt 2L head with new valves / guides / seals - mild port matching and exhaust ports polished. Three angle cut on seats. Added 105480320001 camshafts timed to 102 degrees (will go closer to 99 once car is running well) Clearances 0.40mm (0.016") intake and 0.45mm (0.018") exhaust. NGK BP7ES plugs

Bottom end of engine still in great nick after previous rebuild ~ 15000 km ago.

The dellorto DHLA40Fs I was running on the engine previously were sold with the Duetto. I dug out some Weber 40DCOE32s I had stored for a few years, gave them a quick clean up - checked the accel pumps - re-jetted to 2L specs and bolted up to the engine.

Chokes: 32
Mains: 135
Emul: F9
Idle: 50F8
AC: 200
AP Jet: 35
AP Bleed: 60
Need/Seat: 150


The Cast headers have been replaced with a 4 -2 -1 extractor set from PACE engineering here in AU - really nice kit.

Ignition is a MarelliPlex timed with light to the static advance mark at idle - moves to just under max advance mark when engine is revved over 4k

Cold engine compression test with WOT and all plugs out yields ~180 on all cylinders.

With the engine back together, checked the carb balance using the progression holes, started easily and set the idle mixture by ear the usual way - one screw at a time to get fastest idle, then a 1/8 turn richer.

Once the car has warmed up for a minute or two - the idle starts to wander slowly up and down 200 - 300 revs - seems completely random - sometimes will sit perfectly for a minute or two, then wander again. with occasional stumble and a rare spit back through the carbs (richening that cylinder a little with the idle screw will prevent spitback, but not the stumble.

Holding the revs around 2-3k yields spit back through the carbs / stumbling - but opening up to the mains at 3.5k onwards the engine is perfectly smooth.

In trying to solve the idle area issue I have:

* swapped in an old Bosch JF4 points dizzy + leads + coil
* tried advancing / retarding the ignition a few degrees just in case - has the normal effect but doesn't change the idle / stumble
* Removed the carburettors, disassembled - cleaned out completely and used new rebuild kit to reassemble (including new needle / seat) Confirmed correct float height and stroke (vertical 8.5mm from gasket edge to non-seam edge of float 15mm full travel) - confirmed throttle shafts are straight, butterflies are aligned. Throttle shaft bearings are greased and sealed correctly - no air leaks. Starter jet valves open and close correctly - and seal when closed. Accellerator pumps jets don't drip / leak when car is running.
* Tested fuel pressure regulator - 2.6psi cranking on starter.
* Removed the intake manifold to confirm no air leaks at manifold gasket, and rubber carb flexible mounts are not leaking
* Blocked off brake booster vacuum line and #1 cylinder vacuum port
* Rechecked TDC with dial indicator; Rechecked cam timing, Rechecked valve clearances,
* went up one size idle jet to 55F8 - this did calm the stumbling a little in the 2-3k revs area - but not completely - didn't expect it to solve the idle, but hoped it would settle the mid range stumble.
* the cooling system is holding pressure, the water level is constant - not loosing any through the head gasket to combustion that I can tell.
* Fuel in the tank is fresh / the tank is clean inside, fuel filters are clear.
* Sprayed carburettor cleaner all over the rubber carb mounts / intake manifold etc searching for leaks.... none found.

Really has me stumped at the moment...

Things still on the list to try:

* The idle mixture screws don't have o-rings fitted. The rebuild kits came with them, but the flat washers just deform the o-rings out making them pointless. I know some webers had dished or concave washers here to contain the o-rings. These never had orings - and ran fine on my duetto for a year before i retired the 1750 engine... I guess I could try a little grease on the threads of the mixture screws just to see if it's air sneaking in there....
* Disconnect the Alternator - just in case it's loading / unloading causing the idle to wander (however doesn't explain the stumble) Voltage while car is running is constant - so doubt that's it but just to narrow it down further
* beg / borrow a synchroniser and see what's really going on at each barrell (the compression is perfectly even though.... )
* swap in some other carbs (I have a really grotty set of DHLA40Gs jetted for a hot 2L but they've been thrashed / mixture screws rounded off etc... probably cause more trouble than solve it... still, might help me narrow it down)


If you've read this far - much appreciated. It's probably something really simple I'm missing ....

Grateful for any ideas - I think frustration is blinding me at the moment ;)

Cheers,
 

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Though you have checked al the likely suspects, this sounds like an air leak somewhere. Are the butterfly shafts brass by chance? If so, these can bow and wear oval in the Weber center bushings causing a wandering idle. The bow in the bars center can also cause the butterflies to close differently at different times causing uneven idle. The solution is replacement with the steel butterfly bars. These do not bow from the pump lever, and do not wear oval in the center bushings. This is just a thought.
Another possibility is tired distributor advance springs. You will have to take the distributor apart to check. One weak spring will cause a wandering idle. The total advance will appear correct, but at low speeds the advance may bounce around a couple of degrees, now and then causing the uneven idle.
I hope this helps.
 

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Discussion Starter #3
Hi Gordon, thanks for the quick reply !

I did check the butterflies closely, holding them up to the light - they seem to open and close precisely to my untrained eye at least :)

The shafts are black - the threads at either end are silver - so I assumed steel. The bushed area the shafts pass through between the throats seems a close fit to me, but I didn't load the shaft in the center to check - I'll take them off and have another look to be sure.

In the process I might throw those worn DHLA40Gs on - just in case they idle nicely even if they have other issues... If they wander about too then it's almost *got* to be a manifold or valve issue.

I'm fairly confident the dizzy isn't causing it - only because I swapped in the old JF4 + coil + leads from another car that runs sweetly.


Thanks again for your help - I agree it smacks of an air leak somewhere....
 

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Yes, you have the Weber problem out of the mix with the steel butterfly bars. Another Weber issue I've only seen once, was a blocked bleed-back jet in the bottom of a float bowl. This caused a pump jet to drip and irregularly richened the mixture in one Weber. Searching for a solution, I replaced both bleed-back jets in both Webers and the uneven idle vanished.
Another one is the little dent in the float needle valve arm. As Webers age, the dent can get deeper. On rare occasions, the float needle ball can catch in this dent with a resulting high or low fuel level in the bowls. This usually happens after the float levels are readjusted after replacing needle and seat. A new needle with a new ball catches in an old dent with the lever in a new position. I don't know if the ball rides on the edge of the dent sometimes and other times falls into the dent, but the varying fuel level can cause an uneven idle.
I'm only picking on the Webers and distributors as I restore those for customers and have noted many odd problems with both systems.
I agree that an air leak is an easier and more common solution. You have probably tried checking the insulators for flat, and rubbers for cracks. You probably also sprayed WD-40 or a light oil o the gaskets to check for porosity. It is not the air correction needles, UNLESS you have the wrong style installed. The later emission Webers used a pointed needle on these screws with the rubber "0" ring under a concave retaining washer, The correct ones for your Webers are the blunt needle style without "O" rings under the spring. It is possible, (unlikely) that someone has really cranked down on one of these screws after it is seated, either enlarging the orifice in the body, or displacing aluminum down into the throttle plate bore.
Just a couple more things to look at.
 

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Discussion Starter #5
Thanks again Gordon - armed with a little extra knowledge -> once more unto the breach !


As a side note - I had the radiator recored, I think the overheating problem will be solved.....

here's the core they took out...
 

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Discussion Starter #6
A little update on this - some water under the bridge since the Super was back on the road I know..

The engine still has a miss around idle - I ended up rebuilding a later set of emissions Dellortos with balance ports / screws and a carbtune to see what was going on. Cylinders 1 and 3 are drafting lower than 2 and 4 - requiring a good few turns of the balance screws to bring into line - but on top of that - the top of #4 piston, and the sealing edge of the $4 exhaust valve is spotlessly clean - pointing to water ingress in that cylinder.

I know I originally checked the compression and they came up even, but after checking again last week - the 1 and 3 cylinders take 6-8 revolutions to come up to max pressure, compared to about 4-5 revs on the others - so definitely a valve seal issue there. Might explain why the car is very ordinary at idle / transition - but still howls beautifully under full load...

I think there's a couple of things going on here masquerading as one problem - it's annoying as this is my 4th head rebuild - and I thought I'd taken a great deal of care with it. In hindsight - the engine the head came off had clearly been thrashed and possibly overheated - so I should have had the head checked professionally for flatness / twist (I did go over it with a straight edge.)

A little oil has started kludging on the radiator cap as well - so the head has to come off one way or another. I've sourced another 2nd hand 2L head off a known good motor - and will strip / polish / renew as per the other one and do a swap one weekend as time permits. Once the old head comes off it should be pretty clear where the issues are. I need to do it as a head swap as the car is my daily commute - gotta get to work to pay the bills !

I'll keep you posted with what I find !

Cheers,
 

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It's the "multi-issues" that conspire to confuse.
From my experience.
 
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