Alfa Pages

[Brit01]

I have a problem with the rpm.

When the engine has been working hard on the motorway and it gets up to a good temperature the idling surges to about 1350 rpm when I stop.
If I wait it may take 1 minute or so to get below 1000 rpm (sometimes it stays high when really hot).

I thinking an air leak on one of the carbs - either one of the gaskets between the plastic sandwich or possilbly the carb bearing linkage(hope not this one inside).

I've had an issue with an air leak before but last time it acted differently. It would go up and down while idling.

I thought it was lean as they can 'hunt' but even when rich it does the same.

Spent several times carefully balancing them and it idles nicely for the first 15 mins, but then after some hard driving it starts surging again.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[mg907]

Hi,
If you can find a bit of solace with that I can tell you I've more or less the same behaviour with Weber Carbs
And in addition, reading the Alfa 33 road test on a 22 year old Italian car magazine they complained about the same defect:
"Very unstable idling, variations of about 300 RPM" to tranlsate their words...and I suppose Alfa Romeo factory provided them a car in perfect shape

I was able to improve a little bit the situation after having tuned the carbs a little richer (unscrewing half a turn the mixture screw for each carb) and reducing the idle speed.

Even if the idling is usually enough stable around 1000 rpm, sometimes I get 1500 or even 2000 rpm for half a minute (more or less) without any apparent reason.

Maybe I've some air leak, I'm planning to take the carburettors off to check for worn gaskets or broken plastic sandwiches, but unfortunately I've to buy again a 3/8" socket wrench articulation, because I've lost my one.

[Brit01]

Speaking with Guy Croft I need to check the float heights and needles,

could be overfueling - this is a common result of this.

Never had this issues before. Always dropped to 850 within a second and table.

Something is a miss.

Air leaks normally cause rise and drops of idling not in my case.
It's very stable for the first 15 mins.

I will strip the carbs, lower the floats, check the needles, and change the gaskets.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[Bobkelso]

Hi,
if you cannot find an air leak (may be one that came around
only when metal parts are very hot), another possibility is:

before stripping the carbs, do a try,
put on again the all air intake box parts and see if it is better..

May be without complete air intake parts carbs are
getting too hot and some fuel in the carbs is vaporised
by heat, before getting in the venturi..
..so may be motor is aspiring fuel vapors getting
very the fast idle until carbs temp lower a bit
(fuel evaporation act like a refrigerator machine)

Or if you have a digital thermometer
you can verify this idea controlling if it happen
when carb temp reach an exact temperature.

I think you can reduce this by having carbs
very perfectly revised as you said (float heights and needles)
but may be you cannot eliminate it, also when carbs are perfect
they can permits some vapors go around I think, may bee also
by main jets that instead at idle shouldn't add fuel),

For eliminate it probably best thing is making carbs less hot,

- using complete air intake box parts with fresh air position
intake as you did before, and eventually also put some heat
insulating materials on some pipe and parts (in total with or not
all parts and insulation you can do 20 degree difference at carbs),

- maintaining the plastic insulator at the base of carbs (that give also
some little extra power reducing also air temperature in the carbs),
i think you already have them positioned..

- get some air vents in the bonnet german style
..ohm..ok this may be is not so elegant, best to skip this idea
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Alfa 33

[Brit01]

Hi,

Thanks for the advice.

But I think the opposite would happen with the small air inlets blocked off.
Also less fuel coming in as you mentioned should lower the rpm.

Less air coming in = more fuel which cools things down due to the evaporation.

What happens when the mixture is rich? The cylinders are cooler.
lean = hotter.

I've seen many posts on needles getting slightly stuck and leaking a bit through the progression holes when everything gets hot, more fuel, higher rpm as it burns off.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[Bobkelso]

Uhm, ok, detailed explanation is to be cleared..
(not sure, but if the extra fuel is already vapor
it get cooler the carbs and less the cylinders ?
Normally fuel from ventury is still liquid and
get vapor only at the intake valve cooling it.. )

Anycase It seam rpm problem happen when carbs metal is too hot,
and using all the air intake parts not to aspire hot air
from near the motor you know you can have sensible
(10-20 degree) carbs temperature drop

You are going to discover directly if you have some
needles problem or(and?) some air temperature one,
I'll read here your progress..get some curiosity,
I'm always learning something from these detailed reports
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Alfa 33

[Brit01]

yes I'm expecting it's just a small washer under the needle valve leaking - or some debris in the needle valve, or an air leak.
Maybe the gasket type washer has dried out or something.

As they worked fine before the engine rebuild I did, something just dried out or debris got in during those months sitting in the back of my car.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[Bobkelso]

Ps.: what does it mean in English motor going to 'hunt'?
is like "rpm going up and down at idle"?
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Alfa 33

[Brit01]

yep.

Which is what mine isn't really doing.

it just idles really high when I stop and takes a long time to settle back to normal, sometimes it just stays running high. Not up and down.

Couple of years ago it was hunting up and down, was an air leak on the gasket between the carb and inlet manifold.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[Brit01]

What a silly mistake and the last thing I would have thought of checking.

After removing the floats and needles, cleaning and checking everything, re-installing and re-checking I found the reason behind the high idle when hot.

The expansion tank tube that goes around the left carb linkage was flexing enough to touch the throttle linkage by 1/2 mm to prevent it from returning.

This was happening only when the engine got hot under load and was just enough to make the pipe pliable enough to move sideways.

When cold plenty of clearance.

All of that and turned out to be such a simple explanation.

Chris
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[BigAl]

Doh!, that reminds me of when I had some work done years ago, in the garrage the throttle got stuck, a pipe was stopping the throttle linkage from moving.
Glad you found it and it didnt cause any more head scratching
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x2 33 16v
pictures http://www.alfa-pages.co.uk/viewtopic.php?t=5203

[Brit01]

Cheers guys.

Now just got to put the richer jets back in (55 idles I think) and give them a go.

As it was balanced again and not running rich on one side when letting off on the throttle (as the left carb was running much faster than the right before), last night's journey was popping and crackling a bit too much on deceleration.

Also some pinging under heavy loads more. Mind you that happens sometimes when I get a bad tank of fuel - filled her up last night).

Idle mixture screws are already 4.5-5 turns out on the 53's.
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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)

[Ian M]

Do you know I was going to suggest checking the throttle cable and linkage operation, but then I thought no cos that would of been the first thing he checked.
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Presntly own
Red Abarth 500c

[Brit01]

I did check it Ian several times but it was just one of those things that you didn't see while it was on the driveway.

Just caught it in the nick of time after a hard drive, quick look under the hood and there it was the spring returning slowly the last 1mm, the expansion tube flexing next to the return pivot thingy.

Once cooled down a little the pipe wasn't touching it!!

The alfita really had me thinking and caught me out!


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S2 33 1988 1.7 QV
Dellorto DRLA40 (32mm venturi)