Hanging revs problem, 75 Alfetta GT Webers - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 13 (permalink) Old 10-05-2015, 03:25 PM Thread Starter
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Hanging revs problem, 75 Alfetta GT Webers

Does this symptom ring a bell with anybody?

My GT has a hanging-revs problem, but typically only during/after heavy throttle applications.

Revs will fall back OK under little or no load (not instantly, but OK) - eg revving while stationary, or lifting off from steady throttle, but when the throttle is loaded, for example when accelerating through the gears, revs take way too long to fall back; long enough to have to pause before slotting into the next gear, for example.

The problem SEEMS worse after the engine is fully warm.

The distributor is a new 123 unit
My Timing efforts have been checked by my Alfa guy
Much spraying of carb cleaner around the carbs doesn't indicate an air leak
Carbs are 40DCOEs 72&73 'emission' carbs, with standard jetting
Motor is unmodified as far as I know, certainly not valves etc, which have just been renewed, standard air filter.
Mixture screws are @ 4.5 turns out, below that it spits and farts
With careful balancing using a 4-tube vacuum gauge, adjusting air bleed screws to get under 1 HG (?) vacuum variance, the problem is lessened, but never goes away.

The shop who just performed the head rebuild do not have the balancer, I need to borrow it from elsewhere, and won't have access for a week or two.

Is this problem all about mixture and balance, or is something else at play?

Any suggestions?
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post #2 of 13 (permalink) Old 10-05-2015, 04:25 PM
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Which curve are you using in the 123? Also, poorly synchronized carbs can cause problems like this. My method is to short one spark plug at a time and note the drop in revs then adjust the synch screw to make them all the same.

Ed Prytherch
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post #3 of 13 (permalink) Old 10-05-2015, 09:20 PM Thread Starter
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Ok, thank you Ed,

I used one of the curves that the 123 guys recommended - A. I see after revisiting your "15,000 miles with a 123" thread today that that curves 1, A, E & D suit the 1750/2 litre cars, if I interpreted that correctly. Of these, curve 1 May be a better starting point than the 'conservative' A, then I will perhaps try E.

Also, I neglected to say that the Aus spec GT 1750 Alfettas still ran the 10548 cams. It was fitted with a Bosch 006 distributor. if either of those things makes a difference to your advice. The motor has excellent, even compression.

thanks again...
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post #4 of 13 (permalink) Old 10-06-2015, 01:10 AM
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dellorto carb problem - Alfa Romeo Forum

http://www.webercarbsdirect.com/v/vs...ng%20Guide.pdf

Past 1990 164 V6 in NZ

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post #5 of 13 (permalink) Old 10-06-2015, 02:44 PM Thread Starter
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Thank you Alistair, will follow a couple of those leads.
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post #6 of 13 (permalink) Old 10-24-2015, 01:52 PM
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Quote:
Originally Posted by Ranz View Post
Thank you Alistair, will follow a couple of those leads.
Hey there, were you able to fix it?
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post #7 of 13 (permalink) Old 10-24-2015, 08:12 PM
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Does your car have that gizmo between the carbs fitted to Australian nords around that time that are intended to do exactly what you're describing? I think it started with ADR27a or some such emissions standard.

Alister
1973 105 GTV (Alfa #6 of 19 owned)
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post #8 of 13 (permalink) Old 10-24-2015, 11:41 PM Thread Starter
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Have again borrowed a 4-way vacuum balancer to make sure balance is spot on, but am temporarily stymied due to a waterpump failure.

Aggie - my car is a 1975 model, so pre emissions for Australia - those commenced July 76, so no, there isn't a throttle return damper on the car.

I'll have to get back after I've done the waterpump fix and double-checked the carb balance.
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post #9 of 13 (permalink) Old 10-25-2015, 04:24 AM
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Maybe stating the obvious but the throttle is closing cleanly from fully open isn't it? As in, the actual butterflies closing? Just asking as static revving uses little throttle movement whereas "load" on the road typically has you opening the throttle more fully.

I've had a couple of these 1.8 GT's and they're pretty simple things but there's always the chance something simple is happening, like a sticking throttle.

Alister
1973 105 GTV (Alfa #6 of 19 owned)

Last edited by Aggie57; 10-25-2015 at 04:30 AM.
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post #10 of 13 (permalink) Old 10-25-2015, 05:29 PM Thread Starter
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Will revisit that, Aggie, thanks for the thought.

Throttle seems to return smoothly to the stop, but I guess that's not the same thing as being a bit slower than it should do so. Will check and lube everything thoroughly. Since the problem was worse when the engine was hot, it does point to a mechanical binding issue as a potential source of the problem, either with the linkages or butterflies/pivots themselves.

I did put a kit thru the carbs myself, so it could all be my own fault, of course...
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post #11 of 13 (permalink) Old 10-25-2015, 11:44 PM
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Some useful info here if you need..

http://www.lainefamily.com/images/WeberTuningManual.pdf

Past 1990 164 V6 in NZ
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post #12 of 13 (permalink) Old 10-26-2015, 02:56 PM Thread Starter
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Always nice to have access to that sort of material - thank you.
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post #13 of 13 (permalink) Old 12-14-2015, 01:59 PM Thread Starter
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OK - thanks to all for suggestions.
Balance was indeed part of the problem - a vacuum gauge showed that much. I am going to get me one of those for Christmas.
A very slight binding of the steel accelerator linkage shaft in the last part of its closing action was the other, obviously becoming worse when the shaft expanded when things got hot under the bonnet/hood.
During the discovery process I also determined that I was not getting full throttle. Fixing that makes a nice difference...

Thank you for your suggestion.
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