Weber DCOE Tune after restoration - Page 2 - Alfa Romeo Bulletin Board & Forums
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post #16 of 37 (permalink) Old 11-09-2018, 12:39 PM
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Originally Posted by eas View Post
The funny thing is that at one point the engine revved to around 3000rpm without even touching the throttle. I waiting for around 10 seconds and it did not go down. Switching off the engine and back on solved the over revving.

Driving the car at partial throttle looks good except for another instance where that the RPM climbed up again and had to switch the engine off.

Why does it decide to rev up on its own ?
I don't know anything about "emission" webers, so this advice relates to "normal" DCOES

To me these symptoms are common if the two carbs are not properly synchronised. If this is so, the symtoms you describe will occur as soon as the engine is warm.

Unclip the accelerator linkage and check that the four butterflies of both carbs open at the same time and at the same rate and (importantly) close at the same time. Small adjustments may be required. Set the idle mixture screws to 2-2.5 turns out as a base (advice above suggests that with emision carbs this my be up to 5 turns).

If you have a stethescope start the engine and get the engine to idle and listen to the sucking sound in each trumpet. If he sound is stronger in the two trumpets of one carb than the other carb, adjust the linkage syncronisation screw until the two carbs make a similar sound. After each adjustment blip the throttle and let it retun to idle. Adjust the idle screw from time to time as required.

Once the two carbs are more or less sounding the same, adjust the 4 mixture screws to get all four to sound the same, adjusting the idle screw as required.

Finally reattach the accelerator linkage.

Just my 5c worth ...

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post #17 of 37 (permalink) Old 11-09-2018, 01:46 PM
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I think Mike offers a valid diagnostic here but I've not used that method before. Have you used this method before, Ed?

Jim

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Last edited by papajam; 11-10-2018 at 04:19 AM. Reason: thread continuity
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post #18 of 37 (permalink) Old 11-09-2018, 04:25 PM
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The funny thing is that at one point the engine revved to around 3000rpm without even touching the throttle. I waiting for around 10 seconds and it did not go down. Switching off the engine and back on solved the over revving.
Driving the car at partial throttle looks good except for another instance where that the RPM climbed up again and had to switch the engine off.
Sorry to add a variable but I experienced this behavior once and it was a sticky throttle linkage. I ended up replacing a bushing in the end of the linkage to the webers. It may not be this at all if it stays high with the linkage disconnected and working the throttle from the engine bay. I like Gordon's advice of verifying the butterflies and smoothness of the shafts that holds them.

I'd also make sure the chokes are off.

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post #19 of 37 (permalink) Old 11-10-2018, 01:13 AM
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I think Mike offers a valid diagnostic here but I've not used that method before. Have you used this method before, Ed?
A method common in the SU carb world, absolutely NOT precise enough for Webers.

I agree with Ed, RJ but as well with Gordon.

Main issue in my opinion is the fact the TO needs help to make a proper diagnosis. Means not help from ARBB, but from someone who can check the car.

Maybe it's just a synchro issue, maybe the 'rebuild' went wrong.

There is a point where an individual should realize the need of external support to the car. Not virtual from the web.

Just my 2c of resolving unforced errors created by PO's or forum junkies.

Btw: My experience is that on 85% of all carburetted 105/115's ignition timing setup is wrongly adjusted. And RJ is fully right to concentrate on the M mark.

I've experienced in many cases that folks turned the dizzy by error when fiddling back the carbs to the m/fold. From a pro mechanic point of view it's stupid not to check ignition timing 'just because engine ran well before dismantling'.

Don't limit your diagnosis horizont but exclude unlogical check points first.

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Last edited by hunttheshunt; 11-10-2018 at 01:26 AM.
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post #20 of 37 (permalink) Old 11-10-2018, 05:31 AM
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I bought my first GTV from a doctor in Columbia who took it to a mechanic in Charlotte NC. It had Webers and it ran well. The Doc told me that his mechanic used the stethoscope method so I tried it. It did not work for me as I could hear little difference on a pair that were out of synch. For many years I grounded out a spark plug and noted the rpm drop and now I observe the throttle plate position wrt the first progression hole. I would help the linkage with a finger if I suspected that the plates were sticking.

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post #21 of 37 (permalink) Old 11-10-2018, 07:39 AM Thread Starter
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I appreciate all your opinions, suggestions and help with this issue.

Update:

I tired increasing the turns of the the mixture screw and I did see an improved idle and was able to reduce the idle speed screw to around 1.5 turn. having the mixture screw 5 to 6 turns OUT from home where about the best idling. During this time I did not encounter any erratic high idle which was very positive.

I tried these settings both with timing set at 4000rpm on the M mark and 1000rpm on the F mark. Have not seen any major difference. What I have noticed is it is very hard to get it perfect on the F mark when idle rpm is fluctuating. A slightest change in rpm can throw the timing by a few degrees. Just as a side note, I run on 98Octane fuel, as the only other alternative is low in octane.


Whatever I tried, I did not manage to get a smooth idle below 1000rpm. It was raining today and could not drive her in the wet and therefore I still need to try these settings for day to day normal driving.

1971 - 2000 GT Veloce Euro
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post #22 of 37 (permalink) Old 11-10-2018, 08:39 AM
At 5-6 turns were you getting an acceptable idle (and otherwise good off idle performance)? If so then I think your work is done. That said, 5-6 out does seem excessive even for emission Webers. I would be thinking about a vacuum leak. I would try spraying starter fluid or propane (unlit of course!!) around the mounts and seeing if it changes the idle speed. If so, you likely have a leak.

You might also invest in an AFR meter- very handy tool...

You asked about the advanced weights sticking in the Pertronix dizzy and the answer is yes, since the electronic module only replaces the points and condenser the advance weight mechanism is the same as original dizzy. That is not likely your problem but might be worth checking...
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post #23 of 37 (permalink) Old 11-10-2018, 10:09 AM Thread Starter
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At 5-6 turns were you getting an acceptable idle (and otherwise good off idle performance)? If so then I think your work is done. That said, 5-6 out does seem excessive even for emission Webers. I would be thinking about a vacuum leak. I would try spraying starter fluid or propane (unlit of course!!) around the mounts and seeing if it changes the idle speed. If so, you likely have a leak.

You might also invest in an AFR meter- very handy tool...

You asked about the advanced weights sticking in the Pertronix dizzy and the answer is yes, since the electronic module only replaces the points and condenser the advance weight mechanism is the same as original dizzy. That is not likely your problem but might be worth checking...

I will check for any leaks as suggested.

I do not have a handy AFR meter handy unfortunately. I do think the car is idling slightly rich but I changed the settings so many times that I have flooded the garage with rich fumes which might have effected my head

Thanks about the explanation on the dizzy.

1971 - 2000 GT Veloce Euro
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post #24 of 37 (permalink) Old 11-10-2018, 10:17 AM Thread Starter
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My car runs on Bosch distributor. I did clean the mechanical parts inside the distributor and sprayed some lubricant before placing back the metal plate, electronic trigger and magnet.

1971 - 2000 GT Veloce Euro

Last edited by papajam; 11-10-2018 at 01:16 PM. Reason: delete quote
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post #25 of 37 (permalink) Old 11-10-2018, 10:40 AM
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EAS, glad to hear some progress has been made. There are several differences between "classic" and "emissions" Webers. One of which is the shape of the idle mixture adjustment screws and the thread pitch. On the emissions carbs, the needle has more taper and the thread pitch is finer, at least this is the case on the set of 72/73s I am familiar with. This is what I think Ed was referring to many posts ago. The net affect is they normally need to be opened more turns than "classic" Webers to achieve adequate idle mixture strength.

In your initial post, you mentioned the engine revving to 3000 rpm on its own. IMO, that is a different issue than a delay in returning to a normal idle and is indicative of a vacuum or air leak. Since you had the head off and presumably removed the intake manifold for the head skim cut, it is possible (not probable) that the leak is between the head and manifold. This happened to me many years ago and I don't recall details but replacing the faulty gasket solved the problem. The point is, keep a broad perspective when you check for leaks.
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post #26 of 37 (permalink) Old 11-12-2018, 07:44 AM Thread Starter
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I have been going through the manual to make sure I didn't leave anything out. Port 1 and 4 of the inlet manifold have vacuum feed. See attached marked in yellow

The one on the left is the brake booster supply but I forgot for what the right one is supplying to. Crankcase cam cover vacuum ?? (My distributor does not operate on vacuum)

I could well have a leak from these. Will check tonight.
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post #27 of 37 (permalink) Old 11-12-2018, 08:03 AM
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The fitting on the right has a hose that connects to the cam cover.
On the right front of the cam cover should be a fitting (two 6mm bolts) that has provisions for a large hose and a small hose. The large fitting has a hose goes to the top center of the air cleaner assembly. The small fitting gets a hose that connects to the front 'yellow' fitting on the cylinder #1 intake runner.

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post #28 of 37 (permalink) Old 11-12-2018, 09:12 AM Thread Starter
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Quote:
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The fitting on the right has a hose that connects to the cam cover.
On the right front of the cam cover should be a fitting (two 6mm bolts) that has provisions for a large hose and a small hose. The large fitting has a hose goes to the top center of the air cleaner assembly. The small fitting gets a hose that connects to the front 'yellow' fitting on the cylinder #1 intake runner.
Yes that's right. Thanks.

I will plug/shut them both and see if anything changes. I also have a can of carb cleaner I will use to check for vacuum leaks.

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post #29 of 37 (permalink) Old 11-12-2018, 09:28 AM
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I'll confess that the fittings our GTV are plugged. It idles a bit smoother without an 'induced vacuum leak'.

Jim

Series 2 USA 1750 GTV (in Series 1 European clothing)
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post #30 of 37 (permalink) Old 11-12-2018, 09:45 AM Thread Starter
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I'll confess that the fittings our GTV are plugged. It idles a bit smoother without an 'induced vacuum leak'.

Slightly off topic, what is the original colour of the small thin hose ? Mine is brown and I am assuming it got replaced with a transparent one and due to the oil it sucked throughout its life it turned to brown.

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