Weber DCOE Tune after restoration - Alfa Romeo Bulletin Board & Forums
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post #1 of 37 (permalink) Old 11-08-2018, 02:52 AM Thread Starter
eas
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Weber DCOE Tune after restoration

Hey Guys,

I have dismantled the carbs for a rebuild but having a hard time trying to get the engine to idle smoothly. The carbs are Weber DCOE 72/73 on a 2ltr engine.

Some history.
Car was working fine before dismantling. The only significant hardware I changed was the needle valve (no 83 in the image below) which was replaced with a new one of the same size (1.5) and the washer (no 82) behind it as for some reason there wasn't a washer before. I have re-calibrated the float to 7.5mm. Other insignificant hardware were the bearings, rubber washers, gaskets etc.

I have calibrated both carbs by shining a light into the trumpet hole and checking the butterfly in relation to the progression holes. These carbs have 4 progression holes and the butterfly was sitting right on top of the 4th hole.

Carbs were installed on new rubber mounts and gaskets, and also used a thin layer of RTV on all faces to get a 100% seal.


I started off by turning the Idle speed screw IN 1/2 a turn and mixture screw OUT 3/4 turn.

The car did not want to idle and had to turn the idle speed screw IN 2 1/2 turns to get it idling just around 1000rpm but the engine runs very very rough.

Turning the mixture screws OUT 2 turns from home did not make a difference. 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 have read this thread and some suggested that the mixture screw needs more turns OUT. I haven't tried this yet. Any other suggestions please?

https://www.alfabb.com/bb/forums/spi...ber-setup.html
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1971 - 2000 GT Veloce Euro

Last edited by eas; 11-08-2018 at 02:57 AM.
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post #2 of 37 (permalink) Old 11-08-2018, 02:56 AM
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tune is one thing.
there is also the possibility there is a error in the rebuild.
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post #3 of 37 (permalink) Old 11-08-2018, 03:03 AM Thread Starter
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Just to add on this

The engine was also rebuild with Motronic pistons and 2 thou skimmed from the head. Ignition timing was set on roughly 1000rpm.

1971 - 2000 GT Veloce Euro
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post #4 of 37 (permalink) Old 11-08-2018, 04:55 AM
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The DCOE 72/73 are emissions carbs so you may well need more than two turns out on mixture screws. My limited experience with the emissions carbs says opening the mixture screws four to five turns is within the norm.

Regarding the high idle, an air leak comes to mind. Do you have the proper carburetor support in place? A sticky advance mechanism in the distributor could delay a return to idle.

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post #5 of 37 (permalink) Old 11-08-2018, 05:07 AM
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Maybe not related and checked already but i had the same issue of random increase rpm after i did a rebuild of my DCOE 106/107, mine turned out to be throttle linkage/cable issues (sticky and catching). When in the engine bay working throttle by hand i had no issues but using peddle different story. If yours is LHD then hopefully linkage & cable will be OK, my 1600 spider had a poor RHD conversion hence the issues.
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post #6 of 37 (permalink) Old 11-08-2018, 11:51 AM Thread Starter
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Quote:
Originally Posted by aflp View Post
tune is one thing.
there is also the possibility there is a error in the rebuild.
It could be a rebuild error what is causing the increase in RPM. Perhaps being intermittent, its a little harder to identify.


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Originally Posted by 65Sprint View Post
The DCOE 72/73 are emissions carbs so you may well need more than two turns out on mixture screws. My limited experience with the emissions carbs says opening the mixture screws four to five turns is within the norm.

Regarding the high idle, an air leak comes to mind. Do you have the proper carburetor support in place? A sticky advance mechanism in the distributor could delay a return to idle.
My plan is to try with more turns on the next try. I am not sure what are emission carbs are but the carbs are original to the car and it was sold for the Italian market.

I really doubt I have an air leak as this is very intermittent and not constant. I do have a support bracket.

Can an electronic Pertronix Ignition system inside the distributor stick in advance too ?


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Originally Posted by thorntid View Post
Maybe not related and checked already but i had the same issue of random increase rpm after i did a rebuild of my DCOE 106/107, mine turned out to be throttle linkage/cable issues (sticky and catching). When in the engine bay working throttle by hand i had no issues but using peddle different story. If yours is LHD then hopefully linkage & cable will be OK, my 1600 spider had a poor RHD conversion hence the issues.
When the RPM's went up in front of my eyes, I immediately checked the linkages and all were home.

1971 - 2000 GT Veloce Euro
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post #7 of 37 (permalink) Old 11-08-2018, 12:59 PM Thread Starter
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I have found this post which mentions forward and rear needles.

https://www.alfabb.com/bb/forums/car...ml#post5734258

I am adjusting the two marked in red which I assume are the front. Where are the rear needles ?
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Last edited by eas; 11-08-2018 at 01:05 PM.
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post #8 of 37 (permalink) Old 11-08-2018, 01:56 PM
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If the mixture screws have little or no effect it is likely because you have advanced the throttle plate past the first progression hole.. There are three different types of Weber needle valves and they should not be changed to the non original type. Gordon Raymond knows the details. The correct needles are usually about 4 turns out from fully closed.
Your symptoms could be retarded ignition so check that. Also be aware that your emissions carbs require type 2 idle jets, say 55F17, 50F21, etc. F8, F9, F11 will not work properly in these carbs. Also the emulsion tubes should be F34 or F41. F9, F16 etc will not work properly.

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post #9 of 37 (permalink) Old 11-08-2018, 02:21 PM Thread Starter
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Quote:
Originally Posted by alfaparticle View Post
If the mixture screws have little or no effect it is likely because you have advanced the throttle plate past the first progression hole.. There are three different types of Weber needle valves and they should not be changed to the non original type. Gordon Raymond knows the details. The correct needles are usually about 4 turns out from fully closed.
Your symptoms could be retarded ignition so check that. Also be aware that your emissions carbs require type 2 idle jets, say 55F17, 50F21, etc. F8, F9, F11 will not work properly in these carbs. Also the emulsion tubes should be F34 or F41. F9, F16 etc will not work properly.
Hi Ed,

I had to increase the idle screw a lot to get the engine rpm up to around 900/1000rpm which most probably placed the throttle plate past the first progression hole. With the idle screw not in contact with the arm, the throttle plates are exactly covering the top (closest to engine) progression hole. Is this OK ? As the slightest adjustment, will start opening up the first progression hole.

The new needle valves were supplied with the rebuild kit I got and since I saw them stamped with the same number as the old ones, I change them to the new ones. I still have them which I can swap. Also note that I did not have the washers before as I described in my first post. Is this OK ?

I have adjusted ignition the best I could with the rough idling at around 1000rpm. I will recheck the lighting gun is triggering on the F mark on the pulley.

All other jets, tubes etc. are all original and used the ones fitted when engine was running fine.

1971 - 2000 GT Veloce Euro
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post #10 of 37 (permalink) Old 11-08-2018, 03:35 PM
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Timing

Quote:
I have adjusted ignition the best I could with the rough idling at around 1000rpm. I will recheck the lighting gun is triggering on the F mark on the pulley.
Forget setting the timing at idle. Set it at the max advance mark "M" at 4000 RPM. Let the idle advance fall where it may.
With that done you should be able to set a smooth idle IF you have the carbs correctly syncronized.

Do you have the factory air box or an aftermarket plate tying the two carbs together with a support rod to the motor mount? That is absolutely necessary to maintain syncronization.

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Last edited by Alfar7; 11-09-2018 at 06:37 AM.
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post #11 of 37 (permalink) Old 11-08-2018, 04:47 PM
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Quote:
Originally Posted by eas View Post
I have adjusted ignition the best I could with the rough idling at around 1000rpm. I will recheck the lighting gun is triggering on the F mark on the pulley.
This is EXACTLY what should be done! Adjust the idle timing to the F mark.
The carbs need to be sorted first without the possibility of adding another variable, sticking advance weights or other distributor problems, into the mix.

Jim

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post #12 of 37 (permalink) Old 11-08-2018, 05:29 PM
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Are the front/rear carbs throttle shafts in synch?

Is the ignition in order

Cam timing?
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post #13 of 37 (permalink) Old 11-09-2018, 12:06 AM Thread Starter
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Quote:
Originally Posted by Alfar7 View Post
Forget setting the timing at idle. Set it at the max advance mark "M" at 4000 RPM. Let the idle advance fall where it may.
With that done you should be able to set a smooth idle IF you have the carbs correctly syncronized.

Do you have the factory air box or an aftermarket plate tying the two card together with a support rod to the motor mount? That is absolutely necessary to maintain syncronization.
The engine is set up with trumpets and it does have a metal plate holding both carbs together supported with motor mounts. I have removed the trumpets until i get the carbs in sync.

Having the engine working very rough at idle, it is very hard to get a good reading with the Synchrometer,

I will get the timing on the "M" @ 4000 rpm this weekend.

1971 - 2000 GT Veloce Euro
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post #14 of 37 (permalink) Old 11-09-2018, 12:06 PM
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Ok, SINCE this is now WAY off topic, here is MY OPINION. Now remember, opinions are like some other body parts in that everyone has one. This MUST be understood. The purpose of the BB is shared knowledge. This may be from experience, factory training, learning shortcuts, rumor, innuendo and outright un-truths. It is gathered and filtered by actual application of this "knowledge" by curious, technically oriented individuals to develop their O W N opinion. Sometimes MANY of these are either workable and correct to some degree, and will be used to develop DIFFERENT opinions. This is OK!
While some may be wrong or unworkable in a specific incidence other opinions may have better application.
Here is an example. Above, the non-working emission Webers have had a "minor" part swap, specifically BEARINGS. well and good IF the builder is adept at setting butterfly plates PERFECTLY, but not so good if plates, or throttle plate bore seals are WORN as is often the case with used or abused Webers.
Does ANYONE know if the Webers themselves are rebuilt correctly? I sure don't! So WHY are we talking about adding in another variable, IGNITION TIMING? While there lets do valve clearances, and timing and check compression and leak down. Lets check coil saturation and secondary wiring, and PLUGS!
JUST STOP! If it doesn't run, and the only variable addressed is the WEBERS, and it ran fine before the Weber work, the issue is probably..... thewebers!
If you want to start with an ignition BASELINE you start with factory static advance. That is NOT the way it will be set up when the Weber issue is resolved, it will be at AM as the other OPINION. If you have no idea how it is set, with bum Webers why mess with AM until the Weber issue is resolved?
Now to > MY < OPINION. There is no "my-way-or-no-way" here. There are M A N Y solutions, and we ALL know (or should know) that, and these are based on O P I N I O N S gathered from various individuals EXPERIENCES in different situations. Who is WRONG or RIGHT has little to do with the SOLUTION. In this case, we are not going to fix the Webers by shotgunning diagnostics at the whole engine!
Now, as usual, this is just-my-opinion from my OWN experiences, and everyone else here SHOULD have their own opinion, and with this shared BB knowledge, we can ALL learn something other than telling each other who is "wrong". I remain a master at being confused and incorrect, BUT this is still just MY opinion.


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post #15 of 37 (permalink) Old 11-09-2018, 12:07 PM
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BTW, LETS GET BACK ON TOPIC now. Thank you for your opinions!


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