Uneven running in off-idle position - Alfa Romeo Bulletin Board & Forums
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post #1 of 15 (permalink) Old 04-14-2019, 10:22 AM Thread Starter
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Uneven running in off-idle position

1972 2000 GTV with Centreline 11.1 mm cam and Weber 40 DCOE. Rebuilt head. Experiencing uneven running (slight surge) when running with accel pedal just off idle (f. ex. in 5th on level road below 3000 rpm). Same issue as with previous head. When pushing pedal in for accelerating or slight uphill, the engine runs smoothly. Adjusting mixture screws changes rpm at which uneven running occurs but does not eliminate it. Ignition and carb matching OK and progression holes clean. May be running wrong jets? Could someone advise on ideal jetting? With thanks.
Alain
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post #2 of 15 (permalink) Old 04-14-2019, 11:28 AM
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What is the full model number of your Webers ie. 40DCOE32, 40DCOE116/117, etc ?

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #3 of 15 (permalink) Old 04-14-2019, 12:53 PM Thread Starter
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On the cover, it says:

TIPO 40 DCOE 151
No 242 02
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post #4 of 15 (permalink) Old 04-14-2019, 03:40 PM
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The "normal" idle jets are 50F8. You could try 50F9 if you think it has a lean hesitation. A wideband AFR meter such as Zeitronix ZT3 would enable you to see more of what is going on.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #5 of 15 (permalink) Old 04-19-2019, 05:22 AM Thread Starter
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Thank you for your comments and suggestions. I have now re-adjusted idle mixture using Gunsen Colourtune tool (very handy) and air flow with Synchrometer. I am confident both are properly adjusted. Uneven running has improved but is still noticeable (now at around 2500 rpm instead of 3000). I will investigate the idle jet you are suggesting. I am also wondering if the high lift cam has any bearing on this. Will try to get an answer from Centreline. Cheers. Alain
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post #6 of 15 (permalink) Old 04-19-2019, 07:48 AM
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Which idle jets are you using?
Bigger cams can affect idle but I would not expect them to be giving trouble at 2500-3000 rpm.

Ed Prytherch
79 Spider
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76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #7 of 15 (permalink) Old 04-19-2019, 08:04 AM
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You are pointing him in the right direction Ed. It's not the cams. More possibly the idle circuit on the Spanish 151's. Check to see if they are 3 prog. hole verson.


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post #8 of 15 (permalink) Old 04-19-2019, 09:08 AM
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Would a 55F8 work as well? Just wondering.

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post #9 of 15 (permalink) Old 04-19-2019, 09:28 AM
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Quote:
Would a 55F8 work as well? Just wondering.
Sure, but would they work best?
If you consider an idle jet to be a miniature emulsion tube then the fuel orifice corresponds to the main jet and the air orifice(s) correspond to the air corrector. Both main jets and air correctors influence the AFR throughout the range at WOT but main jets have more influence at low - medium rpm and air correctors have more influence at high rpm. So going back to idle jets, if the leanness is more of a problem at low rpm then going to a bigger fuel orifice is likely to be the better move but if it is at the high end of the progression range then a different air orifice is likely to be the better move.
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Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #10 of 15 (permalink) Old 04-19-2019, 09:58 AM Thread Starter
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The car has 50F8 idle jets and 3 progression holes. Starts well but takes a while for idle to settle, until temp is up to normal. Accelerates very well with no stumble and plugs have the correct color. Still trying to get hold of Centreline.
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post #11 of 15 (permalink) Old 04-19-2019, 10:10 AM
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So if I understand correctly a stumble off idle lets say to 2000 rpm, a larger fuel orifice is the way to go, but in this case the stumble occurs first at 3000 rpm and now 2500 rpm an F9 orifice will have more of an effect due to a smaller air orifice which then richens the mixture in the higher rpm range? Thanks in advance, just trying to make sense and have a better understanding of the black art of Weber tuning.

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post #12 of 15 (permalink) Old 04-25-2019, 06:16 AM
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Have you looked to see what the distributor advance is doing?

Checked timing at the "Massimo" timing mark while revved above 3000 RPM?

Observed the stability of the ignition timing at idle?

And if they're Spanish Webers,make sure the aux venturies aren't loose in the carbs.

I have Centerline's 11.1 mm intake cam in a 1978 Sport sedan and it idles beautifully with Spica.

Also, spray around the carbs and their mountings wit carb cleaner to search for vacuum leaks. Not uncommon to find leaks between carbs and manifold/adapters.
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post #13 of 15 (permalink) Old 04-25-2019, 06:28 AM Thread Starter
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Thank you Slowcreek and others for your input. Since the last post, I have revised the mixture settings and gone richer on all 4 compared to the "ideal" (blue flame) setting given by the Gunsen Colourtune tool. Great improvement! Same situation is described by a Lotus Elan owner with similar results. Now, I will keep an eye on spark plug color, check the venturis and check timing, as you suggest (electronic timing). Will report back. Cheers. Alain
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post #14 of 15 (permalink) Old 04-25-2019, 06:43 AM
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What this thread illustrates is the flexibility of the DCOE system. It also shows it is critical to have both the ignition, and idle circuit set up correctly for transition match, as well as the effect both air leaks and idle mix can have on transition and a good consistent A/F to avoid surging (uneven running). The posters to this thread and the originator have the black-art of fine Weber tuning well covered. It also saves me a lot of effort explaining this procedure!


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post #15 of 15 (permalink) Old 04-25-2019, 06:51 AM
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Quote:
Originally Posted by Scorpion77 View Post
Thank you Slowcreek and others for your input. Since the last post, I have revised the mixture settings and gone richer on all 4 compared to the "ideal" (blue flame) setting given by the Gunsen Colourtune tool. Great improvement! Same situation is described by a Lotus Elan owner with similar results. Now, I will keep an eye on spark plug color, check the venturis and check timing, as you suggest (electronic timing). Will report back. Cheers. Alain
Always worth checking the simple stuff.

You MAY have to go bigger on your idle jets based on the need to open the mixture screws......

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