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Hey folks, need some additional advice here. It seems I got ignition sorted, yet still having problems with cyl #4. When fully closing the idle mixture screw for that barrel, I get no change in RPM (basically the same original issue). Checked spark plug, swapped spark plugs, checked lead from dizzy and everything seems to be flowers and spring from an electrical standpoint.

Had the weber off again to check out the cold start circuit. Someone apparently had caused trauma to one of the cold start pistons and it was stuck partially up. Remedied that and ended up just blanking off the whole system with a kit to try to stay sane.

No real change. I've made sure fuel can successfully feed in through both the progression holes and the idle mixture hole, and after running, both areas do have fuel in them. Idle jets are cleaned out and have been swapped before to no effect. Butterflies at idle are just uncovering part of the first progression hole. I can't find anything that could be keeping fuel out of the cylinder or leaking it in from elsewhere, nor can i find any reason for ignition not to occur.

Odd symptoms:
1. When covering other carb barrels to starve that cylinder of air, the engine loses RPMs precipitously as expected. When covering barrel #4, it starts gaining RPM very quickly.
2. Spark plugs for other cylinders were someone white-ish after idling for several minutes. The plug for that cylinder was light brown and uneventful.
3. Barrel #4 still pulls at about 80% vacuum compared to the other 3 (slightly low). Checked mounts, checked brake booster attachment, but everything seem to be in order.

I feel like symptom #1 is the key somehow. Do I potentially have a cam problem? What other things can I be checking here? Thanks.
 

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It might be drawing air through the brake booster. You have changed the hose but maybe there is a problem with the booster itself, or the plastic hose connector or the check valve at the manifold.
 

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+1 for a brake servo (booster) problem.
I had a similar issue about 18 months ago with number 4 cylinder. I'd discounted the booster as there was no burning of brake fluid through the exhaust.
Found out that the leak in the booster wasn't very big, and there was no noticeable smoke at cold start-up, or any at all when the engine was warm
However, after everything else was checked, there WAS enough leakage to affect the performance of number 4 sparkplug, in that I could never get a good colour on that plug, and adjustment of the airscrew made almost zero difference.
When I disconnected the servo hose, there was just a hint of dampness on the inside, where vacuum from the inlet manifold had sucked it through.
Changed booster, issue disappeared, air mixture screw worked perfectly again.
 

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Thanks for the replies, y'all. I checked the booster hose and there is some slight residue inside it, nothing dripping or anything, but it's there. Technically if I temporarily block the check valve at the intake, I should be able to isolate the issue to brake booster system overall, right?

Does the engine RPMs rising at idle when partially blocking barrel #4 just mean that that cylinder is running extremely lean due to air entering from elsewhere?

Also, any tips on checking out the brake booster itself? I've never removed it or messed with the braking system. Is it possible to service it without replacing it?
 

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Re-tested with the booster hose blocked - engine running quite a bit better and the mixture screw on #4 is now responsive. Closing mixture completely on #4 only brings the RPMs to 800-900 though so I'm wondering if I in fact do have a little air leaking in elsewhere on the manifold, so I'll check for that tonight.

Any recommendations on the booster? I see new ones on classicalfa for about $250, and rebuild parts for about $100 but they appear to be for Benditalias. Or should I just pull the unit and inspect and go from there?
 

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There is a thread about some new aftermarket boosters from Vick Auto giving problems. I would take my chance on a likely good used one. are you sure that the leak is not between the booster and the plastic connector for the hose?
 

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I just finished reading the Vick Auto booster saga. I will avoid that if I can!

I am not sure where the leak originates - the plastic connector on the booster seems finicky to begin with although it doesn't have any cracks. I saw in some other brake servo threads a statement to the effect of brake fluid leaking into the booster, and it makes me wonder if that could be involved given the fluid residue inside the hose. I'll try to isolate further.
 

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It's likely the inner seals. You have an issue that is a copy of mine.
I got a new booster from Classic Alfa and the problem went away. Their p/no BR100R, (now £205)
I sold the old Benditalia booster on Ebay for £75 , as needing a rebuild. That compensated a little for the expense. It has now done 3000 miles + and has been perfect.
 

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GOOD information here!
 

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Hello everyone is a long time that I do not step through here. I use google translate because unfortunately I do not know English well so forgive me if I do not write correctly. Thank you
 

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Giacomo2000 can you specify if all mixture screws have no effect or just one?

Judging by the vacuum readings (i've never used that specific syncronizer) I'd say you at least are out of sync, and at most dealing with some air leaking in as each barrel looks pretty different. Can you start by removing the progression hole covers and checking with a flashlight to see if the butterflies open at roughly the same time?
 

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practically you definitely the cylinder 2 I can not get it down and the cylinder 4 I can not get it up. The butterflies are properly aligned, I'm sure of this.
 

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If the butterflies are aligned and your vacuum reading is very off, that leads me to believe you have an air leak on the mounts or the intake manifold. For Cyl 4 always check the brake booster one-way valve and hose for dry rotting, and to make sure the booster is not leaking air you can remove the hose from the booster and plug it with something (something that won't get sucked in, obviously).

Does anyone know how accurate the meter being used in the photo is? I've only used the one that measures air flow and is placed on the mouth of each carb barrel...

Giacomo2000, was this vehicle running properly before you tried to tune it? Or are you working on getting it running and ran into this.
 
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