small hose on cam cover - Page 2 - Alfa Romeo Bulletin Board & Forums
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post #16 of 54 (permalink) Old 07-22-2018, 04:30 AM
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Originally Posted by malcolm1 View Post
My understanding is that vacuum advance wasn't fitted to any of the GT cars. The link below for a replacement distributor suggests that vacuum advance was only fitted to later engines (after 1984), so that would be on the later Spider models. https://classicalfa.com/categories.p...Coupe-Ignition

Is your engine a later one from a Spider? and converted to carbs? or perhaps someone has fitted a distributor from a later engine?
2 liter donor engine from a 1975 GTV but HEAVILY modified (high lift camshafts, high compression pistons, weber dcoe 45, etc...). The distributor is a bosch with vacuum advance cannister and if memory serves, it wasn't connected to any hose when I acquired it a couple of years ago, nor was the small nozzle on the cam cover (plugged as per 1st photo).
I had been struggling to get proper ignition timing and decent power delivery above 3000 rpm for weeks since the recent rebuild until my mechanic connected the hose from the vacuum canister directly to the intake on the engine block (where the small return hose from the cam cover should normally be fitted) as the weber dcoe carb doesn't have a nozzle for connecting to the vacuum line (like the dellorto on my spider).
I have no doubt that this is wrong and that the 'science' of it is contrary to basic logic since the hose from the engine is most likely feeding pressure instead of creating vacuum at the distributor, and that as such it should be creating more RETARD at high rpm... But in practical terms, it is doing the exact opposite, it is making more ADVANCE at higher rpm and the power delivery is phenomenal in the 3000 to 7000 rpm band... May be the point of this nozzle on the block is to generate vacuum at higher rpm after all, to suck-in the oil vapor from the cam cover???
Happy as I am about reclaiming the escaped ponies, I don't want another engine melt-down on my hand... What do you think?

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Last edited by sbadaro; 07-22-2018 at 05:28 AM.
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post #17 of 54 (permalink) Old 07-22-2018, 05:32 AM
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In my opinion you would get a vacuum pulse on #1 intake stroke. So less than a 1/4 of the time you would have vacuum at your distributor and only at idle and very light throttle and at deceleration and pulsing all the time.Put a vacuum guage on the vacuum supply and see what you get.
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post #18 of 54 (permalink) Old 07-22-2018, 06:22 AM
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In my opinion you would get a vacuum pulse on #1 intake stroke. So less than a 1/4 of the time you would have vacuum at your distributor and only at idle and very light throttle and at deceleration and pulsing all the time.Put a vacuum guage on the vacuum supply and see what you get.
Good advice. Will post shortly.

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post #19 of 54 (permalink) Old 07-22-2018, 01:49 PM
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I would suggest opening up a new thread asking for advice for the knowledgeable experts on the best timing set-up for a modified engine (static timing and speed advance curve)

I guess you want a set-up that is safe but gives the best performance. One of the programmable electronic set-ups (like 123) may be the best option as it gives you a lot of freedom in terms of speed advance.

123ignition

My understanding of vacuum advance is that it gives more advanced timings at part load for driveability and fuel economy. I'm not sure that that the tiny hole in the fitting on the manifold gives a decent signal for your vacuum advance to work on.

I also wonder what the set-up of your current distributor is - do you have a reference number to ID its origin.

1975 GT Junior with 2 litre lump
2006 Subaru Impreza WRX

Last edited by malcolm1; 07-22-2018 at 03:44 PM.
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post #20 of 54 (permalink) Old 07-22-2018, 03:13 PM
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Hi Malcom. It's a bosch 0237 002 018 ignition distributor. I have no clue if side canister operates on both vacuum advance AND retard or for advance only. I lean to thinking that it does both, but I will have to drive the car a bit more with and without it to compare.
The little that I have learned from youtube confirms what you say about vacuum advance affecting driveability, not power, but in real life the car was a totally different beast after connecting the vacuum canister hose to the intake manifold... Go figure...
Anyway, my next homework is to get a hold of one of these and maybe start a new string for more feedback and knowledge base as you suggest.Name:  Screenshot_20180722-194316.jpeg
Views: 272
Size:  91.2 KB

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post #21 of 54 (permalink) Old 07-22-2018, 03:49 PM
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BOSCH Distributor 0 237 002 018 for ALFA ROMEO GTV 2.0

Looks like the distributor is from a later car (Alfetta GTV or similar)

Do you a clearer photo of where the vacuum advance is connected?

1975 GT Junior with 2 litre lump
2006 Subaru Impreza WRX
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post #22 of 54 (permalink) Old 12-30-2018, 06:54 PM
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Greetings all, new to this thread - on my 1750 GTV, that little hose from the T on the cam cover goes to the stub on the front of the inlet manifold, and I think that's the intended place, as I found a photo in the Alfa owners' handbook for the Alfetta GT 1.6 & 2.0 that seems to show that's what they did on those cars.

So, have we got a definite answer as to where the vacuum was supplied from, for those Alfettas etc that did have vac advance distributors? (I'm asking because I'm interested in trying a vac adv dizzy on the GTV, to see the effect on fuel economy.) Was it supplied from this stub on the inlet manifold, via a pulse snubber to even out the vacuum? Or, my GTV has Dellortos, and I read somewhere that these provide a ported vacuum outlet for an advance function - however, I don't know where to find it on my carbs.

Thanks all,
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post #23 of 54 (permalink) Old 01-01-2019, 09:07 AM
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Yes, that small hose goes from the cam cover to the inlet on the front of the manifold. The purpose is to vent the gases into the inlet even when the throttles are closed. If it's plugged the crank case pressure has nowhere to go when the car is at idle. As you press the throttle pedal, the air is drawn through the larger hose into the airbox.

If the small hose is missing or has a leak then the idle will go to pot as air will be drawn into the intake behind the carb and therefore not through the idle circuit. This air will have no fuel mixed in so won't ignite giving you a three cylinder car until the throttle is open wide enough.

Cars with vacuum distributors had a small take off port on the carburettor body just above the distributor.

The stuff about the vacuum pulsing if it's on one port only is a bit of a red herring really, the engine is running fast enough to even any pulses out so they won't cause any problems with the vacuum advance. After all, the brake booster vacuum is taken from no. 4 cylinder only and you don't feel the brake pedal pulsing.


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post #24 of 54 (permalink) Old 01-01-2019, 09:16 AM
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If I've done this right here's a pair of Dellortos intended for vacuum distributors. You can see the brass port at the left.

https://i.ebayimg.com/images/g/6PMAA...muh/s-l300.jpg


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post #25 of 54 (permalink) Old 01-01-2019, 09:50 AM
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This is a timely thread as I'm trying to figure this out on my '74 GTV. It has a 2L engine from later Alfetta with dual Dellortos. There are three small ports:

- The port on the crankcase breather (this was open when I got the car)
- A port on the bottom of the rear Dellorto: intake side, centered on the carb and pointing straight down (also open)
- The stub port on the intake manifold (this is hooked up to the distributor advance)

From what you guys are saying it sounds like this isn't right. But it looks like the port on the Dellortos is ahead of the throttle plates, so unclear if that's actually for a distributor advance?
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post #26 of 54 (permalink) Old 01-01-2019, 01:19 PM
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On my 2L Alfetta engine that small cam cover nipple is is plugged. The vacuum advance tubing form the dizzy runs to the nipple on the engine side of #1 Dellorto carb intake past the butterfly.

Dunno what that cam cover nipple was for. A Euro thing. I suspect it was for idle gas suction form the cam cover like Spica had. I don't think Euros had fuel tank vapor recovery then.

Adder: The small port on the cam cover has slight positive pressure from blowby. It would not actuate a vacuum advanced dizzy bellows AT ALL. I suspect if might have been for Spica variants that had that small tube from US Spica cars running down to the bottom 4x1 on the air filter that provided cam cover suction at idle. Can't see it for vapor via some OVS and certainly not to the gas tank, as scrubbing as that would be in the main line to the air filter housing. Mine on a Euro Alfetta is plugged.
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Last edited by Anfanuts; 01-01-2019 at 04:35 PM. Reason: Adder
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post #27 of 54 (permalink) Old 01-01-2019, 01:43 PM
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The small port on the crankcase breather went to the vapor recovery system in the trunk on US series 2 cars.

Ed Prytherch
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post #28 of 54 (permalink) Old 01-01-2019, 01:53 PM
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Quote:
Originally Posted by Gubi View Post
This is a timely thread as I'm trying to figure this out on my '74 GTV. It has a 2L engine from later Alfetta with dual Dellortos. There are three small ports:

- The port on the crankcase breather (this was open when I got the car)
- A port on the bottom of the rear Dellorto: intake side, centered on the carb and pointing straight down (also open)
- The stub port on the intake manifold (this is hooked up to the distributor advance)

From what you guys are saying it sounds like this isn't right. But it looks like the port on the Dellortos is ahead of the throttle plates, so unclear if that's actually for a distributor advance?
Well you're in luck. I have Dellortos on the GTV with a plugged side nipple/port and a distributor with NO vaccum advance, and Webers with no vacuum ports on the GTJ racer and a distributor with a plugged vacuum advance nipple/port.
What I was told is that you need to connect one hose from the Dellortos to the distributor, and another one from the cam cover to the manifold to achieve proper vacuum advance and ventialtion.
I opted to leave things as is on the junior because I didn't want to drill into the webers, and because the pressure from the cam cover was inconsistant at lower rpm's (it revved more freely for a couple of days when I tried the vacuum advance set-up but I returned it to how I received it from first owner when performance started faltering).

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post #29 of 54 (permalink) Old 01-01-2019, 02:09 PM
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We have to be clear about whether we're talking about fuel injected cars, carburettor cars or fuel injected cars converted to carbs.

1. Carburettor cars work in the way I described above. You won't find a vacuum advance distributor on anything factory built before around the mid 80s. That tube on rhe Dellortos I posted above is angled so it comes out behind the throttle butterfly, it wouldn't be there if it didn't because the whole thing wouldn't work. Trust me on this.

2. Fuel injected cars are entirely different in the way the intake works and usually have some sort of vapour separator and drain back device.

3. Fuel injected cars converted to carbs are usually such a mish mash of parts from many sources that all bets are off!


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post #30 of 54 (permalink) Old 01-01-2019, 02:16 PM
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Peruse this response and photos - LINK. The Bosch JP4 vacuum advance dizzy vacuum connection goes to the engine side of #1 Dellorto carb intake, a small brass nipple coming off that.

PS: You can just make out the black plug on the cam cover nipple.

PSS: it is clear you need either a vacuum or centrifugal or EI module advance. NO dizzy vacuum advance module/tubing means one of the other methods. The cam cover nipple provides NO vacuum.

Last edited by Anfanuts; 01-01-2019 at 02:52 PM. Reason: Add PSS
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