Quattroporte III catch-all thread

Had another go at the carbs today. What a mess....

In our last installment, we saw how the pump nozzles on carb 3 wouldn't squirt, but everything else was ok. When the nozzles were sprayed with carb cleaner and put back, 2 other carbs suddenly started leaking (where they hadn't leaked the day before with the fuel pump on for 10 minutes to test everything).

So I have 3 "issues":

1. intermittent leaking, apparently random and happening to different carbs.
2. am not able to establish an idle, the rpms slow down until the engine dies.
3. tools that will actually allow me to adjust those hard to reach places.

1. gasoline leaks

Came back today, turned on the fuel pumps just to confirm that everything is still ok, and what do I get but a stream of gasoline flowing out of carb 1 (both barrels) and a steady drip from carb 4. :-/

Carb 1 had been a problem the other day too, but carb 4? It had been fine all along! Pulled the carb covers (after trying to tap them to see if the needle valve was stuck), everything looks fine. Nothing is wrong that I can see. I checked float level again, it's still 48mm. Dried the gas off and blew into the fuel inlet pipe, lifted the float, and violá the valve closed just as it should.

I have no idea why it is leaking. First guess would be float level, but they are all set according to spec. Second would be leaking float, but I tested them all before putting everything back together, no leaks. Third would be too high pressure from the regulator, but it's barely 2 years old, and I would expect all carbs would leak, not just two (although perhaps 2 leak and two don't, balancing the excess pressure between them). At any rate I don't have a fuel pressure gauge anymore so can't check that.

Even worse, when I put everything back together, there were *no* leaks! But I didn't "fix" anything, only popped the top off and put it back on again! Ok, I did remove and reseat the emulsion tubes/main jets, but they were fine too. Simply unscrewed and screwed back in.

This concerns me a lot. I don't want to park the car in my space and come back a week later for a drive only to have gas leaking from the carbs! I won't be able to see it when everything's put back together... and it's a real fire-hazard.

Is it possible that the Grose-jets or their gaskets are not performing correctly? I can always pull them and use the overhaul kit needle valves instead. The Grose-jets came with plastic washers, one of which has been shedding fine hairlike particles each time I've checked (carb 1). The needle valves came with copper washers, as far as I can remember.

Since everything seemed kosher again once I did the little carb cover dance, I figured I could at least try and start the engine. First I loosened the linkage locking screws to allow all throttle plates to come to complete closed position. Idle mix screws were all 1.5 turns out from closed as per the manual. Idle speed (throttle stop) screw was backed off so that it doesn't touch the throttle arm at all, again to keep the butterfly plates at fully closed position.

Fired her up, and she started almost immediately! What a sound!

2. car won't idle. How can I tune the carbs when she won't even idle?

Kept my foot on the gas (or hand on the linkage) as I went back and forth between engine bay and driver's seat. After a minute or two I got a nice idle (while still cold) at between 900-1000 rpms. Took a quick reading with the syncrometer and got between 2.5 - 3kg/h over all carbs save no. 1, which was around 5 or 6kg/h! Gave it some gas a few times and let it settle down, then carb 1 was around 3kg/h as well. There was little or no backfiring from the carbs at all, and she sounded pretty smooth overall. Backfiring as I've understood indicates to lean a mixture.

Since the car was still cold (freezing temps today and snow on the ground) I kept my foot on the gas at around 1000 rpm until water and oil temps were both > 50C. When I released the gas, rpms would sink. First to 800, then 600, the a stumble or two before dying. :-(. This happened repeatedly. Thinking that the low rpms might indicate a lack of fuel, I tried giving 1/2 screw more on the idle mixture screws (except for cylinder 6 - carb 2 driver's side barrel - as I didn't have any tool that would reach under carb 1 and over the kick down linkage that's right in front of the mixture screw).

The car started and ran, but worse than before. Lumpy and rough. I had to keep my foot on the gas or it would stall and die within seconds. So I put all idle mix screws back at 1.5 turns, the car started right up and would idle, was much smoother overall, but always progressively slower until the engine died.

Both the breather hose and small vacuum hose that connect to the air box baseplate were disconnected. I noticed that when I covered the end of the vacuum hose, the engine stalled (at least when cold).

The microswitch is in place, but I disconnected it's electrical connection.

So at this point I'm unsure what to do. It doesn't seem that the car will idle with the linkage loosened so that each throttle plate is completely closed, even though all tutorials indicate that it should, and recommend using the idle speed (throttle stop) screw only as a last resort. I tried adding more fuel to the mix, but that made things worse. The only other variable I can think of is adding more air (e.g. opening the throttle plates until the idle is at 900 rpms).

Why would the idle be worse (and slower) when warmed up compared to when cold?

3. tools

I needed 3 different screwdrivers (including a screwdriver bit that I held in my fingers) in order to reach the idle mixture adjusting screws. Thinking it over now, I think the best thing would be one of those flexible bit drivers, with a bendable coil or similar to reach those impossible places.

Also, I put the original "no tamper" caps back on. Maybe at this point it's just as well to remove them? I doubt any concours judge would even be able to see them, with the air cleaner and such blocking all of it anyway... and for anything else, they really don't need to be there. They're really in the way.

I want to get the car out of this rented workshop asap, and had hoped to actually complete the job first. (Obviously at some point I am going to have to buy a house that has a garage so I can work on the car as needed, LOL.)

Any and all tips are welcomed! What now?

 

Hi Tony,

thanks for the quick reply.

I happened to speak with Mike Pierce at Pierce Manifolds as well about the carbs and idling. He had a slightly different take on it than everyone else.

Pierce says that the throttle stop screw (i.e. "idle screw") should be turned 1/4 turn in from contact and left alone. Ideally it should have been set at the factory and cut off so nobody ever tampers with it, he said ;-). It's only function is to prevent the butterflies from slamming into the venturii when going from WOT to idle (i.e. you release the gas pedal while driving fast) and over time making a groove in the venturii or wear down the brass butterfly plates.

He was quite clear that the air bypass screw, which every other manual and web article say is for balancing barrel to barrel airflow, is also for setting idle speed. One turn out from seated as a starting point, which should make everything idle too fast initially, then one brings down the higher flowing barrels to match the lower flowing ones. If the car stalls, open up all the air bypass screws a tad, bringing the total airflow up enough to settle on optimal idle.

So tomorrow I'm going back and will try both methods, his and the one you and most of the other online resources I've seen state.

The linkage was connected, but I had loosened everything first so it didn't interfere with or affect the basic throttle setting of the carbs. It was mainly so I could give gas across all carbs in order to start the car and keep it idling. The linkage was not pulling on the throttle controls of the carbs at all when at rest, only when I pushed on the gas or pulled the linkage manually.

I'm going to see if I can find a flex-screwdriver/bit holder as well tomorrow.

Oh - and I'm still skeptical about the leaking fuel, especially since it seems to be intermittent and "fixes" itself (or by apparently doing nothing aside from removing and refitting the fuel bowl cover)...! Any ideas here?

 

Hi Mike,

you'll find the numbers for the rear bearings in this thread, quite a bit earlier (April/May 2012). Note that they are *not* correct as given in the service manual!

If you don't find the post, let me know and I'll look it up for you and repost it here.

 

Success!

Followed the tips I received mentioned earlier, got the car started and then went from carb to carb tweaking the air bypass to balance everything initially to the *lowest* flowing carb.

The car idled (surprise), but roughly and too slowly. Airflow was between 2.7 - 4kg/hour, uneven between carbs, but pretty even between barrels on the same carb. First I dialed all of them down to just under 3kg/hour, but the idle was really sluggish and lumpy, around 5-600rpms. I opened the bypass valves a quarter turn or so all around and heard the idle smooth out and speed up to between 8-900 rpms, and then dialed in a constant airflow at 4kg/hour across all carb barrels.

This was with the linkage connected, but completely loose so that the carbs' throttle arms weren't affected (unless I hit the gas). I also set the throttle stop screws as indicated, 1/4 turn in from just barely touching.

From what I saw in another article, leaving the butterflies closed to dial in the idle allows the throttle plates the full range of motion from idle to WOT, and the progression holes function as they should, as a transitional system from the idle circuit to main circuit. By using the throttle stop screws to dial in the idle, you effectively set the 0 point (idle point) higher and limit the effective range. So instead of 0-100%, you end up with the starting point at 25% and have a shorter available range up to 100% (WOT). At least in theory. As Tony mentioned, a carb is a controlled leak (wonderful analogy!) - the various jets and nozzles control how much fuel 'leaks' in (idle, progression, main, accelerator) while the air bypass screw together with the throttle plates control how much air 'leaks' in.

Another positive thing to note was that all carbs were dry, both when I started the fuel pump and after I shut the engine off. I hope that lasts. This car has had a carb fire before, I'd rather it didn't happen again. There's no leaking gas outside of the carb, it was the main nozzles that were dripping previously. I'm going to keep a close eye on them. Maybe the new "Grose jets" need a slightly higher float level, say 50mm instead of 48mm, to make sure the bowls don't overflow into the carb barrels.

Thanks again for the tips and help. Depending on when I get the time, I plan on giving the color tune a try in the next few days. The same source indicated that it was of limited use on modern oxygenated gasoline. I want to give it a try all the same, it can't hurt.

 

Failure!

Went back today to do the Colortune routine.

No gas leaks (reassuring), but when I started the car it idled very rough and I had to keep a foot on the pedal to keep it running for a few minutes (the linkage is connected loosely, enough to use the throttle, but not enough to change the base setting on the carb throttle arms). After a while it settled in enough that I could use the flow meter to confirm the settings from a few days ago. Turns out it flows a bit less when cold, around 3.75kg/h compared to 4. I adjusted it up, and later when it was warm it ended up around 4.1kg/h. When warm it idled ok, around 800rpm. Not great, but ok.

After warming the engine up, I killed the motor, removed the spark plug in cylinder 1 and installed the Colortune plug. The car would not idle with the Colortune plug installed. Every time I left the driver's seat (e.g. foot on the gas) to go look, the engine would stall and die. Got my son to help me, and looking at the Colortune, all I can see is a blue spark every cycle - no flame/burn at all.

Replaced the spark plug, and while it idled a little bit better, it was still rough, and still died without a foot on the gas/hand on the linkage.

So I'm at a loss to understand what's going on here. Wednesday it was idling smoothly and steadily at between 8-900rpms after warmup and tweaking the air bypass screws. Today, with nothing changed, it idles horribly and dies if not given extra throttle.

I'm going back tomorrow to try everything again. At very minimum I'm learning from all of this. If I can't get it running acceptably this way, I'll give the other way a try (using the air bypass screws only for barrel to barrel balance, and the throttle stop screws for idle speed setting by opening the throttle plates slightly to expose the progression holes).

One other question I had - for those of you who've done this (Tony, William, CharlesA, maybe Mike and others), what kind of idle should I expect with a cold engine and no choke, in around 0C/32F weather? Should it be lumpy and uneven and need a foot on the gas for a while until warm, or should it settle in and idle smoothly from the beginning? I've disconnected the choke assembly to make it easier to reach the adjustment screws, although I realize that there is a choke mechanism built into these carbs. I've never needed it, even in the cold, 3 pumps on the pedal before starting and it fires right up, although I have had a problem with idling lately in general, especially when the engine is cold. I was hoping to take care of that with the rebuilt and properly tuned carbs, as well as maybe eking a bit better mileage out and of course more power .

Any tips are appreciated!