Carbs pulling 10kg/hr at idle... should be 4kg/hr

hmmm. I set one up literally yesterday evening and it was idling around 5-6kg/hr. Idle ignition timing was set to about 10-11 degrees or so, full timing is 38-39 degrees at 6000. Make sure timing is bang on

 

I should add, I set it up idling around 1000rpm or so.

 

10 Kg/hr airflow in each barrel and 1000 RPM warm idle = you must have a bank that is not contributing IMO!

Even if you have spark on all 8 spark plug wires, those sparks have to occur at the correct time. If you have the "distributor" of a bank set-up with the spark occurring at the end of the exhaust stroke rather than the end of the compression stroke = that bank won't actually be running, and, diabolically, using a timing light, everything will still look "fine". I'd double-check that issue -- JMO.

Is this a euro carb 308 with a single distributor and the double-ended distributor rotor? If so, the proper mounting order of the spark plug wires going around the distributor cap is not the firing order (and if the spark plug wires are mounted on the dist cap in the firing order = one bank will be "dead").

 

Well, always wishing you "good hunting". Your other symptoms so match having a dead bank -- the idle screws don't need to be adjusted so far out because the airflow is high enough in the working cylinders to draw fuel thru the main circuit, and the overly "rich" smell from non-working cylinders. Are you saying that the idle screw adjustment of every cylinder has a direct effect (i.e., you can fully close it to create a obvious miss, and then opening it removes that miss)?

 

You can find a dead bank pretty quick just putting a hand near the exhaust as it warms up ( please don't tought a fully hot header). Or at the exhaust pipe, the run bank will be warm and almost no velocity, the dead bank cold and high velocity...you can usually feel a dead cylinder this way too, it will puff. Also it there is a dead bank it will be lethargic when you blip the throttle.

 

Those IR temp guns (pyrometer) are great for this. They "see" temperature.

Also, doesn't < 2 turns-out sound low for the idle screws. I recall 3+ to get them in the zone. Float height?
I concur that 10 sounds high. I had this happen, and the problem for me was that my idle screws were set too lean, and the engine was dragging dead cylinders.

 

Don't get hung up on ***Kg/hr airflow, there are many factors that can make up a specific Kg/hr number. Having all the same is the goal. What about your air bleed screws, generally all should be closed to start and only used to balance air flow between the 2 throats within one carburetor.

Funny things can happen. A case in point: I have been working (read suckered into working) on a Jensen Healey with twin Dellorto side draft carbs which are very similar to Weber DCOE carbs. The car is owned by a friend of a friend who had just bought it and the car completely died within a few days. I was asked to look at it and after virtually rebuilding a severely hacked electrical system I got the car running. It had a poor idle and low rpm performance so I set about looking at the carbs which were running very rich. The car has and aftermarket electric fuel pump which per my fuel pressure gauge was producing 8 psi at the carbs. Dellorto specs 1.5-2.5 psi and the fuel was literately washing down into the cylinders. Yeah, dipstick was 2" over full, the oil drained out like water. After installing a fuel pressure regulator I tried tuning again. I could get 3 of the 4 cylinders to flow at the same rate while the forth (cylinder #1 on the engine) lagged behind at about 60% flow even with the air bleed screw all the way out. I finally came to the conclusion that something was wrong internally with that carb. I pulled the carb and looked down the throats only to find light coming through a hole in the throttle plate. The problem was that it was in the "good" throat (cylinder #2) that matched the airflow of cylinders #3 & #4 served by the other carburetor. All the while I had been trying to increase the flow of the cylinder with the low flow when in fact it was fine and would have easily matched #3 & #4 using the carb/carb balance screw. The culprit was a missing screw which I was able to get (free) from a nearby VW dune buggy specialty shop, they use a lot of Dellortos. Haven't reinstalled the carb yet so results are pending.

BTW, this Jensen pulled very good money 2 years ago on BAT internet auction. The auction winner sold it to the current owner for about half of what he paid for the winning bid. Going through this car I have been astounded at the amount of poor workmanship, cut corners and cobbled repairs. It sure did look pretty in the photos though.
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Good day Mike,

Did you find the original lost screw? If not, then I hate to say it... but did you check the cylinder's intake port to ensure it was not there (bouncing around the intake valve?).. or maybe scope the cylinder to see if it made its way into the cylinder itself? I would check and double check to ensure that the screw is no where to be found, otherwise it could cause some serious havoc. It is possible someone "forgot" to install this screw... but this is not too likely unless the the past carb person was truly a dolt.

Cheers,

Sam

 

It seems to me that you may have the throttles just slightly open to get it to idle at 1k. This uncovers the transition holes and gives you a high flow rate. There are some good write ups: did you begin by loosening out the throttle cable and linkage so that the plates are resting on the stops? This is how you want to set the idle set screws and balance the flow rates.
Good luck!

 

This is me showing ignorance but in the first post you say you have the spark set at 6 degrees. In the second post Ferraripilot says 10-11 degrees. What is the spec?

On my engine (fuel injected ITBs) the throttle plates don't seem to seal well enough and I ended up setting idle rpm using ignition timing. Without touching the ITBs the difference is roughly 750rpm@6deg to 1600@10deg. If you try to set the same rpm at the 2 different ignition timings you will needing to open the throttles more at 6degs, which means lower vacuum and that probably means open the isle mixture screw to get enough fuel into it.

 

If its an air leak you will need to open the mixture screws, but shouldn't be able get the idle rpm set correctly because you're controlling idle by restricting air plus you are measuring excessive air flow - pointing to an engine not running efficiently and unable to pull the vacuum it should. Could easily be timing as I said above, or cylinders not running or as things on the list get worse valves or rings not sealing.

What year engine are we talking about with what cams?

 

More ignorance from me, all these years I believed the 79 was the end of cars and 80s cars were CIS

Ok, (sad) low duration pollution cams so spec timing is 6 deg....what is this post? just checking cam timing to be sure? but you are checking the timing with a timing light on the flywheel correct? if so I'm out of suggestions other than advance the timing and see if it helps. For sure the engine will run even when its VERY rich so the usual mixture set procedure is turn the screws in until it starts missing then out 1/4 or 1/2 turn depending but if the throttle plate is open too much who knows other than it shouldn't be idling if the throttle is open.

 

More thoughts.

Did this maybe start when you replaced the ignition modules and maybe get the matching coils before going further?

Do you have any way to check if the tach correct and it is actually at 1000rpm?

Do you have anyway to check the vacuum?

 

The OP is in the UK, in markets other than the US 1980 is somewhat of a blended year. They even had 1980 GT4's (with carbs of course) which the US did not.

 

Wait, are you saying there is a world OUTSIDE the US?

Thanks!

 

i.m.h.o.

One more carb overhaul observation I think is worth sharing:

I had 2 issues I wanted to address on my car, which led me to recent work on the carbs. My 308 ran great, making the project bittersweet because I was disturbing a well-tuned machine; however, I noticed a fuel smell and some wet lead plugs, and the idle was inconsistent. Sometimes, it would drop to a perfect ~950 revs; sometimes, it would not.

On this recent carb overhaul (previous to this was approx ~30 years and 20K+ miles ago), I noticed the brass seals at the carb throttle shafts had significant wear grooves. By significant, it had ~ .010" on Ø wear grooves. I could measure from some existing seals what the clearance should've been, and it was .001" on the Ø. BTW, the seal consists of a cylindrical brass inner piece that also functions as a spacer (this inner component had the wear grooves on the OD) and a stamped outer seal piece that has the wavy spring holding the face against the side of the ball bearing outer race. I saw this as an air leak source, so I replaced them. I also addressed the lead plugs tapping a little and sealing with epoxy. I called the epoxy company and spoke with engineering to confirm the application.

When starting up after the work, I too quickly started acting on the idle mixture screws during the flow balancing process, which confounded the balance and made the process clumsy. This is where I noticed dragging cylinders and odd flow readings. The better approach was to keep mixture screws on the rich side, and carefully balance air flow... then start to refine the mixture.

My garage smells less like a refinery after a drive, and the idle is noticeably more consistent.

How we love them so!!