Weber problem: suggestions/ solutions welcome
 

Hi, all.

No; no problems with the V8 this time, but it's related in some ways.

The most brutal UK/ European ford- engine during the 70's was the 3- litre V6. I have one topped off not with IDA's or IDF's, but because of restricted space, the WEBER DCNF or Dino carbs as they are named after the Ferrari Dino V6. It was also used on some italian V12's and also V10's if I remember correctly.

So, this is an IR- setup, just as on my 347 and as on your 289/302/331/347/351/308/327/390/428/429/460 etc.

Problem: When idle adjusted; screws out 1/4 turn (large jets), flow adjusted, 700 rpm and 10 deg advance, the engine runs smooth & nice. But when the throttle is used the engine won't come back to normal idle. It will stay at 1200 rpm and maybe fall back to normal in , say, 2 minutes time. There are also very strong popping noice in the exhaust at this time; not dull but rather hard and very strong sound.

Had same problem even with smaller idle jets and with large jets and screws turned more out.

Other symptoms: at idle there are some poppping from the carburettors.

The holes for the idle jets were cleaned out with compressed air. (But not the idle volume setting screw holes.)

I do not suspect clogged idle ducts/ too lean idle, since popping never comes on when the engine is left alone (throttle not used).

My only suggestion is a leaking inlet manifold gasket. Is there a test, other than spraying fuel or starting gas around the manifold to check this?

Suggestions is more than welcome!

Rune,

Does the intake have any form of inter-runner balance?

What engine rpm does your mechanical advance start at?


Let's say the average manifold vacuum per cylinder at idle is 12 inches, rev the engine up to say 1500 and now on decel the vacuum climbs to 18-20.

The idle circuit now flows hard (more air/fuel) and in combination with possibly more timing at 1200 gives you the hanging idle.

6 vacuum gauges (highly dampened, meant for motorcycle multi carbs) will help you, plot the timing as the engine comes back.

Also fairly common for highly tuned engines to take a little to come back to "idle" depending on cam profile, intake, number of carbs etc.

With an Individual Runner set up, it's critical that you get the balance exactly right in all 6 barrels. Doesn't sound like that's happening. Use a balancing tool to check all 6 barrels, both at idle and part throttle. Check all your linkages and find out which one is binding. There's no amount of tuning that will compensate for a balance issue.

This Family?
http://www.clubcobra.com/photopost/d...rth_440_HP.jpg

http://www.youtube.com/watch?v=3iuKYKwRvcU

These are bad to the bone!

Yes, Rick- that's the one. Although what I have is the standard version with one cam in block only and 3 litres, not 3,4-3,5. It was used alot in the US in fromula 5000 (if remembering correctly).

Barrels have been balanced, but I'll double check it.

Gary; yes- I think you are in to something on the ignition timing. There's no inter- balancing between the runners, no.
I have no numbers, but I know the mechanical advance starts early; maybe at 1000 rpm.

I will experiment with vacum advance removed. If I'm lucky I will borrow a vacum reader for MC from a friend.

But IF the reason for the hanging idle is the hard running idle circuit combined with more mechanical advance, what can be done to make it better?

Maybe put in harder springs? Have some lying around; will do some experimenting here as well.

Thanks. Even though not Cobra, V8 or IDA- related, it's great to have a bunch of people who's willing to discuss weber- carburation and general engine-subjects.

Runs

If the shafts have bearings like the IDA's or DCOE's, often times the leather shields dry up and will allow air to be pulled through the bearings and cause iradic idle. The trick is to use double shielded bearings to eliminate to the potential leak.

throttle shaft brg removal
 

Anyone heard of a tool to remove those bearings without taking the carbs all apart (like a small slide hammer)? I went through a set of carbs I have awhile back and since they are NOS I didn't change the bearings (did put fresh NOS leather seals in). In hindsight it may be better to use modern sealed bearings if the carbs will actually be used, but don't want to disturb too much of what has already taken countless hours to get adjusted right.

Rune,

Any further with your problem?

A dual diaphragm advance can may allow you to retard the timing as the engine is decelerating towards idle, almost "forcing" the engine to idle.

As the engine is say 20-50 rpm above the normal idle speed manifold vacuum would have dropped off enough, and the advance could then be coming back at the same time.

Hope this makes sense.

I have done this with a MSD 6AL-2 6530.

Very good suggestions & tip.
Have not thought of the bearings. Will check to see what they're like. (I remember seeing my IDF's double- sealed bearings, but not the DCNF's. They have passed into the shadow of the V8 fun...)

And on the distributor diaphragm; yes- I agree. As a matter of fact my old man told me that some were supposed to have a hole on the rear side, so no vacum is present on the return- side of the diaphragm.
Gary; on the dual diapragm (Iguess this is the type they use on turbo engines?)- should I route manifold vacuum to both sides of the diaphragm? wouldn't they even out each other?

I have gotten further tuning the carbs: The middle carb need the idle screws 1 1/2 turn out, whereas the front and rear make it with only 1/2 turn out. Since all 3 were 11/2 turns out- soot and unburnt fuel may have made some of the cylinders come on and off? Could explain the popping exhaust as well. - Unburned fuel igniting in the exhaust.

But then I'm curious... Why is the middle carb only sensitive to the idle screws beeing moved inwards? Some blocked fuel passage? Bad filter? Wrong float height? Bad vacum/ leaking inlet gasket? Time will tell, I guess.

Now you're getting there. That balance is absolutly critical, both at idle and part throttle. This should be absolutly the first step. Don't mess with anything else until you get this exactly right.

Rune,

The can I've mentioned has advance and retard with two ports.
The advance plate would be at static timing with no vacuum applied.
The plate would move in either direction ( advance or retard ) depending on which port had vacuum applied to it.
Some have used this can for turbo applications with advance for off boost and retard under boost using just the advance port.

Swap your carbs around and prove your fault to one of them before condemning inlet manifold or an engine fault.