246 Dino webber question....strange problem..

Im really struggling to get the carbs set up having read all the articles I can find....I still get popping and rough running until I do something rather strange....I undo the idle jet a quarter turn (and I mean the actual idle jet...5.5 by the way ) ...all the poping stops (more or less) and that cylinder comes to life together with the others that seem OK in the first place.
I can turn the Idle mixture screw all I want in or out to no real affect...I have changed the idle jets ..cleaned them etc but cannot get that cylinder to work properly until I actually unscrew the idle jet itself a quarter turn....all very odd....ruber O ring is fine by the way....

any ideas ??

 

A leaky inlet valve or a vacuum leak (carb base) on that cylinder would both be possible diagnoses for these symptoms.

 

How can I check if it's a leaky inlet valve
If I do a compression check and its above 120 ( which it is ). Will that prove the valve is ok ?

 

You didn't say if you actually removed the idle jet from the jet holder to insure the jet was not obstructed. Once removed, you should be able to see thru the jet.
Gary Bobileff

 

Jets all new and can see through. All I have to do is turn it s quarter turn and it comes alive ...
Maybe I should pull that carb and just rebuild it and try again but I was trying to identify the problem first ..
I'm still using leaded fuel but don't think that matters.
I will try and test all around for inlet leaks today .
Any other suggestions always very welcome
I have almost rebuilt this car and couldn't have done it without ' Ferrari chat'.

Thanks

 

You need to make sure the carbs are pre set correctly.

By undoing the idle jet screw you are introducing more air into the emulsion circuit and possibly leaning out the mixture enough for that barrel to work.

The idle circuit should be blown through with air also remove the emulsion tubes and calibrated bushes as these can collect fine sediment.

Make sure your fuel is clean

Make sure your idle speed is not to high to allow the next progression hole to come into play, this can give you false info.

Take out the mixture screw and clean carefully, make sure the orifice is clean as well.

You are not going to get anywhere until you start to see some reaction when adjusting this.

Set the carb airflow with the bypass crews at one turn out.

Make sure the linkages are set perfectly as one or two barrels can keep the car running and disguise the true problem

 

I did strip that one carb and clean everything and found a small amount of crud in the bowl....I also found the float level to be only 45mm so I reset it to 48mm. I have read on this site two conflictions...one says the float height does not affect idling and the other says set up the carbs with the air bypass screws right in....I dont have a clue what is correct but I can report that having set the air bypass screws at 1 turn as you said (all six of them) and having reset the float level and cleaned and blown through all jets....she fired up perfectly, no longer backfired at all and seems to run very well...maybe it was just the cleaning, who knows....my only dilema now is should I fidle more with the bypass jets...the book says one on each carb should be closed and mine are all 1t open....the vacume tuning guage that I plonk into the throat shows they are all more or less balanced.....however when I short out each plug for an instant the reduction in RPM is not the same on all the cylinders so I do know that one cylinder is carrying the other slightly....
any comments ...welcome....otherwise I will just leave it as is I thinks....!

 

In my view if you set the bypass screws to fully closed then you have no leeway in balancing up the carbs with each other. If the airflow through each carb is different and with the AB screws fully screwed in then you have no adjustment.

one turn out is fine, then fine tune each carb to get correct airflow thru them.

Check plugs once you have been for a spirited run.

You may also need to consider re-jetting but solely depends on the quality fuel you get.

Shorting out the plugs to tune is not for the in-experienced!!

Tony