Technical Rebuilt carb - now car will not idle!!!

[martinmerc]

I did a simple carb rebuild - clean , replace parts. Now car will only run on the starter circuit (choke) or the main (throttle pushed open). What could be the problem? I removed carb again and blew out all passages with carb cleaner - all seem ok. I am at a loss for ideas? Can anyone help?

 

[Panda Van]

Hi,
Firstly what did you replace in the rebuild?, have you adjusted the mixture screw to see if it runs any better and have you checked there is no air leak between the carb and inlet manifold ?

 

[martinmerc]

Replaced needle valve and float - set levels as stated. New gaskets, blew out passages. Cleaned out dirt in bowl. Tried mixture screw in all positions from full out to full in and in between. No leaks - everything is tight and fits well. I also removed carb again and blew out passages with carb cleaner. all seems to be OK. Before I did this, it idled nicely. I did this to try to fix a small high rpm hesitation in 2nd gear. Now it will not idle.

 

[sixpotman]

Will it Run / idle with gentle pressure on the throttle?

John

 

[martinmerc]

Runs fine with gentle pressure or even more pressure. It just stops totally at idle.

 

[sixpotman]

Has either the cable moved or have you adjusted
the butterfly setting screw on the end of the spindle
usually has a little spring on the shaft of the screw
to stop it vibrating loose.
Or have the choke settings been changed.

John

 

[martinmerc]

Problem appears to be independent of the cable. I can manually move the throttle actuator arm at the engine and get the same effect.

Choke is working fine - again - I can move it at the motor to get the effect.

It appears that the idle circuit is not functional even though I blew out all of the passageways.

 

[sixpotman]

What make of carb is it?

John

 

[martinmerc]

Standard Weber 26 IMB

 

[maddogmorg]

have you tried just increasing the idle speed setting, just to get it to idle?

there isn't any electrical parts to the carb? no throttle closed circuit or anything?

 

[sixpotman]

http://www.carburetion.com/Weber/adjust.htm

I suspect if the carb has only been blown out
without dismantalling then grit has blocked a
jet, or something has been put back together
in the wrong place, there are springs and ball
bearings which act as one way valves.

Not conversent with this carb but some have
an atimalizer bar in the top of the carb venturer
which if rotated block the fuel.

The other thing is check the holes in the fuel
bowl arn't blocked and stopping the flow of petrol
to the mixture screw.

John

 

[martinmerc]

No adjustment of idle speed setting will do it unless you really get the throttle open a lot. This cannot be right. There is a separate idle circuit in the carb. This is what seems to be the problem. If the starter circuit is engaged , it runs fine. Close the starter circuit via the cable, and it stalls immediately.

There are no electrical items to teh carb.

 

[Bambino]

The Weber IMB carburettors fitted to the 500s are a relatively simple devices when compared to some of the other contemporary carburettors fitted to other Italian cars (eg: DCOEs).

I've had a dig around and come up with some information that I hope will help.

The first picture is a general 2D schematic of the IMB series of carbies. They are all constructed similarly with the only difference between say an 26IMB1 and a 26IMB4 being the tunable components (jets etc.)

The idle circuit works thus: When idling, fuel is carried from well 23 through the passages shown in the second picture to the idle jet (13) where it is mixed with air coming in from inlet 5. Through duct 3 and idle speed orifice 18 - which is adjustable by taper-point screw 17 - the fuel reaches the carby throat just past the throttle butterfly 19 where it is drawn into the general mixture. From duct 3 the mixture can also reach the carby throat through transition hole 20 thus permitting a smooth transition from idle to full power as the butterfly opens. The diagram and description are courtesy of the Fiat 500 Shop Manual.

The second picture is of a spare IMB and shows the positions of the idle jet holder (A), the external fuel galleries in the idle circuit and their lead plugs (B) and the idle needle taper-point screw (C).

The last picture shows the idle jet holder and the jet itself.

If your car will not idle, but otherwise runs OK both hot and cold, then the problem is likely to be along this circuit somewhere.

It is possible that the galleries are blocked with general crap and need cleaning (again). You may have to drill out the lead plugs to clean the galleries properly. The plugs can be easily replaced using appropriately sized lead buckshot.

Maybe the jet is blocked - if so, soak it and blow it out with air, do not ream it or you'll ruin it.

The air feed may also be occluded, though this would be unusual.

I think your only alternative is to remove the carby, strip it and clean it again.

Let us know how you get on,
Chris