Bis erratic running

[dadandlad]

I am looking for ideas regarding the running of my Bis engine

I really struggled to find a cure for the rough tickover which turned out to be the O-Ring on the jet with the big flat head in the side of the carb duly replaced with a copper washer

The problem I have now is if throttle is held mid position it misfires

I set the timing static at 10 degrees BTDC and duly checked with timing light and vacume advance blocked, the timing marks align but does seem to fluctuate slightly either side of pointer

It has new points, condenser,spark plugs and leads fitted but still using original coil

The bob weights and springs in the distributor appear to be fine and there is no play in shaft

I have noticed the inline fuel filter i have fitted is never full, would you expect it to be ?

The carburettor has had all flaps and linkages set according to the workshop manual

I should also add the engine has been rebuilt so has good compression and all valve clearances are as they should be

Fuel or timing issue ?

Any ideas greatly appreciated

[mintex]

Mine is doing a similar thing in that if i hold it half throttle it coughs and splutters but if i put a little bit of choke on its fine so mine must be a fuel issue but with using the car for work ive not had chance to look. 

Try it with yours and see if it makes a difference.

[Pete126]

Hi Mark, check you have the vacuum pipes on in the correct place

[dadandlad]

Cheers chaps

Pete I think you may have hit the nail on the head, from memory I think I have the throttle kicker connected to the carb and the distributor advance/retard connected to the manifold 
I  will have a look and play tomorrow evening

Mintex
If the above doesn't work I will give your idea a try

[Pete126]

Fingers Crossed

[dadandlad]

Well didnt get in till 7 this evening but i really wanted to play out

Checked the vacuum pipes and you was right Pete, Carb connected to kicker and Manifold to distributor 

Started her up and she sounds a lot better  smoother tick over and I can shut the choke a lot sooner

Still has a misfire at mid throttle though

[Pete126]

Still has a misfire at mid throttle though

Just a hunch

The problem maybe the transition from the 1st choke to the opening of the 2nd choke it could be opening too soon/late

Look for the Carb section in the manual it has all the measurements of the butterfly positions

[dadandlad]

Cheers Pete

I set all the flaps and linkages the last time I had the carb off using drill bits as accurate measuring devices

The problem is in the first stage at the moment

I haven't really pushed it onto the second stage yet, still running her in and not too keen on over revving her with no load

[dadandlad]

Right boys the plot thickens
 
Fitted electronic ignition and now runs really well on tick over and mid throttle position

When I snap the throttle open it bogs down and then picks up to wherever the throttle is held, at the same time it is coughing back through the carb

Classic symptoms of the carburettor pump either not working or not delivering enough fuel, looks like the carb is coming off again to do the fuel pump metering test.

It had a new diaphragm fitted when I rebuilt it so we will have to see what I find

[Pete126]

Do you mean backfiring through the carb?
 

[dadandlad]

Do you mean backfiring through the carb?

Yes but Iam pretty certain its not a timing issue

The accelerator pump system is simply a mechanical fuel injector that provides enough fuel for a brief period of time for the engine to respond smoothly to the heavy wide open throttle demand.  Its purpose is oh so simple and its function is oh so critical. And, when it isn’t working right, the dreaded “Bog” and the always dreaded backfire through the carburetor are effects of an accelerator pump system that isn’t equal to the task. The “Bog” simply means the engine is not receiving enough fuel when you go to wide open throttle.  While there are other issues that may cause similar reactions within the carburetor, often the problem can be traced back to the accelerator pump system.

[Bambino126]

I have that same issue, that my engine "bogs" down if I open the throttle too quickly. Does that mean my accelerator pump is knackered?
I find it can be dangerous when you pull out in to a gap in the traffic, and the car gulps in air for a couple of seconds and bogs down when you need it to accelerate. My way of dealing with it is to open the throttle more gradually and slowly until the revs rise up before bringing the clutch up to pull away.
Is this accelerator pump easy to fix and where do I get one from?

[Pete126]

Do you mean backfiring through the carb?

Yes but I am pretty certain its not a timing issue

The accelerator pump system is simply a mechanical fuel injector that provides enough fuel for a brief period of time for the engine to respond smoothly to the heavy wide open throttle demand.  Its purpose is oh so simple and its function is oh so critical. And, when it isn’t working right, the dreaded “Bog” and the always dreaded backfire through the carburettor are effects of an accelerator pump system that isn’t equal to the task. The “Bog” simply means the engine is not receiving enough fuel when you go to wide open throttle.  While there are other issues that may cause similar reactions within the carburettor, often the problem can be traced back to the accelerator pump system.

There's something i can't get my head round For the engine to backfire through the carb the petrol needs to be ignited, which can only happen if one of the inlet valves is slightly open?

[dadandlad]

Sounds to be the same problem
If you remove your air cleaner you should be able to see the accelerator pump jet pointing down the barrel of the carb
If you operate the accelerator you should see a jet of fuel squirting down into the carb, if you cannot see a jet of fuel either the pump diaphragm is shot or you have a blockage

Link to carb kit complete with accelerator pump

http://www.ricambio.co.uk/69-fiat/72-classic-fiat-126-parts/149-fuel-system/1368-carburettor-overhaul-kit--fiat-126

[dadandlad]

There's something i can't get my head round For the engine to backfire through the carb the petrol needs to be ignited, which can only happen if one of the inlet valves is slightly open?

Cheers Pete you have now got me thinking, pretty certain that my valve clearances are bang on but I will check them just for piece of mind

Certainly making the brain tick as I have not worked on anything with a carb for donkeys years but really enjoying myself