Erratic idle after idle adjusted for emissions

[JohnFol]

Afternoon all, I have a Weber 34 DATR and MoT centre turned down mixture as emissions too high.
Since then it's prone to erratic idle (both warm and cold) and has stalled at lights. I've now had 2 occasions where upon trying to start the car, I get a blow back of fuel up past the choke plate. I noticed the inline fuel filter is also full of sediment so I may have a few issues.

Filter now changed so I'm after advice on where to look next, i.e. what is causing the blow back ? Where is the idle mixture screw? Is the sediment an indicator I'll need to replace fuel tank?

[Ferrit]

Hell thats not funny.  I assume it has to be from the tank or carb (really?).   Idle mixture screw is on the battery side near the cylinder head

[peteracs]

Idle mixture screw is on the battery side near the cylinder head

On my DATR the idle mixture adjustment screw is on the distributor side (under the auto choke) at the base next to the main choke butterfly lever, may have a sealing plug. You need a long flat bladed screwdriver to adjust it.

And yes you need to look at the state of your tank, at least buy a few filters and clean out the carb which most likely will have sediment in there.

Peter

[JohnFol]

Many thanks for the replies. Good news is the filter in the carb was clear and each jet / emulsion tube looks clean.

Any thoughts on the 'blow back'?
Ie I crank engine but as soon as I stop I get a pffft of air / fuel ejected upwards through the carb.

[Nigel]

John,
I had a similar issue during an MOT a while back. The tester was happy for me
to do the adjustment during the test and said that it may not run right
but nodded when I suggested I'd readjust it when I got home. I did just that.
Your mixture will now be too lean.

Sediment in a filter is normal for our old cars. As said already, keep some spare filters handy.
If you're feeling flush, a new alloy tank will give guaranteed clean fuel.

Nigel

[Ferrit]

get an endoscope into the tank and take a look.  Tbh you can see a lot through the sender hole

[peteracs]

Hi

On the blow back, has the car been running ok before adjustment of the carb?

If unsure double check your timing on the cams and the distributor, both easily done usually, you will need a strobe light for the disti.

Peter

[Neil-yaj396]

I didn't realise that the idle mixture screw had moved over the years as mine is in the 'battery side' too. I guess it depends which DAT you have exactly?

[peteracs]

I didn't realise that the idle mixture screw had moved over the years as mine is in the 'battery side' too. I guess it depends which DAT you have exactly?

Mine is on an early car, so maybe later ones have it changed?

Peter

[lancialulu]

On the VX there are 2! facing the radiator IRC

[HFStuart]

The T/C seems to need a richer than usual micture at idle. When I first fitted the Emerald injection sytem to mine I used a Ford 2.0 Pinto basemap to start the engine. It barely ran. I spoke to Dave Walker who said to apply a +20% correction to the fueling and it was fine.

[WestonE]

On the blow back once you have ruled out a mixture that is too lean. Do look at and preferably blow through (with compressed air) the feed and return pipes to the tank. Also check with engine running if you undo the filler cap do you get a hiss? The crud in the filter is very likely a tank needing to be removed cleaned and sealed. The crud can migrate to block lots of items. The tank hiss with cap loosened is a failed breather valve BTW. Frost Restorations do a decent kit which is fine for a carb tank. The VX & IE tanks have internal baffles and a swirl pot that can get too easily blocked by tank sealant.   

[JohnFol]

Hi

On the blow back, has the car been running ok before adjustment of the carb?

If unsure double check your timing on the cams and the distributor, both easily done usually, you will need a strobe light for the disti.

Peter

Cam belt replaced by Auto Integrate, and distributor/leads/battery/coil/plugs all new or recent reconditioned. All running well before and I start the car every couple of weeks

I think I might be into the realms of compression tests as it now runs lumpy and only with accelerator pressed.

[JohnFol]

Ok, some success.
Found the plugs were ngk bp7's and were covered in soot. Checked compression and all good (so no sticky valves, ring issues etc .. phew!)

Changed plugs to pb6's, and for good measure changed rotor and cap.

Now fires instantly!

Still need to get idle set but very pleased it was a simple fix and engine in good health