Technical What's the likely cause of this fuelling (?) problem

[smart51]

This weekend I have fitted a new Zelmot distributor, and also a new length of fuel between the fuel pump and the copper pipe coming in to the engine bay. The car starts straight away off the key, even if it has been sat overnight. All good.

it doesn't come off the choke well though, even when warmed up. A brief blip of the throttle from idle will make the engine start to die. More so if the choke is off. It is massively down on power at all revs and will barely move under its own steam. It sounds to me like fuel starvation. Turning the fuel mixture further out has no effect.

I've checked that the idle mixture setting, points gap and spark timing haven't changed. They're all good. What's the most likely culprit? Where should I start to look for this one?

 

[bugsymike]

The fuel mixture screw is only an idle adjustment, so once you press the go pedal it is the jets inside the carb affecting the fueling.
Have you contacted anyone on Forum with same type of carb and engine spec. to see what jets their carb has, in case yours isn't jetted correctly?

 

[smart51]

The fuel mixture screw is only an idle adjustment, so once you press the go pedal it is the jets inside the carb affecting the fueling.
Have you contacted anyone on Forum with same type of carb and engine spec. to see what jets their carb has, in case yours isn't jetted correctly?

It was running before, so I don't think it is the size of the jets. My guess is that something happened when I replaced the fuel hose. Something somewhere may have become dislodged and is causing a blockage or partial blockage, or there is air in the system that isn't being cleared.

When the throttle is blipped at idle, it lets more air in. If it is struggling to idle anyway, being already a bit lean, letting more air in would be too much. But being massively down on power at part and wide open throttle would be the main jet not the idle circuit.

Could it be trapped air before / in / after the fuel pump? I would have thought it would prime an empty system without help, so clear any trapped air. General fuel starvation would affect all revs and throttle openings though.

 

[Bleeding Knuckles]

Smart, check that you have not cut the internal hose lining (when pushing it on) and fed it into the pipe causing a blockage.
Ian.

 

[bugsymike]

It was running before, so I don't think it is the size of the jets. My guess is that something happened when I replaced the fuel hose. Something somewhere may have become dislodged and is causing a blockage or partial blockage, or there is air in the system that isn't being cleared.

When the throttle is blipped at idle, it lets more air in. If it is struggling to idle anyway, being already a bit lean, letting more air in would be too much. But being massively down on power at part and wide open throttle would be the main jet not the idle circuit.

Could it be trapped air before / in / after the fuel pump? I would have thought it would prime an empty system without help, so clear any trapped air. General fuel starvation would affect all revs and throttle openings though.

Sorry, I didn,t see that it was a fault that developed after fuel hose replacement, so as @Bleeding Knuckles suggests hose lining or particles in carb.
I have had dirt trapped behind the needle valve only visible when needle jet out completely, giving similar symptoms to a blocked main jet, but not showing by just blowing through the jet, though more on SU carbs in the past.
As you say system should prime from empty so unlikely to have air locks etc.

 

[smart51]

The fuel pipe is fine. I've taken the carb apart and rebuilt it now I know how to remove the emulsion tube. All seems good. It has a 50 idle jet, a 115 main jet and a 190 air corrector jet, which I'm noting here as an aide memoir.

One thing I noticed today, with the engine idling on full choke, the idle mixture screw can be turned all the way in and the engine will cough and splutter but just about stay running. Half a turn out and it will fun but more than that makes no audible difference to the idle.

It still barely runs and is starting not to be fun any more.

 

[Bleeding Knuckles]

Smart, try flushing the pipe sometimes debris can be lodged inside. I would also check your fuel pump operation for full fuel delivery.
Ian.

 

[bugsymike]

Were you able to remove the float and needle jet assembly and check for any debris in the fuel inlet in the carb?

 

[smart51]

Were you able to remove the float and needle jet assembly and check for any debris in the fuel inlet in the carb?

There were a few black specs in the float bowl, which I cleaned out. I didn't think to look on the other side of the needle jet.

I've just done a compression test, this time at WOT. 130 PSI on the front, 120 on the rear. Good numbers, but different.

 

[smart51]

Well, riddle me this? This afternoon I removed the spark plugs and checked that each one was sparking while turning the engine over. Check. I ran the engine to warm it up and did a compression test. Check. Then I removed the brass cover on the carb to see if there was any debris in with the filter. I couldn't see with the carb in situ. Lastly, as always, a road test. It runs. Down on power a bit but it ran. Top speed 53 MPH at the test point on the dual carriage way. It has done 62 there before. On the last two down shifts before getting home, it hesitated when blipping the throttle to rev match. Then it stalled when I stopped to reverse into the garage. It wouldn't start without the choke, which is suspect.

What about what I've done this afternoon could have cleared the problem, even if only for half an hour?

 

[bugsymike]

What about what I've done this afternoon could have cleared the problem, even if only for half an hour?

My thoughts would be along the lines of fit a new fuel filter close to carb, run that through for a few minutes then totally strip and clean the carb 100% including behind float needle jet and main and all other jets and road test again.
It could be particles of rust etc. from the fuel tank.
At least you are getting somewhere.

 

[fiat500]

The fuel mixture screw is only an idle adjustment, so once you press the go pedal it is the jets inside the carb affecting the fueling.
Have you contacted anyone on Forum with same type of carb and engine spec. to see what jets their carb has, in case yours isn't jetted correctly?

Although the following statement possibly doesn't apply to the humble IMB carburettors, I've assumed it correct for several years as it sounds quite authorative and plausible:

"The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation."
It's from the Redline Carburettors website and although the link is to the ICT setup, there is a lot of useful adjustment technique described there.:

CARBURETOR SET UP AND LEAN BEST IDLE ADJUSTMENT

 

[smart51]

Although the following statement possibly doesn't apply to the humble IMB carburettors, I've assumed it correct for several years as it sounds quite authorative and plausible:

"The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation."
It's from the Redline Carburettors website and although the link is to the ICT setup, there is a lot of useful adjustment technique described there.:

CARBURETOR SET UP AND LEAN BEST IDLE ADJUSTMENT

The setup procedure described there is interesting. It says if the idle screw needs to be set 3 turns out or more, you need a bigger idle jet. I have mine set at 3 turns. At 2.5 turns, it can bog down briefly when blipping the throttle. It may not mean that my idle jet is too small, but that the tubes between the main jet and adjustment screw could be partly clogged up. All the bits I can see are clean, those tubes are still an unknown.

 

[bugsymike]

It seems to be related to Webers design of transition holes near throttle valve, interesting to learn, it does seem to contradict earlier design belief and teachings.

 

[fiat500]

It seems to be related to Webers design of transition holes near throttle valve, interesting to learn, it does seem to contradict earlier design belief and teachings.

Those holes can be easily overlooked when cleaning. They might easily be damaged if cleaned without great care.

 

[smart51]

*plaintive howls of despair*

So, I stripped the carb to its individual pieces and put them through an ultrasonic cleaner until they were a medical grade level of clean. It was rebuilt with a new needle valve, new throttle spindle plus new gaskets and O rings. Yesterday I did a quick trip round the block and all seemed fine. This afternoon I attempted a longer drive, but no. The engine starts and idles well and will run well at low throttle openings. Above half throttle, it coughs and splutters and has no pull. What have I done wrong now?

 

[Ant Knox]

*plaintive howls of despair*

So, I stripped the carb to its individual pieces and put them through an ultrasonic cleaner until they were a medical grade level of clean. It was rebuilt with a new needle valve, new throttle spindle plus new gaskets and O rings. Yesterday I did a quick trip round the block and all seemed fine. This afternoon I attempted a longer drive, but no. The engine starts and idles well and will run well at low throttle openings. Above half throttle, it coughs and splutters and has no pull. What have I done wrong now?

Have you checked that the base flange of the carb is flat? They are prone to warping if overtightened and as a result this can lead to air leaks.

 

[smart51]

Have you checked that the base flange of the carb is flat? They are prone to warping if overtightened and as a result this can lead to air leaks.

I did. It wasn't bad but I sandpapered it anyway.

 

[smart51]

Just for completeness, I'll post here that I've replaced the fuel pump just to rule it out. It hasn't changed the fuelling problem.

 

[Goldnrust]

I feel your pain, this engine is really putting you through the ringer! How do the spark plugs look after a run? Do they suggest a lean condition?

I'm half tempted to say that you've really been through the whole fuel system completely, and it might be time to look elsewhere for an issue, possibly a weak spark? A quick test for this would be to close down the plug gaps a bit, if that helps the car take more throttle it would point towards weak spark.

Could you have a fractured wire somewhere which is making an intermittent connection? Have you got a good earth connection between the car and the engine? Things like this could cause the sort of intermittent problems you're describing.

I have to admit I'm clutching at straws for you a bit, it sounds like you've tried many things.