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'flat spot' loss of power accelerating through 4k RPM


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Hi all,

 

Had a nice run out today for our monthly Sunday lunch trip. Did some powering through country lanes and noticed that going through 4000 revs, accelerating in 2nd or 3rd, there's a sudden dip in power (known as a 'flat spot' I think).

 

I know there's been a previous post on this but could not find it. Could some kind soul point me to it or suggest what may be the problem - I assume something interupting fuel supply - injectors?...

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There have been several threads over time so I’ve no idea which particular one you’re referring to but there was a recent one which pointed a finger at a particular after market exhaust manifold which is perfectly feasible I suppose. The first questions to ask are; has it always been there or just developed? Have you modified the car or done any work recently? How well is the motor maintained; 6’s are particularly fussy with regards the ignition system & all the components must be absolutely perfect.

 

When you say flat spot, does that mean you can drive through it & it picks up again as the revs climb or does it just stay flat until you lift off the throttle or drop the revs by changing up? On some cars there was a problem with fuel starvation occurring with a low fuel tank when cornering hard, particularly on long left handers so if you’re navigating country lanes, lots of bends etc! At the time BL issued a modification bulletin to dealers as a fix but I don’t know what it entailed. If it starts & runs OK at lower revs it’s unlikely to be an injector but here are some other pointers on what may be causing it;

 

Choked fuel filter

Blocked fuel line

Dead leaf or other debris in the bottom of the tank!

Blocked in line metering unit fuel strainer

Leaking M/U vacuum hose

M/U diaphragm split

Low fuel line pressure

Knackered fuel pump that can’t maintain fuel delivery rate

Timing too far retarded

Distributor problem affecting the advance curve

Faulty plug or plug lead

Faulty rotor or capacitor

Faulty electronic ignition module

Incorrect non-vented fuel cap fitted

 

I’m sure this list is not exhaustive but you have to try & narrow it down by analysing exactly when it happens & a bit of trial & error.

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what may be causing it;

 

Choked fuel filter

Blocked fuel line

Dead leaf or other debris in the bottom of the tank!

Blocked in line metering unit fuel strainer

Leaking M/U vacuum hose

M/U diaphragm split

Low fuel line pressure

Knackered fuel pump that can’t maintain fuel delivery rate

Timing too far retarded

Distributor problem affecting the advance curve

Faulty plug or plug lead

Faulty rotor or capacitor

Faulty electronic ignition module

Incorrect non-vented fuel cap fitted

 

I’m sure this list is not exhaustive

 

Blimey. :unsure:

This is like reading a medical dictionary.

I'm just about to go out in the 6, I know it's going to die now... :(

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