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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 
Low rev misfire

Carburetor engine, starts easily, ticks over nicely, pulls well and screams loud but at light load/low revs there is an occasional misfire, marginally more pronounced when the engine is cold.
Any thoughts anybody? It is not a Defender, it is a One_Ten

Post #803209 22nd Nov 2019 5:13pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

Cracked dizzy cap or perished / damaged HT lead(s)?

Post #803492 25th Nov 2019 12:32pm
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

Leads are Magnecor and not that old, hall effect unit and reluctor are new, plugs are new. The distributor cap looks OK but given how cheap they are I am going to replace both it and the rotor arm anyway, which leads to my next question.
What are the differences, if any, between the blue and black distributor caps? It is not a Defender, it is a One_Ten

Post #803804 27th Nov 2019 1:58pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

I use the standard lucas leads. Only reason I mentioned the leads was that I noticed one of mine had been damaged the other day from resting on a vibrating engine part and wearing out the outside of the sheath.

With regards to distributor caps, I am not sure as to the colour. But personally I would only order distributor parts from here: http://www.distributordoctor.com/ as there are so many poor quality items out there.

Post #803806 27th Nov 2019 2:05pm
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

Looks like a good site, snag is that they don't appear to stock parts for the 35D distributor.
I have the leads both clipped and clamped to prevent them moving around too much.
I am beginning to wonder if it might be the carb needles. Looking at the profile of the needles there is a step at approx the relevant rev range It is not a Defender, it is a One_Ten

Post #803811 27th Nov 2019 3:00pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

Burlen are your people then: http://sucarb.co.uk/.

I thinkt the doctor can do the v8 standard distributor, parts might have to call though.

Post #803812 27th Nov 2019 3:03pm
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

I have 4 distributors in various states of repair at the moment, as with the rest of the V8 they are stone age simple.
I changed the needles last year after having an unusual fault.
I had always thought that the previously fitted needles were a tiny bit weak right at the top end of the rev range so replaced them with a set which offered more top end but a weaker bottom end/midrange.

BBW is the currently fitted needle
BBZ was the previously fitted needle
It is obvious that the BBZ has a fairly straight line at the bottom end up till about measuring point 4 whilst the BBW needle is weaker right at the bottom end then marginally richer between points 1 and 2 then weaker up to about 7
I noticed during the plug change that the plugs were pale grey, almost white, suggesting that it is running a bit weak It is not a Defender, it is a One_Ten

Post #803862 27th Nov 2019 8:16pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

Makes sense.

Is it easy to tell how many cylinders are mis-firing? If it was carb related, then you would expect four cylinders to be missing I assume.

Spark plug gaps all ok? Each checked for strength out of the cylinder?

Otherwise, don't have much more to offer..

Timing all ok and clamps on the dizzy tight?

Post #803936 28th Nov 2019 11:13am
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

Plugs are new as in 3 days old, all correctly capped. At least part of the problem is trying to diagnose the fault whilst driving.
Typically it happens when accelerating gently from low revs, at which point the engine seems to stutter. Or when rolling gently in 4th gear at approx 1500 revs, at which point it definitely feels like a misfire, but only occasionallydoesn't happen at similar revs in lower gears, presumably because engine is working marginally harder.
New distributor cap and rotor arm are en-route from Simon BBC It is not a Defender, it is a One_Ten

Post #803965 28th Nov 2019 6:48pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

The only similar symptoms I had were when I had my fuel pump replaced and the person swapped over the feed and return fuel pipes (i.e. the engine was starved of fuel). The 110 would pull, but then stutter in the mid/high rev ranges as the return fuel pipe is narrower than the feed and couldn't supply enough fuel.

Perhaps, if you've squared away all of the ignition / electrical parts it might be worth working through the fuel side?

Post #804034 29th Nov 2019 10:59am
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

I have a new distributor cap and rotor arm on the way.
I am trying to go through it one thing at a time, so when the rotor arm and cap arrive, I will change the cap, test drive see if it is better, then the rotor and so on.
The bit that has me puzzled is that if I floor the accelerator, it takes off like a rocket (figuratively speaking) and the engine revs clean and hard.
I am wondering if it is a sticky advance retard mechanism? Though that wouldn't account for the misfire on light load/light throttle. It is not a Defender, it is a One_Ten

Post #804047 29th Nov 2019 1:20pm
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

Sticking float / value in the carb or low dampner oil?

Post #804053 29th Nov 2019 3:26pm
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

Damper oil is OK and I have had a sticky float in the past, it manifests itself in the engine running like carp and a dribble of petrol on the floo.
As mentioned, new distributor bits should arrive either tomorrow or monday and then I will be able to start eliminating things 1 by 1
Just doing a needle comparison chart now to see how much percentage difference there is between existing needle and previous at bottom end of rev range It is not a Defender, it is a One_Ten

Post #804059 29th Nov 2019 4:24pm
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oneten110



Member Since: 02 Jul 2011
Location: North Yorkshire
Posts: 636

United Kingdom 

Still waiting for new distributor cap but I lifted the existing unit last night to lubricate the A/R mechanism and noticed a small blob of silicon grease under the position of No3 HT lead. Pulled No3 lead and the brass is a bit tarnished in comparison with the others suggesting something awry there. Couldn't see any cracks but given that the silicon grease was put into each individual pot whilst the cap was fitted to the distributor, there must be one somewhere in order for the grease to migrate.
I am assured, by the post office, that I will have my new cap today. It is not a Defender, it is a One_Ten

Post #804573 3rd Dec 2019 11:07am
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montyjn



Member Since: 06 Sep 2016
Location: London
Posts: 28

Fingers crossed...!

Post #804724 4th Dec 2019 1:30pm
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