Part throttle misfire

[pharriso]

Car was driving perfectly, then all of a sudden I get a part throttle misfire... idles OK... full throttle is OK... but at light & mid throttle definitely seems to be running on 3.

It also pops on the overrun ... there's definitely an exhaust leak at the collector... from what I've read on here I believe that can cause the popping

Based on "90% of fuel problems are electrical" I've swapped the plugs (BP6ES @ 30 thou), checked the rotor arm, distribution cap & leads. Checked the timing (about 7 deg at idle)... I'm running electronic ignition.

Must be the fuel system? Anyone any idea what to check? I am running 40 DCOEs.

[StressCraxx]

Partly blocked idle jet perhaps?

[Grizzly]

What did the Plugs look like? were they all the same colour or one lighter? (did you make a note of with plug came out of which Cylinder??)

I'm no expert but i had a similar issue that turned out to be crud stuck to the main jet, it would idle fine and if you gave it a foot full it would clear (assuming the Pump jets were adding sufficient fuel). With the dodgy fuel near me i'm constantly cleaning the jets if i don't use it for a week or so at a time. Usually it's obvious which one is playing up by the colour of the plug (often a very light tan compared to the others which are brown) the trick is to pull them out after a bit of a run without letting it idle or all you get is Black plugs.

[pharriso]

What did the Plugs look like? were they all the same colour or one lighter? (did you make a note of with plug came out of which Cylinder??)

I'm no expert but i had a similar issue that turned out to be crud stuck to the main jet, it would idle fine and if you gave it a foot full it would clear (assuming the Pump jets were adding sufficient fuel). With the dodgy fuel near me i'm constantly cleaning the jets if i don't use it for a week or so at a time. Usually it's obvious which one is playing up by the colour of the plug (often a very light tan compared to the others which are brown) the trick is to pull them out after a bit of a run without letting it idle or all you get is Black plugs.

All the plugs were the same, running a bit rich. None was obviously different.

Could the coil cause a part throttle misfire?

[Craven]

Hi,
Is it speed or load related? Engine unloaded will run up to 3000rpm on idle will move to main jets when working at this speed.
So do you have the misfire when stationary, under light load, pulling hard, can you drive through it?
More info needed.
Ron.

[KevJ+2]

I had exactly the same symptoms as described. One Lotus garage changed the plugs and it was better...for a while. But the problem returned. I was convinced it was the idle jets (brand new carbs) but when I got the car on a rolling road, it was found to be definitely electrical. The distributor (TT electronic) was suspected and the cap removed. Inside, the rotor had caught on the terminals and left debris / dust inside. After a clean and blow out + New rotor, it ran sweet. No hint of mis-fire since. A transformation!
Just a thought, in case you have similar electronic dizzy?

[nigelrbfurness]

If it's there all the time I'd replace plug and coil HT leads, then the distributor cap and rotor arm. Worth looking at the carbon contact in the top of the cap, too.

Nigel F.

[pharriso]

Hi,
Is it speed or load related? Engine unloaded will run up to 3000rpm on idle will move to main jets when working at this speed.
So do you have the misfire when stationary, under light load, pulling hard, can you drive through it?
More info needed.
Ron.

Definitely light load... will miss fire when I blip throttle parked... I am thinking bad leads. If I disconnect a lead I can hear a zap ... zap ...zap... from the distributor area.

I have ordered dizzy cap, rotor & materials to make up leads.

[Craven]

Hi,
Always a good thing to keep the ignition in top form BUT if you disconnect a plug lead without providing an alternative discharge path the energy will dissipate somewhere, often inside the distributor cap.
Ron.

[RichardHawkins]

Phill,

In calafornia a man called Keith Franck runs a web site called sidedraft central. If you are convinced the problem is not electrical, then Keith could be who you need help from.

Richard Hawkins

[pharriso]

Update:

So Friday / Saturday, replaced Rotor, no change, changed Cap + Leads no change... Must be a fuel issue!

Today checked the Weber fuel filters - perfectly clean.

Pulled Main + Idle Jets from the rear carb, no obvious issues

Went to pull Main jests from front carb, one stack comes out leaving the emulsion tube & jet in the hole... One comes out leaving the main jet in the hole:

So I get the emulsion tube + jets out the Carb, re-assemble everything, the emulsion tube is a good fit in the stack, as is the jet in the emulsion tube. Check the float valve & setting & re-assemble, now the engine runs perfectly.

Can anyone explain what was wrong?

[pharriso]

So reading the Weber DCOE material available online & the WSM I summize:

1. That the main jets and/or emulsion tube could not come out of the "stack" (air correction jet / emulsion tube / main jet assembly) as the stack is screwed down until the main jet bottoms in the hole. Therefore it coming apart on disassembly really meant that the main jet & main jet/emulsion tube assembly was stuck to the carburetor body, probably with "varnish"

2. Since the "idle" jets work until 3000 rpm & that's where I was having my misfire; there must have been something wrong in the idle circuit.

3. The car was popping on overrun, this would suggest too much fuel.

Could the float have jammed down allowing too much fuel into the carb bowl, making the engine run rich? I'm grasping at straws here...

[pharriso]

A quick update... misfire is still gone so the issue must have been crud in the idle jets.

Backfire on deceleration is still there , as is the exhaust leak under the car. Took a good look & I have a hole in my exhaust manifold where 1/4 meet 2 & 3:

I have some J-B Weld 37901 Extreme heat metal paste on order & will also wrap with Glass Fibre exhaust insulation tape. Time to save up for some nice shiny Paul Matty Stainless Steel manifolds.

p.s. My experiences exactly match Archigator's in this thread- http://www.lotuselan.net/forums/lotus-elan-f19/severe-misfire-t23952.html