120 engine missing under load

I have exhausted all avenues as to why my 51 120 is missing so badly under load. Carbs are correct and rebuilt by Joe Curto. They are balanced as per manual. Starts well and starter lenoid works very well. Timing is spot on (static). Plugs are all identical in color, maybe a little rich. Distributor is very good with no play in bushings. Points set at proper .012 .
Have installed dizzy from early Mk2 with identical results.
Car will miss terribly when under load ( accelerating) . If gently applying throttle much better response. If you put your foot in it, it is just terrible. It is fine when at speed. Has anyone experienced this or any further ideas?
Any help is greatly appreciated.

Hi Jagmg:

Have you checked/cleaned/changed the fuel filters? The one in the tank, any in-line ones you may have added and the small ones in the fuel lines to the float bowels. The last time something similar happened to me it was debris partially blocking those small filters in the float bowels, which prompted me to put an in-line filter before the carbs. I have also experienced a miss under load (going up a long gradient) with other vehicles–which otherwise ran fine–and it was a clogged/overloaded fuel filter.


Hi Jagmg.
Sorry I’m no expert but have recently done the distributor on my Mark V and reset the timing.
Static timing would be fine to get the car running but a timing light will allow finer tuning.
Also once it is timed with a light at idle check to see if the spark advances as you rev up the engine.
Just a thought.
Regards, Graham.

Hi Graham, yes have checked timing twice and used timing light to check advance upon revving engine. Advance worked 30 degrees or so, no wheel.

A real fast thing to check for, is high-pressure spark plug misfires. Use a new set: if the problem persists, likely you have eliminated the plugs.

About 35 years ago, I began having lots of Champion plugs do this very thing, which switched me to using MGKs, a move I’ve never looked back on.

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.012 gap would be too small? I thought .016 is the recommended. If the plugs don’t solve it next step would be the leads.

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Standard ignition should be between 0.016” to 0.020”.

Jaguar manual page P.15 contact setting max opening of .012 (.31 mm). This is what I have always used. Car was running very well until late last fall. Thanks for the input

If 0.012 works… excellent!

The gap within that range is not that critical.

Fuel feed is fine. I did a flow test and it was more than sufficient. Screens in fuel bowl are clear.

Probably already checked this but oil level in carb?

Another thing: those #%€&ing condensors can emulate all manner of problems.

I would experiment with a thicker oil in the dashpots since it sounds to me like a too-lean condition is induced when you stomp on it…certainly could explain the difference you’ve noticed between light vs. heavy throttle application.

Oil level in dashpot is good. Viscosity issue is one I never thought of. I have ATF in it now. Maybe put in some 10/30. Cant hurt.
Have switched complete dizzy with no change. Condensor I think is OK. I did swap coils as well. It is a hairpuller.

If the condensor doesn’t help, definitely try another coil.

On the electrical side, things to check: bad plugs, bad condensor, bad coil or positive ground coil used on car converted to negative ground (assuming there is only one ballast resistor, around 4 ohms and not one in the coil and one external). The spark plugs need a bit higher voltage to fire under load, compared to no load, a bad condensor or improperly-configured coil can still fire the plugs under no load, but not fire the plugs under load.

On ignition timing: improper advance curve under load (vacuum issue, distributor springs incorrect).

On fuel side: floats too low or other something else producing insufficiently rich mixture (too lean under load).

bet its the condensor

That, or plugs: I am SOOOOOO glad I got rid of the entire antiquated ignition on the Jag.

Next EDIS will be in my Rover.

Any of the HT ignition parts

I had a similar problem with my 4.2 E Type particularly once the engine was hot. It turned out to be the plug leads. It was the last thing I suspected because they were not very old and had come as a set from one of our well known suppliers. Changing them again was really an act of desperation but I had changed everything else and it fixed the problem.

You might be able to see some sparks or corona discharge in a darkened garage.

Shropshire, UK