Still missing under load

As per old post I am still trying to find cause of my 1951 120 missing badly under load. Carbs are tuned fine, timing is spot on, advance is great etc. Have tried diff dizzy and HT leads. Same result. Under takeoff with moderate go pedal it will miss terribly. If easing on throttle it will accelerate nicely through the gears albeit very slowly. However when at speed on highway and you put your foot in it it responds great with no missing. Only 300 miles on total rebuild. When first finished resto it was marvellous. Suddenly I am faced with this. I might add that compression is good and even, not broken it yet either. I am now considering whether the timing chain tensioners are giving me grief. Has anyone experienced timing chain problems and what the symptoms may be. Also I will say a very experienced Jaguar mechanic has been through it extensively last year and he was stumped as well. Been with british cars since 1971 and never been stumped. Any sage advice out there .

I know this may be a long shot, but I spent weeks trying to sort out what sounds like a similiar issue…Maybe I was silly, but I was setting the timing from the tach and it ended up that the tach was way off…

Pull dist cap and see if rotor springs back. If not pull out and lube mech advance. If that’s not it check vacuum advance is operating.

Pat H

Thanks Jeff. I did static and dynamic timing. All good.

Thanks pat. Did all that as well as different distributors. Tried electronic ignition ( Petronix). No change. Maddening.

Have you tested fuel pump pressure and flow?

Have you tried hotwiring the ignition from a separate battery?

Don’t think it would be the timing chains tensioners , if the chain’s are that lose you would know !

Sunk floats
Wrong needles
Wrong oil in carb pistons
Dirt in float bowls
Intake manifold air leak
What fuel pump are you using?
Do you have the 1/4" thick insulators between carbs and intake manifold?

Clogged fuel filter, clogged fuel line.

Pressure and flow up to spec.

It sounds like the ignition timing is off to me. Please explain how you dynamically timed a 1951 XK engine. There are ways to do it, but there are no factory timing marks on the engine for use with a timing light.

Sounds to me like your problem exists under low vacuum/low total air flow(engine rpm) condition…so…in line with what Rob has mentioned, I would try installing heavier dashpot springs, or simply “weight” the dashpots by slipping a metal washer over the dashpot tube to make it heavier. The idea is to purposely richen the mixture under low-vacuum condition. Of course, this should be done only after checking the oil level and viscosity of the dashpot oil.

Hi Mike, seems to me at TDC we marked crank pulley and oil pan with whiteout and then made 45 degree mark from that. Not totally accurate just to ensure it is advancing. Somewhat around 30 degrees. The Jag mechanic confirmed it was good. I assumed he had a degree wheel.

Thanks for that Lee. It’s just that it was running just beautifully and suddenly developed the problem. Nothing gradual…

Just trying to find out if it is fuel, or ignition.

Will try that and the possible manifold leak. Thanks everyone.


Done, difference.

If it improves when you feather the throttle it’s consistent with weak mixture. Try giving it a bit of choke when it next happens: either it will drastically worsen if already rich (unlikely) or it will run much better while you find the choke ‘sweet spot’

When we tuned up my HS8 carbs it ran very nice in the garage , tick over was spot on , revs smooth from 700 to 3000 , soon as we took it out for a test drive , it completely changed , was missing cutting out , we enriched the mixture and it was fine !
Got a colour tune after that , takes the guess work out of what the mixture is !
Don’t think because you have replaced something with a new part , the new part is good ,
Could be fuel or electrical , prosses of elimination , don’t give up , it will be very satisfying once you have found the fault , and kick yourself for not identifying it sooner :blush: