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Rough running 3.8 liter

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Rough running 3.8 liter
#1
  This topic is about my 1956 Jaguar XK140
140JAG Avatar
140JAG Silver Member Bob Hardison
Huntsville, AL, USA   USA
1956 Jaguar XK140 "Old Red"
1961 Jaguar Mark 2 "White Mark 2"
1965 Jaguar Mark 2 "Old Blue"
1969 Jaguar E-Type Convertible
I have an engine problem that is driving me crazy. My 140 has a 3.8 E-type block and the C-Type head with HD8s. 50 psi oil. It starts easily and smoothly and drives great until it starts an uphill pull when warm (65C). Then it loses power. It's not missing but when I add throttle, I get pre-ignition and low power. I have been able to limp it home, and I have checked timing at 12 BTDC at idle, and color-tuned the carbs to 'bunsen blue'. I suspect something breaks down (Condenser? Coil?). I can't find a vacuum leak. What am I missing? What procedure would you go through to find the problem?

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lmuzza Lindsay Musgrave
Country Victoria, Australia   AUS
Hi Bob, normally I would suspect weak spark- try a different coil or spark plugs, otherwise you may have a partially blocked fuel filter?

140JAG Avatar
140JAG Silver Member Bob Hardison
Huntsville, AL, USA   USA
1956 Jaguar XK140 "Old Red"
1961 Jaguar Mark 2 "White Mark 2"
1965 Jaguar Mark 2 "Old Blue"
1969 Jaguar E-Type Convertible
I'll give the coil a change. The car is on a recent rebuilt (>2 years), and the glass sediment bowl I added is clean, so it's probably not the gas.

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johnshep44 Avatar
johnshep44 John S
Tewkesbury, Glos, UK   GBR
Hi Bob,

I am assuming you are running a standard distributor. I had a similarly annoying problem on my 4.2 S1 E Type. It turned out to be a cheap Chinese rotor arm was the cause. How? I hear you ask. Well it turns out that these rotor arms are made of cheap red plastic which fits too tightly on the shaft. When the engine gets up to temperature, the expansion of the shaft causes a crack to open up in the rotor arm which leads to tracking and weak spark/misfire.

It was a maddeningly difficult fault to find but once I replaced the rotor with a high quality one from "The Distributor Doctor" http://www.distributordoctor.com the problem was solved. Also I had a subsequent failure of the equally cheap and nasty Chinese condenser which I replaced with a high quality one from the Doctor. I carry a spare of each now as they are so cheap as well as a set of his points but I have not had to replace them.

hope this helps

best of luck
John

140JAG Avatar
140JAG Silver Member Bob Hardison
Huntsville, AL, USA   USA
1956 Jaguar XK140 "Old Red"
1961 Jaguar Mark 2 "White Mark 2"
1965 Jaguar Mark 2 "Old Blue"
1969 Jaguar E-Type Convertible
Actually, I'm running a new Mallory dual point distributor,so I don't think that's it. I have checked the rotor and cap and they are relatively new. I'm going to replace the coil and see if that cures it. Thanks for the input.

PeterT Peter T
Oxford, Oxford, UK   GBR
It is worth checking the top hose for a very slight leak. Jaguar strategically placed the distributor under the hose such that, in the right conditions, the most insignificant drop of water can cause misfires.

140JAG Avatar
140JAG Silver Member Bob Hardison
Huntsville, AL, USA   USA
1956 Jaguar XK140 "Old Red"
1961 Jaguar Mark 2 "White Mark 2"
1965 Jaguar Mark 2 "Old Blue"
1969 Jaguar E-Type Convertible
Thank you, Peter. I have had that same issue on another car, and if you look closely, you can see where I have double clamped that upper hose. It absolutely does not leak. I am waiting for a break in the weather to try out the new coil which, like the old one, starts and runs fine (in the garage). I need to warm it up on the road and see if that fixes the problem.

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DAWTRIJAG Avatar
DAWTRIJAG David Warr
St. George's, NL, Canada   CAN
Hi Bob,
I had a similar problem with my 3.8 E-Type and it turned out to be two cracked jets in the two rear carbs. The jets were new but very finely cracked. When the engine was cold it started and ran great but once it warmed up it was stumbling. The jets were sticking in the choke position because they were no longer round. I had to remove the jets and sand them down so that they could still slide in the brass bushing. It was a very strange problem.
David

140JAG Avatar
140JAG Silver Member Bob Hardison
Huntsville, AL, USA   USA
1956 Jaguar XK140 "Old Red"
1961 Jaguar Mark 2 "White Mark 2"
1965 Jaguar Mark 2 "Old Blue"
1969 Jaguar E-Type Convertible
Peter, I was going back through all the advice, and I had let the enginge idle in the garage for a pretty long time, and guess what? I saw a drop or two of water from that upper hose, even with the double clamps. So I removed both of them (one was an original Cheney clamp) and replaced them. No drips from the second effort, so I'll hit the road and hope that will solve the problem. Thanks so much.

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PeterT Peter T
Oxford, Oxford, UK   GBR
Bob, only too pleased that my idea was even relevant. It's one that sticks in your mind for decades after it's caused you to push a Mk2 uphill to get it away from a T-junction. More to your situation, it's not necessarily a matter of effective clamping, the rubber at the end of the hose can degrade especially under and close to the clamps/clips. Water can then leak through the rubber even along the length of the hose under the clamp.

These days I prefer to follow the old Rolls-Royce policy and treat water hoses as a 'service item' - change them all if one looks a bit iffy/every few years/whenever there's an access opportunity for the ones that are difficult to reach.

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