Issue: XK140 MC (SE) engine. Starts up fine, idles smoothly and runs well until the engine gains sufficient heat to permit revs to the upper ranges. Engine pulls smooth then begins to break up. Gets progressively worse the longer I drive it. At the beginning of the drive the engine breaks up once I go over 4,500 RPM and toward the end of a 20 minute drive the engine breaks up once I go over 3,500 RPM. Engine will not rev much over 5,000 RPM. Distributor is a Pertronix Flame Thrower. When stopped I can rev (carefully) the engine over 4,000 RPM without any issues. It is only when the car is driving in gear that the engine breaks up.
I called Pertronix tech support and relayed these issues. Answer was that if the engine starts and runs it is not the distributor that is the issue. However to them it sounded like a poor ground connection resulting in loss of spark at higher revs and suggested that I check the engine to chassis ground. When I checked I found that the PO during the restoration had not installed ANY ground wires to the engine.
Thinking that I had found the needed “aha” moment, yesterday I installed the two ground wires in the two proper locations (rear RH motor mount and starter mount to chassis) removing paint when needed to make sure it was a good direct-to-metal ground.
I then test drove the car…and…there was absolutely no difference. Exact same engine break up as before.
Very frustrating.
So now…where to go from here? What do I check next?
It might be time to look at your carburetors, as it sounds like the engine is starving for fuel in the upper revs. You also could pull the sparkplugs and look at gaps and condition to see if some are running lean or rich. I am surprised that you still take that old engine to those kinds of revs. They were red-lined at 5000 back when they were new.
Guys…I generally don’t rev it over 4,000 ish. I was trying to diagnose what the engine was doing to see if it was only breaking up in a range of RPMs (i.e. clears up when it goes over xxx). I guess I also was not really clear in that it never got over 5,000 only that it broke up to the point that it wouldn’t reach 5,000…
As indicated the problem exists at much lower revs…which affects the “normal” daily driving range.
Well at least that portion of my post got your attention!
The PO did all the engine work and he did install a new pertronix coil at the same time as he installed the pertronix flame thrower distributor. Not sure about the resister wires though.
What is really confusing me is why it would only start to happen once the engine reaches operating temperature and then get worse the longer I drive it (and why only under load)?
I would try a new 3 ohm coil and make sure you are not running the original style copper conductor spark plug wires. The low resistance wires cause the coil to heat up more due to higher current flow.
it sounds like a fuel capacity issue i had, the tank was silty and actually plugged in the tank strainer and the big pump was full of crap. i used to take that 3.4 to 5500 + regularly but it needs lots of gas even for hs6 carbs in my opinion a stock early 3.4 is very strong engine with better con rods than the heavier later rods. just my 2 cents
Some progress to report. I happened to read an installation instruction for the Pertronix coil that said that the (positive) ground needs to be connected to either the battery positive terminal or the chassis. When I looked at my car, my coil had been grounded by the PO with a short wire to one of the bolts in the engine holding on the inlet for the radiator hose. I remembered that the tech support for Pertronix had told me that their system needed a strong ground to get the kind of spark needed at higher revs. So I ran a ground wire from the positive terminal of my battery to the coil and disconnected the one going to the engine.
I took a test drive - same route and amount of time as before - and the engine pulled smoothly with no missing under load . However - although it was MUCH better than before - now I had slight backfire occurring upon lifting the throttle (sounded like it was coming from the exhaust and not under the hood) and the engine seemed a little rough and slightly down on power.
But definitely an improvement.
Since the only thing I changed was the ground wire I’m thinking that with the coil/distributor now operating with the right amount of ground, there is clearly something else that I need to change.
I’m going to start off by changing to a different (new) set of spark plugs since the ones in there now look a little sooty and see if that changes anything.
Any suggestions on what would cause the backfire? If now the timing was a little off would that cause it?
Backfire issue is solved! It’s not the car’s fault - it’s the loose nut behind the wheel.
Apparently when I reinstalled the original plugs to test drive the car after changing the ground wire I inadvertently swapped the #2 and #3 wires…
So I put the new NGK’s in and the wires back where they belong and did another test drive. This time there was no backfiring and no breaking up under load. Engine pulled smoothly and rev’ed freely.
When I came back in I pulled the #1 and #6 NGK’s and they looked a little rich to me so I’ve still got some fine tuning to do. But the car felt great!
Exact same engine break up as before.
and did another test drive. This time there was no backfiring and no breaking up under load. Engine pulled smoothly and rev’ed freely.