Rough engine that has me completely stumped

Re: Item number three in your procedure, for future reference.

I would think the device in series with the plug wires is a resistor. These have a damping/supression effect on the waveform which reduces the harmonics therin and reduces the radio interference radiated by the system.

If you're using an analog multimeter, try using a higher ohmmeter range. If it's digital autoranging, I would have thought it would have taken care of that unless there is a complete open circuit which depends on the high voltage to jump the gap. Tom Hughes

Gentleman...I'm happy to report a tentative resolution to my woes. The short answer is that my exhaust valve timing was off by 15-20 degrees due to the camshaft "adjustment sprocket" exhibiting stripped teeth and the exhaust cam subsequently rotating out of adjustment. Just for interest, here were the trouble shooting steps:
1. Carbs rebuilt by me and second checked by a pro carb shop...AOK.
2. Replaced all ignition electrical items, coil, cap, plugs, rotor, points, wires...AOK
3. Double-checked firing order by "running" the ignition wires from plug to distributor (a side note: my wires run through a cardboard manifold or sleeve, and I discovered that you cannot check this via a multi-meter as there appears to be a diode? or small fuse looking item in the sparkplug boot that prevents a continuity check).
4. Ignition timing checked and double checked at 10 degrees BTC at 1,000 rpm.
5. Dis-assembled, cleaned, and oiled the distributor with it's centrifugal advance mechanism. Other than a little "stickiness"...AOK.
6. A visual check of the plugs revealed 2,3,4,5 looking light tan and clean whilst 1 and 6 were black and sooty...obviously poor burning of the fuel/air mixture. As the electrics were previously checked okay and carb function would affect all cylinders, I decided to check the valve timing.
7. Timing chain tension was checked...AOK.
8. The last thing to check: with #6 cyl at TDC, the inlet cam was checked AOK with the Jag cam tool...the exhaust cam was not even close. Huh? While trying to adjust the exhaust cam sprocket, it was found to rotate rather freely when torque was applied to the nuts with a wrench. Further investigation revealed nicely rounded teeth on both the adjuster ring and the adjusting plate.

Parts are on order and I'm confident that things will be back to normal shortly. Thanks again to all who offered advice...it definitely helped throughout all the troubleshooting.

Cheers, Eric.

While your at it - Make sure that there is not a crack in the distributor cap -allowing a "spark jump". Happened with me on a Morgan -with very similar problems.

Jerry Ellison
Fayetteville, NC
XK140 OTS

I'm not convinced about "if the engine revs up tp 4000 in neutral fuel is fine" comment. I've had fuel pump or filter etc... issues before and there was no problem rev'g it up. granted that's on a 12 but the logic is the same. it doens't take much fuel to ge the engine to rev up, it's when you put a load on it and you need the fuel that problems will occur.

how about firing order ? that's a little too obvious but it's often the obvious that comes and bite you!

I woudl also try to spot the misfire with an IR thermometer. if you have a jug that's not firing, chances are its EGT or even the plug will be cooler... may not show if the misfire is very short but worth a try.

I'd also check or even replace the dis before tearing up into the chain...
Pascal Gademer

Removing the head is a little drastic. If the lower chain is loose, you can hear it. If the upper chain is loose, you can feel it after removing the cam covers. Have you checked dwell?

Okay gentleman, here is my plan of attack...
I'm planning on correcting the timing problem first. Driving with the gas cap off had no effect, staring into the guts of the distributor showed no obvious dfects, but the timing mark definitely jumps about way more than 1/4", especially whilst revving. A trusted friend from our Landrover club (and professional mechanic) suggested that a loose timing chain would be worth looking into. So I've ordered the timing chain tensioner tool, cam setting gauge, and all the appropriate gaskests for the job. I'll launch into the job shortly and will report back on this forum with the results. Any advice or pitfalls to know about ahead of time?
Cheers, Eric.

My '68 (built Feb '68) sounds like it is set up like yours and it does not have a vacuum advance. I have installed the Pertronix unit in my E-Type and 2 other cars and it works great and is very easy to install and fits all inside the original distributor. Probably easier to install than new points. Like Stew said my timing mark advances smoothly as engine is revved and does not jump around. One other thing the Pertronix unit let me do is easily install an in-line ignition kill switch for some theft protection without altering any of the original wiring etc. I do not think this would cure all your problems though if there is a problem with the centrifugal advance. I have had weights sieze and springs break on other cars. Hope this helps.

David Barnes '68 FHC

Eric
The jump you observe in the timing mark when checking timing concerns me. Is it more than a 1/4" inch from spot on? This indicates wear in the distributor - usually at the cam that the points use to open and close. Wear at this location can be overcome by use of a Petronix pointless ignition upgrade for about $90.00 from all the usual suspects. Your distributor is correct without the vacuum device. If you do not want to go to the Petronix upgrade, have the distributor rebuilt. I don't know who rebuilds these. My Series 2 Variation 1 is eaxcatly the same as yours and my timing mark is spot on - 10 degrees before top dead center at 1000-rpms with no observable jump using the original points. I believe my distributor was checked out at the time of engine rebuild in 1990, but less than 3000 miles ago. How old are your spark plug wires? The problem can be there too.
Stew Cleave
JOCO Chief Judge
'69 E-Type 2+2
'01 S-Type
and other LBC's

Sorry gentlemen for the delay in replying to your helpful suggestions/questions...I've been away on some recurrent flight training and just returned home. So herewith my responses:
1) Yes, two new floats during the carb rebuilds and set correctly to 17mm.
2) Yes, the engine does rev freely in neutral up to as high as my nerves will allow, but there is a definite roughness or stumble most of the way up. Does this then suggest ignition as Mr. Cameron suggested? Perhaps ties into the timing mark jumping about? Should it be basically rock steady at idle and up through higher rpms / advance? All I've dealt with before were my old Landrovers where it is routine to deal with wear and things moving about, but they still plug along.
3)I like the tips about fuel pressure as I have not checked the tank's pickup line or general sump condition yet. I'll eliminate the plugged gas cap idea tommorrow by driving a bit with the cap off.
4) I'm not sure what to make of Mr. Brady's question? Yes it's a real Jaguar, complete with Heritage Certificate and real oil puddles on my concrete floor.
The last thing I'd like to ask is...is it correct for the 1968's distributor to NOT have a vacuum advance pot? All that is there is the vernier adjustment for the centrifugal weights. Perhaps this was their first attempt at US smog compliance? Thanks again for all your suggestions.
Cheers, Eric.

The very small vent hole in your gas cap is often overlooked. It usually gets clogged with polish and can result in a vacuum that keeps the fuel pump from providing adequate pressure to the carbs.

Patrick McLoad
1966 RHD OTS E-Type
#1E1445

Is this a real car or a model? Jaguar affectionado and etc.

Mike Leicester

Check for fuel pressure (flow) at 3,500 RPM under load when it starts to stumble, may be a fuel pick up or screen problem in the tank resulting in you going lean. This can be done by adding a 'T' in the fuel line and monitoring a remote fuel gauge.
If it is a lean condition under load, be careful you don't burn a piston.

Regards
Mike

Here's the best troubleshooting tip I know: if the engine will rev up to say 4,000 with no load (in neutral) the fuel is fine: it's the ignition. If it won't, it might be either fuel or fire. So does your car rev up properly with no load, or not?
1969 E Type

Did you install new floats when you overhauled the carbs?