Submitted by alan.barc@veri… on Wed, 09/10/2008 - 12:03

Pulled the head for a valve job on this just-acquired 4.2 due to lack of power and rough running at all rpm's. Cylinders 1, 4-6 have a heavy carbon build up around the ports, while 3 and 4 are spotlessly clean. All the piston heads have carbon build-up. My jag-man says it's oil getting past the rings which therefore must be replaced and while we're at it let's do a complete rebuild at big bucks (the engine was supposedly rebuilt about 1990 and odomoter now shows as 65K). Non-jag guy says it could be an ignition problem (previous owner installed Petronix). Either way, there's no noticeable exhaust smoke.

Any similar experiences, comments or ideas would be appreciated. Thanks

Submitted by alan.barc@veri… on Fri, 09/19/2008 - 11:46

You guys are just awesome. Thanks
Biggest problem with mechanic is he restores Jags (and other Brit cars) so he likes to tear down and rebuild. Other than his ears and eyes (40+ years experience with Jags) he's not good at diagnostics first then correct specific problem, . I've learned my lesson. Most of the 'simple' things had been done. Compression test AFTER the valve job (the exhaust valves were shot) shows 123 on all cylinders which seems low for a 9 to 1 engine. Runs well at 3,000 RPM and higher but idles too fast and misses at low rev pick up. Plugs 4-6 are good but 1-3 show heavy soot which I attribute to a rear carb problem (choke adjusment is OK). So looks like the next job will be to rebuild the carbs .

Patrick - Do you have any experience with Motorcars Ltd? - They have a Dallas location as well as Houston. Thanks

Submitted by wcjssj@comcast.net on Wed, 09/10/2008 - 21:50

Hi Again

More thoughts. Every classic car I have bought has been rough running and carboned up. I replaced plugs, wires, distributor cap, rotor, and coil. Timed the ignition. Rebuilt the carbs. Cleaned out the gas tank. Run a few tank fulls thru with extra Techron. Had a leak down test. Has all this been done? How did you know you needed a valve job? More details would be helpful.

Is your jag-man highly recommended?

Most oil leaks on these cars are thru the valve stem seals.

Nice looking car.

Curt

Submitted by wcjssj@comcast.net on Wed, 09/10/2008 - 20:53

Hi Alan

A leak down test (about $100) before the head was removed would have told you the condition of the rings, and the valves, and the leakage around the valve stems, separately. Your mechanic should have suggested this. Carbon can be caused by many things, including idling and driving around town, cheap gas, and rich carburators. Most owners use premium Chevron with Techron. Techron removes carbon and prevents build up. If you do remove the engine, be sure to put in a new clutch. The engine and transmission must be removed as a unit in this car, even just to put a new clutch in. I have a Pertronix unit and love it. No carbon build up in my engine.

Good luck

Curt

Submitted by SC38-21185J on Wed, 09/10/2008 - 20:03

Well sure, could be any number of thigs causing rough rpms. What did the spark splugs look like? That would be my first indicator. More than likely the carburetors were incorrectly tuned, running too rich. One has to wonder why some rings were leaking while other were not. You really shouldn't have to be doing this at your mileage. I would personally ditch the Petronix, but a lot of people like them.

But on the other hand, you will have a great-running engine after all is said and done. Just for the sake of it, I'd have the carbs completetly cleaned and rebuilt. I assume the guy doing the engine is a good mechanic...let him get the engine started and running well. Don't forget to run the engine in. Use Mobil One after run-in.

Patrick