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1973 XJ6 Series 1 wants to stall


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Old car question. I have a classic I purchased recently because I want to maintain the vehicle as an original. It is one of the best condition originals of this model still in existence. Of course it's a Jag with many gremlins. My issue has been hard to diagnose. The shop that services "fixed" the problem with the previous owner but it still has the same issue. The car has 108,000 miles. Distributor, 1 fuel pump (it has 2), plugs and wires all replaced and timing was done recently. The issue: After driving for more than 20 minutes and at speeds over 40 the engine losses power and wants to stall. It's not correlated to transmission as that runs smoothly. I get no backfire it just wants to lose power after up at operating temperature over 40. If I take foot off accelerator I can overcome it for a bit but still wants to stall a few seconds after. If I go below 35 it never loses power or stalls. The idle is a tad bumpy but doesn't stall. I also have two symptoms related or unrelated 1.) Smell of fuel (normal of jag because hoses/pumps are in the trunk cabin) 2.) heavy exhaust fumes penetrating cabin


I'm not sure where to start and I don't want the mech to "fix" the wrong thing. 


I'm leaning on a back pressure issue? Perhaps exhaust system clog/leak or if this year has a catalytic converter-not sure?

So many possibilities- Fuel, Spark, exhaust? I think I can rule out spark or electrical as thats fairly new and now change on dash or other electrical components. 


Please advise!


Thanks Scotty!

2 Answers
Posted by: @brian-hawaii

if this year has a catalytic converter-not sure?

Catalytic converters were not used until 1975 in the U.S. and some cars I've seen didn't have them as late as 1980.

What kind of shape are the carbs in? Sounds to me though like the ghost of British Leyland...

Carbs/AED were tuned recently and good condition.

Posted by: @brian-hawaii

Carbs/AED were tuned recently and good condition.

Does it have an EGR valve? That's something that typically would start operating after the engine reaches operating temperature. If it's dumping too much exhaust gas back into the intake when engaged that would choke the engine down.

Or there might be some other temperature-sensitive, vacuum controlled emissions gadget causing a problem. 1970s cars tended to have quite a few of those. (I don't know the details of your model. When it comes to British cars I've only worked on stuff like like MG and Triumph sports cars.)