Our shop was in a “economically challenged“ part of town and we did a lot of work for used car lots. Instead of replacing valve guides more often we knurled them and reamed them to size. We even had a guy that would ask us to only regrind the damaged journal on crankshafts and sell him only one set of bearing shells when an engine would spin a rod bearing. My boss went along with it... I was younger then and allowed that to bother me. This guy also reused head gaskets. He would spray them with that copper spray stuff. He would pay $5 bucks for a can of that but not $20 for a head gasket.
In reference to Skip‘s comments on the valve guide and likely bent valve, I have more recent and relevant experience. Not long ago I acquired a 4.0 Gems engine that had been stored at least 10 years (probably closer to 20) after only covering 27,000 miles. The outside of the engine looked like hell but the inside was like new. I know because I took it apart to inspect it and replace the head gaskets as well as every seal and other gasket. I was surprised that several of the valves were oddly tight in the guides. I opened them up a tiny bit to assure they didn’t get stuck. I do suspect you have a bent valve. if that is all that is wrong, its not too bad. The shop that did the work might even fix it for free. Tearing it down and putting it back together is a bummer but if you have the time to do it at least it isn’t expensive.
if you have a bore scope you can look through the spark plug hole. If you don’t have one they are fairly cheap and is a great tool to have on hand for many reasons. You will have to look very carefully as it doesn’t take much “bending” to cause such a problem,
I had a friend who over revved his old Triumph. It had very thin valve stems. It ran like crap right after. He knew he bent some valves. His solution... drive it. “They will straighten themselves out.” Much to my surprise the problem got better. I’m not saying it was 100% better, but it actually ran pretty well after a while. It was kind of a junker and we were poor teenagers. Our old $300 cars didn’t have ECUs and the only trouble lights were temperature and oil pressure. Ignorance was bliss...
Also, while I did work as a machinist, Skip is actually much more experienced and knowledgeable on this specific issue. I wasn’t aware until I read this post this was a typical Rover V8 issue. If I was having such a problem, he is who I would ask for advice,
I have a brand new set of 4.0 heads I was going to use on my next build. I now plan to take them apart and measure the valve stems and guides.
Also, on lapping the valves, we didn’t do it when we did valve jobs unless it was requested. We also didn’t do it the way you are “supposed to do it.” We used a hand drill on the valve spring side of the stem. The machinist that taught me that, was in his 70s and had been an automotive machinist since his teen years. Never had one come back with or without lapping.
.Lapping does make it a better job, that can’t be argued. . If you are just replacing a valve or two, it works well. It also can reveal if you have a problem.