There may be a more serious problem here.
It sounds like the problem MIGHT be that the valve lifter is acting on one cylinder only, and not both. (Hence, the shuddering and continuing to run - it's running on ONE cylinder!)
This could be caused by failure of the link between the two "dog bones" in the timing chest or perhaps the big end roller has fallen out of one of the "dog bones" and it no longer lifts the cam follower.
If this is the case, it should be fixed quickly or contents of the timing chest could be seriously damaged by the loose bits.
This can be checked easily by removing the inspection caps and observing that the valve lifter is actually lifting the valve when operated.
It sounds like the problem MIGHT be that the valve lifter is acting on one cylinder only, and not both. (Hence, the shuddering and continuing to run - it's running on ONE cylinder!)
This could be caused by failure of the link between the two "dog bones" in the timing chest or perhaps the big end roller has fallen out of one of the "dog bones" and it no longer lifts the cam follower.
If this is the case, it should be fixed quickly or contents of the timing chest could be seriously damaged by the loose bits.
This can be checked easily by removing the inspection caps and observing that the valve lifter is actually lifting the valve when operated.