Start with a list
Hi Warren
Start with a list of possible causes once you have a list of possible put the list in order of difficulty to check and fix. Once you have done that start with simple end of the list.
Now for my basic list for loss of power:
1. Bad gas or water in the fuel
2. Plugged or nearly plugged fuel filter, this fits your runs but doesn't take a load
3. One at a time unplug and replug each spark lead from plug and distributor (if you have had any of the leads off just before this problem started check the firing order)
4. Loose or broken spark lead, quick check for this is to ground a screw drive with a jumper lead, then with engine running ground out each spark plug one at a time see if find a plug that when you ground it out doesn't make a difference. If you find a plug check or replace the lead you can check it by unhook in it from the plug and holding it 1/8 - 1/4 from block if you get a good spark pull the plug.
5. If the plug looks good swap it with a another and see if the problem moves.
Now to start checking for internal
6. Connect a vacuum gauge and look for a quick fluctuation, which would be an indication of a problem.
7. Do a compression test look for low readings - if you find one low cylinder do not assume it is time for rebuild another test.
8. Check the valve clearance, you are looking for exhaust valves in particular that have no clearance. If you find that you have less than what the manual calls for you may get lucky and adjusting the valves may restore the power. The problem is that with no clearance the valve doesn't seal. This assumes that your engine doesn't have hydraulic lifters.
All of the problems I've described have happened to one of my trucks.
Cheers Phil
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