The 250 is in a 68 truck with lots of miles. Engine is covered in a thick coat of crud so the engine hasn't been apart in long time if at all. #5 cylinder doesn't fire at low speeds but when the engine is reved it runs on all six. I can feel it and hear it at the exhaust pipe, misfire and then sounds very smooth when reved. I shorted the spark plugs and confirmed it's number 5.
Points ignition and it all seems ok with the usual slightly sloppy distributor shaft. Engine starts instantly but when cold runs very poorly (like only on 4 cylinders) until it gets some temperatures. It has a stock hand choke and I know how to use it
One warmed up it idles steady with the number 5 misfire.
The past owner tried to fix it and in the process replaced the tune up parts and put on a correct factory rebuilt carburetor.
I checked the compression, the cylinders are between 105-120 PSI, with number 5 being 105 psi along with a another one. Low but enough to fire the cylinder.
I adjusted the valves. The head was quite clean and all rockers were moving ,push rods spinning and oil flowing . When backing off the rocker until it clicks, the engine seems to run smoother. Tighten the rocker nut slowly just to the point of eliminating the clicking and the idle gets rougher...and will stay this way despite not being tighten down the usual 1/2- one turn more. Number 5 does this as well as number one cylinder but number one doesn't misfire.
Vacuum gauge reads pretty steady at 16 inches at a 500 rpm idle with the timing set at 4 degrees. Rev the engine and the vacuum drops to 5 and then 22 and the engine comes back to idle.. But just as the engine returns to idle the gauge needle fluctuates between 16-17 inches and stays that way for maybe 5-10 seconds before going back to a steady 16 inches at idle.
There's no vacuum leaks to be found and only a PCV and ported distributor vacuum.
No excessive blowby or exhaust smoke.
Because of the strange lifter reaction to adjustment I'm thinking there's a few lifters pumping up at idle speeds and one pumps up enough to hold open a valve on number 5...But the vacuum gauge needle should have been dropping on that cylinder... What do you think?? Thanks
