Veteran
Posts: 143
 
Location: Denver, Colorado, USA | udoittwo - 2024-03-16 9:40 AM I've been GM all my life until I bought my 60 Plymouth 318/powerflight. I never had a car with solid lifters so I never had to adjust valves. What causes them to loose adjustment and happens when they get out of adjustment? I'm guessing some springs get tired especially on a car such as mine that was in a barn for 30 years? Anyway, my car is idleling and running a little rough. I've yet to check compression but have done a tune up including replacing the carb with no difference. It's not making any noises in the valve train or backfiring through the carb or exhaust but I was wondering, what are the symptoms of it needing adjustment and could that make a rough running motor without being noisy? I just started wondering about valve adjustment as I probably have 30,000 since I rebuilt the motor and haven't touched the valves. Never thought about it before. After camshaft and lifter break-in, the primary cause of a change in valve lash is seat wear and valve face wear. As the seat and valve face wear against each other, the valve creeps up closer to the rocker arm tip, tightening the lash. This wear is especially pronounced on the exhaust with unleaded gas if hardened exhaust seats are not used. If lash is allowed to tighten too much, the valves become preloaded and hang open leading to a number of issues including but not limited to rough running, loss of performance, and burnt valves. In the scenario of lash too tight, the valvetrain won't be noisy since there is a lack of clearance. There is very little wear to the A318 camshaft lobes, lifter feet, valve stem tips, rocker arm tips, rocker arm bores, and rocker arm shafts after initial break-in on a rebuilt engine with properly set geometry, seat pressure, and good oiling. The limited wear at these locations after break-in loosens lash, which can lead to a number of issues when out of spec. However, in a healthy engine without faulty parts the seat and face wear tightens lash faster than the wear to components loosens it, so loose lash is unusual and a sign of a problem. For solid lifters, the lash should have been set to cold spec on the engine stand when the engine was rebuilt. The cam grinder spec takes precedence over the service manual, but A318 COLD specs for 1960's factory cam grinds and OEM-type lifters are generally 0.015" intake and 0.023" exhaust. After the engine is run for 20 - 30 minutes for camshaft break-in and while the engine is up to temp, the valves are checked/readjusted to the hot spec of 0.013" intake and 0.21" exhaust. Note that these specifications were tighter in earlier manuals at 0.010" intake 0.018" exhaust, but Mopar loosened them by the mid-1960s. The lash is then checked hot again at 500 and 1,000 miles. At the 1,000 check, if the lash hasn't changed since the 500-mile check the valvetrain is now at its baseline. From that point forward, the interval depends on the build and use. If it's a low-performance factory spec build that doesn't see over 4,500 RPM, lash should be checked at least every 12,000 miles. If it's a performance build that is driven hard and sees over 4,500 RPM, lash should be checked at least every 3,000 miles and more frequently if the engine sees regular track time (I check every 1,000 miles if I'm regularly racing the engine). If you haven't checked lash in 30,000 miles after the rebuild, I'd definitely get the engine up to running temp, shut it down, and check lash before running it any more. If you find that the lash is tight and adjusting it doesn’t resolve your rough running issue and you have ruled out other factors, I’d perform a leak-down test. Since you mentioned you've never had solid lifters before, I have a tech article that might help you in the adjustment: https://poly318.com/poly-318-valve-lash-preload-adjustment/ |
? about tappet adjustment
