I recently replaced my head gaskets (one side was leaking but I did both). All seemed to go smoothly and upon firing the engine it ran quite nicely — at low (under 3500) revs. When running above 3000 rpm, as revs increase the engine feels like it is suffocating or otherwise straining to "put out". At idle all is fine.
There are no obvious (to me anyway) odd valve noises or other internal engine sounds, but when driving for a while it now wants to run much hotter than before. It doesn't seem to get unusually hot at idle and was the same before.
Since I had not removed the distributor, I didn't initially re-do the timing, but after driving and feeling something wasn't right, I re-set it. Stupidly, I did NOT check to see what it was at before I loosened the dist. bolt.
One thing I found very strange was when setting the timing, I was stunned at how little impact rotating the distributor up to 30 degrees made to the engine running. When I've set timing on other cars (small 4 and 6 cyl engines), turning the distributor only 10 degrees would stall the engine. Not here. The timing can be altered to 20 degrees (indicated) and still idles reasonably well. Is that normal?
Other Potentially Relevant Information:
• high compression 1971 351C w/4V heads
• before the gasket replacement, the engine ran very strong and smoothly
• FelPro Permatorque blue gaskets were used and oriented correctly (same gasket for both sides, but flipped—front is forward on both), properly step-torqued to spec.
• valvetrain is completely stock non-adjustable and showed virtually no wear (engine has 17,000 miles) to valves, seats, rods or rockers. Lifters were not removed from block.
• timing is currently set at 6 degrees BTDC (and was set with dist. vac hose off and pugged).
• new NGK UR4 plugs
• vacuum at 2000 rpm is 18 in hg and stable. I did not measure it at higher revs.
• Holley 650 carb is fairly recent (5 years) and was running perfectly before gasket replacement.
• turkey pan gasket (with ends) was also replaced and seems (visually) to be sealed well.
• Ignition is Ford Duraspark (2?) installed by previous owner about 6 years ago
• dist cap and rotor are recent and in good shape
• cooling system was running about 180+ degrees before vs. 220+ after.
• water pump is 1 year old Edelbrock
• thermostat has not been changed recently but was working fine last year
The most plausible theory I've had suggested so far is some sort of ignition advance problem. Unfortunately for me, I do not know what or how to check/test for that.
Any help and suggestions would be greatly appreciated.
Mark
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