I've had a miss that seems to have gotten significantly worse since the end of last year - looking for thoughts or suggestions. Best way to describe it is between 1100-1200rpms (just off idle) to about 1800-2000rpms, it acts like it has the biggest cam in the world. While it does have a relatively aggressive cam (Comp Thumpr), previously just off idle it would smooth out as one would expect. WOT throttle performance does not seem to be impacted. A little detail on my setup. Original 401, rebuilt in 1997 or 1998, about 25k on the rebuild. Its been flawless in its current state since 2011 or 2012. Has seen it's fair share of nitrous over the last 3-4 years (75-100 shot, aftermarket fuel system that is up to the task). I initially suspected it was ignition related, so I replaced the plugs, checked plug wire resistance (all ok), new MSD Blaster 2 coil, new cap/rotor, and new Pertronix Ignitor III ignition module (receiving battery voltage via triggered relay). None of this helped. Did a compression test (cold), highest was 160, #4 was 120-125, all other cylinders were 135-150. Pulled the valve covers, does not appear to have wiped a cam lobe. Leaning towards a burnt valve at this point. Planning on doing a leak down test next. Anybody see something obvious I am missing? Thanks for any help!
Check for vacuum leaks. They can show up especially with a big cam at lower rpm and then smooth out once the engine is at cruise speed.
Are you still running a stock intake manifold? Check closely for cracks, I had that problem on my 364.
Will have to double check for vacuum leaks, but I didn't find any the first go around. Not running vacuum advance, so should be good there. The intake is an original piece that has been ported - where did you see your cracks? Other reason I am leaning toward a burnt valve is that although it smooths out over 2000rpms, I think I can hear a faint popping in the exhaust. Hard to tell thought since the dumped Flowmasters tend to make quiet noises hard to hear.
When is your timing all in by? Not having vacuum advance at part throttle could negatively affect the transition between initial and mechanical advance, leaving the overall timing becoming overly retarded in that RPM range before the mechanical kicks in. This could give the impression of a heavy miss as the engine struggles to keep itself running. Just an idea
I had a similar driveability issue on my Mustang; one of the exhaust valves was sunk. It's probably less likely on a Nailhead, but it's an easy check (start with the low compression cylinder). If one valve stem tip is higher than the rest, there's your problem. Just something to check.
A burnt valve would also sack the compression on that cylinder. If they all have 125 or better the valves are sealing. I would lean towards a carb issue. Maybe a vacuum leak causing the power valve to open too quickly and causing a rich condition?
Too much variation in compression for a completely smooth running engine. Popping? possible worn valve guides. Idle mixture too rich and will cause stumble when transitioning to just off idle (jets above the throttle plates). Set the idle mixture to max rpm then turn in (leaner) to get a 50-100 rpm drop on each screw..reset the idle speed.
That compression seems extremely low on the lower cylinders for something with 25k miles. Also too much variation. My stock 401 with 109k miles has a low of 145 and a high of 160, with the others at 150 or 155 psi.
Thanks for the suggestions guys! Have not gotten around to doing much yet, hopefully this weekend I can tackle some of the additional suggestions. Responses below! 34* total timing all in by 2200 or 2300rpms. Ends up being around 20* initial. Ran it this way for years with no issues. Originally disconnected the vacuum advance because it provided too much advance at highway speeds, and never got around to fabbing up something to limit it. Haven't put a gauge on it, was jumping straight to a leak down test. I've thought there was an off chance it could be carb related. I like the idea - will add that to the list! Yes, around the carb base and intake base. No change. Actual compression numbers are as follows. 1: 160psi, 3: 145psi, 5: 135psi, 7: 140psi. 2: 140psi, 4: 125psi, 6: 135psi, 8: 150psi. Test was done cold. Would prefer less variation, but its not awful. One thing to remember is that with my cam, it's probably bleeding off some cranking compression due to the valve timing/overlap, especially compared to a stock cam, since my static compression is still only 9.3:1 or 9.4:1.
You're correct that the aggressive cam will somewhat affect max compression readings. Could be that testing the compression when hot might even it out a bit. FYI "engine restorer" additive has worked great for my car. Mine used to have compression readings much like yours before adding it to my oil back in 2011. IDK how using a big cam affects timing, but the factory dual quad had an aggressive cam and initial advance was 12*. Max should be about 30* at 3000 rpm or before. My car is set up that way and runs great on 93 pump gas. Any more than 30* advance will detonate under load, but did not cause what you are describing. It sounds to me like a vacuum leak or possibly an internal issue with the carb. Good luck and keep us posted.
Andy, i had a bad pertronix III it would break up at a certain rpms. any way to throw a different one in or put the points back in just to make sure its not the pertronix. for the carb just take it apart and spray all the passages and blow them out. if your running ethanol gas it will gum up the passages.
Was the compression test done with all the plugs out? If not it's easy to miss a blown head gasket inbetween cylinders!
My cam needs a lot more initial. Ideally I'd be closer to 30 or 32* total, but to get adequate initial with the re-curve kit, it ended up at 34*. I've never had an issue with detonation though, since my dynamic compression is lower. Hey Joe, I did throw a brand new one in, didn't change at all. Unless I've got the worst luck in the world?! I try and avoid ethanol gas at all costs, but every so often end up with a few gallons. Pulling and cleaning the carb is on the list after I throw a vacuum gauge on it, and do a leak down test. Yep, all the plugs were pulled.
125 to 160 lbs psi is too much variation. Cam or not, the cylinders on a 25,000 mile engine should be closer together then that in compression. I stand by what I said earlier - something ain't right.
