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Need some diagnostic expertise please: Engine/ignition?

markcharltonmarkcharltonOld enough to know better and still young enough not to care. My '71 Pantera
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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George PGeorge PAdmin.
Mark,

this doesn't sound like a fuel supply problem, so I'm ruling that out for now. If the motor was starving for fuel it would "buck" and not just run out of steam.

Here's some things you can do:

Check to verify the secondary butterflies or their linkage is not rubbing on the intake manifold somewhere, preventing the secondaries from opening. If the carb has mechanical secondaries you can pull the carb linkage while you look down the carburetor and verify all 4 butterflies go wide open.

Pull a spark plug, connect it to its distributor wire and lay it on a metal part of the engine where it will be visible while the engine is being cranked. Have the engine cranked while you watch, look for a fat blue spark. If the spark is weak and orange in color, you got ignition problems.

Crank in some more static advance into the distributor, 18 to 20 degrees, and check if the engine will rev higher.

With a timing light hooked to the motor, start the motor & rev it, you should see the timing advance as the revs of the motor increases.


cowboy from hell
markcharltonmarkcharltonOld enough to know better and still young enough not to care. My '71 Pantera
I went through everything again today. Vacuum is good. 18 at idle, climbs to 25 when rev'd and no fluctuations. A manual pump hooked up to the distributor verified that the advance does respond well to vacuum. Testing the vacuum at various locations showed no leaks in the system.

Checked the timing again and it was set at 10. Did it move after I set it last time? Who knows, but when re-set to 6 it ran fine right up to 5000. Hmmm.

Spark seems good and indeed, the engine does respond when the advance is changed. I just expected it to be more affected by such large changes in timing at idle.

I bled the cooling system again (got a bit more air out) and now she runs great and pretty much stays under an indicated 200 on the water temp gauge. Still a bit warmer than the 190 she used to run before, but maybe a tired thermostat or some crud is impeding flow. I'll look into that next.

Maybe it was just bad timing and air. Seemed fine after an hour long run over the back roads here. Up to a hundred and tons of loping around at 25—30. I am happy again.

Thanks for the input George and Ron.
sickcatsickcatMember
Hello Mark,
Had the same problem in the past, had my initial advance @ 8*, was running a little warm & no real get up and go.
I verified Top Dead Center & the Marker was a little off. This is done by bringing #1 piston to the top with the spark plug out " clockwise & either making a tool to stop the piston or using an indicator.
You then mark the dampner & then go counter clockwise and mark the dampner at the very same reading on the indicator or against the stop. the engine should not rotate all the way threw with the stop and this is ok! Now you should have 2 lines from the procedure and in the exact middle is true TDC. if your pointer does not line up to this line you should adjust the pointer or a least adjust for your timing and record it.
I set My Pantera to the Plate in the Engine Compartment Spec's which are 16* BTDC and at 900 rpm Idle. This is with the Vacumme hose disconnected @ the distributer and plugged.
You'll need an RPM meter and an Timing strobe light, Rpm on dash is usually off, you will more than likely have to go back and forth a few times. Once you get this set then its time to adjust the carb idle mixture.
watching the rpm gauge & screw in the small needle screw real slow on the side of the Carb, some have 2 and some have 4. when you see the idle starting to drop, back out slow and when the idle stops climbing, back out 1/4 turn more and do the next, continue till all are complete.
Now recheck all your setting one more time And redo the idle screws again. rehook up vacume & give her a test drive
You'll find that the Cleveland responds well and crisp after.

Mark
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