When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Over the winter, I rebuilt the engine in the 78 F-150 4WD 351M C6.
I installed a:
Summit RV Cam
Summit double-roller timing set with straight-up timing
Ford 400 heads
Edelbrock Performer intake
Holley 4 barrel vacuum secondaries - 600 cfm.
Once back together, I set the ignition timing to spec, and it ran like crap! I then adjusted the ignition timing with a vacuum gauge and tach, and recorded the following values:
After a week, I repeated the test. Same results. Truck runs great, plugs look good, truck starts quickly and there is no pinging on acceleration (preignition?). There is, however, a ticking sound on DEceleration - I think it's an exhaust leak but haven't found it yet.
I realize I'm outside the realm of factory spec with this build, but doesn't 28 degrees seem a bit "off"?
Your timing marks could be off... sometimes the damper ring slips or other problems. Suggest you check TDC with the positive stop method. The basic process is outlined in the link.
Once you know where true TDC is on the damper, you can get more accurate timing and trouble shooting.
Last month, I pulled the engine. When putting everything back together, I lifted the engine by putting a block under the harmonic balancer and used a floor jack. The outer ring of the harmonic balancer came completely off the inner hub, so it's possible that it had slipped when I had lifted the engine previously
When I had the engine out, I verified the straight-up timing, and installed a new harmonic balancer. I re-timed the engine, using a vacuum gauge and tach, and it still runs best at about 26 degrees
When setting the timing, I had the vacuum to the distributor disconnected and plugged. The rest of the time, the distributor gets ported vacuum off the carb. I used full manifold vacuum for the testing
I know the 351m family had the timing retarded several times over the life of the engine. Is it possible that the timing marks are set to account for the retarded timing, and my straight-up timing set has negated the factory mods?
i don't worry about it anymore, but I am curious about it
NO, the timing chain sprocket just sets the timing of the camshaft in relation to the crank shaft. The timing marks on the harmonic balancer are timing in respect to piston position on #1. TDC on the HB is TDC of #1 no matter how you set the cam timing with the chain set (assuming the HB is marked correctly and outer ring hasn't slipped).
re Ign Timing:
Mine has a quick advance kit in my distributor so to make sure that's not affecting my overall timing in a detrimental way, I block vacuum, set timing at 34-35 degrees BTDC with rpms elevated where the centrifugal is all in ..... so max is 34-35 degrees under load / no vacuum. Then I let her back to idle, pull plug on vacuum and hook up to ported.
If I were to attempt setting at idle and any centrifugal is maybe in, my totals would be less at WOT and a matter of luck.
I think I have the original distributor. Would the springs weaken over 38 years and allow the centrifugal weights to change the advance at low RPMs-say 800ish? How could I check?
I think I have the original distributor. Would the springs weaken over 38 years and allow the centrifugal weights to change the advance at low RPMs-say 800ish? How could I check?
OEM springs were pretty stiff compared to the quick advance kit. You could pull the vac hose and plug it, put a light on the timing marks and watch for when it starts to advance and when it stops (rpm) as you slowly bring rpms up from low idle.
Mine is all in pretty early, before 2,000 anyway.
Back in the day shops had distributor machines that let you chuck a distributor in it and check such things, adjust, etc .... hard to find now.
Even if your's is totally stock, you can still set timing at a total with vacuum plugged as I did, then simply hook vacuum back up. 351M or 400 s don't like over about 34-35 degrees BTC under WOT.
The vacuum method is a good way to double check timing.
That said, if she runs best with 24 degrees advance at idle measured with timing light, something is off. Even with a hot racing cam 20 degrees should be plenty. OE was 8-12 degrees.
Even if the vacuum advance is fast/early 24 degrees measured at idle is far more than normal... however it got there.
As posted, full mechanical advance should not be more than 34-36 degrees (3000 RPM, vacuum advance disconnected).
I have seen more than 10 degrees error using a replacement timing cover and pointer. Or maybe the damper is off. Or the timing chain is off a tooth or two.
I think it is worth re-checking TDC with the positive stop method.
The harmonic balancer is now, but reads the same as the original. I took a picture last week of the timing set (dots point at each other) and that's good
Could this be a symptom of the distributor being off by a tooth? I don't think it is, but I guess it could be. The vacuum can is in the correct general location
I have seen more than 10 degrees error using a replacement timing cover and pointer. Or maybe the damper is off. Or the timing chain is off a tooth or two.
I think you maybe didn't mean that like it come across .... whether a timing chain is off or dead on (or even whether present or not as in building a bottom end on a stand) has nothing to do with the alignment of timing pointer to harmonic balancer when #1 is at true TDC. The timing chain only affects the timing of cam shaft (valve events) in relation to crankshaft position. Even ignition timing is set by the HB-pointer relationship independent of how the cam is timed.
I think it is worth re-checking TDC with the positive stop method.
Any time in doubt, that's the way to check it for sure ....
.... but it can be checked with some degree of accuracy with plugs out and a depth gage of sorts inserted through the plug hole with care to determine when #1 is really at TDC.
As far as the pointer goes, if it is in the wrong position, it can show 20 degrees advance, for example when the true value is 10 degrees. The pointer is just pointing to the wrong location. I have had this problem. Variations in the damper can cause the same problem. With factory tolerances plus re-pop parts the errors can stack up pretty fast.
If cam timing is off, the engine usually wants different ignition timing to run somewhat decently. In this case the pointer/damper could be correct and the engine just wants unusual timing.
Well, I don't yet have a TDC tool, but I used a long 3/8" drive extension in the plug hole while a (very carefully) rotated the engine. Watching the pointer, and loosely supporting the extension, I could no longer determine upward movement after about 3 degrees BTDC. And I could feel the tool start to drop around 3 ATDC. Granted, it's not as good as a TDC tool, but I think I can rule out a very large discrepancy
i drove it quite a bit yesterday (to Mexico for tacos) and noticed the idle was getting rough. This morning, I found a leak on the climate control vacuum tube. Fixed that and all is well
i broke out the digital tach and distributor wrench and set the timing to factory spec. Of course, had to adjust the idle speed. And yes, disconnected and plugged the ported vacuum while setting.
as a test, I couldn't see any appreciable advance at low RPMs, so I don't think there's an issue with the advance coming in too soon
i reset the distributor to the factory spec, and the idle speed under full electrical and a/c load, and went for a drive. Runs great!
04-13-2016, 12:45 PM
