I have a 1966 F100, with a straight 6. I've had the timing set by hobbyist, and pro's. The pro's set and marked TDC last time. Slowly the truck kept performing worse over a 6 month period. Last week it finally just quit running.

I ordered a timing gun and used one for the first time, utilizing the mark the pros made I set the timing where it should be. The truck ran horrible, and barely could move out of the driveway. Out of frustration, I just ignored the marks and turned the distributor until it sounded good. Now it performs great, but the marks are way off!

What would make the TDC mark change? or am I just using a timing gun wrong?

 

Look at the harmonic balancer which has the timing marks painted on it. Look closely next to the timing chain case and see bits of ancient dried out rubber oozing out between the balancer and the hub (due to old age). When this happens the balancer can rotate on the hub and the marks will no longer sync with the timing indicator. This balancer should be replaced because if it deteriorates much more there is potential for disaster when /if it flies off the hub. Unfortunately these are really hard to find. There are a few services which claim they can rebuild them but I have no experience with that. Hope this helps.

 

While it's true the timing marks can shift the actual timing doesn't change and doesn't explain the performance loss over time. Maybe the distributor hold down bolt was loose?.

You'll want to replace the damper when you get a chance though.

 

Thanks for the ideas, I made sure the distributor was tight, so that was noan issues. This truck is very very original, I'm nearly certain nothing majorhas been done to the engine aside from servicing it. It’s so old someonethought something may be slipping inside the engine. I was starting to wonder ifI was just loosing so much compression from age, it was time for a completerebuild, which I am not interested in doing. It’s just a cheap old fun truck tome, so I’m not doing a restoration. I’ll keep it running as long a as I can

I've found the harmonic balancer in the LMC catalog, and I’ll probably givethat a try. But what is the damper and what does it do?

 

Hi - As the others have mentioned, changing the balancer is a good first step. Once you get that changed, and before you spend any money on anything else, here is what I suggest:
1) With new balancer on, remove all spark plugs.
2) Make sure ignition is off, wheels are chocked, transmission in neutral if manual, Park if automatic.
3) With all the plugs out, and nothing else to hold back the engine, just try moving the harmonic balancer back and forth (clockwise / counter clockwise) a little bit by hand. See if you have play in your timing gears. You will know when there is contact as the crank will get a little harder to turn in either direction once the timing gears engage the camshaft and try to open the valves.
4) If the play is excessive, it points to worn timing components. Next step is to check compression on all cylinders since you have plugs out anyway. A cheap gauge at Napa is like $30.00. Check the psi of each cylinder and see if they are all close to the same psi.

Now you can move forward based on what you find out. To answer your question on the balancer: http://en.wikipedia.org/wiki/Harmonic_balancer
Post again when you get the balancer on and can do those tests...

 

I've heard good things about these folks. If they don't have a damper for you, they can probably rebuild the one you have. I've purchased one from them, but haven't yet installed it.

DamperDoctor

~Steve

 

One other thing you might consider. I'm not sure how old the motor is. Has it just been painted to clean up a used motor or is it a new rebuild?

IF it is indeed a used (not rebuilt) motor, the timing chain could very well be stretched out and continuing to stretch. And, if it is worn, it could have very well jumped a tooth/gear. I've seen em jump 2 or 3 times and still run but only when the distibutor was turned more and more.

If this is an older build, you can try this:
Remove the valve cover. Use a long bar with the appropriate socket for the center crank pulley bolt and slowly rock the pulley left to right. While doing this watch the rocker *** move. There should be 'very little', if any hesitation in the movement of the ***. If there is a lot - chain is shot. And could be stretching more every tim you run the motor.

 

check the pin that holds the gear on the distributor shaft.

not sure when ford started using plastic coated timing gears, but iv'e seen the plastic break of, but the motor would still run. check for slop in timing chain.

 

the beautiful painted up motor picturedabove, is not mine.
Mine is old and ugly, likely with the original timing chain. Someone else suggested it could be stretched and slipping a tooth. So I'll test it and see how much play there is,

 

TDC never changes. When the no. 1 piston gets to the top of the compression stroke that is TDC. The only thing that will change timing is a chain slipping on the timing gears, a distributer not being tight and turning or possibly a spring breaking on the mechanical advance in a distributer which will cause the timing to advance. If the balancer is worn and slipped that will not cause timing to change, that will only make it look like it changed.

 

skidoorulz is absolutely correct. A slipped balancer will not change the timing of the cam/crank.
I will say this, I couldn't really see it very well when on my cell phone earlier, but now that I'm on my home computer and I see how bad that rubber Is on that balancer on that red motor - man, that is scary. That's an accident waiting to happen...
Will be very interested to see what you find.

 

OP says truck started running worse and worse over 6 month period until it actually quit running. Then he changed distributor position and truck ran well. For the most part, once your timing is set, you really shouldn't have to monkey with it for many miles, typically checked during tune-up intervals. IMO - Something in the distributor / timing components (points? or other items as Skidoorulz points out) would "seem" to be the place to look, however, until he can get a consistent mark on where TDC is for a baseline, he could be chasing ghosts as moving the distributor could be compensating for a different issue in order to get the truck running. Always good to start with a solid baseline and then follow a flow chart to find route cause -TDC correct (yes / no) - Compression good and within range psi of other cylinders (yes/no) - Timing set correct (yes / no) Point gap correct (yes/no) Vacuum advance operational (yes/no) Vacuum leaks (yes/no) etc. Thats just spark, timing and compression diagnostics, not even talking fuel issues. Reading spark plugs is also very helpful. Hope we all find out what the issue really was in the end...

 

I had this happen years ago to a 66 I had with the 300 I6,,, motor had a lot of miles on it, and every so often, it would start hard and run worse,,, kept doing it, ended up being the gear on the bottom of the distributor,,, the roll pin had sheared over time, and the gear would slowly slip like when I revved a bit to much, or when hot, and I would have to advance the timing again,,, pull your distributor and check,,, you can get a new one from NAPA for not to bad,,, had to put a new one in my current 66 Shrek. And NO, TDC should not change,,, if it does,, bad things like valves hitting pistons happen,,,, and on the 240 and 300, the cam gear and crank gears are in contact,,, no timing chain to slip or jump,,, had a lot of 300's,, never had the timing jump, one of the durability features of them!

 

Reading the possibilities, that would be the "best" problem to have?

Easy to check, easy to fix - though would be concerned, if it happened once it would probably happen again. What typically causes roll pins to break?

 

pickupmanx2 is correct, there are timing gears, not a chain. been awhile since i was into one of those, but iirc some came with a phonalic(hard plastic) cam gear. i have seen those strip teeth off, but when that happens you're dead right there.