Ok, I have been fighting a problem, that i have blamed on both the carb and ignition, so to date ive replaced both the carb and the entire ignition systems, Truck runs better but the origial problem still exists. All started after replacing linkage to carb, engine was again capable of WOT and problems began shortly after a few fun blasts down the highway. Tonight after changing the coil, the last thing not replaced, I noticed that the timing mark moves when I rev the engine (while timing) by more then 15 degrees. And yes I have the vacum advance unhooked. Does this sound like a timing chain that has steached? The truck wants to run best around 20 degress btdc. But that seems to change from time to time. Once it wouldn't even run or start at 10 degrees, but thats were its at right now. Going down the road, when the engine is placed under a load it will stumble and back fire through the carb, it also seems to lack power and accelerates poorly

 

The timing is supposed to move when you rev the engine. There are weights down in the distributor that advance the timing according to the rpm of the engine.

 

Franklin is correct when he says the timing marks changes when the engine goes from idle to being revved up.

Although,
Here's a way to tell if the timing chain and gears are stretched.

This is just to check ,for piece of mind.

Take the cap off the Distributor.

Now, put a power bar and socket on the harmonic balancer
(crankshaft bolt)
Now, While you turn the crank one direction,
Watch the rotor to see how many degrees the power bar travels before it starts to turn.
Now turn the crankshaft back the other direction, see how many degrees you turn the crankshaft before the rotor turns the other direction.

Get back with this amount.

**Remember 360 degrees to a complete circle
90* is a 1/4 of a turn
180* is 1/2 turn.



Dennis



Please Don't Ask Me Any Tough Questions As:
"I'm Saving My Memory For When I Develop Alzheimer's"


78 F-150 429CJ,Silver
w/Explorer Pkge.
641/2-Mustang 260,Pre-World's Fair Car.
64-Fairlane500 S/C waiting for a 390-4spd.
68-Mustang 289-Sunlit Gold 80,892Mi
78-Buick LeSabre 403 4V
84 Volvo DL Wagon

 

I'll get that hopefully within an hour or so, is there a way to tell if it has jumped a tooth?, without a teardown

 

since it was difficult to tell exactly how far it moved, I rotated the engine with the bar to TDC on Balancer going in one direction, then I reversed direction and it went 10 degrees exactly before the rotor moved. I say it was difficult because until i used the balancer for refrence I couldnt really tell how far i had gone. I'm completely at a loss with this one. After replacing all the componets related to ignition and the carb, the truck runs really pretty good except for this backfire through the carb, I just drove it again and am noticing the backfire escpecially after shifting from first to second and then starting to accelerate again. Thats when it really likes to do it. If I'm real easy it wont do it, but if I push it at all it backfires. It also likes to do this when running down the highway in third at say 70mph and I lay my foot into it. The only other peice of info I can offer is that with a vacum guage I get a flutuating needle reading at idle it hoovers around 15 but fluctuates rapidly a few pounds either way It smooths out at high rpm. But I don't have a miss.

 

Turn the damper and set the timing mark to the proper
BTDC spot just before the power stroke.

Take the cap off and * Eyeball * if everything lines up visually,
rotor to the metal contact in the cap.

I forget how many teeth are on a dist gear.
10 degrees says to me, it's a tight chain and gear.



Dennis



Please Don't Ask Me Any Tough Questions As:
"I'm Saving My Memory For When I Develop Alzheimer's"


78 F-150 429CJ,Silver
w/Explorer Pkge.
641/2-Mustang 260,Pre-World's Fair Car.
64-Fairlane500 S/C waiting for a 390-4spd.
68-Mustang 289-Sunlit Gold 80,892Mi
78-Buick LeSabre 403 4V
84 Volvo DL Wagon

 

Not sure exactly what your asking me to do. I did replace the dist after the problem appeared and now when the balancer indicates 10 degrees its right on the #1 cylinder in the cap. Is 10 degrees BTDC right? My shop manual ,a chilton, doesnt show a 360 in 68 only a 361, and I know thats for the big trucks. The motor in it now is from a 72 and the manual says 6B for the timing??? I'm asuuimg thats 6BTDC. I figured the diff in the two was mostly emmisions required.

 

Here's a little story. See if it sounds like your problem.
My distributor wore out and started to drag internally. This caused it to shear the drive pin. Not knowing what happened I reset the timing and it was fine. Then the pin slipped some more and I timed it again. Same thing, only this time when it slipped and back fired it seperated the balancer (timing marks moved on the crank).

Get number one cylinder up to TDC and see if the balancer timing mark shows zero.
It's worth considering.


Bear Tracks:

 

tongie607,
Sounds to me like you may want to do a compression test, too. That fluctuating needle on the valve guage is usually an indication of a bad intake valve. Either the intake valve seat is shot and it's not sealing properly, or, more likely, one of the intake valve guides is worn and the intake valve is not seating "squarely" on the seat everytime. If this happens, the bad valve/seat/guide bleeds off vacuum erraticaly at lower rpms, which could affect the vacuum advance, not to mention allowing the engine to backfire thru the carb. IF you do a compression test and come up with a low cylinder (alot lower than the rest), pull the valve cover and pull the rocker are for that cylinder/intake valve. See if you can move the valve stem type side to side. That would tell you if you have a bad guide causing the problem. If not lateral movement, it is probably a burnt intake valve or burnt seat. Good luck. - Don

 

I ONCE HAD A TRUCK THAT SPIT THROUGH THE CARB AT LOW RPM AND SOME WHILE I WAS ACCELERATING.
THE ENGINE TEARDOWN SHOWED A VERY WORN OUT LOBE ON THE CAM.
HOPE YOU FIND OUT WHAT IT IS.


68 TORINO
94 EXPLORER

 

TYPING IN ALL CAPS IS CONSIDERED YELLING and is against the guidelines here just to let you know.