ground test to plugs
 

well heres one , i have been running my old truck for a yr and a half and have always felt it was a little under powered have set timing and am using a decent zenith carb everything is set up as well as they are going to be like point gap and spark plug gap i have also changed to a b cam for the distributor and find the truck seems to run smoother with . heres the problem when i do a ground out test to the plug wire copper strip using a screwdriver grounding to a headnut i find cyl 1,2,and 3 to drop in idle and the engine runs noticeably off but on number 4 nothing changes when grounding . spark seems weak red yellow color instead of blue white but when not grounding the engine does not seem to be missing , so what am I MISSING.


thanks

Re: ground test to plugs
 

Re: ground test to plugs
 

Manifold leak! Agree Good and Flat!

Re: ground test to plugs
 

My first thought was a bad cap where the spark jumps between 4 and 3 as Bill mentioned. I've seen it twice in the past couple years. Also, I'd do a compression check.

An UNLIT propane torch can be used to sniff out a vaccum leak. Just open the valve and hold it next to the manifold ports. Don't go too tight and break the manifold ears when you check them.

Re: ground test to plugs
 

I LIKE Tom's idea of the unlit propane torch. A similar idea I have used is to drip oil around the manifold connections - the oil vaporizes and adds to the fuel loading if there is a leak.

But you have to clean up the oil you drop by this method. Propane MUCH easier.

Joe K

Re: ground test to plugs
 

You mention weak spark, is that only on #4 or all of them?

Re: ground test to plugs
 

mitch on the spark its just #4 i will check tonight in the dark for wandering spark issue the manifold is tight as i dare ..i have broken ears off before so not really wanting to be looking for another manifold i will find another cap to try in case theres an internal failure
really appreciate the suggestions and will keep you posted number 4 plug looks to be burning lean grey soot not much the # 3plug was brown, hmmm ?

Re: ground test to plugs
 

The spark should be the same intensity on all so nix the vacum leak scenario for now

Re: ground test to plugs
 

Re: ground test to plugs
 

Spark intensity is hard to judge by eye. Make a spark tester out of an old plug, widen the gap to about 3/8", ground the body to the head,connect the plug lead and crank, key on. If it jumps that wide gap you're barking up the wrong tree. May be a compression problem (stuck valve?)

Re: ground test to plugs
 

rich thanks for thatdescription on making a spark tester i will give it a try i checked at night no wandering spark looked again at the dist cap which is a repro the fit is not tight as it is slightly bigger in diameter than the distributor so what i noticed is that as it is clipped on it still has a little wiggle room which means that it can throw off the distace between the rotor and the electrodes on the cap so looking close at that i found that the gap was off on the #4 electrode , so i changed to an old ford original that i have that has a hairline crack on #1 post but still works once changed i checked the rotor gap set it at .025 and also replaced the rotor as it had a small tear at the base of the metal flap that transferes the spark from the coil . that done i fired the old truck up and could tell a noticeable when i did the ground test , so between the rotor gap and rotor , that seemed to clear up the problem . thanks to all for the info ive learned a lot from you!

best

Mike

Re: ground test to plugs
 

mike
glad you got it figured out. i like the inline style spark testers easier to determine the intensity and pattern, and your actually running the engine without killing the cylinder or can be put inline of the coil also