ground test to plugs

[rollingsculpture]

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

[BILL WILLIAMSON]

HMMM! Doesn't REALLY sound like the old problem of # 4 spark jumping to # 3 in/at the main distributor cap body, kinda' odd! If it were mine, for a quickie, I would tighten the 4 manifold nuts, in case there's a SLIGHT leak, & try another distributor cap body. Usually when # 4 jumps to # 3 it goes out the top of the cap to # 3 connector strap & can be seen in the dark. If you look at the cap setup on www.modelabasics.com you can see that when cap jumping happens, it's always from #4 TO # 3 because of the routing of the conductors inside the cap body. Bill W.

[Gary WA]

Manifold leak! Agree Good and Flat!

[Tom Wesenberg]

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.

[Joe K]

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

[Mitch//pa]

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

[rollingsculpture]

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 ?

[Mitch//pa]

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

[BILL WILLIAMSON]

Originally Posted by Joe K

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

A cheap stethescope with the probe removed, just the open hose, is great for finding vacuum/exhaust leaks. Harbor Freight sometimes has them for $1.99. Bill W.

[RichPA]

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?)

[rollingsculpture]

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

[Mitch//pa]

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