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stumped on spark issue

N

normmack

Member
I have been trying to figure this out for longer than i care to admit. I am working on 4 cylinder gas engine with firing order of 1342. #2 and 3 cylinders will not fire correctly. I had always assumed it was my distributor but today I took the the #1 wire and held next to all plugs and 1 & 4 jumped a nice strong spark and 2 & 3 had very weak spark. did the same with the #2 wire with same result. and the did same with #3 wire with same result.
Ok, I then put a new plug in the #3 hole and tried again with the same result.
I feel that i ruled out a faulty distributor as three different wires/lobes/rotor positions give me the same result with weak spark on 2 and 3 cylinders.
I also feel that since i tried a new plug it cannot be fouled plugs.
I am not sure how its possible but its like the plug is not grounded while screwed into the head. I am out of ideas.
My next plan is to take off carb and burn out cylinder- put all new plugs in and try same again, in case it does have something do with fouled plugs.
any suggestions ???
thanks
norm
 
Have you checked the shaft in the dist the bushings mite be sorn enough. I hook the timing lite to each wire when its running and watch for constant spark on each wire. Sounds like your shaft needs a new bushing.
 
You did not say how old the plug wires are. Of course I have seen nearly new wires that would not carry proper current. But if they are a few years old, this could be a possibility. Also, the distributor cap could be faulty. Just a couple of thoughts.
 
To eliminate the plug possibility, just simply take #1 and 4 plugs out and change them with #2 and 3. Then see if problem moves with the plugs. Doubt its a plug problem but that is how I would check it out. I just don't have much faith in some of these brands of new plugs. Next, if your plug wires are long enough to be changed out with each other, do the same with them and see if problem moves. Best way to do both of these tests is with tractor running, pull one plug wire off at a time to see if that is where the miss is at. If problem don't ever move, and stays at #2 and 3, and the problem is spark, process of elimination points back to the distributor.
 
if it was the wires, I would assume that I would never get a strong spark on any of the plugs , its the consistency of good spark on 1 & 4 and weak spark on 2 & 3 that leads me away from those causes.
a bad wire would be bad 1 thru 4, same with a bad cap. at least thats where i am leaning.
norm
 
thanks but please reread my original post, i think everything i did would correspond to everything you suggested, I appreciate the ideas and please keep them coming.
norm
 
Try sparking the individual wires to GROUND, rather than to their specific spark plugs, and compare the spark.

I'm gonna guess all 4 are gonna look the same.

When you are jumping the spark to the plugs, you introduce more variables... the sparkplugs themselves, the compression in the cylinders, and the fuel mixture in each cylinder.

By the way, have you done a compression test of all 4 cylinders... a blown gasket between #2 and #3 is a COMMON cause of a paired misfire like that.

(BTW, the one guy GUESSED you are working on a Ford... WHY not post the model, it's costs NO more to include all the pertinent information!)
 
I re-read your post. I don't see where you said anywhere in there about swaping the existing plugs to different holes. Just new one in number 3. Which, if perhaps the new plug in number 3 was faulty, proves nothing. Like I said, some of these new brands of plugs in my opinion, suck. I doubt its plugs, but the chance is still there. Most likely distributor or cap, but I would be sure before spending more money. My 2 cents.
 
OK Norm, I went back and re-read your original post a couple more times, and I understand about the wires, but there is still the possibility of the dist. cap. I had a truck once that ran fine under normal conditions, but when it would rain, it would develop a miss. Found that the dist. cap was cracked and dampness would form along the crack and attract the spark. A new cap fixed it. Good luck and let us know when you solve the problem.
 
A bit more info on the engine might be helpful. There are many different 4-cylinder engines and almost ALL have the same firing order. Weak spark on the 2 and 3 cylinders could indicate a worn distributor cam or a worn shaft bushing. An analog dwell meter might shed some light on the problem. A dial indicator on the movable point arm might also give some info. If the points are not opening the same amount for each cylinder, it could cause the spark to be weaker on some and stronger on others.
 
It sounds like you've checked all of the most obvious stuff with what you've done so far. That said, check the bushings in the distributor. Usually when they wear they do so concentrically, but it is possible yours did it on just two sides, basically 180 degrees apart. If that's the case it will shorten the time the points stay open and shut, which controls the saturation time of the coil, and ultimately the strength of the spark.
 
It may be that there is a compression difference between the cylinders. Spark jumps at different voltages based on compressed or not compressed fuel air mix . I would use the new plug grounded to the chassis with a jumper cable to test each wire (and each cap position) to assure each is about equal in strength. If so, the issue is internal to the engine. I do not believe the carb is at fault.
A compression check is needed (at full throttle with a well charged battery) Jim
 
Simple the problem is in the cap since the # 2 and 3 wires have low spark. It can't be the plugs as they are after the cap and wires. Replace cap points and all wires with solid wire you can get these at Napa. Your old tractors don't do well with the carbon core wires.
Walt
 
Are you running this test while cranking the engine? If so, and there is a compression difference, the cylinders with good compression will draw more battery voltage away from the ignition, showing as a weaker spark.

Run your spark test with the engine running. Check the spark one wire at a time at the distributor with a grounded piece of wire. Do the same at the plug end, one wire at a time. Then you can narrow the problem to the cap, wire, worn shaft, etc... If there is a running spark problem!

There is no possible way a plug screwed into the head is not grounded.

Even a totally fouled plug will still draw a spark from the wire, it just won't ignite the fuel.

Any possibility some of the plugs are resistor plugs and some are not?
 
Its been stated already but I will repeat again. Check compression, a low comp cylinder does not require as high of a voltage to jump the gap therefore the spark will appear weaker. A lean cylinder can do this too "lean misfire" Go ahead and check spark by allowing the wires to jump to a ground (not spark plug) they will probably look the same, if not then check cap, dist shaft/bushings, wires etc.

This is the same thing as a low compression engine will not benefit from a high output coil as the spark voltage needed to jump the gap is lower. There might be a small benefit if you also extended the gap of the plug but only if a hotter spark was actually needed.
 
I had thought about that before, but just had the distributor to a pro shop and it checked out ok for stuff like that, (although he does not have a way to test spark output)
thanks to all for your suggestions
norm
 
Its a Case 440 tractor. has new head gasket ,rebuilt head. valves were checked by three different people (who have mechanical experience) I do have a slight bent connecting rod on number 2 cylinder giving it 90 psi compared to 120 psi on other cylinders. I just never imagined that would affect spark in two cylinders even if they share a wall, they do not share an intake port on the manifold.
norm
 
also the connecting rod has been bent for at least ten years. we rebuilt motor about that long ago and noticed it then.
 
Thanks I am probably on my third cap third condenser, fourth rotor, third coil, 3/4 sets of all plugs (champion, ac, bosch) and several individual plugs)
I switched out working carb in the spring, ( I did not do all this stuff with the working carb on tractor, but assume the same)(just wanted to eliminate the carb, i think that is why next step is to take carb off tractor and do all again, with no fuel mix in cylinders.
 
i am not smartest guy in the world , but since researching on the net about similar problems, and suggestions here it sounds like its more in the compression/fuel side, which thanks to my limited tunnel vision always says spark is ignition related.
Thanks to all, will keep checking here for a couple of days and keep trying (about 3 years in) I can only work on it sporadically as I have to make a living.
thanks
norm
 
Check out what Walt Davies and NCWayne said to do and most likely you'll find your problem.A sparkplug fires the same with 0 compression as it does with 200lbs compression if one doesn't think so then pull the plug out of the engine and hold the plug in your hand while the engine turns over(LOL)
 
I originally did not think compression would have anything to do with it either, and still not convinced it does but I have tried all other options I can think of. The problem with the cap Idea is that I have the same condition on 2 and 3 plugs from all holes of the cap (I did not do a 4th hole as three was enough to convince me it was not the cap), So my logic (as cloudy as my wife says it is,) says problem has to be in the cylinders. I would think if its the cap it would be the other way around 1 hole firing on all plugs, 2 hole missing on all plugs, 3 working etc. In my mind, the same hole in the cap is either working or not. should not matter if I go to 1234 plug. Its not going to pick or choose which plug it wants to work on, therefore it has to be cylinder related. (something in those two cylinders keeping the plug from grounding correctly (which I find impossible)or somehow keeping the plug from sparking correctly.
 
I don't normally comment but this is an intriguing problem. I had the same problem on a 8n Ford many years ago. I found that by taking a solid wire sparkplug wire and putting a gap in it (heat shrink) the spark had to jump two gaps and the missing cylinder came back to life. Jumping a sparkplug gap is affected by many variables including compression, fuel air ratio and even fuel octane. If the problem is cylinder related replacing electrical components will not solve it
 
we have done that also on things in the past and have tried it this time also. I am not sure of the science behind it, but that does not seem to help sometimes but not here, sorry. Like I said this has been an on and off again fight for 3 years including some very good local legend type backyard mechanics involved. (but I am coming to the point where I need the tractor so its getting attention again.)
 
1-3-4 have 120 psi and I know #2 has a bent connecting (but it has had it for a confirmed 10 years and hard to say how long before that.)so its at 90 PSI.
norm
 
You need to check your plug wires to make sure they're in the correct firing order on your cap.
1-3-4-2 if the rotor is rotating CW. Hal
 
Does the 440 have points and condensor?

I experience a similiar issue with a JD 40, but it happened to be the shaft bushing in the distibutor and a weak condensor.
 
You need to go back to BASIC ENGINE to diagnose and repair this problem. You are focusing in on fine points that may or may not be the problem. Back in the "old days" a problem like this would warrant the use of a scope, a vacuum gauge, compression tester, dwell meter, and possibly other diagnostic tools.

You mentioned having a bent connecting rod. You also said that the compression on that cylinder was lower. That will cause a loss of output from that cylinder, and will to a lesser extent affect the other cylinder in the pair (cylinders normally operate in pairs - one on compression, the mate on exhaust).

Of course, the bent rod brings up another question. You also stated that the engine was rebuilt some time ago, and the bent rod was noticed then. The question: WHY would ANY competent rebuilder/DIYer not replace an OBVIOUSLY BAD component??????

There are a multitude of problems that can cause the condition you describe. Among them:
head gasket leakage
ring leakage
intake manifold leaks - from gasket or cracked casting
Distributor issues
spark plug issues
And the list goes on to less obvious and more minute things. I suggest going back to basic diagnostics, and taking an orderly and thorough approach. Fixing obvious things like a bent connecting rod would be high on the list.
 
done by a very incompetent DIY with a very limited budget. who was overextended at the time as it was. thanks for your input
 
Switch number 2 and 3 wires. If you are trying to wire the dizzy backward this will fix the firing order. I showed my old man this when I was 13 and it would have been ugly if I hadn't outrun him. He had been working on it all day.

Aaron
 
im gonna take a real wild guess on this. pull the distributor cap, look down by the points. check and see how many lobes are on the distributor shaft that open and close the points. if you see 6, somebody put the wrong distributor in there at some point in the past.
 
One thing that can be easily checked out. Spray ether along the intake manifold gasket while the engine is running. Engine speeds up you have a leak. I chased a problem with a Datsun for a long time until I found the problem was a bad manifold gasket. Question. Would a worn timing gear or chain or cam on the camshaft cause it? probably not just thinking out loud.
 
Unlikely because the timing gear/chain assembly would most likely affect ALL cylinders similarly. The engine in question already has a known defect- a bent connecting rod. That leads me to believe that the engine was not competently rebuilt, and likely has had many other corners cut.
 
I would systematically test to confirm the last place I had good spark. As you say you have good spark on 2 cylinders start with a known good wire and check to ground on engine for spark at each tower on the distributor. if those all test out then test each plug wire and then each plug. I would not worry about the plugs not being grounded when they are screwed in. That spark should jump 3/8 of an inch. And you want blue spark, not yellow or orange.
 
thank you, I am sorry if I did not make sense with my original explanation.
dist. hole one and wire 1 = good spark 1-4 weak on 2-3
dist. hole 2 and wire 2 = good spark on 1-4 and weak on 2-3
dist. hole three wire 3 = good spark 1-4 and weak on 2-3
I did not do wire 4 because the pattern was fairly obvious.
This was done with all others grounded to frame while turning engine over.
also it has been mentioned about the solid wire, it is solid wire (learned this long ago too, but still appreciate any help.)
norm
 
sorry, no, just had it to professional beginning of the week, points open at 0 90 180 270 up to 4000 rpm (is what shop guy told me) right on time.
thsnks
norm
 
I already told you was not done by an engine guy (I despise motors) give me a corn binder or manure spreader any day. I did happen to have a melted candy bar handy for gasket material though. It was easier to work with than taffy.
thanks (you must know my wife she calls me incompetent too) (she might be a little cuter than you though)but maybe not.
norm
 
Two or three guys mentioned holding the wire to the head instead of the plug and it would probably give the same spark no matter what wire you used or what cap hole you put it in which would eliminate everything but the plug and/or compression. I havn't seen your results of that test yet. I am 100% certain that going to a bare spot on the head that you will see no diff. no matter what hole or wire. And since you said you tried a new plug in one of those cylinders and made no diff. then that leaves cylinder compression + or - as the probable culprit.
 
that is what I assume also, but which do I tackle and how. I am not sure how the two cylinders would be affected like that since I know one has 90 psi (which is low ) but the other has the normal 120 psi which the others have. even if my new head gasket was bad, I still am not sure how that is creating a weak spark. My mind also says it has to be cylinders some way, but would like to know before ripping apart things.
thanks
norm
 
Has anyone ever determined just WHAT bent the con rod?? Did it hit a valve? Was cylinder full of liquid and someone tried to tow start it? Probably would help determine the spark problem if one knew what bent the rod and why. Is it that it just doesn't jump as far on those two cyl. or is it that the tractor actually misses when running on those two? I havn't really heard how it actually runs. If it runs good with the problems it has then let it be. You said it's been this way for 10 yrs?
 
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