The Stovebolt Site Forums The Shop Area Engine Shop non-firing cyinders, trouble shooting help

We need some help in trouble shooting. I'm really hoping HotRod Lincoln comes to the rescue again.

We've got a '51 1/2-ton with a '54 235 that is only running on 4 cylinders. The #3 and #4 cylinders are not firing. Here's what we've done so far.

Check the compression:
1: 165
2: 160
3: 165
4: 165
5: 160
6: 165

New plugs. We have swapped plugs from different cylinders with no change.
New wires. We have swapped wires from different cylinders with no change.
New cap and rotor. We rotated the cap with no changes.
We checked that the bushings in the distributor are tight and there does not seem to be any wobble in the rotor.
Since #3 & #4 share the same intake, we checked the manifold bolts and all is tight.
This is a new truck frame off so we even pulled the carb off the manifold and stuck a dental mirror down the throat to make sure we didn't leave a rag in the intake.
We have adjusted the valves.

Now what? Anyone else run across this?

Thanks.

Maybe paint on the spark plug holes keeping from making good contact? That's a stretch but it's all I got.

Dennis

Have you double checked you have the correct firing order?

Since those are paired cylinders, could you have the wires opposite of where they should be?

Firing order- 153624

how did you determine 3 & 4 weren't firing?

Fuel, air, spark, and compression are needed to fire - you say you've got compression verified which indicates the rings and valves are OK in 3 & 4. If the engine is running on the other cylinders, then fuel and air are verified in a general sense.

1) It could be ignition.. have you confirmed spark at the plug for both cylinders? That is: pulled the plug and ground the housing and seen a spark?

2) If there is spark, then it is timing or firing order which ApacheFiend suggested

3) A cylinder filling issue, which could be a blockage as you mentioned. However I would expect that to show up on your compression test.

4) your compression test was faulty (gauge or method, etc) and you really don't have good compression in 3 & 4. If you didn't indicate that compression was good in those two cylinders, i'd tell you to check the head gasket between 3 & 4

good luck

#1 in tomctomc's post above. If no spark start tracing backwards until you do find spark. New caps have been known to be defective from the supplier.

We started the trouble shooting by pulling plug wires independently. If you pull #1, the engine runs but rougher, etc. When you pull the wire on #3 or #4, nothing appears to change. There is spark from the end of the plug wire over to the tip of the plug.

Our compression tester is pretty simple. You screw the end into the plug hole, connect the gauge and turn it over. We did it with all six plugs out but it was so consistent across the cylinders I ruled that out. I was pretty happy to see that it was so consistent.

I have verified the firing order as 153624.

We did adjust the timing a small amount but it was not off enough to affect the other cylinders.

The only thing that I haven't tried is what DennisM noted. I have not pulled a plug and turned it over to verify the plug is actually sparking. I suppose that could be it. We will try that next.

If you have an inductive timing light, try clipping it on each of the six wires to see if each will fire the light. If you have any cylinders that don't light, try swapping wires and spark plugs with ones that do using a process of elimination to determine if you have any bad wires or plugs.

I recently had a unique problem tuning a 235 with brand new AC plugs. I couldn't get my timing light to fire when connected to #1. I figured that my timing light had gone bad. Spent a half hour trying to figure out what had happened to the light before I clipped it to #2 and it discovered that it worked perfect. Turns out that the brand new spark plug on #1 was a dud. Never had that happen before. We take spark plugs for granted except when they get fouled and sometimes forget that they can be defective. I was lucky that the dud was on #1 because a dead plug on another cylinder that wouldn't have had the timing light attached to it might have gone undiscovered for a while.

Matt

You didn't mention points. If the gap is too small a little runout in the dist. can cause some plugs to not fire. I have also seen valve seats recessed to cause miss fire but not show up in a compression test. Check the height of the valve springs with the valves closed.

Thanks Guys. Here is the latest update.

We have swapped plugs and wires from working cylinders to #3 and #4. That makes no difference. I have an inline plug tester where you insert it between the plug and the plug wire and then run the engine. when it sparks, the light on the tester lights up.

The distributor does not have a set of points. It came with an electronic trigger. A Petronix I think.

I tried DennisM's suggestion to make sure that we have ground at the plug hole and it is not painted over. I checked it with an ohm meter and all have the same reading.

We just pulled the plug out of #3 and #4 plugs, hooked it up to the wire and started the engine while grounding the plug. There is spark at the plug confirmed.

This leads me to the valves. we are going to pull off the cover and re-set the valves.

Anyone else got any ideas?

Most of the bases have been covered by the above suggestions. Have you tried checking for vacuum leaks by using an unlit Propane torch around the intake manifold gasket for the center port in the cylinder head? If the engine smooths out or speeds up, you've found the problem. It would be hard to run fast enough to give me a Pertronix system, but some guys love the things. If you have a regular distributor with points the next step would be to swap it in there temporarily and see if the problem still exists.
Jerry

Look for a broken valve spring.
George

I vote for the vacuum leak. 3 and 4 share an intake runner.

As Jerry mentioned, I had the intake manifold gasket slip slightly when I reinstalled an intake manifold. Caused the exact problem you are experiencing.

Rick

If the manifold happens to get installed without one of the alignment rings, or the ring is a little too wide, a vacuum leak is a very likely result. Sometimes if a manifold has been milled or leveled on a belt sander, etc., the ring needs to be shortened as well or it prevents the gasket from sealing.
Jerry

Well first off, those are pretty impressive compression numbers! I used one of those testers for live house wires to check the spark at each plug on my 38. It didn't flash when held near #5. I replaced the wire with no change. I then replaced the plug and the miss went away the the tester flashed. The plug was shorting.

Check the compression:
1: 165
2: 160
3: 165
4: 165
5: 160
6: 165

Check for vacuum leak on the intake runner for those two cylinder, and it the spark plug is shorted inside, the indicator will flash. So replace the two plugs with two new ones and see what happens.

I two have seen the rare bad spark plug out of the box.

Jerry and those who suggested vacuum leak are the winner!

We checked the valves and all was well and went back to the manifold. The bolts were tight but we realized that the intake manifold inserts were not installed so that there was a big leak at the gasket. Once we installed those, torqued it all back down it now runs like it should...purring.

My next trip to town was going to be for a new set of points.

All is right in the world again. Thanks for all the help from everyone.

Keep the points handy, but I've had very good success with the Pertronix Unit. I've installed several, and not had any issues.

Glad to hear you guys got it resolved. So, what happens with an intake leak? Does it prevent a full fuel charge to get into the cylinder?

Dennis

Usually enough air gets drawn into the cylinder that it dilutes the fuel charge enough to cause a lean miss. On a V8 with a dual plane intake a leak at one intake runner will sometimes cause a miss on the other side of the engine.
Jerry

That makes sense to me. Thanks Jerry.

Dennis

I had a Dodge 440 pull truck engine on the dyno one night that developed a sudden air leak at one intake runner while running at full load and 6500 RPM. The max HP we got on that pull was 714 on one 4 barrel and gasoline. I heard a slight detonation rattle, and before I could chop the throttle and kill the engine, it had burned a half-dollar sized hole in one piston. It took less than 5 seconds. Gasoline engine detonation is one of the most destructive forces short of a nuclear explosion. It's got more power than an equivalent weight of dynamite or TNT.
Jerry