Should be an easy one, but I am lost. No change in RPM when I remove #1 plug wire. Plug looks brand new. Swapped out plug and wire to see if it follows, but miss stays on #1. Fire is getting to the plug. Compression is good, and within 10% on all cylinders. Vacuum is 19 with a slight flicker, but less than 1. Pulled valve covers, and the valve train seems to be moving good, pushrods all rotating. Timing is on. Tune up no help. I am not sure what to do next. I guess I can do a leak down test, but it seems like with this kind of miss it would show up as compression or vacuum! Any ideas? 77 350.

Tommy

 

Wow seems like you have covered everything.I would do a leak down to rule out internal engine,valves,rings.I assume you have a timing light because you said timing is on.Do you see the light flash faster when you raise rpms to 2500-3000 and you have it hooked up to #1 plug wire.You said it has fire-do you have a plug tester to test the diff between a different cylinders spark against #1.You have pulled each wire from cap 1 at a time and noticed a difference between all of them except #1 right.WHat were your compression #s on that side.

 

Only thing left is the distributor cap.

 

Swap the plugs in #1 and #2 cylinders to see if the miss 'moves'. It could be as simple as a bad plug. You can get juice to a bad plug...but if it doesn't fire, your timing light won't function.

 

With the engine running, use a spray bottle and spray some water into the carb. This should break up and expel any deposits that might be keeping a valve from properly closing.

I also agree the cap or rotor might be a problem as well.

 

Check for continuity on the #1 distributor cap post. Definately put a timing light on #1 wire to verify actual spark.

Did you verify that the #1 intake valve fully opens? You mentioned the P/R rotating, but not valve opening. If you wiped the #1 intake lobe, you would see no real change in your vacuum reading and compression would check fine. However, you would not get enough fuel to support meaningfull combustion in that cylinder, therefore no impact by removing spark. Do you smell raw fuel in your left tail pipe?

Good luck

 

When the vacuum needle is flicker it shows that one of the valve stay open, do you have mechanic stethoscope to hear the valve clicking to see if there is a different between #1 to the others.
Avner

 

If that plug is firing, the valves are working properly and the cylinder has good compression, the only other possibilities are getting enough air and getting enough fuel. Is there any possibility that something could be obstructing the intake manifold passage at #1? Any work done recently in that area (gasket replacement, etc.)?

 

I'm not sure I understand your issue?

Are you sure problem IS with #1 cylinder?
After running motor from cold for bit, can you detect an obvious temperature difference in the manifold exiting each cylinder....
If you have individual pipes from each port (headers) it would be easy to confirm if a particular cylinder is firing or not.

You say that "Plug looks brand new" and then say "Fire is getting to the plug"

If the plug is firing, you should see some evidence at the plug tip, maybe slight colour change on the porcelain. If it's NOT firing, then it should come out wet/oily due to NO flame and unburnt fuel. What is it?



How did you check that plug? Did you replace it with a new one, or swap it with one from another cylinder? Maybe you have a faulty batch of plugs? What are your gaps? Is the plug shorting out at the tip?

Have you tried visaully checking for spark? What happens when the plug is removed, connected to the lead, and body of plug is up against the block (grounded).... If you can see a strong spark at the plug tip when cranking the motor, then you can elimninate the dissy, the points and the cap and the plug. Wriggle the lead near each boot end and see if you lose the spark.... you could have a bad termination of the plug lead.
OR
Do any of your leads come very close to the exhaust manifolds.... you may have compromised the insulation and have a short. Check the leads at night time with motor running and see if you have any visible arcing to the block or pipes.

Sure seems strange that you can't isolate the problem.....

But then again, I had an issue with an after market alarm module with an ignition cutout relay which was intermittently creating ignition issues.
I was chasing my tail with plugs, leads, coil and HEI module before I found it I now have a full complement of spares.

Hope you sort it out.

 

Here is what the Chassis Service Manual says in part:
"It is often difficult to locate a weak cylinder. A compression test, for example, will not locate a leaky intake manifold, a valve not opening properly due to a worn camshaft, or a defective spark plug.
With the cylinder balance test, the power output of one cylinder may be checked against another, using a set of grounding leads..."

PM me if you would like the entire test and an image of the grounding leads. I'll scan the section and email it to you.

Cheers,
Pete

 

Any hairline crack in plug not seen could cause the plug to fail under any load. If you pull the plug out it may fire but once submitted to a compression stroke it may fail.

 

Thanks for all the good responses. Some very good ideas that I didn't think about. I believe I have either a bad intake gasket, or cracked manifold. After troubleshooting and cleaning etc, I started to notice a small whistle noise, not easy to hear, because of the exhaust. I sprayed some carb cleaner in the area, and it stopped. I had a loose bolt on the intake a while back so I removed it, sealed it, and reinstalled it (just the bolt). Don't know how long it had been loose, but maybe over time the gasket in turn deteriorated. Anyway, I am going to pull the intake today. Thanks again for your advise. Cheers.

Tommy

 

It's very possible that the gasket 'separated' and is partially blocking the intake passage. Sounds like you are on the right track.

 

The whistle noise stopped or the miss stopped.If you had a vaccum leak and sprayed carb cleaner on the leak the idle would raise immediatley.

 

All of the above. I removed the intake, and it looks like some of the intake gasket is missing. I am thinking maybe it was drawing in too much air, and basically nuetralizing the fuel. The manifold looks OK. Does this sound logical to you guys? Should have it back together in a couple of days. Thanks again.

Tommy

 

 

One more thing that hasn't been mentioned: take a very close look at your valve springs. Cracked/broken springs are hard to spot, but if you have one, it'll produce a pesky miss that is hard to diagnose.