no contribution from cylinder number 8
 

My '57 T-Bird 312ci has a problem I can't identify. If I pull off leads one at a time at the dizzy cap, the rpms drop about 40-50rpm, except on cylinder #8. I've changed the lead, swapped leads with ones that I know are good, swapped plugs, bought new plugs, etc. I've also replaced the distributor cap and rotor, just fitted new points and condenser (it was running a Pertronix electronic ignition module - so I removed that just in case). I've run a compression test - getting around 130psi or so on #8. Pretty much the same on all cylinders. I've done a leak down test - nothing leaking past the valves, although there is leakage past the piston rings, same effect on other cylinders. I can't get a percentage leakage as my leakdown tester gauge is dodgy. I'm at a bit of a loose end as to what else to check. I haven't checked the valve clearance on my solid lifters, but I did take the rocker cover off and there was a similar amount of play when the valves were closed, like other cylinders. I'm missing something! Can anyone else suggest what I can look at or test. I'm assuming an inlet manifold leak wouldn't affect a single cylinder. Maybe a valve isn't opening?? This one has got me befuddled!

If you are getting a reading similar to the other cylinders then the engine would be mechanically functioning correctly. You know you have fuel because it will run. You are correct that a vacuum leak would cause issues on more than just one cylinder. So.... My guess would be to go back over the ignition system. I remember recently on another forum where I read about an engine not firing on #1 but otherwise okay. (Sounds like much the same issue.) I will go look at his post (at H.A.M.B. - How is this possible?) and will come back here to tell you if he found a solution and what it was.

It looks like that guy never posted again. Maybe he fixed it and forgot to tell anyone. Anyway - all indications were pointing to the distributor. Try checking the point gap on every lobe. Set it to minimum on the lowest lobe and see how it runs. You may have a smaller lobe and the points are not opening to make the spark. I personally feel the problem is probably the breaker plate that the points bolt onto. I would try to find someone with a distributor and try that in the engine. If it solves the problem you should get yourself another distributor.
let us know what you find.

Thanks for your suggestion. That sounds like a possibility although if the cam lobe on #8 had an issue, then I shouldn't get spark at the plug. Following on from what you said, I could also verify this by moving leads on the cap. I think there's enough play in the distributor rotation to shift the leads around to the next position on the distributor cap then rotate the distributor accordingly (45deg). I might try that.

I've checked the gap on the cam lobe associated with cylinder #8. Everything looks good there. Gap looks fine. Of course there is spark at plug lead for cylinder #8. My idea of rotating the dizzy body and shifting the leads accordingly is a means of checking the cap, but of course I end up with the same cam lobe (duh!). At least I know it is not the cap. I did block off the vacuum port near cylinder #8. I'm wondering if my compression reading is misleading me somehow. The leakdown test (I don't have a percentage loss) shows leakage past the rings, but not the valves, although I'm sure there would be some regardless.

Oh. I didn't realize you are getting spark at the plug. I assumed where it didn't affect the RPM when disconnected but the other seven did..... that #8 was not getting spark. Hmmmmmm - gotta think about this one a little bit. Odd that it should not stumble when you pull off #8 wire and it IS getting spark.
Yes I agree. I would try to move everything by one position in the distributor (hopefully there's enough room to do that) Then you will know if #8 still seems different or if the other cylinder is acting that way with everything in the new position. Let us know what you find after switching the stuff around. In the meantime I'm going to keep thinking this one through. If I come up with something I'll let you know.

I've been pondering this one for a few minutes now. You have fuel, check! you have spark, check! it's timed okay because it DOES run, Check!. Only other thing I can think is something mechanical in the valvetrain. What is baffling is that you have compression. At this point I would pull the valve cover and measure lift. I suspect you have a valve (or two) on cylinder #8 that is/are not moving open the correct amount. I would also check valve / tappet adjustment while it's open. I'll keep pondering this poser.

Hi Steve. I pulled the rocker cover off yesterday to see if there was anything obvious amiss. I will take some measurements this morning to see if clearances are ok and lift is sufficient. Although I have fuel, there doesn't seem to be an easy way of seeing whether fuel is getting to cylinder #8 although I guess enough fuel should wet the plug. Thanks for your time on this.

Tricky doing accurate measurements, but I did use a rule across the tops of the rockers and eyeballed the travel of each rocker at the spring side. It all looks good. Cranking the engine over for about 20 seconds didn't wet any of the plugs on the right bank (where #8 is) but wet a couple on the left bank. Maybe there's a vacuum leak at the inlet manifold to the head right at cylinder #8? Hard to verify without removing the inlet manifold replacing the gasket, etc. Being in Australia, it'd probably take a week or more to get another gasket from the US.

This is what I do when I suspect a vacuum leak that is localized like your thinking; I put some water into a spray bottle and spray it around where I think it may be leaking. Usually if there is a leak the idle speed will change a little giving you a good idea of where the leak may be. Only time this didn't work for me was when my leak was on the valley side of the intake and it was leaking from the crankcase area (if you get what I'm trying to say).

I tried spraying water around the intake near cylinder #8. No real response. I may try something more volatile when I get time later today. I re-ran compression tests on all cylinders - all got within 135-140psi, so pretty good. I've checked tappet clearances, screw adjuster positions, etc. - all good too and valve lift seemed fine. I also used my leak down tester to check that both inlet and exhaust valves open and close, which they do.

It really has to come down to a vacuum leak near that cylinder. I've ordered a gasket set from the east coast (of Australia). Hopefully I'll get that later this week. I guess the inlet manifold will get a new coat of paint in the meantime.

At this point I would have to say that I agree it's sounding like a vacuum leak. Everything else checks out as okay. I think it would affect compression and leak down tests but if everything mechanical checks good there isn't much more that it could be.
Good Luck! I hope the gasket set solves your problem.