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By Freoway57 - 7 Years Ago
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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!
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By Hoosier Hurricane - 7 Years Ago
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There is a vacuum port on the back of the intake manifold, where the power brake booster hose is connected when equipped. If the booster hose or diaphragm leaks, there is a vacuum leak primarily at #8 cylinder. Try unhooking the hose and plugging the manifold port and check #8 again. Another possibility is a leaking intake gasket.
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By Freoway57 - 7 Years Ago
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Hi John. I'll give it a try tomorrow morning and see what happens. I'll let you know. Cheers, Lloyd
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By Freoway57 - 7 Years Ago
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I plugged the vacuum pipes to the booster and to the wiper unit on top of the fuel pump. No difference. I even rotated the distributor 45 degrees (about the extent of the timing adjustment) and shifted the leads around to see if a different cylinder misbehaved. No change.I reckon I can hear a light pop-pop-pop sound - possibly coming from cylinder #8 - while the engine is idling. That may be related.
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By Hoosier Hurricane - 7 Years Ago
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If the pop, pop, is regular and not intermittent, I would suspect an exhaust valve is not opening, due to a pushrod off the lifter, or heaven forbid, a lobe on the cam worn flat. With the exhaust valve not opening, the trapped air in the cylinder would blow back into the intake when that valve opened, causing a pop each time. This condition would not show up on a compression or leakdown test. Crank it over and watch #8 exhaust valve to verify that it indeed does open and close.
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By Vic Correnti - 7 Years Ago
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Have you checked for a flat cam by watching the valves and rockers operate on that cylinder? Does the plug get wet like it is miss firing?
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By 1960fordf350 - 7 Years Ago
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I had a 351m in a truck that had similar issues. It would make a pop pop through the carb. Didn't matter what rpm you were running. It was intermittent and drove me nuts. Tune-up, carb rebuild, and a timing chain later I still had it. Turns out I had weak valve springs and a couple exhaust valves were beat into the heads.
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By Joe-JDC - 7 Years Ago
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Questions? Does it run rough? Does it idle rough, is it down on power, is it low on vacuum? Does the spark jump to the plug with the boot close with the engine running? Joe-JDC
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By Freoway57 - 7 Years Ago
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Sounds like a possibility - worn out cam or dodgy valves. Nothing else seems to make sense as everything else checks out. Haven't seen a wet plug out of that cylinder, but I'll check. I presume if the valves are stuck in the head it'd be pretty obvious with the rocker cover off. Cheers, Lloyd (Fremantle, Western Australia)
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By charliemccraney - 7 Years Ago
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Pull the valve covers, look for anything obvious. If nothing is found, adjust the valves and, once adjusted, take note of the adjuster screw positions. They should all be in the same position. If one is in farther than the rest, then there is probably wear somewhere between the valve and cam, possibly including the cam. If the screw is out farther, it probably means that the valve has worn into the seat.
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By Sandbird - 7 Years Ago
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Twice in my life I have seen where too long of a bolt was used in the last hole back on the left side of the intake manifold. The bolt hole is drilled all the way through, when too long a bolt is used it goes all the way through and hits the push rod on #8 cylinder.
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By Freoway57 - 7 Years Ago
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Thanks for all the advice. It was tricky actually measuring valve lift. In the end I ran a long straight rule across the rockers above the springs. They all seemed to move equivalent amounts, but I'd need to confirm that. It doesn't run rough, although it could be a little smoother. It accelerates well, but driving it around it sounds like there's a miss. I'm going to accurately measure all compressions to be absolutely sure. I cranked the engine for about 20 seconds. No wetness on any of the plugs on the right bank - a little on the left bank. I'm wondering whether there's a vacuum leak around the inlet manifold to the head near cylinder #8. Would that cause this problem?
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By Hoosier Hurricane - 7 Years Ago
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Yes, a gasket leak could cause it. Try spraying some volatile liquid, like choke cleaner around the area with the engine running. If the engine reacts, there's a leak. Do this outside, you'll be spraying near the distributor where there are sparks. I usually use an unlighted propane torch to find a vacuum leak, but have had an occasional flash of flame if I allow the gas to accumulate. No harm done, but my heart rate jumped a little.
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By Freoway57 - 7 Years Ago
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Thanks guys. A few things. As Sandbird mentions, there is a longer bolt than acceptable in the last back hole of the intake manifold. Doesn't hit the pushrod, but is a little close. John - I'll try spraying some volatile liquid around the manifold near cylinder #8. I re-ran compression tests on all cylinders - all got within 135-140psi, so pretty good. As Charlie suggested - 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.
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By Freoway57 - 7 Years Ago
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I recently received intake manifold gaskets and fitted those. I cleaned up the faces of the heads and intake manifolds until they were sparkling. However, the problem was still there when I started her back up - no drop off in revs at #8 cylinder when I disconnect the lead. Spark and lead both good. I then started spraying around with some "Start Ya Bastard" (a highly volatile engine starter in a can). Nothing around the manifold to head connection. I then noticed a change in revs when I sprayed the mushroom-like fitting on the driver's side. I believe that's a cap to the hot air tube that runs across the manifold to the choke assembly. Is it possible that the pipe could corrode inside the manifold? But then why would it affect the intake and not the exhaust? It's hard to tell exactly where it runs through. I removed the mushroom cap, cleaned it solvent and sealed it with solder. When I refitted the mushroom cap, there did seem to be a noticeable improvement. That is, the revs dropped when I disconnected the lead to cylinder #8. Does it run through the inlet plenum as well? Maybe I'll cap the side to the auto choke and fit a manual choke, based on advice received.
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By oldcarmark - 7 Years Ago
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If You want to keep the Auto Choke there is an aftermarket which simply runs a hot inlet pipe to the Exhaust Manifold like a lot of 1960's Fords did. This was a Kit sold to do away with the choke Pipes through the Intake setup. Probably find One on Ebay.
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By Gene Purser - 7 Years Ago
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The carburetor pulls a vacuum through the choke, so maybe that would explain the RPM increase when the start fluid is sprayed on the intake for the choke vacuum. But I don't see how it would connect to the miss on #8. Maybe there is some defect in the intake manifold? I hope someone can fill in the blanks.
The choke might not release correctly since you stopped the intake air to it, unless it is rusted through on the inside of the manifold. If you remove the choke, be sure to plug the vacuum port on the carburetor that draws the hot air into the choke.
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By KULTULZ - 7 Years Ago
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Freoway57 (5/30/2018)
I then started spraying around with some "Start Ya Bastard" (a highly volatile engine starter in a can). Nothing around the manifold to head connection. I then noticed a change in revs when I sprayed the mushroom-like fitting on the driver's side. I believe that's a cap to the hot air tube that runs across the manifold to the choke assembly. Is it possible that the pipe could corrode inside the manifold? But then why would it affect the intake and not the exhaust? It's hard to tell exactly where it runs through. I removed the mushroom cap, cleaned it solvent and sealed it with solder. When I refitted the mushroom cap, there did seem to be a noticeable improvement. That is, the revs dropped when I disconnected the lead to cylinder #8. Does it run through the inlet plenum as well? Maybe I'll cap the side to the auto choke and fit a manual choke, based on advice received.
What you experienced with the rise in RPM when hitting the hot air choke fresh air intake was as a result of an enriched mixture to the carb. The tube rides entirely in the exhaust gas cross-over and should have no influence over the miss even if the tube is corroded and allows exhaust gas into the choke fresh air inlet. If the tube is compromised you should have a lean fuel condition and also hear exhaust noise @ the tube on either side.
Have you put a vacuum gauge on the engine? When you blocked off the booster and vacuum pump from the fitting on #7 and #8 intake runner, the fitting or it's mounting may be defective also. When using propane, I use a special wand to control flow. The valve is turned withe the wand @ the carb air-horn until RPM is stabilized and then you go on search and destroy with that setting. Probe the fitting itself and both #7 and #8 runner sealing and the actual runners in case there is a stress crack or something similar.
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By KULTULZ - 7 Years Ago
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By KULTULZ - 7 Years Ago
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By KULTULZ - 7 Years Ago
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By Hoosier Hurricane - 7 Years Ago
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Is it possible that your flammable liquid was actually being drawn in by a vacuum leak at the carb/manifold juncture or a worn throttle shaft/ bore? An air leak at the primary throttle shaft will sure foul up the idle mixture. Like Kultulz, I don't see how the choke heat tube can affect the mixture to that extent.
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By oldcarmark - 7 Years Ago
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I have a Question along similar lines as this Topic. On my 56 I just purchased the Choke Pipe plugs into the Teapot on Left side and goes down to the crossover Pipe through the Manifold to Passenger Side and up to the Choke. The fittings on the ends of the cross Pipe are not tight. Just sitting on the ends of the Cross Pipe. Is there any Engine Vacuum being drawn through this Hot Air setup? In other words should this setup be air Tight? Having trouble getting it to Idle decently in Drive. Just got the Car and I am sorting out what needs to be fixed.
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By KULTULZ - 7 Years Ago
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Hoosier Hurricane (5/30/2018)
Is it possible that your flammable liquid was actually being drawn in by a vacuum leak at the carb/manifold juncture or a worn throttle shaft/ bore? An air leak at the primary throttle shaft will sure foul up the idle mixture. Now there's two good points I forgot about.
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By KULTULZ - 7 Years Ago
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oldcarmark (5/30/2018)
I have a Question along similar lines as this Topic. On my 56 I just purchased the Choke Pipe plugs into the Teapot on Left side and goes down to the crossover Pipe through the Manifold to Passenger Side and up to the Choke. The fittings on the ends of the cross Pipe are not tight. Just sitting on the ends of the Cross Pipe. Is there any Engine Vacuum being drawn through this Hot Air setup? In other words should this setup be air Tight? Having trouble getting it to Idle decently in Drive. Just got the Car and I am sorting out what needs to be fixed.
Yes, it is a fresh/heated air source and operated by manifold vacuum. It has to be air tight.
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By Freoway57 - 7 Years Ago
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OK. That makes sense. So I shouldn't have blocked off the mushroom fitting. I will remove the brass fitting on the inlet manifold that connects to the booster, etc. and find and make sure it seals properly with a thread sealer or similar. I have directed a propane spray (with wand) around it when the engine is running but it doesn't appear to make a difference.
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By oldcarmark - 7 Years Ago
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KULTULZ (5/30/2018)
oldcarmark (5/30/2018)
I have a Question along similar lines as this Topic. On my 56 I just purchased the Choke Pipe plugs into the Teapot on Left side and goes down to the crossover Pipe through the Manifold to Passenger Side and up to the Choke. The fittings on the ends of the cross Pipe are not tight. Just sitting on the ends of the Cross Pipe. Is there any Engine Vacuum being drawn through this Hot Air setup? In other words should this setup be air Tight? Having trouble getting it to Idle decently in Drive. Just got the Car and I am sorting out what needs to be fixed. Yes, it is a fresh/heated air source and operated by manifold vacuum. It has to be air tight.
OK so where is the Vacuum Source? Through the Choke? Where do I seal it to check if its a vacuum leak Source? Cap the threaded fitting on the Choke Housing. I did try the Propane Test and didn't find any change in RPM.
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By Sandbird - 7 Years Ago
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Joe-JDC (5/5/2018)
Questions? Does it run rough? Does it idle rough, is it down on power, is it low on vacuum? Does the spark jump to the plug with the boot close with the engine running? Joe-JDC
I didn't see any response to observing the quality of the spark as suggested in this post. I think it would be a helpfull clue to know this.
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By KULTULZ - 7 Years Ago
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oldcarmark (5/30/2018)
KULTULZ (5/30/2018)
oldcarmark (5/30/2018)
I have a Question along similar lines as this Topic. On my 56 I just purchased the Choke Pipe plugs into the Teapot on Left side and goes down to the crossover Pipe through the Manifold to Passenger Side and up to the Choke. The fittings on the ends of the cross Pipe are not tight. Just sitting on the ends of the Cross Pipe. Is there any Engine Vacuum being drawn through this Hot Air setup? In other words should this setup be air Tight? Having trouble getting it to Idle decently in Drive. Just got the Car and I am sorting out what needs to be fixed. Yes, it is a fresh/heated air source and operated by manifold vacuum. It has to be air tight. OK so where is the Vacuum Source? Through the Choke? Where do I seal it to check if its a vacuum leak Source? Cap the threaded fitting on the Choke Housing. I did try the Propane Test and didn't find any change in RPM.
The vacuum source is within the carb. It pulls filtered outside air through the tube in the intake manifold where the air is heated to operate the choke. The carb is calibrated for the extra vacuum draw and any leakage will upset the factory carb fuel trim.
If yours is 56 H4000, simply plug with a rubber cap the air intake side (@ the carb air-horn) and remove the inlet fitting to the choke housing and check for outside air leakage with a MITY-VAC to see if it will hold vacuum. Service the system until it will hold vacuum and re-attach.
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By KULTULZ - 7 Years Ago
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Sandbird (5/30/2018)
Joe-JDC (5/5/2018)
Questions? Does it run rough? Does it idle rough, is it down on power, is it low on vacuum? Does the spark jump to the plug with the boot close with the engine running? Joe-JDC I didn't see any response to observing the quality of the spark as suggested in this post. I think it would be a helpfull clue to know this.
One would a$$-u-me the secondary IGN operation has been verified. And it seems all the mechanical's have been verified.
If it has, it sounds to me as if #8 is running too lean and will not fire the mixture or the vacuum leak is blowing out the flame.
But GOD knows, I have been wrong before. Just ask the wife... 
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By KULTULZ - 7 Years Ago
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By Freoway57 - 7 Years Ago
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Just thought I'd provide the result of the original post. After refitting the intake manifold with new gasket, the intake manifold improved slightly. My first run indicated that I may have solved the problem of poor contribution from cylinder #8. Things definitely improved as the manifold vacuum reading increased into the low side of a normal reading. I then rechecked all other vacuum fittings - including the brake booster. I discovered that the booster diaphragm did have a hole in it which was a big part of the problem. Interestingly, the first time I plugged the booster pipe, the problem didn't disappear, since the intake also had a leak. Given that there were two issues - an intake manifold gasket fault and a booster diaphragm leak - it makes it harder to diagnose the problem. One naturally assumes there is one fault, not two or many! I have previously worked in maintenance engineering, so I'm annoyed I got fooled by this issue of multiple failures.
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By oldcarmark - 7 Years Ago
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So have You solved the Problem by fixing both Issues?
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By Freoway57 - 7 Years Ago
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Hi there. Been away a couple of days. Sorry for the late reply. I reckon I have although I haven't been able to take the car for a test drive - the wet weather has just set in. I'm 99% confident. If not, I'll report back.
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