Need some 1965 engine troubleshooting advice

[48Super]

For a while now, I've been unable to get a nice idle on my '65 GS but it would run fine on the road. When I recently drove it for the first time this Spring, it ran really rough and I could tell there was a very pronounced miss.

 

The first thing I did was to remove the plugs and right away I found the #8 plug was black with carbon as if it had not been firing. I checked the ignition wire which had OK resistance, but to be sure I installed a new wire, a new plug and spare distributor cap. I ran the engine again and I could tell there was still a miss present at all RPM's.

 

I disconnected the #8 ignition wire and hooked it up to the original plug, laid it on the exhaust manifold and ran the engine. I could see a good, strong spark so I'm sure there is spark in that cylinder.

 

I pulled all the plugs and did a compression test and found that #8 cylinder was 165 psi, all others were around 170-185 psi. (Note: this engine has 99K miles on it)  I also removed the valve cover and cranked the engine and both valves on the #8 cylinder appear to be moving normally.

 

I've run the engine several times while trying to figure out what is going on and I noticed that when I look the plugs, they got sooty as you would expect while the engine is running with the choke on while warming up but the #8 plug is still clean like there is no fuel getting to that cylinder. I also noticed a hissing sound which I originally thought was just the normal sound of the air being sucked into the carburetor. I used a piece of hose to listen around the carburetors and manifold and found that hissing seems to coming from underneath the intake manifold. I had the manifold off a few years ago when I cleaned and detailed the engine and the core plug was secure and in excellent condition and the intake manifold gaskets are installed correctly.

 

Before I get into pulling the carburetors and Intake manifold I'd like to know if it is possible to have vacuum leak that would affect the #8 cylinder in such a way that would cause it not to fire?

 

Sorry for the rambling post but I thought it was important to detail what I've tried so far. Thanks for any suggestions you might have.

[Seafoam65]

 If the car was in my shop i would put a smoke machine on the intake and verify where the vacuum leak is coming

from.....it certainly sounds like you might have one......You might want to check out the pcv valve and make sure 

it isn't gummed up and causing a vacuum leak....if that's OK i'd recommend doing a smoke test.

[lrlforfun]

OK Mike: Winston is on the right track. Vacuum leak first, next, fuel dump.  That's it!  If there is a fuel problem start with a super duper inspection of the tank. They can get caca in the system which is very fine and can pass the fuel filter gumming up the needle and seat and other internal components.   Our mutual friend Father Time misses nothing. Mitch

[telriv]

IF I remember correctly there is a fitting on the intake manifold that supplys vacuum to the vacuum canister on the right fenderwell for the HVAC system.  IF that hose is broken/leaking it will affect #8 clylinder only.  ALSO, the vacuum line screwed into the right rear of the rear carb. is for the vacuum modulator for the trans.  This is ALSO right by the feed for the #8 cly.  It coud be cracked OR fallen off at the modulator down at the trans.

Just a couple other things to check BEFORE pulling things apart for no reason.

OH, AND, the plug under the intake manifold IF rotted out/leaking would be exhaust noise NOT a vacuum leak.

 

Tom T.

[gungeey]

if memory serves correct no plug under a 2x4

[48Super]

Thanks for the advice so far. I did some more investigating which included running the engine and measuring the temperature of the exhaust manifold ports. What I found was that the #8 port was much cooler that the others on that bank. I just wanted to verify that the cylinder was not firing at all before I got deeper in the disassembly.

 

I removed the front carburetor and saw evidence of a vacuum leak at the mounting gasket. The manifold has no heat crossover channel for the front carburetor but the the standard mounting gaskets have a cut out for the channel. You can see in the picture that the cut out extends beyond the mounting surface and definitely causes a leak but that didn't explain the problem with #8 cylinder. Also, what is the correct gasket for this location? All the ones I've seen look like what I have.

 

Next, I removed the intake manifold and took a good look at all the mounting surfaces and the gaskets. The #8 cylinder surfaces looked more discolored than the others which might be a sign of a vacuum leak. I plan on installing new gaskets. The existing ones were Felpro, can anyone suggest something better. I did not use any type of sealer on these gaskets, should I and if so, what?

 

While I had everything apart, I also removed the valley cover and took a look at the #8 cylinder cam lobes and they appeared normal so I think my issue was probably due to a vacuum leak at the manifold port. 

 

Any further information or suggestions will be appreciated.

[telriv]

The carb. gasket is backwards, should be turned the other way.

 

Tom T.

[48Super]

This gasket, Felpro #9803, has a gap in the area shown regardless of orientation. Looking at pictures of mounting gaskets from some other manufactures, it appears that some are made more closely to the contour of the heat crossover channel in the manifold and those might work correctly. Is a stainless gasket also needed on top of the mounting gasket to cover the crossover channel on the rear carburetor? 

 

[1965rivgs]

Mike,

  Tom T. is correct. The cut-out in the gasket for the heat crossover should be in front of the primary butterflies. Your pic displays the cut-out behind the secondaries. Dont know if that`s the way you had it installed?

  There is no need for the stainless gasket because the exhaust never contacts the carb base on the dual 4 manifold. The parts book does spec out the stainless gasket for the rear carb however.

  The lack of a pattern on the intake other than the runner in question, the all shiny ones, is weird. Usually all the runners have the pattern displayed on the rear runner but with a better port match... if you look closely the raised bead of the gasket that is the sealing point stops leaving a pattern on the front edge of the runner. Not saying that your intake gasket was installed upside down in this case, but it is possible to do so. Whenever I do intake gaskets I always double check to be sure the gasket matches the intake runners in the head.

Tom Mooney

[48Super]

Sorry, you are both correct. I misunderstood what Tom said, thinking the gasket was flipped side to side. I did have it installed incorrectly. The rear carburetor did not have the steel heat shield installed but I have the correct part for it. The intake manifold gaskets were correctly installed, my picture was a little misleading as I had wiped the surfaces a little beforehand. The #8 port was definitely different in appearance however. Thanks for the input. Hopefully, once I correct these errors, the car will run properly. 

[telriv]

As Tom mentioned the stainless steel gasket is NOT needed for the 2x4 manifold as it has NO open passages for exhaust heat.

 

Tom T.

[RivNut]

If you're in doubt as to what the correct gaskets are for the carburetors, contact Jon Hardgrove, owner of The Carburetor Shop, Eldon, MO.  Joh does nothing more that make correct gaskets for carburetors by carb number.  

                                                                                                     Carburetor        Kit#

     1964     8   401 A/T                                     Carter    AFB               3633   1362638          859

     1964     8   401 S/T                                    Carter     AFB               3635   1362640          859

     1964     8   425                                          Carter     AFB               3635   1362640          859

     1964     8   425 (Front of 2) A/T              Carter     AFB                3645   1363267          860

     1964     8   425 (Rear of 2 - S/T)             Carter     AFB               3634   1362639          860

     1964     8   425 (Rear of 2) A/T               Carter     AFB                3646   1363267          860

     1964     8   425 A/T                                  Carter     AFB                3665   1362639          859

 

www.thecarburetorshop.com

 

 

[48Super]

Here is an update on my engine situation. I installed new intake manifold gaskets (composition type from Best Gaskets), correctly installed the carburetor mounting gaskets and put everything back together again. The engine behaves the same as before, #8 cylinder is not firing judging from the appearance of the plug compared to the others. The engine has a noticeable shake and I can feel the miss in the exhaust for that side. The cylinder compression tested good at 160 psi and I checked the cam when I had the valley cover off and it looked good and the valves seemed to move normally. I'm am out of ideas on this one. Any suggestions would be appreciated.

[TKRIV]

If #8 Cylinder was not firing then the spark plug would not be black. 

It would be clean.

Black plugs can only be if there is some sort of combustion! 

Would badly worn valve glide allow excessive oil to enter the cylinder and foul the plug?

 

Also may pay to have the intake manifold pressure tested so to absolutely rule out internal casting failure.

 

My 2 bobs worth.

Tom K

Edited April 20, 2021 by TKRIV
Typo (see edit history)

[48Super]

Yes, the #8 plug is clean and the others are not. I examined the intake manifold very carefully and it was in excellent condition,no cracks and the sealing surfaces were flat and smooth. This problem has really got me stumped.

[1965rivgs]

Hi Mike,

  Does the miss go away as the engine warms up? Did you confirm that cylinder #8 is carrying no power by shorting the plug and observing any change in RPM? Have you checked for spark at the #8 plug? If not, try using a new plug, or switching a plug with another cylinder. Also, try switching the wire with another and see if the miss moves. Have you checked for a vacuum leak associated with the vacuum fitting on the manifold, as Tom suggested, which may be leaning out the #8 fuel charge?

  It is possible to pass a compression check and still have a bad valve. A leakdown test is more likely to reveal a valve seat problem.

  If all else fails, just sell the car to me! lol...never mind, I doubt I could afford it!

Tom

 

[TKRIV]

I had a similar vibration problem on my 65GS. It had my mechanic going nuts trying to work out what the problem was.

You can see by the photo that the non working cylinder was clean. He figured that some exhaust gases are getting from

the exhaust passage through to the intake passage (within the manifold) and due to the exhaust gases having little or

no oxygen the fuel could not ignite.

Mechanic then completely blocked of the exhaust cross-overs (see photo) and the vibration problem was gone.

Strange story but true.

So now whenever my mechanic rebuilds a nailhead (he has built dozens of nailheads) he

always pressure tests the inlet manifold and blocks the inlet manifold exhaust cross-overs.

Again, Very strange story but true.

Tom K

 

[48Super]

7 hours ago, 1965rivgs said:

Hi Mike,

  Does the miss go away as the engine warms up? Did you confirm that cylinder #8 is carrying no power by shorting the plug and observing any change in RPM? Have you checked for spark at the #8 plug? If not, try using a new plug, or switching a plug with another cylinder. Also, try switching the wire with another and see if the miss moves. Have you checked for a vacuum leak associated with the vacuum fitting on the manifold, as Tom suggested, which may be leaning out the #8 fuel charge?

  It is possible to pass a compression check and still have a bad valve. A leakdown test is more likely to reveal a valve seat problem.

  If all else fails, just sell the car to me! lol...never mind, I doubt I could afford it!

Tom

 

Tom,

 

The miss stays there when the engine warms up and does not go away as the RPM's increase. I know I have spark as I disconnected the ignition wire and connected it to another plug which I laid on the exhaust manifold and I could see a healthy spark when I ran the engine. Previously, I installed a new plug, wire and cap, of course the problem was still present. It seems that I have compression and spark so that leaves fuel in question.  The intake manifold internal leakage problem posted by TKRIV is very interesting. I wish I had seen it before I reassembled everything. I did notice when I removed the manifold and I was cleaning it, a big chunk of carbon came out of one of the exhaust crossover ports so maybe there is a problem there. Last fall the car ran well and I experienced the current problem when I took it out for the first time this Spring (after just spending over $1000 on a brand new set of Diamondback triple whitewalls!)

 

I may just decide to sell the car as is rather than go through the pain and expense of rebuilding the engine. I find that as I have gotten older, I am losing my enthusiasm for dealing with problems like this.

[60FlatTop]

I had an untenable miss in my '64 Riviera a little over 20 years ago. I thoroughly checked everything I could without removing the head. It was just a slight shake, not severe and the plug was firing. In the end I am pretty sure it was a cracked piston. I have seem that happen on a few nailheads. It took my 16 year old daughter (at the time) to find it. She started it at full throttle.

 

I don't know if it would have shown if I had inspected it on top with the head off, but the first picture is what I found in the pan.

 

My first thought was a cracked vacuum hose on a manifold runner to that cylinder. Once you have done all the routine tests you have to figure it is something outside the normal and you have identified the cylinder. The head is not hard to pull. The oil pan is a bit of a pain. But you may want to side that piston out and play it safe.

 

Bernie

 

[TKRIV]

Mike, Don't give up on your investigation on why your nailhead has the vibration.

We all want to know the reason just in case in the future we have a similar problem.

Tom K

[48Super]

Sorry to say it but I'm about ready to give up on this problem. Nothing I've done so far seems to have any effect on the cylinder misfire. Now I have to decide if I want to sell the car the way it is and take the loss or spend a fortune to have the engine rebuilt.  With 100,000 miles on it, if it needs to come apart for anything, I should just do it all and get it over with. I don't have the capability of pulling the engine myself anymore so I will have to find someone in my area qualified to do the work and that will probably not be easy.

[60FlatTop]

I am a proponent of fixing the single broken or worn part. Sometimes you can replace a part and give it a little wear to match the existing wear. Years ago I had a 1956 Olds that had been in storage a lot longer than the previous owner remembered. The first week I drove it one of the valves was sticking and closed too slow, got smacked by a piston and bent. I bought one new valve and asked the shop to just do a single angle grind and lap the new valve with grinding compound. Nothing else. I drove the car another 4 years or so and sold it.

 

I your instance you know something is "amiss" with number 8 cylinder. Yes, that was a bad pun. Focus on it and let the other 7 be. Like the Olds, I have had engines with just one valve or guide. A nailhead is easy to spot. Remove the rocker arm assembly and wiggle the end of the valve stem with your finger. Bad ones are noticeable. If you have a wobbler the seat will probably be rounded and not sealing. Fix the one valve and lap a little "wear" into it. Sort of create the same volumetric efficiency of the other 7. It's worth a try.

 

If you check the valves and still find no pinpoint problem you really could pull #8 rod and piston for a thorough check without tearing it all apart.

 

One thought while typing; have you put a dial indicator on the cam lobes for #8? Don't get discouraged if it takes lots of little bites to isolate the problem. I have seem guys who thought taking everything apart and making it all new would fix it. Now there's real discouragement when it all goes back together and still doesn't work.

[48Super]

Curiosity got the better of me and I took the intake manifold off again to check it for an internal leak as previously suggested. I made a pair of simple plates to block off the exhaust crossover and pressurized the manifold to 40 psi with no leaks detected. At least I now know another thing that is not the problem.  I had used composition material intake gaskets thinking they might seal better than the original steel type gaskets. They might seal OK but they self-destruct when you remove the manifold and the next guy to disassemble things will hate you for using them (especially if you are the guy) as they are a real pain to scrape off. Not sure what to try next.

[TKRIV]

Speaking to a friend of mine who is a mechanical engineer about this problem... and then he opens a cabinet and hands me over a device that lets you look inside the combustion

chamber while the engine is running. You can see if there is combustion 

and also by the color of the flame you can adjust the A/F mixture.

I have never seen this device before.

Has any body used this diagnostic tool?

[48Super]

Here is the latest update, still no joy. While checking everything again, I found the #6 plug had a cracked insulator on the electrode end and I thought for sure that was part of the problem. Today I reassembled everything again with all new gaskets and a replacement spark plug. Engine still runs the same and the #8 plug still appears like it is not firing. I hate to sell the car but I don't think I want the hassle of dealing with it anymore.

[RivNut]

Different distributor cap. Could be cracked. Before giving up, take it to a professional.

[48Super]

I have changed the cap, wire and plug and verified that there is spark in the affected cylinder. The compression is 160 psi in that cylinder and the valve action appears normal, cam lobes are in good condition also. Its a mystery.

[telriv]

Mike,

 

   Please keep plugging away.  Has to be something simple. Could be so easy BUT we ALL miss some of  the easiest things sometimes. When I run into this I always think to myself

Use the :KISS" prinsciple "Keep it simple stupid".

    I have run into things like this MANY times throughout my carreer'

It can be so simple you almost wonder what your over looking that turns out to be something was overlooked.

 

Tom T.