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Discussion Starter #1
Hi, I looked all over and dis not see this mentioned elsewhere, so.....

1. I have the order sheet from the Dealer and the the car was built in October 1968. The heads are 48, i assume all 48 heads are "D" port, right? The date code is H208. H is the 10th letter, so does that mean the heads are October 20, 1968? Seems odd that the heads would not predate the car since I would expect those would be an inventories sub-assembly.

2. The car is trim code 250 which is blue. Does anyone make a paint that would match interior trim colors. Also, the softer parts (Seats and door panels) are noticably a darker blue than the harder parts (console, dash and kick panels) which are brighter. Was that normal?

3. I have a popping sound that is present every 5 to ten seconds or so at idle (kinda sounds like a fart) and on deceleration sounds more like a 'pop' almost like a backfire. Probably not a question for here because it can be anything. I am chasing it down, but I figured since I was here.....

Thanks all!
Mike
 

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I think H is August so your heads were cast in Aug 20th 1968.
Not sure about the blue colors.....hopefully someone else can offer some help with that part.
How do your spark plugs look?
 

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Hi, I looked all over and dis not see this mentioned elsewhere, so.....

1. I have the order sheet from the Dealer and the the car was built in October 1968. The heads are 48, i assume all 48 heads are "D" port, right? The date code is H208. H is the 10th letter, so does that mean the heads are October 20, 1968? Seems odd that the heads would not predate the car since I would expect those would be an inventories sub-assembly.

2. The car is trim code 250 which is blue. Does anyone make a paint that would match interior trim colors. Also, the softer parts (Seats and door panels) are noticably a darker blue than the harder parts (console, dash and kick panels) which are brighter. Was that normal?

3. I have a popping sound that is present every 5 to ten seconds or so at idle (kinda sounds like a fart) and on deceleration sounds more like a 'pop' almost like a backfire. Probably not a question for here because it can be anything. I am chasing it down, but I figured since I was here.....

Thanks all!
Mike

Popping sound & backfiring could be a couple things. Bad timing. Sloppy timing chain causing bad timing. Plug wires crossed. Needs a complete tune-up. Bad exhaust leak/gasket. Improper adjustment of the valves. Cam lobe worn off. Burned valves. Weak valve springs.

Check your timing, do a compression check and put a vacuum gauge on it and see what you get. Then go from there with your findings. Kinda a generic question and hard to give a solid answer.
 

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Discussion Starter #4
Thanks Jim! Yeah, I know its like asking why I have a fever. I had the car on a scope and they said it was pretty good, but number 2 cylinder was a not as happy as the rest at idle. It has MSD ignition, but you are right, I have no idea as to the health of the wires or plugs. I know I have exhaust leaks, so I will start down the road of triage.
 

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Discussion Starter #5
Thanks "Shake". H being August makes no logical sense, but that's how things are.
 

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Thanks Jim! Yeah, I know its like asking why I have a fever. I had the car on a scope and they said it was pretty good, but number 2 cylinder was a not as happy as the rest at idle. It has MSD ignition, but you are right, I have no idea as to the health of the wires or plugs. I know I have exhaust leaks, so I will start down the road of triage.


Sounds like a plan. Just have to go a step at a time and sometimes know what the problem isn't in order to learn what it is.
 

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im pretty sure "H" is the 8th letter = August.
BTW my 400, TH400 auto car shipped 10/5/68 and my #62 heads are dated H298 (August 29, 1968), and I118 (Sep 11, 1968), within 2 weeks of each other I have heard could have been normal, so I believe they are the originals, I do wish they were marked same day though.
CPG
 

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Discussion Starter #8
Thanks Cleveland, apparently I was drunk in my earlier comments, or forgot how to count to 8. Ugh, Yes H is August. Ha ha, not my finest hour!
 

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What is the compression on #2 cylinder compared to the others? In my experience with these cars, lower compression and popping out the exhaust has always been burnt exhaust valve(s). Not saying it is, but a compression test and a leakdown test of that cylinder will confirm it or rule it out. If you are running a cylinder leakdown test and hear air escaping from the exhaust pipe, it's an exhaust valve.
 

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Discussion Starter #10
What is the compression on #2 cylinder compared to the others? In my experience with these cars, lower compression and popping out the exhaust has always been burnt exhaust valve(s). Not saying it is, but a compression test and a leakdown test of that cylinder will confirm it or rule it out. If you are running a cylinder leakdown test and hear air escaping from the exhaust pipe, it's an exhaust valve.
Thanks! It will bum me out if I have to pull a head. Then again, I want to put a hotter cam in it. I will do a compression test when I am able and triage the issue. Hopefully driving it is not hurting it.
 

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Discussion Starter #11
Geeteeoguy and Pontiac Jim,

I have an update, I have replaced the plugs (the ones there seemed OK, but were gapped at .035), Wires, cap and rotor. i gapped the plugs at .045 (I have more modern GM HEI distributor). The timing is a 6 BTDC and about 29 total and is steady, does not jump around. Compression is good and there is no leak down - the compression holds until I release the pressure on the gauge. Compression is between 150 and 161 except for #3 where it is 140 and 7 where it is 145. I hear no noise in the valvetrain, but I can hear a knock which I suspect to be something in the lower end. When I stop #3 cylinder from firing, the knock is less audible.

I suspect I will have to pull the motor and then in the process of checking the lower end, I can better inspect the valves and springs. I have #48 heads on block born as a 2 barrel carb motor.

I will bump the timing up to 8 and adjust the carb some, but I suspect these will be bandaids to the bigger issue, being the knock.

Any thoughts you have hearing this related to my popping sound (muted put-put every few seconds in the tailpipe) would be appreciated, but I mainly wanted to just update you since it looks like pulling the motor is in my future - damn.

Thanks,
Mike
 

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How many miles on the engine?

With your heads, you should have 10.5 compression which is the factory rating. I show for 1968 a 2 Bbl 400 CI with 10.5 Compression rated at 290 HP/428 Ft Lbs of torque - if this is like your engine. PSI for 10.5 - 10.75 compression engines is listed as 185 - 210 PSI @ a cranking RPM of 155-165 RPM's. The '68 Service Manual says the lowest cylinder reading should be 80% of the highest cylinder reading. If you use the 185 PSI factory number, the lowest allowable number would then be 148 PSI. Number 3 & #7 cylinders falls below the lowest allowable cylinder pressure based on the 80% reading.

Initially, it does sound like you have a rod bearing going/gone. The test is as you did, pull the plug wires until the one cylinder knock changes /lessens to determine which rod bearing is going bad.

Not uncommon, and one of the most common problems, is indeed rod bearing failure on a GTO versus mom's station wagon. Many reasons, but wear is high on the list because you don't buy/own a GTO to drive it like a little old lady on her way to church. Hard driving, some over revving, burn-outs, overheating, and poor oil/filter maintenance are contributing factors. If you have an oil pressure gauge, you should also notice low oil pressure when the engine get to operating temp if it is a rod bearing.

There are other things it could be as well, but any kind of "knocking" is never good. I would not drive it until I found out what it is.

As stated earlier, a vacuum gauge will identify a burned valve - which could be the popping sound. You could also have a valve adjusted too tight - holding the valve slightly open and causing the popping noise - which would also lower compression and might be something to look into on your 2 lowest compression cylinders.
 

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Discussion Starter #13
Thanks Jim, you raise some sobering points. I had some people tell me driving it would be OK as long as I didn't push it, but my big show is coming up and requires a trip over a mountain range. I have another car, but it is a Ford and I will never hear the end of this if I don't take my Pontiac. Between funds and available time, this needs to be a Winter project. There are two 2 bbl engines, one is 10.5 CR and the other is 8.6. I have 48 heads, but I have an XM block which is not present in the 69 book, but is in the 68 and indicates it as the 8.6 compression motor. I am not 100% sure this is the original block, it is a 1969 built in 1968, so I wonder if they had some XM blocks left over.

What I had heard about the 80% rule was that if the lowest cylinder is within 80% of the highest cylinder rating and not what the factory number is. Still if this is an 8.6 compression engine, then 161 (my highest reading) does not seem too bad. Problem is, the engine was claimed to have been gone through and I have no idea what was done. An XM block would not have left the factory with 48 heads, so I guess I have a mix of components. Who knows what pistons and other parts were used....

I also thought if a valve was slightly open, there would be leakdown after reaching max compression reading. None of the 8 cylinders had any leakdown.

I just want someone to wave a magic wand so I can drive the car until I can rebuild the motor....

Thanks for the advice, i guess I have some thinking to do!
 

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The XM code for 1969 is listed as a 265HP 400CI. The correct heads should be #45 which is the small valve 8.6 compression heads. Pontiac compression is determined by the chamber cc's of the head. By adding a set of #48 heads, they will yield a higher compression UNLESS the engine was rebuilt with aftermarket pistons having a dish to lower the compression. With stock flat tops, the compression is 10.5. But one could use the smaller chambers/bigger valve #48 's, and add dished pistons to drop the compression to a more streetable compression of let's say 9.5 or less - which could explain your compression numbers, but you would not be able to confirm this without a tear down. By using the 48 heads you get the bigger valves and screw in rocker studs, so that is a plus.

The 265HP would have used the 2Bbl carb as you have pointed out. A Q-jet and corresponding intake could be easily added (if the engine doesn't already have this) with your combo if you simply wanted a little more performance. I suspect with the 2Bbl carb that the engine is crisp and responds well, but runs out of steam in the upper RPM's. The Q-Jet would still give you the crisp throttle and provide more air/fuel throughout the entire RPM range and it would not run out of steam as you hit the upper RPM factory limits.

So pulling the engine to identify where the knock is coming from could provide you with a lot of info on the engine build. I suspect you should be able to use many of the parts if the rebuild is fairly fresh and this will keep costs down. If you plan on keeping the car long term, then I personally would not hesitate to pull it and tear it down and see what I really have and then go from there. You may even want to upgrade/replace a few things when you rebuild it. :thumbsup:
 

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Discussion Starter #15
Thanks Jim,

I do believe the car was born a low horsepower 2bbl car. It does have the XM block, but I can't find the VIN on it (I believe 69s were stamped on the block) so I don't know if it is the original. I assume someone swapped the heads and put the better #48 heads on it, and someone added an Edelbrock Performer manifold and a 4bbl Holley carb. You are correct (and are thinking the same as me) in that I have no idea what they did on the bottom end. I am told the motor has 25k miles on it, but the car sat a lot and I don't know how it was treated when it was driven.

So, while I would prefer to spend the money elsewhere, I do think pulling the engine would solve a lot of unknowns. I would like to be around 10 to 10.5 compression so I could assess the pistons as well as the heads (checking the valves and springs). Bigger pluses are I would like to it to have a little lope at idle, so a cam set with lifters and push rods is in order and I have headers that I think hit the crossmember, so I want to do RA III manifolds. So pulling the engine may not be too bad if it really was done 25k miles ago, it was done right and this rod (or whatever it is) has not messed anything up. Perhaps I can get away without replacing everything and end up with an engine that gives me piece of mind and runs how I want.

Thanks for your input.

Mike
 
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