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Discussion Starter #1
I just bought a 65 GTO with 398 engine. Seller said he always used 93 octane without ethanol. I put 9 gallons of Shell V power nitro+ in with the half full tank. 5 miles later engine started acting starved then as I reached my driveway it started popping and running rough. Never heard any pinging. Any idea what happened. Obviously it had something to do with the new gas.Thanks for any help you can offer.
 

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Did the car set awhile. The problem can be many different things. I just purchased a 65 weeks back and it stopped pulling gas from tank. Long story short it had rust in it. I replaced it and sending unit along with rubber hoses and filter. I did have to clean carb out as well. First thing to do is pull filter out of carb. If its good work back to tank. If your good there look for a vacuum leak. PVC or brake booster hoses are a good starting point. Hope this helps. Good luck.
 

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Discussion Starter #3
Seller filled tank six weeks ago and had used about half when I bought it. In line filter is clean as can be. The popping sounds were like mini backfires. Thanks for your input.
 

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Have you considered fuel boiling in the lines or in the float bowl? Vapor lock issues can make my brother's 64 Tri-Power GTO exhibit similar symptoms under the right conditions.
 

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Discussion Starter #6
Update. I drained the gas tank and found nothing foreign in the fuel. I decided to try and start the engine to see if a cool engine made any difference. It started and immediately began coughing quite a bit and blowing mist back out of the carb at every cough. Could this be a timing issue with the 93 octane?
 

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Today I got all the fuel lines empty and air flowed freely thru all lines. I pushed compressed air thru the pickup into the gas tank. All good. Replaced the fuel hoses that I pulled loose to drain everything and put a mixture of 2 gallon of cam2 with 2 1/2 gal. of new 93 octane. Couldn't get the fuel pump to prime and pull fuel from rear to front. Car started but still backfired and wanted to quit running. I noticed that the clear plastic fuel filter was showing empty. Pulled the hose loose from carb and unhooked coil wire. Turned the motor over and saw fuel pumping in to my catch container. I am going to have the timing checked and maybe try a different number and see if that takes care of the problem. Any good ideas would be appreciated.
 

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Discussion Starter #9
Started the motor again. It soon became a somewhat steady pop about 1 per second at idle speed. Shut it back off.
 

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Started the motor again. It soon became a somewhat steady pop about 1 per second at idle speed. Shut it back off.
Could be a timing issue or even the timing chain.

Could be a crossed spark plug wire - causing a misfire.

However, could be a valve needing adjustment. One valve may be hanging open just enough to cause the popping back through the carb. Hopefully not a burned valve as a burned valve will also do this.
 

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I'm no expert but I might start by looking for a cracked distributor cap. You would see signs of arcing if it's cracked. An inspection of the spark plugs can tell you a lot about what's going on. Pull them one at a time so you don't cross a wire.
 

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Two things to check are the rubber fuel line at the gas tank. They crack with age and are often overlooked. Another quick and easy check for a valve that may be bent or not closing is with the engine running hold a dollar bill by one end at the exhaust tip. It should just flap but if there is a valve problem it will try to suck the bill in to the exhaust tip. On my 65 I also wrapped the fuel line with heat shielding tape it’s just over the exhaust pipe and as we know heat travels up. All this said it sounds like your carb may need a rebuilding. Check the voltage at your coil too as you may have a low voltage to the coil problem or electrical issue.
 

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Discussion Starter #14
Found the problem but not the cause. Pulled the right side valve cover and had two side by side broken rocker stud bolts. I am told they are pressed in with a special tool. Do I want a quick fix or pull both heads and have them checked properly by a machine shop? Was this coincidence putting 93 octane shell fuel in and breaking down 5 miles later? Could detonation cause this?
 

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Found the problem but not the cause. Pulled the right side valve cover and had two side by side broken rocker stud bolts. I am told they are pressed in with a special tool. Do I want a quick fix or pull both heads and have them checked properly by a machine shop? Was this coincidence putting 93 octane shell fuel in and breaking down 5 miles later? Could detonation cause this?

Bad news.

Never heard of detonation being the cause. In theory, it is possible that detonation busted off a ring land and it got jammed between the top of the piston & valve. You might consider purchasing an inexpensive bore scope and pulling a plug to look into the cylinder.

Two things come to mind right away. Was a high lift cam was installed? The lift may be too high IF long slot rockers were not installed. I am pretty sure your stock rocker arms will have a shorter slot. The high lift cam forces the rocker arm slot right into the rocker arm stud, and will bust it off. Photo #1 .

The next thing is if the engine was bored out and 400CI pistons used. The valve reliefs in the top of the piston are different from the 389CI. The angle of the valves are different as is the spacing. This changed in 1967 with the introduction of the 400CI. So pistons could have slapped the valves if the matching valve reliefs were not incorporated in the 400CI pistons.
Pic #2 & #3 .

If the valve guides were sized too tight it is possible for the valve to seize - and something is gonna let go. A bent valve can also seize up with the same results.

Removing the broken studs and replacing them is not for the amateur. I suggest pulling the head and having a shop do this - BUT after you determine what the cause was/is.

Just my thoughts on the whole thing, but it could be something completely different. :yesnod:
 

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Discussion Starter #16
My plan is to remove the heads and have a machine shop replace the rocker arm studs with threaded studs...if I find a good machine shop in the Columbus Ohio area
 

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That is an excellent plan. You may have gotten a stuck valve due to old fuel and that could have caused the problem. Odd that it happened when it did to you, but not related to your install of fresh gas, IMO. Get the studs installed and have the heads and valves thoroughly checked out.
 

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Discussion Starter #18
Spent some time trying to identify what I can. Rockers are Comp cams H 1451. Identifies as magnum steel roller tip long slot hi lift rockers with approx. 8 11/16 inch push rods. Heads have raised numbers 9778776 and A285 date. Intake looks like 977877. Block looks like 9778788 and same A285 date. I couldn't see too well but looks like two freeze plugs per side. Only numbers I could see on tranny were 8640239-1. That's all I found so far.
 

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Spent some time trying to identify what I can. Rockers are Comp cams H 1451. Identifies as magnum steel roller tip long slot hi lift rockers with approx. 8 11/16 inch push rods. Heads have raised numbers 9778776 and A285 date. Intake looks like 977877. Block looks like 9778788 and same A285 date. I couldn't see too well but looks like two freeze plugs per side. Only numbers I could see on tranny were 8640239-1. That's all I found so far.

Looks to be all 1965 pieces. The block might be 9778789 and not 9778788?
A285 = January 28 1965.

The heads are not GTO heads. The "76" 389CI heads were used on the 290/325/333 HP engines found in the full size Pontiac. GTO heads are the "77" heads. The "76" heads are listed as 10.5 compression, the "77 heads were listed as 10.75 compression.

I don't see your intake number, 977877. For 1965, the casting numbers I have are 9778817 - 2 bbl intake, 9778816 - 4 Bbl, and 9778818 - tri-power.

What is your block code? You might have the original GTO's 389 block and someone installed the big car #76 heads on it - unless the engine has been replaced with a big car 389. WT- 389/335HP manual trans, YS - 389/335HP automatic. WS - 389/360HP tri-power, YR - 389/360HP automatic.
 

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Discussion Starter #20
The block has YC stamped on the front of the passenger side and looks like numbers 262085 or 6. What does the YC mean? Thanks for all of your input.
 
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