I just bought a 64 Coupe with a 350 NOM engine with 10:1 compression. It is a Southern California car, and came with about 1/4 tank of S. California gas. It ran fine around the block several times before I put in 5 gallons non-ethanol 91 octane. I noticed it started backfiring a little after this. So I added 10 gallons of 93 Octane (with ethanol) and one can of octane booster. Now it is really running bad, especially on acceleration. The fuel was not mixed just filled in the tank as I just described. I just sucked about 14 gallons back out of the tank. My question is, would you try 5 gallons of straight 93 Octane (the highest I can get here in Owasso, OK), try the 93 Octane and add 1/4 can of Octane Booster, or is it possible I threw too much Octane at it? What I was told is this is a 350 10:1, comp cams roller cam, 2.02 heads, steel crank, Arias Pistons, Pink rods, Holley Carb, and original Winters aluminum intake. It sounded very strong before I started adding gas and I have done nothing else to it. Thank you ahead of time for any thoughts and ideas. - Rob

 

10:1 compression should not be a problem with 91 or 93 octane gas. I have an LS and a stock C4 with that compression and no issues. I suspect the problem is somewhere else.

 

How long has it been sitting?

 

Agree.

Time to start troubleshooting. 10:1 shouldn't be an issue, dirt in the carb, clogged fuel filter, plugs, wires, coil, dwell, timing, etc.......

Tom

 

I have a 350 cu. in. with 10:1 forged flat tops and 64 cc heads (1.94" and 1.60" valves) with a 350hp cam in a heavier car (Gen 1 Camaro) than a C2. I can run 89 octane ethanol with no detonation problem.
I can run 87 octane ethanol in the stock 327 / 300hp in R66.
I also think you have another problem contributing to the sudden performance issue. You may have gotten a bad load of gas with water or excess alcohol or you have crap in the tank or plugged fuel filter etc..

Just my guess, worth a little less than $.02 in copper, but much more than $.02 in paper.

 

Thanks guys, it seemed to run rich in the little bit I drove it before adding any gas, so I will start with the 93 octane and the carburetor and start diagnosing. This is my first SBC with a high compression engine. I appreciate your responses!

 

Before I got it I am not sure. But since I got it it was not sitting long before the issues started. It ran fine until I added the 91 octane gas.

 

It's possible that when you added gas, you stirred up some crud that was sitting on the bottom of the tank and it's now in the carburetor.
Just food for thought.
Ted

 

10:1 is not an issue ...unless you got some bad gas. I would stick with 93 though. When you drain it out look for water. I also dont like octane booster.

 

I agree I think by adding gas you stirred up something in the tank. I would drain the take thru a strainer and see if anything comes, my bet is it will.

 

you might be able to just look in the tank.

 

You might also want to do an inspection of the rubber hoses and steel fuel line. Cracked hoses and pin holes in the fuel line will cause rough run condition. Also check your fuel pump pressure while you are at it.
Just recommendations.

 

I have a 1965 L76 which is 11:1, I run 89 octane with an octane booster that I buy from Amazon, half a bottle per tank. I also use 90 octane ethanol free at our local Sheetz station when they have it. I don't have any issues at all with the the cars performance and you shouldn't either. I would heed the advice and recommendations above. By the way, my plugs are nice and tan, your's should be too.

 

Could it have backfired and blown the carburetor power valve, that will make the engine run rich and very hard to drive.

 

I too worry how old the original tank of fuel was. Gasoline today older than 8 months old can be a problem. It turns into a gel like product that plugs up fuel lines, carbs and can even make valves stick. It cleans out pretty easy if you are cleaning the tank and lines.
If you smelled a strong odd smell from the exhaust, not a normal exhaust smell that would be one indicator. In the really bad cases the valves and pistons get gummed up with a shellac like shiny substance from old gas trying to burn.
If old gas was burned, the smell sticks around. Can't describe it but it is an odd strong smell. Once you smell it you remember it.
My hope is you have none of these issues and your fix is simple.

 

When I first put EFI on my car....it was done as part of an engine freshening and dyno sessions. The tank had been filled before I pulled the engine and was about 6 mos old when I got it running again. It fired up OK the night we dropped it back in and wired the EFI into the car. The next morning we headed off through Dallas traffic on the way to North Carolina. As the sun came up and we idled in traffic it started running terrible and missing and belching, dying etc. I finally made it to a gas station. When I opened the cap you could literally see the fuel vapor rising out of the tank with it idling (has a tank return for the EFI). The fuel seemed to be churning/boiling even after shutting it down. I got about 5 gallons of fresh fuel in it and it immediately ran better. It got better and better as I went 50 miles or so and added some more fresh. Soon it was running great and never had another issue.

The point is modern fuels don't age well..and even more...they are designed for EFI systems and to be under pressure at all times. Not sucking from a mechanical pump and dropping the boiling point on it with the lower pressure. When it was carb'd, and using a regular mechanical pump, I'd notice when I shut it off to fuel up and then restarted it would immediately do well...but after leaving the station and stopping at a light...it would try to die...as if it was vapor locking. I also noted some fuels were better than others in my area. Shell was terrible, Chevron was best, Valero and even Wal Mart did OK.

Another thing to consider is what was the volume of 93 octane usage at that station? Often that fuel sits in tanks a lot longer. I use high volume stations where the fuel is likely to be fresher.

JIM

 

I run my 1970 LT-1 (11:1) on California 91.. No additives - no problems.
"Backfire" is not a symptom of low octane.. Pinging (pre-detonation) under acceleration is.

Fred

 

So do I. It may be the gasoline but it's not the octane. You need to determine from what she suffers before you can get to the cause. Bad fuel varnish up the carb? Dirty fuel you accidentally introduced now clogging up the carb? Or an ignition or some other running problem, such as a vacuum leak or other defect that happened to rear its ugly head coterminous with your adding fuel. Many cars decide to give trouble unrelated to what you last did. That is the human condition.

Run a diagnosis. If you are not sure how and don't have help get out the shop manual (63 manual with 64 supplement, must haves) and turn to the engine diagnosis chart. Even a non-mechanic can use this to narrow down your problem to ignition, fuel, or something else.

I like to pull spark plugs and have a look. Pull one at a time, mark them so you know from what cylinder they came. Post a photo. There are some fellas here smarter than I who will help.

Dan

 

That makes ALLOT of sense. Check the fuel filter and take a sample. Also that gas station might have water in the gas. Take that sample.

 

The 327 (.040) in my car is also 10:1 compression. No problem running on 91 octane. It will run with 87 octane no problem, detonation or otherwise…….just have to be careful and go easy peasy on the go peddle.



John