Internally stock 1978 Chevy 350 with an Edelbrock manifold and carb and dual exhaust in my '53 Chevy. The engine has around 70,000 miles on it. Turbo 350 trans and 3.42 rear end. I have always run 87 or 89 octane gas and no problems except under load occasional ping with 87 in the tank. I have been driving this car for at least 10 years with no problems at all. Sometime this summer I drove the car after it sat for almost 2 months (pulled out the rear end and then didn't have time to get it back together; you know how it is) and it was detonating almost continuously. I thought it was the gas so I put in a can of seafoam and topped off with super unleaded. That solved the problem Last week I filled up with 87 octane and it's pinging a lot under load again. Two nights ago I retarded the timing a couple degrees and that helped but on the way home from work yesterday it was pinging again when I was pulling off the highway and accelerating up to 55. (After driving at 70 for maybe 5 minutes on the highway.) What would make a low-compression engine that has always been fine on 87 octane suddenly ping under even mild loads? I can run 89 octane but it seems like something must be wrong to create this sudden change? It is not running hotter or colder than usual and everything else is the same. I drive the car regularly, ranging from once every week or two to every day depending on the weather and my mood, but I live less than 10 miles from where I work so I put less than 1,000 miles per year on the car.
1978. Any Chevy that came originally with EGR has way too much part throttle advance if EGR is not used or if it clogs up. If it has the original HEI, you need to get a vacuum can from a 1-to or up truck or an aftermarket one with about half the advance.
It is the original HEI although I replaced the advance weights and springs when I swapped the motor into my car. I will look into changing the vacuum advance can but it's been on there the last 10 years without this problem. I'll check the advance weights. I can see how they might be sticking and need lubed. How would valve stem seals affect this? (I'm not being sarcastic, I'm trying to learn.)
Well, if it was a local guy who wasn't into hot rods I'd say that there was carbon built up on top of the pistons but I don't think that is the case here. I make a habit of going only to the newer stations and to stations that sell a lot of gas so the gas isn't sitting in their tanks or getting water in it. There are also certain stations I won't buy gas at because of issues I've had with their gas in the past. You might try getting gas at one of the newer stations that sells Texaco or Exon or Shell or at Costco if you have a card. My 350 in my daily driver 71 likes Texaco and that is what it usually gets. It doesn't get gas from those cute old mom and pop stations that have had tanks in the ground for 40 years as that is usually where you get the bad gas around here.
oil leaking past the seals/ guides can dilute the mixture, and effectively lean out the cylinder due to different burn rates.
Common issue I have found is the advance springs get weak and let the mechanical advance come in too quick. Other items as mentioned -stuck mechanical advance -stuck vacuum advance can -distributor lock bolt loosened up and advanced the timing. Verifying the timing curve with a light would be the way to go.
Yeah it is a common issue, happened to my on two different vehicles .... it ended up being a cracked distributor cap ... hard to see but both times a carbon trail on the inside. Caused by cap hitting the firewall ... too close on one car, sloppy motor mount on the other .... ten bucks later all was good.
Could also be a vacuum leak, clogged jets, anything else to make the mixture too lean. High temperature also does it, either in combustion chamber (EGR likely), or just engine temp, though I guess you'd have noticed that.
What is the ethanol content of your fuel? Starting to see stations with e15 here. If the fuel has more ethanol than you use to see, may need to richen up your carb settings. Vehicles running higher ethanol contents need more fuel to have a good stoic. Also check your rubber fuel lines, when they switched from strait gas to e10 blend in the 90s, I used to **** the fuel line shut when the engine got hot as the e10 made my fuel lines very soft. Caused it to ping like crazy when I got on it or loaded it.
Our gas is not as good as it once was. We had to replace the fuel systems in six new crown vics after we got contaminated gas. Our samples looked like muddy water.
Wow! I am learning today. I will replace the advance springs (likely culprit) and vacuum advance can and look for vacuum leaks (also a possibility). Gas quality I can't exactly fix but at least I can keep it in mind. My car does not have the EGR valve anymore (Edelbrock manifold & carb) so that can't be it, the dist is at least two inches from the firewall so it definitely didn't hit and the dist was locked down tight when I checked and retarded the timing. As for the curve, I was able to see it advance mechanically when revved with the vacuum line plugged, but it's near impossible to actually see the numbers and it did seem a little sluggish to me so I'm starting to suspect the weights are worn or just need lubed. Speaking of which, what should one use to lube the advance weights? White lithium grease? Valve stem seals are also a definite possibility - I get that lovely blue puff when I fire it up in the morning and never really cared but didn't realize it could affect the car's running. I will also check the rubber portions of the fuel line to see if they've gone soft. Damn, I love the HAMB. Thanks guys!
And the winner is...sticking advance weights!! It took a few weeks before I had time to actually get out and look into things. There did not seem to be any vacuum leaks but when I pulled the distributor cap the weights had a light coat of surface rust and would hang up a bit when I worked them back and forth by hand. I cleaned and lubed everything and put it back together and the one time I drove it to work it did not detonate on the way home in the usual place. All is well. The car does feel sluggish, though, so I'm guessing I need to retime the distributor but I haven't had a chance to do so yet. (I am REALLY crunched for free time lately!) Thanks to all for the helpful advice.
And the winner is...sticking advance weights!! It took a few weeks before I had time to actually get out and look into things. There did not seem to be any vacuum leaks but when I pulled the distributor cap the weights had a light coat of surface rust and would hang up a bit when I worked them back and forth by hand. I cleaned and lubed everything and put it back together and the one time I drove it to work it did not detonate on the way home in the usual place. All is well. The car does feel sluggish, though, so I'm guessing I need to retime the distributor but I haven't had a chance to do so yet. (I am REALLY crunched for free time lately!) Thanks to all for the helpful advice.
Next thought...rusted weights and pivots are oft caused by electrical erosion from secondary current...rotor may be going bad and allowing some misfire thataway. Center of rotor on bottom will show burning if this is going on.
One of the first things I do if timing is the suspect, I see if the weights are moving freely, check the vacuum advance as well. Always a winner to find a no $$ fix. Bob
Along these same lines... You don't have some kind of an exhaust restriction going on do you? Car sat undriven for a couple months... Maybe some critters are building a nest or storing food in the tailpipe???
My 94 Chevy PU had the same problems. Done the same as you are speaking. Never over heated... Head gaskets leaking. Fixed it. That was my experience. Keep us posted Sent from my XT1093 using The H.A.M.B. mobile app
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