MMJ

I will start from the beginning and hope you have time to read all of this. When I bought my 69 Vette a few months ago, it ran fine. It has an L46 in it. I knew almost no history on the car, but I knew that I should probably run high octane fuel in it, so I put about 5 gallons of 93 octane and added some seafoam to help clean out the carb. It still ran fine on that. One night my son had one of his buddies over to the house and they wanted to hear it run. It has headers and side exhaust so it is pretty loud. I had started it a few hours before so it wasn't completely cold, but it wasn't warm at all. I started it up, and let it run just a minute but then revved it up pretty loud to impress this kid. After that, it started running a little rougher. So the next day I backed it out of the garage to see if I could smooth it out some. It just got rougher and rougher and then started afterfiring bad. I mean loud, flames out the exhaust and everything. So the first thing I thought was the carb must need adjustment (which I later learned should not have been my first thought). It does not have the original Rochester, but a big 4 barrel Holley. So I adjusted it a little, but still afterfire. Only now it wouldn't even start. So I read up on afterfire and found that it was usually caused by a timing problem. So I checked the distributor, in which someone had installed electronic ignition. It was a Pertronix unit and it had a broken wire. I just knew that was the problem, so I ordered a new unit. When I got it in, I installed it, and the car did the exact same thing. So I pulled the number one plug got TDC on compression stroke, made sure timing mark was at zero, made sure rotor was pointing to correct wire and that all wires were in the right spot. They were. So played around with moving the distributor to advance and retard. Still nothing. So then I thought it must have jumped time somehow. So I took off fan, water pump, all pulley's, harmonic balancer and timing cover. The marks on the sprockets were right. So while I had all that off I sandblasted and painted parts and ordered a new correct timing cover. The one that was on there was a cheap chrome one with the bolt-on timing marks. So I finally got all that ready to put back on and reinstalled everything. The car had not been touched since I took all that off, but when I installed the harmonic balancer, the mark was pointing to advance. I mean exactly at the A. That kind of worried me, but I went ahead and finished reinstalling everything. I moved the mark to the zero on the new timing cover and reinstalled the distributor, on which I replaced all the electronic stuff with points and condenser. Also installed new plugs, new rotor and new distributor cap. I did not replace the coil, it still has the "flame thrower" coil that was used with the electronic ignition. I didn't think it would matter, but I don't know that. So after I finally got all that back on, it was he moment of truth. I put the key in the ignition and.... POW! Afterfire. I moved the distributor some and then would get backfire out the carb. The starter sounds like it is laboring, and I've heard that means it's too advanced, but I don't know. I feel like it should have at least been close enough to start, even if it ran rough. But it's just not happening. Does anybody have any advice on what to do next? Could I have the carb way out of wack to where it won't start? I have it set to initial setting. I checked the plugs, there is fuel getting to them. Could something have happened that night when I revved it up without it being warm? Any thoughts or ideas would be appreciated.

Mark

TimAT

Pull the #1 spark plug, put your finger over the hole and have someone bump the starter until it comes up on the compression stroke and blows your finger out of the hole. Then verify where your timing marks are and that the #1 plug wire is in the correct place on the cap. GM used a couple different balancers and front covers, and also had some timing tabs that bolted on. It's possible that someone scrambled the balancer/pointer combination in the past and then just did a "by feel" ignition timing.

If that's no help, it's time to pull the rocker covers and take a close look at the valve springs and rockers, along with the pushrods. Broken spring, bent pushrod, galled rocker can all be on the possible list.

Lourendo

How did you do the seafoam? Ive heard stories of people putting it in too fast, causing it to fill up the cylinder, which causes way too much pressure blowing valves and rings even to the point of cracking a head. Probably hyperbole but still possible.

MMJ

Thanks, guys.

I will check timing again (about 25th time) today. If nothing, I'll check rockers, rods, etc.

I mixed seafoam with gas then poured into tank.

Any other thoughts from anybody?

CA-Legal-Vette

Have you checked the spark plug wires? That was my first thought in the first part of your description. I'm a little less confident after reading he rest. Worth a look though.

I'd follow Tim's suggestions. You need to make sure that your starting point is correct.

jnb5101

After-fire can be caused by a lean condition. If you are satisfied that the dizzy is set up properly, check the carb for clogged filter, float level and debris that may been dislodged. How did you "adjust the carb"?

68/BB

99 times out of a hundred the situation you described, i.e., car runs fine, you start it, rev it, 'suddenly' starts running poorly, continues to deteriorate, etc....is going to be a moved distributor. And I say that because of the 'suddenly' part. However, it appears that you have done everything to check that out. Just curious, was the distributor clamped down good and firm when you first checked it? I guess a vacuum leak would be the next thing I would look for.

MMJ

No, I haven't checked the plug wires, but I will today. Thanks

MMJ

I just ran the adjustment screws that are on each side of the carb all the way in then back out 1-1/2 turns. The plugs are getting fuel, I checked that. Is it the right amount of fuel, though? I don't know. I guess the seafoam could have loosened up some trash in there also.

MMJ

Thanks, yes the clamp was super-tight. It was hard to get it broke loose. I have been wondering about the vacuum too. There's 14 miles of vacuum hose in these cars. I need to get a little pump-up vacuum tester and check that.

tokim

This^^, also, once it has been established you have good fuel flow, float level set right (on a Holley, remove the site plug on the fuel bowl, the fuel level should be right below that level, so when you slightly bump the car the fuel will trickle out of that hole).
If that all checks out, remove the ignition rotor and make sure your advance weights have free movement (they can become rusty, semi-stuck, weak springs, etc.), squirt them with your favorite lubricant, to allow free unrestricted movement.
Good luck, and be sure to let us know, how this all turns out.

Mike Ward

Get rid of it. Known to be a very unreliable POS.

Bagpype

I had the exact same situation you're having on a 1965 ford f100 I had years ago. I installed a pertronix points eliminator system on the distributor and it ran fine. I took it out and low and behold, the thing was doing the exact same thing yours is doing. I went through EVERYTHING! Found nothing then went after the timing. Adjusting it did nothing. I finally took the dist apart and found that the rotor was sitting a fraction of an inch too low in the cap. The pertronix came with a small rubber o ring to keep the rotor up a little higher. It sounds like you're having a timing issue. I know you replaced the electronic stuff but I'd have another go at the distributor with fresh eyes.

Dustup7T2

I agree that this section remains suspicious.

Since everything else suggested, has been tried, I haven't read that the valve covers were removed and valvetrain inspected.

MMJ

ok, I will check the level in the bowl. I already cleaned the weights in the distributor. They move freely. I guess they could nave weak springs. Thanks

MMJ

I can try another coil, I have a new 12v one in a box that's for another car.

MMJ

I had the valve cover off on the #1 side and didn't see anything suspicious, but have not checked the other side. I think that's the first thing that I'm going to do when I start on it this afternoon. What should I look for specifically?

Sunstroked

I'd try a new coil as Mike Ward suggested.

MMJ

Just checked under valve covers. Everything looks good. Changing coil next.

georgiaboy

Backfiring with a Holly Carburetor often blows the power valve and it will run very very rich. Ask me how I know. This don't sound like it started the problem, but with it backfiring so much now, it might be a new problem.