I built the 400 in my 79 a couple years ago and it runs very well...until yesterday. I'm about 80 miles into this tank, and on my way home from work, I thought I felt a vibration (not in the steering wheel, but in the front oc the truck). Then the engine became evident it was running rough and was skipping and surging. I stopped. It idled pretty rough, then died. Fired right back up, ran rough, and died. Sometimes it would idle smooth for several seconds (20-30) then start surging and skipping and die. I could hold a little throttle and it would run about the same, except some backfiring in the exhaust (more like a big hiccup).

The carb is a 670 Street Avenger with nothing really done to it. The bores are really black (not sooty, more damp or so). I have always used the air filter, so that's odd.

I took the carb off and removed the bowls. Nothing inside has a dirty or clogged appearance at all.

Not sure if I should completely rebuild it. Any ideas of a more lkkely problem?

 

Might be the needle and seat hanging up. Sounds like the needle is sticking open causing the carb to flood.

 

I found it's when the engine warms up. And this just suddenly happened.

I built the engine in my 79 in 2014. 400, .030 over, 9.3:1, 218@.050 cam, Weiand Action +Plus, Holley 670 Street Avenger. I have put 8000 miles on it. I drive it to work every day and that's about all. It has run like a dream!

I was driving home from work (15 miles), made it 5 miles and it felt like I had a vibration. I got to a stop sign and could tell it was the engine running rough and sputtering. I continued on and got about a half of a mile and it was getting worse and wouldn't run right, so I pulled over. It sputtered and died. It would start, but run rough and hesitant, and die. I could give it some gas and it would hold a higher rpm, with coming-and-going rough running. But as soon as it returned to idle, it would get choppy, rough, and die. Sometimes there was a little bit of backfire with revs or rpm cycles. Just a single half-assed backfire.

In looking for the problem, I found that the distributor cap and rotor contacts were in pretty bad shape, so I scraped the globs of build-up off them. The throat of the carburetor was COMPLETELY caked in black funk. Not really sooty or gummy - kind of in between. So I figured SOMETHING must have happened to really gunk up the carburetor.

I removed the carb and took off the primary bowl to find that it is really really clean and tidy inside. No fouling anywhere and no clogged ports or vents. So I put it back on and changed the cap and rotor.

Started right up and idled what seemed a bit high. SO I let the choke warm up, and the idle still seemed high, but I found it was only around 1000 in Park. BUT, I could hear a whistle like a loud vacuum leak.

So I started checking all the lines and plugs and found that the filter on the oil cap was doing it. It's a Ford Racing cap for their aluminum valve covers with a parts-store chrome filter...one of those cone-shaped ones about 1.5" in diameter. I wrapped my hand around it and it stopped. I took the cap off and used a piece of paper to find that there's a lot of suction on that hole. So the PCV is sucking pretty good.

BUT...I was working with the idle speed as the engine warmed up. Once it warmed up it started surging and stumbling until it died. I could rev it and keep it running, but it will not idle warm. Temp was 175 and steady.

SO I've found that it suddenly does this when it gets warmed up. Any ideas????

 

Did you remove both the float needles and check for trash and reset the float level? You don't have to remove the bowls to do this. You could go to Holley's web site, they have a great support page with lots of vids on tune up and set up. Keep after it, you'll find the key!! Good luck

 

I'll do that... (??unless the fuel level is correct in the sight windows??)

 

I think I found the problem. I DO need to replace the pickup in the distributor, but there may be more to that story too. I was going to replace it anyway, and change my 13 mechanical advance to the 10 I have in the process. I brought the engine to the #1 rotor alignment and noticed the reluctor was 1/3 of the way between teeth instead of lined up with the pickup.

I went to move the pickup side-to-side and turned the reluctor! Closer looking, I found there was no pin! I was able to pull the reluctor off by hand, and the pin was down in the groove, just about even with the cover plate over the mechanical advance.

SO, my reluctor has been able to rotate on the shaft a little bit. THAT would cause the problem I have. But it has also had 2 teeth rub on the pickup. I need to figure that one out too.

Here are a few pictures. To take 1 less picture, I let the face of the pickup hold the pin so you can see the end of it worn down.

 

I experienced similar problem like this in my 86 f150 and it turned out to be a partially sheared pin on the drive gear of my distributor. I did not know what it was until the pin sheared the rest of the way about 50 miles later. The bad part about a partially sheared pin is that even if you have the distributor out and are looking right at it its hard to tell.