In need of a Miracle (AKA you guys)

[Siempreloco]

My 620 has been inoperable for a few months now and i CAN NOT figure out what's wrong with it.

Here's the story:

As most of you know I have two 620s, one is a little shit kicker that i got and the other one is a very clean 73 with a rebuild l16 and 4 speed, I was driving the 4 speed, having no issues, came to a stop light let off the gas pushed in the clutch hit the brakes, noticed the brakes were hard, turned down the music and realized my motor had cut off. Fired her back up with no trouble, revved it up and took off, stuck behind a car making a turn off the road, dies again when I brake. I crawl it to a friends house and look everything over, nothing is obviously wrong. Decide I can limp it home and work on it in the morning, it starts having issues when i press the gas i hear the carb jet but the engine shutters like its not firing correctly, it seems to clear up and start driving home, same problem, cuts off, wont start, starts, carb and timing seem off, crawls more and dies and wont start.

 

I brought it home and the carb looks clean, got a new starter cause i burned mine up, unrelated, new coil new condenser new rotor new distributor cap new points rewired everything checked and rewired grounds, tried my electronic ignition dizzy from my other 620 and still the most i can do is get it to fire, very rarely, idle it roughly, and if i drive, the engine and tranny still haul ass and have the power, but if i brake and the rpms go down at all, it dies, and usually it wont restart.

 

Me and my grandfather are at our wit's end, I need all the opinions and help I can get here guys.

 

I WANT MY TRUCK BACK IM GOING INSANE WITHOUT IT!

 

 

 

Edit- Also did a new ballast resistor, bypassed everything with a wire from the pos terminal on the battery to the pos terminal on the coil, still wouldnt start, it seems to start once in a blue moon under no special circumstance

[68Datsun510]

Firing order? timing correct? Is the cam set properly?

[bonvo]

stock hitachi carb?

[HRH]

Put a timing light on it while it's idling like shit, probe all four wires, see if the light pulse seems normal regardless of if the motor is running poorly. Next, pull all four plugs, what color are they? After that, do a compression test. That will give you the basic information to figure out which direction to go.

 

Report back after doing this.

 

Next, check your fuel bowl in the carburetor. Does it have fuel, and as Bonvo mentioned, what is it? Weber or Hitachi?

[Siempreloco]

Timing should be correct, though we mostly timed by ear, firing order is correct, dizzy has been replaced with top dead center position, weber 32/36 progressive carb with no issues that we have found. All plugs look clean with white tips. I don't really know if the cam is set properly, and I havn't done a compression test but I would think my compression is fine due to the engine being rebuilt and the way it runs when i'm on the gas. If we get it to start as long as we stay on the gas a little it will run like a champ forever.

Anything else to move on based on this info?

[bonvo]

white tips? sounds like your running lean to me what weber do you have? does it have the antideaseling valve? if that is disconnected it would do what your mentioning

[Siempreloco]

My grandfather is dead set that its an electrical issue and hes a pretty decent old time mechanic, but it almost seems like a giant vacuum leak or something to me that I just cant find, I've been told that perhaps there's a vacuum leak inside my brake booster, but my grandfather told me that was stupid and we didn't even check it....however if you guys second the idea I will test for it. While we were testing electrical current, tracking the movement of the 12v all the way to the points with a light up screwdriver probe looking thing of which I don't remember the proper name, he said he saw an arc down INSIDE the dizzy and he got shocked with his hand on where the hood latches, so he sworeup and down it was the dizzy, but like i said we tried the electronic ignition dizzy from the other 620 with no luck.

[Siempreloco]

white tips? sounds like your running lean to me what weber do you have? does it have the antideaseling valve? if that is disconnected it would do what your mentioning

Its a 32/36 like I said, im unsure of the antidieseling valve, i would have to see an oic of it, Im still learning a lot about working on vehicles, all i know is what i have done in my life, if i do it once i know it and ill be good at it and never forget, but like I said im still learning.

[bonvo]

give me about an hour to dig threw my photobucket and find a pic lol

[wayno]

Sounds like the antidieseling valve to me also, but mine would start no problem, just wouldn't idle till I connected the wire, then it ran fine.

[fourfiveten]

new fuel filter? I know it's basic but hey, start with the basics

[Siempreloco]

new fuel filter? I know it's basic but hey, start with the basics

its a glass cleanable one, its clean.

[HRH]

I would also check your fuel pump. Diaphragm ones can fail and suck fuel out of the bowl on shutdown, then be a pain to start back up. The anti-dieseling valve is rare on the 32/36s I've seen. Most don't have it. If it does, check 12v going to it with key on and off.

 

On the plugs, is it a white insulator with a brownish tinge to it, surrounding the electrode?

 

With the wires, the main thing you're looking for is steady spark, not an erratic light flash here and there. Basically indicating the wires are doing their job, then move down to the plugs.

[Boaty]

Well for one, brake boosters *can* cause problems. Easy check, remove brake booster vacuum hose, plug it. Problem go away? No? Replace it. Probably not a vacuum leak at the booster, then. Simple stuff - not stupid.

[Trophy24]

I vote for brake booster as well. I have seen that problem on several cars. Same symptoms. Runs ok until you try to idle. Pinch the vacuum line to the booster and see if engine speed changes while you are keeping it running off idle. It should not change. Easy test with no $ invested.

[moparvwfreak]

sounds like a massive vac leak to me as well. if it happens when you hit the brakes, check the booster. eliminate that as a possible problem first. if it still persists, then go on to other troubleshooting.

[Siempreloco]

I do have an antidieseling valve, i'm about to head to work but I updated my grandfather on everything and hes gonna check it all out, then i'm headed over after work, i'm pretty sure that's what its going to end up being, if not ill check the booster and get back to you guys asap. Thanks for everything so far.

[littlejason]

I had an intermittent problem with an anti-dieseling valve once. It was in the wiring. When you step on the brakes, the wiring would move.

[ggzilla]

The clue to the cause is the fact that the brakes immediately loose boost. The only thing that can cause that is a booster/line/checkvalve problem. This can cause a vacuum leak, poor idle, engine dying.

 

Concentrate on that.

[Ranman72]

I second the check valve for brake booster

easy just plug booster vacuum line problem goes away booster or check valve problem

problem stays booster or check valve not the problem

[Siempreloco]

Looks like a little of everything, antidieseling valve was shit, some wiring was shit, i got a new distributor too, everytihng together seems to be working, just gotta replace that valve, which i ordered already, and i also ordered some nice 8mm red plug wires for my pertronix flamethrower 2 coil and ngk plugs....goood.