351m/400(tbh, not sure. Heavily modded engine). HEI distributor. Holley 670 truck avenger Carb.

So my engine ran the other day, but it was running really rich. Drove it to work fine, then on the way home from work it started up, died shortly after and then proceeded to miss badly and occasionally make a small backfire sound. Felt like it wanted to die when not giving it gas.

Got it home, pulled the sparks and they had a lot of carbon buildup. Cleaned them off and put them back in. The idle mixture screws weren't having an effect last time I had tried adjusting them and though it might be because of a vacuum leak.

Replaced ally vacuum lines, plugged unused ports and also I stalled a pcv since my engine had 2 breathers from PO.

Fast forward to today, tried to start my truck... Cranked, but not run. Checked the Carb, one of the floats was a bit high, so I lowered it, the other was a bit low so I raised it a hair. Returned idle mixture screws to base (1.5 turns out from all the way in). Cranked a few more times, still nothing.

Carb rebuild kit comes tomorrow and figured I'd give it a good cleaning and rebuild it. Any recommendations on what might be causing the engine to not start?

 

Yep.. As of last night I have a "no start" condition too. She cranks and cranks but no light-off at all...

Fuel is okay and I already swapped ICMs... I'm thinking it's a dead coil. ...Time to install the in-line tester to see if I am getting any spark... then trace the ignition system backwards to the dizzy, then to the coil.

I'm hoping not to chase my tail with two bad ICMs... figure the odds!

 

One of the field tests for a defective power valve is, turning the idle mixture screws all the way in, will have no effect. Most of the carbs have power valve protection built in by now, but a backfire through the carb used to be a dependable method of destroying the power valve. This would lead to an overly rich condition. May be a factor.

 

Did some research, the symptoms of a blown PV sounds exactly like what I have going on. Rebuild kit should be here tomorrow. Will pull the Carb and check the PV. I'll give it a quick crank or two before pulling and see if I can get it to start with the throttle wide open and the idle screws all the way in.

Just reminded me that the mechanic that looked over my truck had said that a truck should normally die when the screws are all the way in but mine didn't die.

Edit:
Let's say I had a blown PV that was dumping tons of fuel, would a vacuum leak give it enough air to run while no vacuum leak wouldn't run?

 

Rebuilt the carb, everything looked fine inside, replaced everything. Only issue I saw was that one of the Carb vacuum ports had a cover on it that had cracked pretty bad, how much of an issue do you think that would have caused? It was definitely leaking.

One weird thing... My power valves are 2.5 (new and old). Should I order a higher vacuum power valve for my 670 truck avenger?

 

Yes, the rubber port caps are prone to cracks and leaks. Can cause a big ole vacuum leak, run crappy, etc. What happens is peeps try to tune around it, or in spite of it.

The proper power valve size or vacuum opening is determined by application and, elevation. The carburetor can be somewhat tailored and used for several different engines and applications. So, assuming you are using the stock OEM carburetor, the Truck Shop manual in Specifications will list the part # (or color) of the correct power valve for your carb and truck combination, and accounting for your elevation.

If you have a modified engine, hot camshaft, custom carb, you'll have some experimenting to do. Need to see your idle RPM vacuum reading, and, at say 50 mph cruising on the highway.

2.5 will probably work, but may not be optimal. It's low enough that it won't flutter at idle and contribute to a crapp-ier idle quality, but it may not be opening quite soon enough for good performance.

Shoot for a power valve that is 2" below your steady cruise vaccuum indication and see how that works. Make sure you do this after everything else - jetting and timing and idle mixture etc, is dialed in.

 

Set the floats using the "upside down float" adjustment method and put it at 1 1/2 turns out for idle mixture.

Will be throwing the Carb back on tomorrow and cranking. Not sure what I'll do if it still won't start, the spark plugs are clean, wires are new and the hei is new too and worked before.

Is it possible the timing was set for it to run with the really rich condition and would be unsuitable for a normal mixture? What would be a good place to set the timing to?

I know these questions are kinda jumping the gun, but I've had horrible luck getting this truck running smoothly

 

If you had a vacuum leak as you mentioned, it would have been running lean. Factory spec for timing is OK, or maybe 2 to 4 degrees advanced from that, or say even 10 to 12.

Is the timing chain, cam, and crank sprocket set new? One common cause of sudden ****ery is "jumped time".

One quick way to get an engine and carb tuned well quick and accurate, is to use a Mechanic's vacuum gauge connected to a source of constant manifold vacuum. Check for steady vacuum 18" to 20" at factory idle RPM. It indicates engine health at a glance - valve issues, bad gaskets, vacuum leaks, late ignition timing, restricted exhaust etc, all kinds of stuff. This would be handy for what you're doing. They are inexpensive and easy to use.

 

The description you gave definitely sounds like its timing related. i have no experience with the HEI dizzy but i would take a closer look at the dizzy in particular. I would bet heavy its somehow directly distributor related.

 

Ugh, I don't know crap about timing. I had the timing set at a shop before but the guy said he thought I had a carb issue since the idle mixture screws were unresponsive and didn't kill the engine when all the way screwed in. This is after the previous shop I sent it to also failed to properly time it (they're the ones that installed the HEI)

Basically this all started back when I bought the truck, had a blow out of the rear intake manifold gasket. I replaced the seal but mistakenly pulled the whole dizzy and I don't know anything about timing a truck.

Sent it to a shop to get it running again after I gave up, they said it was fixed, I ran into some money issues and took me a few months to get the truck back. Got the truck back and it still didn't run. Sent it to another shop down the road and they got it running, but it ran super rich. Now, after. Replacing all the vacuum lines and rebuilding the Carb (what the previous shop thought was wrong) I'm here today trying to figure this out still -_-

 

When I mentioned "jumped timing" was referring to the timing set including chain between crank and camshaft. On high mileage engines the chain becomes stretched and will eventually jump or skip teeth on the sprockets.

Ignition timing, distributor and rotor orientation is pretty simple. Line up #1 piston at TDC on the compression stroke. Observe distributor rotor, it should be pointing at the location of the #1 plug wire terminal. Makes sense, no?

 

How would I check to see if the timing is jumped?

 

First make sure the distributor is installed correctly. Sounds like you may have had some difficulty there.

Not sure I'd necessarily trust a shop to do it correctly? Trust, but verify. It would depend on the shop. To test for worn chain:

If you remove the plugs the engine is easier to turn over with a wrench at the crank bolt. Normal rotation is clockwise. The distributor rotor itself turns counterclockwise. As you turn the crank back and forth, the distributor rotor should also turn almost immediately. If there is a lot of slop or delay before the rotor changes direction, that is an indication of a stretched timing chain. Can also remove the fuel pump and detect the amount of deflection on the chain too. Back to ignition timing though.

The #1 piston is the forwardmost on the passenger side. There are two TDC events in a 4 stroke cycle, this is where people make a mistake sometimes, one at the compression stroke, and one at the exhaust. Turn the engine over with your thumb over the spark plug hole until the compression stroke pushes it away. As you approach TDC, the "0" mark on the damper should align with the pointer. You'll have to mess around a bit and make sure the piston is exactly at TDC. When you're certain of that, observe where the distributor rotor points. Should be just shy of the #1 terminal of the distributor cap. Then make sure all the wires are in the correct firing order from this #1 point in a counterclockwise direction. The distributor body itself should have free range to be adjusted without the advance canister hitting anything. If you look at pictures of other engines of the same type, they'll all have the distributor in basically the same spot and angle.

 

You can also grab a paper towel, run some water on it, squeeze all the moisture out you can with your hand, put it in the plug hole making sure its in there but not actually down past the threads where you can lose a piece, on the compression stroke it should build up enough pressure to audibly (most of the time) pop it out of there. Just make sure its in well enough to seal but not too tight. The Chilton/Haynes repair manual have good step by step info like Tedster9 just gave but THE BEST THING TO DO IS CHECK OUT VIDEOS ON YOUTUBE, even if you don't find a video of timing your exact engine you should get the theory down fairly quickly and then have the repair manual with you to ensure you are going through all the steps. Its really easy stuff after you have done it a time or two. The thing that threw me was just how many times i had to turn the damn thing to get to the compression stroke of the first piston. Its a little like a jack in the box when you have the paper towel in there, you don't know when it will pop out and sometimes there's enough force to make a good pop and surprise the heck outta ya.