Sbc wont idle or start unless wide open throttle

In reply to pres589 (djronnebaum) :

Could this possibly Be a result of a bad distributor cap?

Can you be more specific about what you actually did when you 'rebuilt' your engine? And please post the year and model of your engine and vehicle. I also notice that you are jumping to conclusions that several factors are not the problem. You need to double check these factors rather than simply saying that they are not the problem.

The big three are proper fuel/air delivery, compression, and ignition (timing, distributor indexing, and firing order).

So I recently put the fuel pressure regulator on and turned down the fuel pressure and that helped it enough to idle somewhat. I had to advance the distributer as far as it would go to idle somewhat smooth. Now when I give it throttle it bogs right as it hits about 1500 rpm. 

Did you put a timing light on it?

Did you put a vacuum gauge on it?

That's where I'm saying the distributor is out a tooth - if your rotor is actually pointing at #8 tower instead of #1 tower, your timing is hideously retarded and you won't be able to turn the distributor enough to get it to where it needs to be.  And it will run like crap.

In reply to SkinnyG (Forum Supporter) :

Can't you get the timing close by using a vacuum gauge and turning the distributor for max vacuum reading at idle?  Saw it on an old school site somewhere, don't know how valid it is.

You could, but what really matters is your max spark advance is what the engine wants.  There is no guarantee that setting the distributor for max vacuum at idle is the perfect timing at WOT.

If you pay attention to what you are doing on the distributor install, it's not hard to set up the base time pretty much bang on from the get-go.

^^^This

Idle timing is completely inconsequential.  The only reason the OEM says 10 degrees initial is because the dizzy is curved for 26 mechanical for a total of 36.  It's common to tune the weights/stops for (let's say) 20 mechanical so you can set initial to 16 depending on the engine combo.

If he's using a stock distributor, he should use the correct initial simply because the engine will want 36 total.

But again... unless his dizzy is pre-74, there is no such thing as being "off a tooth."  HEIs have 8 magnet points on the trigger, 8 posts, and it doesn't matter where it goes in.  If the #1 wire is plugged into the post where the rotor points at that wire at TDC, it's correct.  That's one of the joys of the HEI.  You can drop it in at any clocking to clear firewalls, intakes, or brake boosters and it will work exactly the same.

If he's using points, THEN he needs to have in on the right tooth because the housing's clocking is referenced to when the little cam breaks the points.

FMB42 said:

Can you be more specific about what you actually did when you 'rebuilt' your engine? And please post the year and model of your engine and vehicle. I also notice that you are jumping to conclusions that several factors are not the problem. You need to double check these factors rather than simply saying that they are not the problem.

The big three are proper fuel/air delivery, compression, and ignition (timing, distributor indexing, and firing order).

Agreed.  We need more info.  What all did you do? What year?  Stock assembly (other than the Holley)? Stock distributor?

Also.... Are you sure you got the ignition vac line on the correct source?  Depending on the year and the dizzy, it could require manifold vacuum or ported.

Erik_K said:

In reply to pres589 (djronnebaum) :

Could this possibly Be a result of a bad distributor cap?

Unlikely.  They rarely just "go bad."  They slowly degrade over time as the brass corrodes.

Curtis73 (Forum Supporter) said:

If the #1 wire is plugged into the post where the rotor points at that wire at TDC, it's correct.

Totally!

Unless he's working off a diagram from the interwebs, where #1 is a -supposed-to-be- a specific post, and doesn't know that the rotor is somewhere else and hasn't moved the wires accordingly.

There is a benefit to doing things in an "expected" way - it's easier to diagnose any issues.

Also why shops don't really like modified cars: "Where do I get the book on -this-?"

Rebuilt carbs almost never work right without fiddling, especially Holleys, which are right bastards to get the float height right because it seems like every time you adjust them, the gaskets tear, so you need to pull it apart again to reseal it.

Wait wait wait waityou said a Double pumper?

Replace both power valves. 

How big is the cam?  Let's say you are getting 15 in of vacuum, you will need 7.5 power valves on both ends. Bigger cam, less vacuum, smaller numerical power valve. 

You may need to remove the bowls 3 or four times as your vacuum will change a bit as you get it running properly. 

Having a cylinder cold and the rest warm to hot doesn't sound like carb tuning though.  It sounds 100% ignition OR it's a dead hole for some reason (really badly hosed up valve or piston or something).  Compression test might be a place to start.  OR swap components around and see if the dead cylinder follows.

There's probably some really good decision tree to follow on for an HEI ignition carb'd engine which might be better than 20 different suggestions all hitting at once.

Or cracked plug, or plug that got dropped and the gap closed, or bad wire...

There has to be a process of elimination, starting with all of the fundamentals.

NEVER assume something is good just because it is new.  New is the most likely time for a thing to be bad, because it is untested.

Check compression, check ignition timing and firing order, check everything.

99% of carburetor problems are ignition related.

I'm not helping any further until the O.P. has stopped spinning the "wheel-o-parts" and does some proper diagnostic.

Cylinder #1 to TDC using something in the spark plug hole and checking to see if both rockers have a little play.

Checking to see that the mark on the damper is now close or at zero on the scale.

Double-checking the firing order, verifying that it matches the distributor rotation and verifying that the rotor points to whatever terminal is being used as number one.

We know it's getting fuel.

We know some cylinders aren't firing.

This tells me that the ignition is all screwed up.

Hell, I can make an engine run with no carburetor and a can of brake cleaner. The fueling isn't THAT critical to simply keep it turning over.

In reply to ShawnG :

All true words! Assumptions are troubleshooting's worst nightmare. 

Ya, rebuilt carbs are almost always a nightmare. And using the term 'almost' is being kind.

Strange, I must be doing something wrong then. I rebuild carbs frequently for work and seldom have an issue unless something is worn out.

In reply to ShawnG :

There's a guy in New Jersey who rebuilds Quadrajets and his work is impeccable.  We sent them the carb from a Caddy 500 that was getting a cosmetic rebuild (reseal and repaint, basically) and when I started it up, it fired immediately and settled into a proper-speed idle.  Didn't even have to touch the idle mixture screws.

IIRC he had some type of carb dyno that he would use to calibrate everything before shipping out.

In reply to Pete. (l33t FS) :

Sounds like Cliff Ruggles. He really knows his Q-jets. I've bought parts from him in the past.

It's all relative, lots of mechanics don't have a clue about carburetors, points ignition, etc these days and for good reason. Hardly anyone in a dealership environment needs to know about them.

We just finished up a '78 Bronco that the local Ford dealer gave up on so the owner brought it to us. One of the complaints from the dealer mechanic was that the truck "wouldn't idle". He didn't know that you have to press the gas pedal once to set the choke. Truck started and idled just fine for me.

The latest thing I've seen from the factory with carburetor was a 1991 Toyota pickup, I'm sure there were others that held on a little longer but it's really no surprise that people these days are confused and intimidated by them.

Like Colin Chapman said: "Make it adjustable and they'll adjust it wrong"

To start, I teach my students both carbs, points, and both mechanical and vacuum advance. I use EFI terminology in doing so so leading them to the modern age is a more progressive step.  Many of their dirt bikes and lawnmowers have carbs, so I can build on what they already know.  Teaching the old stuff, the kids understand the "why," which (I believe) help them understand diagnostics better.

I enjoy carbs.  I enjoy making them run really well.

I really like Rochesters, too, because they are quite tuneable.  I have Ruggles' Rochester book, too; excellent resource.

FMB42 said:

Ya, rebuilt carbs are almost always a nightmare. And using the term 'almost' is being kind.

Not if they're rebuilt correctly.  I've been messing with engines for four decades, have rebuild hundreds of carbs and I've never had to replace one to get an engine to run well unless it was the wrong Carb for the application when I got involved.  I've had to machine new throttle shafts and bushings, seal up leaking plugs and replace the occasional float or other component  but most of the time a complete disassembly followed by a thoroughly cleaning, carefull reassembly with new gaskets and O-rings and correct adjustment is all that's needed.

I'm talking about store bought rebuilt carbs. Sorry that I wasn't clear about that. I've successfully rebuilt a large number of carbs both as a pro motorcycle mechanic and long time auto tech (probably about 100 in all). Sure, there were times that I had go back in to correct a problem or mistake that I made.

Meanwhile, the OP has yet to clarify exactly what he is working on and what he has actually done or had done (and in what order).

O.P. is probably long gone.