Technical Holley backfiring, stumble

I have a situation re: carb stumble and backfire under acceleration. The engine is a Chevy 350 with a Holley that is not the original engine or carb in my Chevy. The previous owner had a friend build him the engine when he totally rebuilt the old girl from the frame up. Yes, I know, that makes the engine a wild unknown. Building an engine is not a problem in my opinion but although I'd like to build a small journal 327 for it for fun, time and space are limited for that project. I may do it next year. That is a long term issue.

My short term is this: I am having off-idle stumble and WOT (and close to WOT) backfires. I am a Q-jet and AFB type guy, this is my first Holley.

Holley Street Avenger model 83760-02.
Vacuum secondaries.
Dual fuel bowls.
Electric choke.
Lists as a 670 cfm carb which I think is too big, my math says a 600 is better.
Previous owner had set the sight glasses showing fuel level at a bit above halfway
Accelerator pump set to .015" gap
I have 'four cornered' it. I do not know the cam specs but it's a bit bumpy and the engine prefers around 950 - 1000 rpm for idle.
Pulling about 13" Hg, shaky needle.
Best vacuum is with the secondary mixture screws at only about 1/2 turn out.
Mechanically the secondaries will open by hand if the throttle is pulled to WOT first.
I have not checked for base plate vacuum leaks yet. I know this should be high on my list.
I have not jammed a paperclip on the secondary vacuum rod yet, too see if that moves. I know I need to.
It had a little dieseling after shut down. After four-cornering that seems to have stopped. Could be simultaneous events proving nothing...
Balancer has lost paint at mark and in a 6" area around it. Chipped and flaked off. I painted the mark white but there's so much shiny stuff when I hit it with the light that I think another "first thing" is to spray it orange and then get white paint in the mark and triple check timing.

Even at idle, when I manually blip the throttle, it will stumble. This made me check the accelerator pump, and yep, it was pushing on the arm. I fixed that. I took it for a drive and still had crackle in the pipes on hard accel and then a backfire in the carb at WOT.
Timing too advanced? Lean condition? Vacuum secondaries not opening? That's all I can think of. Power valves, squirters...I am an AFB and Q-jet guy so you're talkin Greek lol.

That's my situation. Burn it? Demolition derby?

 

Since you can't see the marks stick your vacuum gauge back on it and adjust the distributor until you get the highest reading, I would drop the float a little also.

 

Good idea.

I don't think I can drop float level just with the two screws though, can I? I recall seeing something about you can raise flat that way but not lower. Need to take the block off, correct?

 

Check the intake gasket and carb/intake gaskets. Super simple with some aerosol.

 

Have you checked to see that the accelerator pump actually squirts fuel when you open the throttle? With the engine not running look down the carb throat while opening the throttle. You should get a strong even squirt.

I'm not sure what you mean by a "crackle" in the exhaust. That leads me to think along the lines of ignition or a weak cylinder. Have you done a compression test. I would do one before I started checking other things.

 

Can do

 

Yes, it squirts. Blasts, even.

I can hear it popping off in the side pipes. Like popcorn pops but sharper, four or five of them. Didn't do it twice lol

You don't feel there's a carb issue here?

 

Look at the plugs. Ohm check the wires , what ignition ?

 

I’m sure it’s not your problem but none of my Holleys have a gap on the accelerator pump arm. The .015” clearance is overtravel on the arm at WOT
(wide open throttle). Incorrect interpretation of this can cause a stumble according to the Holley trouble shooting.

 

Yes, the .015 clearance is at WOT, the slightest movement of the throttle should produce gas from the discharge nozzle. And the best advice I've gotten on vacuum secondary springs is heavy car=heavy spring. My recent woes, that sounds the same at yours, was lean out due to apparent crud in the secondary metering. That said, I'd switch the accelerator pump discharge nozzle to a 35 after adjusting the pump lever.

 

I hope this is legal to post here. If not, please delete. But it does address this apparently common issue with the Avenger carb
https://www.hotrodders.com/threads/holley-670-vs-street-avenger-hesitation-fix.507033/unread

 

I am constantly running into this very issue, with guys who should know better, but they are insistent that there is supposed to be a gap at the pump lever at idle. Makes my head hurt.

 

Check your vacuum advance on the distributor.
Where is it connected?
I had a points conversion distributor that back fired when the vacuum advance was connected to the carb... never knew why

 

Why not relieve the hurt in your head?
State the facts of what you know and how you deal with it.
Right here

 

I regret to say I'm one of the guys who don't wake up in the morning knowing everything LOL

Like I say, I'm a Q-jet and AFB type of guy. I found several sources that state the .015" gap for this. If that's wrong that's wrong. I am not insistent; I don't know who you talked to about it recently, but I am as good as my info. Used to be there were ten or twelve local guys who could tell me this. Well. They are dead now. I use the internet.

It seems that this is info you might have. If I knew everything...I wouldn't be having this issue.

 

Can do but not for a while. Previous owner installed a Pertronix.

 

Carb base

 

Thanks, wow. Like I say, I think this is the wrong carb for the engine anyway so I'm not married to it.

I'm not sure that it's wise to start work on a carb that's probably always going to be too big. PO should have just used the existing Holley from the L79 he removed in my opinion, I know the mag writers disagree with that.

 

I’ve NEVER had an engine run better with manifold vacuum to the vacuum advance can on any ignition. Everyone of them ran and especially idled better on venturi vacuum.

 

The WOT backfire is the part that sticks out to me.

I have never had a problem with backfiring (either out of the exhaust or up into the intake), that wasn't timing related, except for the one time I had a bent intake valve. Other than that, every issue was ignition related, and more specifically, timing related since the ignition system was actually working fine. In fact, even the last time I started a thread on my issue getting my dual quads to run right, was mostly an ignition issue, which was brought on by me using the wrong balancer and having the incorrect TDC mark. Many a "carb problem" is actually a timing/ignition problem.

For you, I'd start with the absolute basics since it sort of feels like you and the rest of us are pissing into the wind. Put a vacuum gauge on the engine and see what kind of readings you're getting. How many inches is it reading? Is the needle holding steady or is it jumping all over the place? Then do a compression test. Are you getting acceptable compression in all cylinders? If you have a weird number there, I'd proceed to a leak down test.

Assuming all of that is good, then reset your TDC. Like, really reset it. Use a piston stop, and check your marks. From my own personal experience, are you sure you're using the proper balancer? Chevy made 2 different balancers with different timing marks. They both fit and will work, but the marks are different. You could be 20-30 degrees off just there.

Outside of that, make sure your wires and cap are in good shape and aren't crossfiring.

 

Yes, this is an unknown for sure.

13" Hg. I report a 'shaky' needle but that's subjective. I am much more used to cams that behave closer to stock, like the split duration in my old 462. It's not bouncing 2-3" either way, more like a .5" range, total.

Not sure where my compression gauge is. I swore off this old car nonsense a dozen years ago, and I gave away stuff.

I'm also hampered by no longer having a shop to work in. I'm in a one-car garage built in the '50s. So enough room to open one side door or the other, not both. I do have an option with a storage space that is almost two bays wide...but this car is so loud I'm afraid of pissing the neighbors there off, and now also everyone knows where a stealable old car is...

No idea about the balancer. I didn't install it or build this engine. But from it's appearance, it matches the age of the paint job on the engine so I'm of the opinion it was the one on the engine before the re-build. Checking vacuum while setting timing should give me a good hint re: 30* off.

I recently had multiple ignition problems from the key to coil. That's re-wired, all good to go. But I did check the wires and cap at that time. Not to say visually good parts aren't a failure point though.

 

Easy to swap and see

 

Without knowing what cam is in there, 13 inches of vacuum seems very low. Even with a pretty steady needle, the total vacuum seems low. That could be indicative of a vacuum leak, but could also be indicative of timing that's off.

 

Manifold vacuum and venturi vacuum are two different things. You don't get much venturi vacuum until there is a lot of air passing through the carburetor venturi. Venturi vacuum is what Holley uses to open the rear throttle plates on the vacuum operated 4 bbl. Ford used it on the old Load-O-Matic distributors as an indication of engine speed.

 

if the rain holds off today I'll pull the heap out, sand the balancer orange and put a couple 1" black stripes about 3" away from the mark on each side. I'll get the mark bright white, and see what's really what.

Why does the vacuum seem very low? Definitely not a stock cam. I wasn't surprised at all to see a bit less than 14".

You have made me really wonder about that balancer. I'm a reformed Buick guy. I was always happy to find a balancer lol

 

My crate 350/290 hp SBC runs 18.5 inches of vacuum. It's not a "stock" cam, but it's not a big cam either. Smooth idle, 8:1 compression.

 

I have never used anything but manifold, but then again, my knowledge is Q-jets and AFB.

My understanding is that I can use shop air to blow over those holes on the Holley, to see if my vacuum secondaries work. I don't have shop air here...I wonder if the canned air I have for interior detailing will do the trick.

 

Ah. The 30-over 455 I had built had a pretty big cam (for a Buick with stock-type pistons it was pretty big) and 10.5:1. That would do 15".

Not knowing the build of this 350 annoys me a lot.

 

I had an off-idle stumble when trying to really punch it. My 455 Buick was pretty sleepy. A whole bunch of carb changes, distributor repairs, and all kinds of other stuff improved the power but didn't fix the issue. Finally I thought maybe it was the intake gasket, but before changing it I got a compression tester and checked it. I had no compression on #4. So I added oil to the cylinder, and pressure came up to around 50. Conventional wisdom says that means it's the rings, so I pressured up the cylinder, and air was coming out of the exhaust. I removed the head and found #4 exhaust valve was carboned up and holding open. It hadn't burned the valve yet, luckily. It turned out all of the exhaust seats were hammered out of both heads, and I had almost zero close pressure as a result on all exhaust valves.

I had a lot of very knowledgeable and experienced guys trying to help me, and nobody could figure out what it was until I ran that compression test. I spent A LOT of time and a bunch of money chasing this issue.

Do a compression test. If it doesn't tell you what it is, it will at least tell you what it's not.

 

You should see the port in one of the front venturis. It takes a pretty good stream of air directed past this port to get the secondary's to open. I don't think canned air will do it.

You know the only difference between ported vacuum and manifold vacuum is that you get full advance at idle with manifold vacuum. This typically gives a smoother idle. Once you open the throttle plates the vacuum signal to the vacuum advance is exactly the same. At wide open throttle you have no vacuum advance on acceleration. Vacuum advance is there to add timing during part throttle operation to improve fuel economy.