Carburetors, what can go wrong?

[david craft]

1. The throttle blade shaft can wear in the bore. This allows air to leak past at idle causing poor atomization. Need to replace base or put a bushing the bore.

2. Idle screws can be over tightened and destroy the lands where they seat. You'll never get a good adjustment.

3. Quadrajets can leak fuel at the bottom center of the main body at the plugs. Old hot rodders use to sand this area and epoxy.

4. The base can be bolted down so tight that the throttle blades wont close.

5. Choke coils can burn out, choke kickdown linkage bent, a whole host of other stuff.

Be sure to check rebuilts out of the box, I've seen sloppy shafts that never should be on the shelf. Folks use too much return spring because of throttle linkage friction or bad geometry. Ideally the spting should be 180 degrees from the linkage and just enough to return to idle.

A random simplified carb adjustment procedure;

Bring engine up to temp. Change oil if dirty. Make sure choke unloader had functioned and choke is off. Plug vacuum line to distributor and set timing. Be sure that RPM is below 1000. Reconnect vacuum line. Kill engine. Count how many turns (full and partial) to lightly seat idle mixture screws and return to initial setting. Record data. Fire engine. Adjust engine idle to factory setting. Slowly adjust screw inward till rpm just begins to drop and record. Open back up to full rpm and add a quater turn. Adust idle down to specs again if it has increased and repeat process. Do both sides. There should be at least a turn or so between lean drop and rich stumble, otherwise the screw seats may be compromised. This thread is open to comments, questions, and fine tuning...lol

[Lakota Wolf]

Exactly as stated about Run on. The HEI's are known for electro welding the fly weights and the standard vac. advance did have some issues. I have gone to adjustable vac. advances and have put dialectric on the weights.

[Jim Stoltz]

Thanks - I'll check that out. I used the distributor from the "new" engine, but it had been sitting for about the same amount of time (6+ years) as the motorhome.

[Bill Shockley]

I went through hell with the Holley on my '74 Dodge 360. I rebuilt it twice and it still wouldn't come of idle right.

Finally in desperation I pulled it off, completely disassembled it and boiled the component bodies in dish soap and water for one hour and then soaked them in Gunk one hour, washed them clean with clear water and took fine brass wires to all the passageways I could get to.

I put it all back together TIGHT, especially the two body sections. I used  a new carb to engine block gasket.

Now the engine purrs. I also had the intake manifold off, the gaskets were leaking and it was full of coke, some of which took a small chisel to remove.

bill

[Jim Stoltz]

I don't have a heat riser or vacuum lines to worry about. It's an extremely simple arrangement. Power brakes are hydraulic from the factory. I took out the heat riser when I put the new exhaust collectors on. Long story but I couldn't use it anyway. I have an open element air cleaner with no provision for it. Yeah, it probably takes a little while longer for it to warm up but I'm not worried about it. The electric choke works, but something is binding in the linkage so I have it propped open.

The more I read the more I think my air horn is warped. It has a "whistle" that I thought was normal, but I've read that's a tell-tale sign of it sucking air through the base and the air horn. Plus it looks like that fuel puddling down in there is spilling out of the bowls past the air horn gasket.

I'd never have guessed, but one person said it's from the design of the air cleaner hold down. The stud only goes into the horn. Some people tap them further into the body of the carb so there's not constant pressure pulling up on the middle of the air horn (which is where it seems mine is leaking). I dunno.

There are ways to straighten them out, but if I do find it's bent I'm just going to get an Edelbrock and sell this one for whatever I can get for it.I did order some pretty thick airhorn gaskets from quadrajetparts.com that I haven't installed yet so I guess I can try that first.

[david craft]

Wow ....great investigative analysis on your part. Never heard of the airhorn issues. I guess if the whole casting was flat you could mill a little. But, who could be that lucky? Drill and tap sounds good if the material is there. Just oversize the airhornj bore and jam nut that sucker down on reassembly. Drill press anybody? I'm sure Pat will issue a GORV mechanical merit badge for this regardless of outcome....you earned it!

[Bill Shockley]

Straightening the airhorn

https://www.youtube.com/watch?v=l_SgQiVf3_0

? Maybe this helps.

    b

[david craft]

Way to go bill....got me looking at carb vids for half an hour....guess i got out cheap...lol

[Jim Stoltz]

Well, I bit the bullet and sought professional help for my QJet. When looking at the Edelbrock, it's a square bore which would require an adapter on my stock, spread bore intake. And after reading about adapters, the consensus seems to be that you need to replace the intake with a proper square bore in order to get the fueling proper. And, I like the spread bore design of the QJet. When those secondaries open it sounds like the world is coming to an end. 

So....I brought it to a guy that a local engine builder recommended. The builder had done a few Pontiac engines (400 and 455 GTOs) for me so I trust his judgement. So I drove 40 miles to this guy's shop. It was immediately obvious this guy was a pro. There were carbs EVERYWHERE. All shapes and sizes, chrome ones, big ones, little ones. Tri-powers, dual-quads...anything and everything. I gave him my inspection report and he couldn't believe it. CO was 11.11% with an allowable value of 3%. He said he'll try to work with the one I gave him, but if he can't he'll dig me up another.

The list of things to do for the upcoming camping season is too long - I needed to outsource this one.

[Rich Thomas]

I don't blame you a bit Jim, From 8-10 yr's old till present I've probably rebuilt, adjusted, jetted and cleaned hundreds of carbs  very few of them with the proper tools to do the job and very few I would say ran optimal; very close i will say but not peek. It's just one of those things that a proper shop can do better than me. "Damn that's hard to admit" LOL

[Jim Stoltz]

It is tough to admit defeat. I like to *think* I can do anything. But some stuff I just need to leave to people that have seen it thousands of times before. That's why there are "pros".

I am looking forward to a good-working QJet though. Improved fuel economy and no eye-stinging-super-rich exhaust fumes.

[david craft]

I think you made a good move, back it up with a good filter away from any heat source, especially that heater hose. Id use that ceramic in the q-jet as a backup.

[Jim Stoltz]

I have an inline filter back at the tank. 

Speaking of heat - it had an electric pump at one point. I can see the empty spot on the frame rail and wiring where it used to be. They were installed to pump past vapor lock that the block-mounted pump couldn't handle. I drove it in some really hot weather last year, and in some really trying stop and go traffic without any problem. I'm not sure whether to install a new electric pump (I have one) or just roll the dice. 

My dilemma is that the electric pump manufacturer said that if used as a supplement to the stock mechanical pump to only run it in a vapor lock situation. The reason, they say, is that if the diaphragm in the stock, block-mounted pump were to fail it would fill the crankcase with fuel and destroy the engine. 

But...adding the inline electric pump also introduces a restriction that could (or probably would) strain the stock pump and lead to vapor lock that otherwise wouldn't have happened.

So I dunno what I'm going to do. This vintage 454 was infamous for vapor locking, but I don't know what the threshold is. It doesn't get super hot around here.